1. INTRODUCTION
⌅The
construction sector is growing at an unprecedented rate around the
world, and this trend is anticipated to continue. This rapid growth in
the construction industry in recent years, together with the need for
technically superior characteristics and values for building materials,
has resulted in the use and application of a large number of novel
building materials (11. Vairagade, V.S.; Parbat, K.; Dhale, S.A. (2015) Fly ash as sustainable material for green concrete - A state of art. Int. J. Res. Eng. Sci. Technol. 1 [2] , 17-24.
).
Over the next 40 years, the world is forecast to add 230 billion square
feet of new construction, with the building and construction industry
accounting for 39% of worldwide final carbon dioxide (CO2) emissions (22.
Dean, B.; Dulac, J.; Petrichenko, K.; Graham P. (2016) Towards a
zero-emission, efficient, and resilient buildings and construction
sector. Global Status Report. 1-48.
).
Concrete, a common construction material composed of aggregates and
cement to bind them together, is the third-largest source of
human-induced greenhouse gas emissions, behind the transportation
industry, for the main pollutant of carbon dioxide in the atmosphere (3-93.
Jaya, N.A.; Yun-Ming, L.; Cheng-Yong H.; Abdullah, M.M.AB.; Hussin K.
(2020) Correlation between pore structure, compressive strength and
thermal conductivity of porous metakaolin geopolymer. Construc. Build. Mater. 247, 118641. https://doi.org/10.1016/j.conbuildmat.2020.118641.
4.
Rifaai, Y.; Yahia, A.; Mostafa, A.; Aggoun, S.; Kadri, E.H. (2019)
Rheology of fly ash-based geopolymer: Effect of NaOH concentration. Construc. Build. Mater. 223, 583-94. https://doi.org/10.1016/j.conbuildmat.2019.07.028.
5. Cai, J.; Tan, J.; Li, X. (2020) Thermoelectric behaviors of fly ash and metakaolin based geopolymer. Construc. Build. Mater. 237, 117757. https://doi.org/10.1016/j.conbuildmat.2019.117757.
6. Singh, N.B.; Middendorf, B. (2020) Geopolymers as an alternative to Portland cement: An overview. Construc. Build. Mater. 237, 117455. https://doi.org/10.1016/j.conbuildmat.2019.117455.
7.
Ling, Y.; Wang, K.; Wang, X.; Hua, S. (2019) Effects of mix design
parameters on heat of geopolymerization, set time, and compressive
strength of high calcium fly ash geopolymer. Construc. Build. Mater. 228, 116763. https://doi.org/10.1016/j.conbuildmat.2019.116763.
8.
Wu, J.; Zhang, Z.; Zhang, Y.; Li, D. (2018) Preparation and
characterization of ultra-lightweight foamed geopolymer (UFG) based on
fly ash-metakaolin blends. Construc. Build. Mater. 168, 771-779. https://doi.org/10.1016/j.conbuildmat.2018.02.097.
9. Živica, V.; Palou, M.T.; Križma, M. (2015) Geopolymer cements and their properties: A review. Build. Res. J. 61 [2] , 85-100. https://doi.org/10.2478/brj-2014-0007.
).
Developing enduring construction and building materials with a
decreased environmental footprint through both manufacturing and
management steps is presently a focal point in the worldwide housing and
construction industry (1010. Singh, N.B. (2018) Foamed geopolymer concrete. Mater. Today Proc. 5 [7] , 15243-15252. https://doi.org/10.1016/j.matpr.2018.05.002.
).
As a result of this predicament, it is imperative that sustainable
alternatives to cement and building materials containing cement be
utilized in construction in order to preserve the long-term viability of
the environment (1111.
Srividya, T.; Kannan, P.R.; Sivasakthi, M.; Sujitha, A.; Jeyalakshmi,
R. (2022) A state-of-the-art on development of geopolymer concrete and
its field applications. Case Stud. Construc. Mater. 16, e00812. https://doi.org/10.1016/j.cscm.2021.e00812.
).
The Paris Climate Agreement, which was signed in 2015, also requires
all countries to adhere to the emission targets specified (1212.
Rashidi, S.; Esfahani, J.A.; Karimi, N. (2018) Porous materials in
building energy technologies-A review of the applications, modelling and
experiments. Renew. Sustain. Energy Rev. 91, 229-247. https://doi.org/10.1016/j.rser.2018.03.092.
).
Alkali-activated
materials (AAM), generally called geopolymers, are a relatively new
class of attractive construction material that offer a solution to limit
the adverse results of cement manufacturing. These materials are
gaining popularity due to their environmental and performance advantages
over standard Portland cement concrete and cement-based construction
materials (1313.
Glasby, T.; Day, J.; Genrich, R.; Kemp, M. (2015) Commercial scale
geopolymer concrete construction. The Saudi International Building and
Constructions Technology Conference 1-11.
). Alkali
activated concretes (AAC) have become an issue of great importance due
to their widely accepted identification as “environmentally friendly” or
“green” construction materials compared to cement-based concretes due
to their low CO2 emission potential and ensuring recycling of industrial by-products (14-1814.
Dong, M.; Feng, W.; Elchalakani, M.; Li, G.K.; Karrech, A.; Sheikh,
M.N. (2020) Material and glass-fibre-reinforced polymer bond properties
of geopolymer concrete. Mag. Concr. Res. 72 [10] , 509-525. https://doi.org/10.1680/jmacr.18.00273.
15.
Aziz, I.H.; Al Bakri Abdullah, M.M.; Heah, C.Y.; Liew, Y.M. (2020)
Behaviour changes of ground granulated blast furnace slag geopolymers at
high temperature. Adv. Cem. Res. 32 [10] , 465-475. https://doi.org/10.1680/jadcr.18.00162.
16. Kalaiyarrasi, A.R.R.; Partheeban, P. (2019) Mechanical and microstructural properties of metakaolin geopolymer. Emerg. Mater. Res. 8 [2] , 275-282. https://doi.org/10.1680/jemmr.17.00019.
17.
Zhang, J.; Li, S.; Li, Z.; Gao, Y.; Liu, C.; Qi, Y. (2021) Workability
and microstructural properties of red-mud-based geopolymer with
different particle sizes. Adv. Cem. Res. 33 [5] , 210-223. https://doi.org/10.1680/jadcr.19.00085.
18.
Dadsetan, S.; Siad, H.; Lachemi, M.; Sahmaran, M. (2019) Construction
and demolition waste in geopolymer concrete technology: A review. Mag. Concr. Res. 71 [23] , 1232-1352. https://doi.org/10.1680/jmacr.18.00307.
).
There are many groups in the scientific and commercial communities
trying to improve new binders with the alkaline activation of industrial
by-products (3-83.
Jaya, N.A.; Yun-Ming, L.; Cheng-Yong H.; Abdullah, M.M.AB.; Hussin K.
(2020) Correlation between pore structure, compressive strength and
thermal conductivity of porous metakaolin geopolymer. Construc. Build. Mater. 247, 118641. https://doi.org/10.1016/j.conbuildmat.2020.118641.
4.
Rifaai, Y.; Yahia, A.; Mostafa, A.; Aggoun, S.; Kadri, E.H. (2019)
Rheology of fly ash-based geopolymer: Effect of NaOH concentration. Construc. Build. Mater. 223, 583-94. https://doi.org/10.1016/j.conbuildmat.2019.07.028.
5. Cai, J.; Tan, J.; Li, X. (2020) Thermoelectric behaviors of fly ash and metakaolin based geopolymer. Construc. Build. Mater. 237, 117757. https://doi.org/10.1016/j.conbuildmat.2019.117757.
6. Singh, N.B.; Middendorf, B. (2020) Geopolymers as an alternative to Portland cement: An overview. Construc. Build. Mater. 237, 117455. https://doi.org/10.1016/j.conbuildmat.2019.117455.
7.
Ling, Y.; Wang, K.; Wang, X.; Hua, S. (2019) Effects of mix design
parameters on heat of geopolymerization, set time, and compressive
strength of high calcium fly ash geopolymer. Construc. Build. Mater. 228, 116763. https://doi.org/10.1016/j.conbuildmat.2019.116763.
8.
Wu, J.; Zhang, Z.; Zhang, Y.; Li, D. (2018) Preparation and
characterization of ultra-lightweight foamed geopolymer (UFG) based on
fly ash-metakaolin blends. Construc. Build. Mater. 168, 771-779. https://doi.org/10.1016/j.conbuildmat.2018.02.097.
).
The
term geopolymer was first used by the French scientist Joseph
Davidovits to measure the tridimensional structure of inorganic
polymers, which are formed by low-temperature polycondensation of
aluminosilicates (1919. Davidovits, J. (2008) Geopolymer chemistry and applications. Institut Géopolymère, Saint-Quentin, (2008).
).
There are a wide variety of potential solid aluminosilicate raw
materials for the synthesis of inorganic polymers such as fly ash, blast
furnace slag, metakaolin, construction and demolition wastes, and red
mud (19-2219. Davidovits, J. (2008) Geopolymer chemistry and applications. Institut Géopolymère, Saint-Quentin, (2008).
20.
Giannopoulou, I.; Panias, D. (2007) Structure, design and applications
of geopolymeric materials. Conference: 3rd Int. Conf. on Def. Pro. and
Str. of Mat. 5-15.
21. Provis, J.L.; Deventer, S.J. (2014) Alkali activated materials state-of-the-art report, RILEM Melbourne, Australia, (2014).
22. Abdollahnejad, Z. (2016) Development of foam one-part geopolymersUniversidade do Minho, Portugal, (2016).
).
Geopolymer-based inorganic materials have a wide variety of application
areas, including fire-resistant materials, thermal insulation, building
materials, water purification, and so on (1919. Davidovits, J. (2008) Geopolymer chemistry and applications. Institut Géopolymère, Saint-Quentin, (2008).
, 2323.
Lemougna, P.N.; Wang, K-t.; Tang, Q.; Melo, U.C.; Cui, X-m. (2016)
Recent developments on inorganic polymers synthesis and applications. Ceram. Int. 42 [14] , 15142-15159. https://doi.org/10.1016/j.ceramint.2016.07.027.
, 2424.
Provis, J.L.; Van Deventer, J.S.J. (2009) Geopolymers : structure,
processing, properties and industrial applications. Woodhead Publishing,
Cambridge, (2009).
).
Masonry units are an
ancient and fundamental construction and building material used all over
the world. Earth masonry units (adobe or compressed earth blocks)
account for a significant portion of the built environment today.
Earth-based buildings house from one-third to half of the world’s
population (2525.
Pacheco-torgal, F.; Lourenȯ, P.B.; Labrincha, J.A.; Kumar, S. (2014)
Eco-efficient masonry bricks and blocks design, properties and
durability. Woodhead Publishing, UK, (2014).
) mainly
for houses and apartment buildings. These construction technologies are
extensively employed world- wide both in developed and developing
countries, sometimes due to strong cultural aspects, long time tradition
or even as a symbol of solid construction. Moreover the good
performance of masonries is well known by the users, including its
strength, ther- mal and acoustical properties. In developing countries
besides the former reasons, brickwork or blockwork are the only suitable
methods for self-construction in non- industrialized circumstances.
However the traditional masonry units are usually non-eco-friendly
products, mainly because of their high energy consuming components due
to their production method (fired-clay bricks. Bricks can be composed of
a variety of materials, including calcium silicate, concrete, and clay,
while clay and concrete bricks are the most common. Clay brick
manufacture, on the other hand, necessitates high-temperature (900-1000oC) kiln burning, which emits a significant amount of greenhouse gases (2626. Ahmari, S.; Zhang, L. (2012) Production of eco-friendly bricks from copper mine tailings through geopolymerization. Construc. Build. Mater. 29, 323-331. https://doi.org/10.1016/j.conbuildmat.2011.10.048.
).
According to the World Business Council for Sustainable Development
Agency (2017) and the cement technology roadmap by 2050 research, the
contribution of ordinary Portland cement (OPC) production worldwide to
greenhouse gas emissions is estimated to be approximately 1.35 billion
tons annually or approximately 7% of the total greenhouse gas emissions
into the earth’s atmosphere (2727. Schneider, M.; Romer, M.; Tschudin, M.; Bolio H. (2011) Sustainable cement production-present and future. Cem. Concr. Res. 41 [7] , 642-650. https://doi.org/10.1016/j.cemconres.2011.03.019.
).
In light of all this information, sustainable concretes should be
created with readily available resources on earth, recycled materials
with low energy requirements, and generated with little or no waste, in
order to have the least possible environmental impact (28-3028.
Ahmed, H.U.; Mohammed, A.A.; Rafiq, S.; Mohammed, A.S.; Mosavi A.; Sor
N.H.; Qaidi, S.M.A. (2021) Compressive strength of sustainable
geopolymer concrete composites: A state-of-the-art review. Sustain. 13 [24] , 13502. https://doi.org/10.3390/su132413502.
29.
Luhar, I.; Luhar, S.; Abdullah, M.M.A.B.; Razak, R.A.; Vizureanu, P.;
Sandu, A.V.; Matasaru, P-D. (2021) A state-of-the-art review on
innovative geopolymer composites designed for water and wastewater
treatment. Materials. 14 [23] , 7456. https://doi.org/10.3390/ma14237456.
30.
Ghisellini, P.; Ripa, M.; Ulgiati, S. (2017) Exploring environmental
and economic costs and benefits of a circular economy approach to the
construction and demolition sector. A literature review. J. Clean Product. 178, 618-643. https://doi.org/10.1016/j.jclepro.2017.11.207.
).
Mined raw materials and non-renewable energy are used in the cement
industry. The utilization of industrial by-products can greatly reduce
the amount of energy used in these processes, as well as waste
management. Through the use of waste materials, geopolymer technology
reduces greenhouse gas emissions and lowers raw material costs (1111.
Srividya, T.; Kannan, P.R.; Sivasakthi, M.; Sujitha, A.; Jeyalakshmi,
R. (2022) A state-of-the-art on development of geopolymer concrete and
its field applications. Case Stud. Construc. Mater. 16, e00812. https://doi.org/10.1016/j.cscm.2021.e00812.
, 3131.
Almutairi, A.L.; Tayeh, B.A.; Adesina, A.; Isleem, H.F.; Zeyad, A.M.
(2021) Potential applications of geopolymer concrete in construction: A
review. Case Studies Constuc. Mater. 15, e00733. https://doi.org/10.1016/j.cscm.2021.e00733.
).
Economic
and environmental restrictions are becoming more prevalent nowadays,
and one of the implications for the construction sector will be a demand
for more energy-efficient building and construction materials (3232. Malhotra, M.V. (2002) Introduction: sustainable development and concrete technology. ACI Conc. Int. 24 [7] , 22.
). When it comes to energy leaks in buildings, it’s estimated that the walls account for about 40% of the heat loss (33-3533. Asadi, I.; Shafigh, P.; Abu Hassan, Z.F.B.; Mahyuddin, N.B. (2018) Thermal conductivity of concrete - A review. J. Build. Eng. 20, 81-93. https://doi.org/10.1016/j.jobe.2018.07.002.
34.
Yatsenko, E.A.; Goltsman, B.M.; Smoliy, V.A.; Kosarev A.S. (2016)
Investigation of a porous structure formation mechanism of a foamed slag
glass based on the glycerol foaming mixture. Res. J. Phar, Bio. Chem. Sci. 7 [5] , 1073-1081.
35.
Abdollahnejad, Z.; Pacheco-Torgal, F.; de Aguiar, J.B. (2015)
Development of foam one-part geopolymers with enhanced thermal
insulation performance and low carbon dioxide emissions. Adv. Mat. Res. 1129, 565-572. https://doi.org/10.4028/www.scientific.net/AMR.1129.565
). When masonry units (adobe bricks or compressed
earth blocks) used as construction wall elements are examined for
thermal insulation, it is well known that they have very low results.
Thermal insulation materials are the most powerful approach to reducing
heat loss from buildings, hence diminishing heat energy requirements and
contributing to the near-zero energy objective (3636.
Pacheco-Torgal, F. (2014) Eco-efficient construction and building
materials research under the EU Framework Programme Horizon 2020. Construc. Build. Mater. 51, 151-162. https://doi.org/10.1016/j.conbuildmat.2013.10.058.
).
As a result, insulating materials are required in addition to
construction materials. Despite XPS and EPS providing excellent
insulation and having extremely low-density values, their limited fire
resistance makes them unsuitable for use in buildings. Aside from the
fact that glass wool and stone wool are useful in terms of density,
thermal insulation, and non-flammability, the materials employed for
bonding during application cause non-flammability to be compromised (37-4037.
Bicer, A.; Kar, F. (2017) Thermal and mechanical properties of gypsum
plaster mixed with expanded polystyrene and tragacanth. Ther. Sci. Eng. Prog. 1, 59-65. https://doi.org/10.1016/j.tsep.2017.02.008.
38. Singh, B.; Gupta, M.; Chauhan, M.; Bhattacharyya, S.K. (2020) Lightweight geopolymer concrete with EPS beads. IOP Conf. Series. Mater. Sci. Engineer. 869, 032048. https://doi.org/10.1088/1757-899X/869/3/032048.
39.
Zhao, Y.; Jow, J.; Cai, X.; Lai, S. (2015) Fly ash-based geopolymer
foam technology for thermal insulation and fire protection applications.
World of Coal Ash (WOCA) Conference.
40. Zhang, Z.; Provis, J.L.;
Reid, A.; Wang, H. (2014) Geopolymer foam concrete: An emerging material
for sustainable construction. Construc. Build. Mater. 56, 113-127. https://doi.org/10.1016/j.conbuildmat.2014.01.081.
). Polystyrene and polyurethane are also poisonous, and they release poisonous smoke when they burst into flames (4141. Pacheco-Torgal, F.; Jalali, S.; Fucic, A. (2012) Toxicity of building materials. Woodhead Publishing, USA, (2012).
).
Geopolymer foams have drawn the attention of researchers and studies
were carried out for many years due to better properties (thermal
stability, inflammability, and green production process) compared to
conventional building insulation materials (42-4642.
Samson, G.; Cyr, M. (2017) Porous structure optimisation of
flash-calcined metakaolin / fly ash geopolymer foam concrete geopolymer
foam concrete. Eur. J. Env. Civil Eng. 22 [12] , 1482-1498. https://doi.org/10.1080/19648189.2017.1304285.
43. Novais, R.M.; Pullar, R.C.; Labrincha, J.A. (2019) Geopolymer foams: An overview of recent advancements. Progress Mat. Sci. 109, 100621. https://doi.org/10.1016/j.pmatsci.2019.100621.
44. Yang, T.; Chou, C.; Chien, C. (2012) The effects of foaming agents and modifiers on a foamed-geopolymer. Adv. Civil. Env. Mat. Res. 905-914.
45.
Bajare, D.; Vitola, L.; Dembovska, L.; Bumanis, G. (2019) Waste stream
porous alkali activated materials for high temperature application. Front. Mat. 22 [6] , 1-13. https://doi.org/10.3389/fmats.2019.00092.
46.
Azimi, E.A.; Al Bakri Abdullah, M.M.; Ming, L.Y.; Yong, H.C.; Hussin,
K.; Aziz, I.H. (2015) Review of geopolymer materials for thermal
insulating applications. Key Eng. Mater. 660, 17-22. https://doi.org/10.4028/www.scientific.net/KEM.660.17.
).
Wall elements with thermal insulation capabilities must have high
closed porosity, low density, and low water absorption to achieve the
necessary thermal insulation performance (4747.
Mattila, H. (2017) Moisture behavior of building insulation materials
and good building practices. Conference: Rakennusfysiikka 2017 -
Building Physics.
). Aside from that, lightweight
masonry modules are frequently used in the construction sector to reduce
dead load and improve earthquake safety. Although density values range
from 300 to 2000 kg/m3 depending on the application, those with a density of 300-1200 kg/m3 are favored, with a strength value of 1-100 MPa desired (4848. Vandanapu, S.N.; Krishnamurthy, M. (2018) Seismic performance of lightweight concrete structures. Adv. Civil Eng. 2018, 2105784. https://doi.org/10.1155/2018/2105784.
). For the foaming of lightweight geopolymer foams, the direct foaming approach was recommended in various research (4444. Yang, T.; Chou, C.; Chien, C. (2012) The effects of foaming agents and modifiers on a foamed-geopolymer. Adv. Civil. Env. Mat. Res. 905-914.
, 49-5249. Vaou, V.; Panias, D. (2010) Thermal insulating foamy geopolymers from perlite. Minerals Eng. 23 [14] , 1146-1151. https://doi.org/10.1016/j.mineng.2010.07.015.
50.
Novais, R.M.; Buruberri, L.H.; Ascensão, G.; Seabra, M.P.; Labrincha,
J.A. (2016) Porous biomass fly ash-based geopolymers with tailored
thermal conductivity. J. Clean Prod. 119, 99-107. https://doi.org/10.1016/j.jclepro.2016.01.083.
51.
Korat, L.; Ducman, V. (2017) The influence of the stabilizing agent SDS
on porosity development in alkali-activated fly-ash based foams. Cem. Conc. Comp. 80, 168-174. https://doi.org/10.1016/j.cemconcomp.2017.03.010.
52.
Cilla, M.S.; Morelli, M.R.; Colombo, P. (2014) Open cell geopolymer
foams by a novel saponification/peroxide/gelcasting combined route. J. Europ. Ceram. Soc. 34 [12] , 3133-3137. https://doi.org/10.1016/j.jeurceramsoc.2014.04.001.
).
Porous materials are created by adding air into a suspension or liquid
medium through direct foaming processes. The amount of gas contained in
the geopolymer slurry influences overall porosity, and pore size is
linked to setting characteristics (5353. Studart, A.R.; Gonzenbach, U.T.; Tervoort, E.; Gauckler, L.J. (2006) Processing routes to macroporous ceramics: A review. J. Am. Cer. Soc. 89 [6] , 1771-1789. https://doi.org/10.1111/j.1551-2916.2006.01044.x.
). The size and distribution of the cured foams are influenced by the blowing agent used and the amount added (5454.
Masi, G.; Rickard, W.D.A.; Vickers, L.; Bignozzi, M.C.; Van Riessen, A.
(2014) A comparison between different foaming methods for the synthesis
of light weight geopolymers. Ceram. Int. 40 [9] , 13891-13902. https://doi.org/10.1016/j.ceramint.2014.05.108.
). Foaming is commonly accomplished by mechanically mixing H2O2 (4949. Vaou, V.; Panias, D. (2010) Thermal insulating foamy geopolymers from perlite. Minerals Eng. 23 [14] , 1146-1151. https://doi.org/10.1016/j.mineng.2010.07.015.
), metallic Al (4040.
Zhang, Z.; Provis, J.L.; Reid, A.; Wang, H. (2014) Geopolymer foam
concrete: An emerging material for sustainable construction. Construc. Build. Mater. 56, 113-127. https://doi.org/10.1016/j.conbuildmat.2014.01.081.
), metallic Si (5555.
Medri, V.; Papa, E.; Dedecek, J.; Jirglova, H.; Benito, P.; Vaccari,
A.; Landi, E. (2013) Effect of metallic Si addition on polymerization
degree of in situ foamed alkali-aluminosilicates. Ceram. Int. 39 [7] , 7657-7668. https://doi.org/10.1016/j.ceramint.2013.02.104.
), and sodium perborate (5656. Svingala, F.R. (2009) Alkali activated aerogels, Rochester Institute of Technology, Rochester (2009).
)
into the geopolymer sludge. The degradation of the additives in the
alkaline sludge results in the formation of a bubble and a porous layer.
In Equations [1] , [2] , and [3] , a bubble and porous structure are created because of the degradation of the additives in the alkaline sludge:
Due to the
general high gas-liquid contact area, liquid foams are thermodynamically
unstable. As a result, several physical processes like drainage,
creaming, and Ostwald ripening destabilize the slurry, causing large
pores to form. Surfactants are required to maintain system stability,
manage the pore size distribution, and lower the angle at the gas-liquid
interface. Surfactants keep gas bubbles stable in the system by
preventing them from aggregating. Surfactants such as butyric acid,
valeric acid, butyl gallate, propyl gallate, hexylamine, and calcium
stearate are commonly utilized (57-6257.
Gonzenbach, U.T.; Studart, A.R.; Tervoort, E.; Gauckler, L.J. (2006)
Stabilization of foams with inorganic colloidal particles. J. Am. Cer. Soc. 22 [26] , 10983-10988. https://doi.org/10.1021/la061825a.
58.
Maryoto, A.; Setijadi, R.; Widyaningrum, A.; Waluyo, S. (2020) Drying
shrinkage of concrete containing calcium stearate, (Ca(C18H35O2)2), with ordinary Portland cement (OPC) as a binder: Experimental and modelling studies. Molecules. 25 [21] , 4880. https://doi.org/10.3390/molecules25214880.
59. Nemati, C.M.; Naseroleslami, R.; Shekarchi, M. (2019) The impact of calcium stearate on characteristics of concrete. Asian J. Civil Eng. 20, 1007-1020. https://doi.org/10.1007/s42107-019-00161-x.
60. Maryoto, A. (2015) Improving microstructures of concrete using Ca(C18H35O2)2. Procedia Eng. 125, 631-637. https://doi.org/10.1016/j.proeng.2015.11.086.
61. Kurtulus, C.; Baspınar, M.S. (2020) Effect of calcium stearate on the thermal conductivity of geopolymer foam. J. Turkish Chem. Soc. 7 [2] , 535-544. https://doi.org/10.18596/jotcsa.660727.
62. Zhang, X.; Bai, C.; Qiao, Y.; Wang, X.; Jia, D.; Li, H.; Colombo, P. (2021) Porous geopolymer composites: A review. Comp. Part A: App. Sci. Manufac. 150, 106629.
).
Another
technique to improve energy efficiency in buildings is to incorporate
lightweight aggregates to increase porosity and acquire low density
construction material at the same time. One of the materials used for
this is expanded perlite (4949. Vaou, V.; Panias, D. (2010) Thermal insulating foamy geopolymers from perlite. Minerals Eng. 23 [14] , 1146-1151. https://doi.org/10.1016/j.mineng.2010.07.015.
, 63-6563. Demir, İ.; Baspınar, S.; Kahraman, E. (2018) Production of insulations and construction materials from expanded perlite. Lecture Notes Civil Eng. 6, 24-32.
64. Sriwattanapong, M.; Sinsiri, T.; Pantawee, S. (2013) A study of lightweight concrete admixed with perlite. Suranaree J. Sci. Technol. 20 [3] , 227-234.
65.
Sengul, O.; Azizi, S.; Karaosmanoglu, F.; Tasdemir, M.A. (2011) Effect
of expanded perlite on the mechanical properties and thermal
conductivity of lightweight concrete. Energy Build. 43 [2-3] , 671-676. https://doi.org/10.1016/j.enbuild.2010.11.008.
).
Expanded perlite is a siliceous volcanic glass with a large volume that
expands significantly when heated. Its volume grows 4-20 times when
heated above 870 oC (6666. Chandra, S.; Berntsson, L. (2002) Lightweight aggregate, Noyes Publications, New York, (2002).
).
The thermal insulation qualities of expanded perlite are greatly
improved as a result of this increase in volume and porous structure.
Furthermore, compared to the density of conventional perlite, expanded
perlite has very low density. Compared to alternative materials
including exfoliated vermiculite, expanded clay or shale, pumice, and
mineral wool, the low cost of expanded perlite may be a significant
advantage (6565.
Sengul, O.; Azizi, S.; Karaosmanoglu, F.; Tasdemir, M.A. (2011) Effect
of expanded perlite on the mechanical properties and thermal
conductivity of lightweight concrete. Energy Build. 43 [2-3] , 671-676. https://doi.org/10.1016/j.enbuild.2010.11.008.
).
The
current consensus is that there are many variables affecting the
properties and performances of fresh and hardened geopolymeric
materials, including alkaline concentration, curing temperature, curing
duration, Na2O/SiO2 ratio, SiO2/Al2O3 ratio, H2O/Na2O ratio, and additives (1919. Davidovits, J. (2008) Geopolymer chemistry and applications. Institut Géopolymère, Saint-Quentin, (2008).
, 67-6967.
Gunasekara, C.M. (2016) Influence of properties of fly ash from
different sources on the mix design and performance of geopolymer
concrete, Engineering and Health RMIT University, Australia, (2016).
68.
Tchakouté, H.K.; Rüscher, C.H. (2017) Mechanical and microstructural
properties of metakaolin-based geopolymer cements from sodium waterglass
and phosphoric acid solution as hardeners: A comparative study. App. Clay Sci. 140, 81-87. https://doi.org/10.1016/j.clay.2017.02.002.
69. De Vargas, A.S.; Dal Molin, D.C.C.; Vilela, A.C.F.; da Silva, F.J.; Pavão, B.; Veit, H. (2011) The effects of Na2O/SiO2 molar ratio, curing temperature and age on compressive strength,
morphology and microstructure of alkali-activated fly ash-based
geopolymers. Cem. Conc. Comp. 33 [6] , 653-660. https://doi.org/10.1016/j.cemconcomp.2011.03.006.
).
Although many studies were performed about improving the mechanical
properties of geopolymer materials and geopolymer foams, different foam
forming methods (4949. Vaou, V.; Panias, D. (2010) Thermal insulating foamy geopolymers from perlite. Minerals Eng. 23 [14] , 1146-1151. https://doi.org/10.1016/j.mineng.2010.07.015.
, 7070.
Medri, V.; Papa, E.; Mazzocchi, M.; Laghi, L.; Morganti, M.;
Francisconi, J.; Landi, E. (2015) Production and characterization of
lightweight vermiculite/geopolymer-based panels. Mater. Des. 85, 266-274. https://doi.org/10.1016/j.matdes.2015.06.145.
, 7171.
Zhang, Z.; Wang, H. (2016) The pore characteristics of geopolymer foam
concrete and their impact on the compressive strength and modulus. Front. Mat. 3, 38. https://doi.org/10.3389/fmats.2016.00038.
), stabilizing foam cells (7272.
Bai, C.; Colombo, P. (2017) High-porosity geopolymer membrane supports
by peroxide route with the addition of egg white as surfactant. Ceram. Int. 43 [2] , 2267-2273. https://doi.org/10.1016/j.ceramint.2016.10.205.
, 7373.
Cui, Y.; Wang, D.; Zhao, J.; Li, D.; Ng, S.; Rui, Y. (2018) Effect of
calcium stearate based foam stabilizer on pore characteristics and
thermal conductivity of geopolymer foam material. J. Build. Eng. 20, 21-29. https://doi.org/10.1016/j.jobe.2018.06.002.
), and examining the thermal properties of geopolymer foams (6161. Kurtulus, C.; Baspınar, M.S. (2020) Effect of calcium stearate on the thermal conductivity of geopolymer foam. J. Turkish Chem. Soc. 7 [2] , 535-544. https://doi.org/10.18596/jotcsa.660727.
, 7474. Bai, C.; Colombo, P. (2018) Processing, properties and applications of highly porous geopolymers : A review. Ceram. Int. 44 [14] , 16103-16118. https://doi.org/10.1016/j.ceramint.2018.05.219.
),
there are no guidelines and comprehensive publications examining the
mechanical, physical and thermal properties of many samples.
So
far, investigations of alkali activated materials generated utilizing
slag and fly ash as raw materials has continued, significantly in
relation to microstructures, workability, mechanical performance, and
durability (1717.
Zhang, J.; Li, S.; Li, Z.; Gao, Y.; Liu, C.; Qi, Y. (2021) Workability
and microstructural properties of red-mud-based geopolymer with
different particle sizes. Adv. Cem. Res. 33 [5] , 210-223. https://doi.org/10.1680/jadcr.19.00085.
, 7575.
Abdollahnejad, Z.; Nazari, A.; Pacheco-Torgal, F.; Sanjayan, J.G.;
Barroso de Aguiar, J.L. (2015) Prediction of the compressive strength of
one-part geopolymers. Int. Con.f Eng. 1226-1234.
, 7676.
Shill, S.K.; Al-Deen, S.; Ashraf, M.; Hutchison, W. (2020) Resistance
of fly ash based geopolymer mortar to both chemicals and high thermal
cycles simultaneously. Construc. Build. Mater. 239, 117886. https://doi.org/10.1016/j.conbuildmat.2019.117886.
).
Alkali activated materials were used in some real situations in Asia,
Australia, and Europe with established technical specifications (11. Vairagade, V.S.; Parbat, K.; Dhale, S.A. (2015) Fly ash as sustainable material for green concrete - A state of art. Int. J. Res. Eng. Sci. Technol. 1 [2] , 17-24.
, 1414.
Dong, M.; Feng, W.; Elchalakani, M.; Li, G.K.; Karrech, A.; Sheikh,
M.N. (2020) Material and glass-fibre-reinforced polymer bond properties
of geopolymer concrete. Mag. Concr. Res. 72 [10] , 509-525. https://doi.org/10.1680/jmacr.18.00273.
, 1515.
Aziz, I.H.; Al Bakri Abdullah, M.M.; Heah, C.Y.; Liew, Y.M. (2020)
Behaviour changes of ground granulated blast furnace slag geopolymers at
high temperature. Adv. Cem. Res. 32 [10] , 465-475. https://doi.org/10.1680/jadcr.18.00162.
).
Nevertheless, due to a shortage of clear guidelines that may be
extensively recognized, alkali activated blend formulation is almost
always difficult to evaluate and recreate. For this reason, determining
the working range with a detailed study is important research for alkali
activated concrete. The properties and necessary performance conditions
of foam concrete (CLC) masonry units used in all types of masonry
walls, whether load bearing or not, are presented in TS13655 (Turkish
Standard). According to this standard, the strength value of all kinds
of walls, including monolithic walls, sandwich (double-layer walls)
walls, partition walls, retaining walls and foundations, as well as
general applications underground, should be >1 MPa and <450-550
kg/m3 density property.
Experimental design methods
have gained popularity in recent years for various reasons, including
obtaining more information with less laboratory effort, saving time and
raw materials, and examining the variables that influence the critical
attributes. When an investigation is conducted without employing the
experimental design approach, modifications are made to the first
variable assumed to influence the outcome, and the first variable is
used until the best values are found. When trying to find the best
values for the second variable, there will be a shift from the first
variable’s optimum value. Every modification performed at the variable
level will lead to changes at all levels. Although the influence level
of each variable will be understood separately, the process will not be
fully understood, and the effects of the variables on one another will
not be resolved. The effect of one variable can be evaluated with
varying degrees of other variables in studies that follow the
experimental design method, and more data can be gathered with fewer
experiments (7777.
Kathirvel, P.; Kaliyaperumala, S.R.M. (2017) Probabilistic modeling of
geopolymer concrete using response surface methodology. Comput. Concr. 19 [6] , 737-744. https://doi.org/10.12989/cac.2017.19.6.737.
, 7878.
Ferreira, S.L.C.; Bruns, R.E.; Ferreira, H.S.; Matos, G.D.; David,
J.M.; Brandão, G.C.; et al. (2007) Box-Behnken design: An alternative
for the optimization of analytical methods. Anal. Chim. Acta. 597 [2] , 179-186. https://doi.org/10.1016/j.aca.2007.07.011.
).
For
the subject of geopolymers, there are many researchers who obtained
findings using different experimental design methods. In their studies,
they generally focused on critical parameters such as water/binder
ratio, fly ash/alkali ratio, alkaline concentration, and raw material
ratio. Li et al. worked on slag-based concrete using the Taguchi method (7979.
Li, N.; Shi, C.; Zhang, Z.; Zhu, D.; Hwang, H.J.; Zhu, Y.; Sun, T.
(2018) A mixture proportioning method for the development of
performance-based alkali-activated slag-based concrete. Cem. Conc. Comp. 93, 163-174. https://doi.org/10.1016/j.cemconcomp.2018.07.009.
).
In addition, Nazari et al. prepared geopolymers containing rice husk
ash and fly ash using the Taguchi method and determined the optimum mix
compositions (8080.
Nazari, A.; Sanjayan, J.G. (2015) Hybrid effects of alumina and silica
nanoparticles on water absorption of geopolymers: Application of Taguchi
approach. Measurement. 60, 240-246. https://doi.org/10.1016/j.measurement.2014.10.004.
).
Hadi et al. conducted studies on geopolymer-based concretes using the
multiple regression model with four variables related to strength abd
slump set time, and developed formulations including alkali /binder,
sodium silicate/sodium hydroxide, and water/binder ratios (8181.
Hadi, M.N.S.; Zhang, H.; Parkinson, S. (2019) Optimum mix design of
geopolymer pastes and concretes cured in ambient condition based on
compressive strength, setting time and workability. J. Build. Eng. 23, 301-313. https://doi.org/10.1016/j.jobe.2019.02.006.
).
Lokuge et al. studied fly ash-geopolymer concrete using a multivariate
adaptive regression spline model. During their studies, they collected
all the literature on the subject and analyzed the variables of
water/binder, alkali/binder, sodium silicate/sodium hydroxide, and
alkaline concentration (8282.
Lokuge, W.; Wilson, A.; Gunasekara, C.; Law, D.W.; Setunge, S. (2018)
Design of fly ash geopolymer concrete mix proportions using multivariate
adaptive regression spline model. Construc. Build. Mater. 166, 472-81. https://doi.org/10.1016/j.conbuildmat.2018.01.175.
).
Onoue et al. performed an experimental design about fly ash-based
geopolymers using the Taguchi method. They studied the sodium
silicate/sodium hydroxide ratio, alkaline concentration, fly ash-slag
ratio, mixing time, curing temperature, and cumulative temperature with
two different fly ash types purchased from different lots (8383.
Onoue, K.; Iwamoto, T.; Sagawa, Y. (2019) Optimization of the design
parameters of fly ash-based geopolymer using the dynamic approach of the
Taguchi method. Construc. Build. Mater. 219, 1-10. https://doi.org/10.1016/j.conbuildmat.2019.05.177.
).
This article presents comprehensive results about the physical properties, mechanical properties, and thermal conductivity properties, which will guide the preparation phase of fly ash-based geopolymer foams that are planned to be used as building materials. This study is also crucial in terms of evaluating fly ash, which is a thermal power plant waste, in the context of the circular economy and transforming it into a value-added product in the field of construction.
2. MATERIALS AND METHODS
⌅Solid
main raw materials used for the preparation of foam geopolymers are fly
ash and metakaolin. Fly ash was supplied by Seyitömer Thermal Power
Plant (Kütahya/Turkiye). It is classified as Class F according to the
ASTM-C618 (8484.
ASTM C618-03 (2003) Standard specification for coal fly ash and raw or
calcined natural pozzolan for use in concrete. Annu B ASTM Standard C,
3-6.
). Metakaolin powder (MEFISTO L05) was purchased
from the Czech Republic. Sodium silicate solution and sodium hydroxide
particulates were used in the preparation of alkaline solution. In the
foam formation phase, hydrogen peroxide (30% concentration) was used as a
foaming agent. and calcium stearate was used as a surfactant.
Additionally, chopped polypropylene fiber (1 cm long and 30 µm diameter)
and expanded perlite (1-3 mm diameter) were added to samples. To
improve the geopolymerization properties of the solutions, a small
amount of metakaolin was added. To abide by the circular economy goal
while also improving geopolymerization, a maximum of 15% metakaolin was
added (8585.
Luukkonen, T.; Abdollahnejad, Z.; Yliniemi, J.; Kinnunen, P.;
Illikainen, M. (2018) One-part alkali-activated materials: A review. Cem. Conc. Res. 103, 21-34. https://doi.org/10.1016/j.cemconres.2017.10.001.
, 8686.
Görhan, G.; Aslaner, R.; Şinik, O. (2016) The effect of curing on the
properties of metakaolin and fly ash-based geopolymer paste. Compos. Part B. Eng. 97, 329-335. https://doi.org/10.1016/j.compositesb.2016.05.019.
).
Calcium stearate was chosen due to its outstanding surfactant
properties while being affordable, odorless, and having low toxicity (8787.
Izaguirre, A.; Lanas, J.; Álvarez, J.I. (2009) Effect of
water-repellent admixtures on the behaviour of aerial lime-based mortars Cem. Concr. Res. 39 [11] , 1095-1104. https://doi.org/10.1016/j.cemconres.2009.07.026.
).
It is a white powder, that is slippery and insoluble in water. Chopped
polypropylene fiber was added to the samples to prevent cracks that may
occur during drying (88-9488.
Ranjbar, N.; Talebian, S.; Mehrali, M.; Kuenzel, C.; Cornelis, M.H.S.;
Jumaat, M.Z. (2016) Mechanisms of interfacial bond in steel and
polypropylene fiber reinforced geopolymer composites. Composites Sci. Tech. 122, 73-81. https://doi.org/10.1016/j.compscitech.2015.11.009.
89.
Abdollahnejad, Z.; Mastali, M.; Dalvand, A. (2017) Comparative study on
the effects of recycled glass-fiber on drying shrinkage rate and
mechanical properties of the self-compacting mortar and fly ash-slag
geopolymer mortar. J. Mat. Civil Eng. 29 [8] , 1-11. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001918
90. Ranjbar, N.; Zhang, M. (2020) Fiber-reinforced geopolymer composites : A review. Cem. Conc. Comp. 107, 103498. https://doi.org/10.1016/j.cemconcomp.2019.103498.
91.
Sukontasukkul, P.; Pongsopha, P.; Chindaprasirt, P.; Songpiriyakij, S.
(2018) Flexural performance and toughness of hybrid steel and
polypropylene fibre reinforced geopolymer. Construc. Build. Mater. 161, 37-44. https://doi.org/10.1016/j.conbuildmat.2017.11.122.
92.
Mastali, M.; Kinnunen, P.; Dalvand, A.; Mohammadi Firouz, R.;
Illikainen, M. (2018) Drying shrinkage in alkali-activated binders-A
critical review. Construc. Build. Mater. 190, 533-550. https://doi.org/10.1016/j.conbuildmat.2018.09.125.
93.
Kheradmand, M.; Abdollahnejad, Z.; Pacheco-Torgal, F. (2020) Drying
shrinkage of fly ash geopolymeric mortars reinforced with polymer hybrid
fibres. Proc. Inst. Civ. Eng. Constr. Mater. 173 [1] , 28-40. https://doi.org/10.1680/jcoma.16.00077.
94. Amran, Y.H.M.; Farzadnia, N.; Ali, A.A.A. (2015) Properties and applications of foamed concrete; a review. Construc. Build. Mater. 101 [1] , 990-1005. https://doi.org/10.1016/j.conbuildmat.2015.10.112.
), and expanded perlite was added to improve the thermal insulation properties (4949. Vaou, V.; Panias, D. (2010) Thermal insulating foamy geopolymers from perlite. Minerals Eng. 23 [14] , 1146-1151. https://doi.org/10.1016/j.mineng.2010.07.015.
, 6363. Demir, İ.; Baspınar, S.; Kahraman, E. (2018) Production of insulations and construction materials from expanded perlite. Lecture Notes Civil Eng. 6, 24-32.
, 6565.
Sengul, O.; Azizi, S.; Karaosmanoglu, F.; Tasdemir, M.A. (2011) Effect
of expanded perlite on the mechanical properties and thermal
conductivity of lightweight concrete. Energy Build. 43 [2-3] , 671-676. https://doi.org/10.1016/j.enbuild.2010.11.008.
, 9595.
Jedidi, M.; Benjeddou, O.; Soussi, C. (2015) Effect of expanded perlite
aggregate dosage on properties of lightweight concrete. Jordan J. of Civ. Eng. 9 [3] , 278-291. https://doi.org/10.14525/jjce.9.3.3071.
).
Table 1 summarizes the chemical compositions of solid raw materials measured by XRF, and Table 2 gives specific surface area and specific weight of raw materials, respectively. The specific surface area of fly ash and metakaolin were detected by the BET method. Density values of aluminosilicate raw materials were measured with a pycnometer.
| SiO2 | Al2O3 | CaO | Fe2O3 | MgO | K2O | |
|---|---|---|---|---|---|---|
| Fly Ash (%) | 50.30 | 19.10 | 4.55 | 12.40 | 4.67 | 2.16 |
| Metakaolin (%) | 54.10 | 41.10 | 0.13 | 1.10 | 0.18 | 0.80 |
| Properties | Fly Ash | Metakaolin |
|---|---|---|
| Specific Surface Area (kg/m 2 ) | 7.91 | 17.08 |
| Density (kg/m 3 ) | 2.58 | 2.50 |
In order to prepare the alkaline solution, NaOH plates were completely dissolved in sodium silicate solution. To ensure the homogeneous distribution, the fibers added to the alkaline mixture were mixed for another one minute. Solid content was homogeneously mixed by adding calcium stearate into the mixture containing metakaolin and fly ash. The alkaline solution was poured onto the solid mixture without waiting for it to cool and the geopolymer content was mixed for five minutes. After that, H2O2 was added to the geopolymer mixture and the blend was mixed vigorously for 1 minute. Finally, the geopolymer mixture was poured into 10x10x10 cm molds and cured for 24 hours in an oven (Figure 1). The hardened geopolymers were demolded and stored at room temperature for 28 days.
The samples prepared for thermal conductivity analysis were produced as presented in the flow chart in Figure 1. Since the authors preferred the guarded hot plate method for thermal conductivity analysis, the samples were produced with 30x30x5 cm dimensions.
Compressive strength, bulk density, and thermal conductivity analyses of the prepared foam samples were performed on the 28th day. Analysis of the large amount of data obtained was performed with the SPSS program, and the results obtained were confirmed with statistical data.
2.1 Experimental design procedure
⌅Geopolymer experiments were conducted with Box Behnken experimental design method. A detailed study was carried out to obtain comprehensive information about compressive strength, density, and thermal conductivity properties of fly ash-based foam geopolymers. The maximum (1), minimum (-1) and average (0) values for six variables were determined for the experiment design. Maximum and minimum values were provided by using preliminary studies. The average value (0) represents the mean of the maximum and minimum value. The values determined for the experiment design are presented in Appendix 1. Oxide molar ratios of all mix design compositions of samples are shown in Table 3. Contour plots were used to interpret the complex data collected.
| Fly Ash-Metakaolin Ratio (%) | Alkaline Concentration | H2O/Na2O | SiO2/Al2O3 | Na2O/SiO2 | Na2O/Al2O3 |
|---|---|---|---|---|---|
| 85-15 | 6M | 13.42 | 5.19 | 0.28 | 1.40 |
| 8M | 11.75 | 5.19 | 0.33 | 1.63 | |
| 10M | 10.47 | 5.19 | 0.37 | 1.85 | |
| 90-10 | 6M | 13.40 | 5.44 | 0.28 | 1.47 |
| 8M | 11.73 | 5.44 | 0.33 | 1.71 | |
| 10M | 10.45 | 5.44 | 0.37 | 1.95 | |
| 95-5 | 6M | 13.37 | 5.72 | 0.28 | 1.55 |
| 8M | 11.71 | 5.72 | 0.33 | 1.80 | |
| 10M | 10.44 | 5.72 | 0.37 | 2.05 |
2.2 Instrumentation
⌅Compressive strength was tested with 10x10x10 cm specimens. Samples were cured at room temperature (25 ºC±1 and 50±5% relative humidity) for 28 days. The strength of geopolymer foams was tested with a loading rate of 3 kN/s. Bulk densities of samples cured at room temperature for 28 days were measured in accordance with ASTM C642. At least three samples were examined, and average values were recorded. Scanning electron microscopy (SEM) with back-scattered electron (BSE) images were performed to analyze the surfaces of the specimens and investigate porosity on a LEO 1430 VP device. Secondary electron images were obtained with an acceleration voltage of 20 kV. Information about bonds formed in geopolymer samples and their amounts were obtained by the FTIR method. FTIR analysis was performed to correlate with drying shrinkage. The samples were characterized by diamond ATR spectroscopy and absorption spectra in the 4000-400 cm-1 spectrum range. The thermal conductivity of samples was tested using a TA Instruments FOX 314 Thermal Conductivity Analyzer. The guarded hot plate method was used to measure the thermal conductivity of low conductivity construction and insulation materials at a temperature of 25 °C. The samples were dried in an oven at 105 °C until they attained a consistent weight for thermal conductivity analysis.
All samples were subjected to strength and density tests. Thermal conductivity investigation was done on samples chosen from among those that showed the best strength and density performance. As a result of these analyses, contour plots were created. Only selected samples were subjected to XRD, SEM and FTIR analyses. The formulations of the selected samples are depicted in the figures that accompany the analysis findings.
3. RESULTS AND DISCUSSION
⌅3.1 Compressive strength analysis
⌅In the study, the effects of all variables on strength were examined with the help of contour graphics and the results are presented in Table 4 and Figure 2. As the indicator on the right side of the graphics turns from cold colors to warm colors, the strength value increases. As shown in all figures, the strength values vary between 0.57 MPa and 2.75 MPa. The graphs with the most transitions between colors were examined visually, and H2O2, expanded perlite, and curing temperature were the variables that have the most effect on strength.
| Model | B value | Standard Error | Beta value | p-value | R2 | Binary r |
|---|---|---|---|---|---|---|
| Constant | 0.614 | 0.767 | <0.001 | 0.424 | ||
| H2O2 (%) | -0.779 | 0.177 | -0.479 | -0.624 | ||
| Curing Temperature (oC) | 0.022 | 0.009 | 0.269 | 0.518 | ||
| Alkaline concentration (M) | 0.101 | 0.044 | 0.248 | 0.340 | ||
| Expanded Perlite (%) | -0.073 | 0.032 | -0.248 | -0.616 |
Data obtained from compressive strength tests were analyzed using statistical analysis methods. Multiple regression was carried out with the help of stepwise linear regression in order to investigate which variables used in geopolymer composition contributed more to strength development (Table 4). Stepwise linear regression is a method of regressing multiple variables while simultaneously removing those that aren’t important. SPSS ordered the variables in the model according to their correlation strengths with the dependent variable, as shown in Table 4. The model was constructed by measuring the effect of each variable entered into the model. The model shows that H2O2, curing temperature, alkaline concentration, and expanded perlite amount all influenced strength. The beta value describes the effect of the independent variable on the dependent in multiple regression. Regardless of sign, large numbers represent the most impactful parameter on the property. The relative importance of the variables on strength, according to the standardized regression coefficient (β), were H2O2 amount, curing temperature, alkaline concentration, and expanded perlite quantity. H2O2 and expanded perlite were negatively connected to strength and strength was positively related to curing temperature and alkaline concentration. A positive relationship suggests that the change is directly proportional, while a negative relationship shows that the change is inversely proportional. When the significance (p-value) of the variables and models is assessed, the association between the variables was statistically significant because the values are less than 0.05.
The negative and moderate value (r=-0.624) correlation between H2O2 and strength values was obtained using binary correlations between the strength value and the variables. The curing temperature and strength values were found to have a positive and moderate binary (r=0.518) correlation. The alkaline concentration and strength values were found to have a moderate (r=0.340) positive binary interaction. Finally, a moderate binary (r=0.616) negative association was calculated between expanded perlite content and strength development.
The table shows that H2O2 and expanded perlite reduce strength, whereas curing temperature and alkaline concentration enhance strength. The numerical size of the beta value, regardless of sign, also provides the most information about the effect of variables on strength. This means that H2O2, curing temperature, alkaline concentration, and expanded perlite all impact compressive strength.
In this context, the authors created a ternary strength graph by selecting three different variables that have the greatest impact on strength. In this case, the authors chose three variables that had the greatest impact on strength and created a ternary strength graph to estimate the operating range.
The area covered by red, yellow, and green on the triangle graph suggests that strength values at and above the intended value (1 MPa) can be obtained, which was determined at the start of the study. As an outcome, samples made at maximum 1.0% H2O2, minimum 70 ºC, and containing any amount of expanded perlite can achieve >1.0 MPa strength. The coloring changes to turquoise and dark blue in locations where the curing temperature is lower than the established value and the H2O2 amount is higher than the indicated value, where strength values below the target value are feasible.
While
the maximum compressive strength was achieved at 80 °C, the minimum
compressive strength was obtained at 60 °C, and the strength gradually
varied as the temperature was increased. The compressive strength
increased when the curing temperature was raised, according to this
finding. Although Hardjito et al. (9696.
Hardjito, D.; (2005) Development and properties of low-calcium fly
ash-based geopolymer concrete, Curtin University of Technology,
Australia, (2005).
) asserted that increasing the
curing temperature over 60 °C does not considerably increase compressive
strength, temperature increased strength in fly ash-based geopolymer
foams in our work.
The degree of porosity mainly controls the mechanical strength of the geopolymer foam (5555.
Medri, V.; Papa, E.; Dedecek, J.; Jirglova, H.; Benito, P.; Vaccari,
A.; Landi, E. (2013) Effect of metallic Si addition on polymerization
degree of in situ foamed alkali-aluminosilicates. Ceram. Int. 39 [7] , 7657-7668. https://doi.org/10.1016/j.ceramint.2013.02.104.
). The increase in H2O2 contributes to thinning and weakening of the pore walls and decreasing
bulk density values cause a decrease in strength. Geopolymer foams
containing 1.0% H2O2 reached a strength value of >2.5 MPa. While the strength values of samples containing 1.5% H2O2 decreased to 1.5-2.0 MPa, it even falls below 1.0 MPa for samples containing 2.0% H2O2.
When expanded perlite is added to a geopolymer mixture, its porous and weak structure causes weak zones (9797. Tsaousi, G.M.; Douni, I.; Panias, D. (2016) Characterization of the properties of perlite geopolymer pastes. Mater. Construcc. 66 [324] , e102. https://doi.org/10.3989/mc.2016.10415.
).
As a result, expanded perlite in the combination has a negative impact
on the mechanical characteristics. While samples without any expanded
perlite can reach a strength value of about 1.5 MPa, when 2.5% expanded
perlite is added to the mixture, the strength value drops from one to
0.5 MPa.
Using the triple chart in Figure 3, H2O2 must be less than 1.5%, curing temperature must be greater than 70 °C, and perlite must be less than 2.75% to achieve strengths greater than 1 MPa required by the TS13655 standard.
3.2 Density analysis
⌅The findings for density analysis are shown in Appendix 1, statistical results in Table 5, and the contour plots in Figure 4 were utilized to observe the variables that affect sample densities. Figure 4a reveals that, regardless of the amount of calcium stearate used, sample densities vary directly with the amount of hydrogen peroxide. Figure 4b shows that hydrogen peroxide has a greater effect on density values than expanded perlite. The density values were somewhat affected by the temperature increase in Figures 4c and 4d, and the density values may increase with the increase in alkaline concentration. Figure 4e demonstrates that the amount of fly ash has little effect on the density values.
| Model | B value | Standard Error | Beta value | p-value | R2 | Binary r |
|---|---|---|---|---|---|---|
| Constant | 687.370 | 56.011 | 0.000 | 0.874 | ||
| H2O2 (%) | -148.667 | 12.983 | -0.788 | -0.789 | ||
| Alkaline concentration (M) | 15.667 | 3.246 | 0.332 | 0.332 | ||
| Curing Temperature (ºC) | -1.667 | 0.649 | -0.177 | 0.177 |
Multiple regression was performed using
the stepwise linear regression approach, as in the compressive strength
study, to determine the variables impacting the density of the foam
geopolymers. Table 5 shows the parameters that have the greatest impact on density. The most
effective variables on density in this model were peroxide, curing
temperature, and alkaline concentration. When the significance (p-value)
of the variables and models presented in the table are examined, the
relationship between the variables was statistically significant since
the values are less than 0.05. Along with low density, expanded perlite
is commonly used in manufacturing lightweight concrete (9898. ASTM C332-17 (2017) Standard specification for lightweight aggregates for insulating concrete.
, 9999. ASTM C330 (2017) Standard specification for lightweight aggregates for structural concrete, Annual book of ASTM Standards.
).
Even though expanded perlite was employed in the study, it was not one
of the variables affecting density in the SPSS analysis. The small
amount in the composition is considered the main cause. As a result, as
can be followed by the beta values, hydrogen peroxide, curing
temperature, and alkaline concentration all had a greater impact on the
density value, respectively.
As presented in Table 5, the variables of hydrogen peroxide, curing temperature, and alkaline concentration had a significant and robust relationship (R2=0.874 and p<0.05). Together, these three variables explain approximately 87.4% of the total variance of the density values. There was a negative and robust (r=-0.789) relationship between H2O2 and density values. There was a positive and low binary correlation (r=0.177) between the curing temperature and the density values. A moderate (r=0.332) positive binary correlation was calculated between alkaline concentration and density values. Thanks to model three, a ternary diagram was drawn, and the operating range was determined by considering the parameters that most affected the density value (Figure 5).
Although Hardjito et al. (100100. Hardjito, D.; Wallah, S.E.; Sumajouw, D.M.J. Rangan, B.V. (2004) On the development of fly ash-based geopolymer concrete. Am. Conc. Instit. Mater. J. 101 [6] , 467-472.
)
claimed that raising the curing temperature above 60 °C does not
increase the compressive strength significantly, in this study, the
temperature increased the strength of fly ash-based geopolymer foams.
To produce a sample with the density of 450-550 kg/m3 required by the TS13655 standard, the triple graph in the figure indicates that H2O2 must be less than 1.75%, curing temperature must be higher than 70 °C, and alkaline concentration must be greater than 8 M.
3.3 Thermal conductivity analysis
⌅The Figure 6 shows contour plots created from the thermal conductivity analysis
findings. The thermal conductivity values of fly ash-based geopolymer
foams ranged between 0.089 to 0.1328 W/mK, according to the results of
the investigation. The effect of H2O2 and Ca-stearate on thermal conductivity is seen in Figure 6a. Ca-stearate and H2O2 both appear to improve the insulating capabilities gradually. The
decrease in heat conductivity was induced by the presence of more air
gaps in the geopolymer matrix caused by the increased amount of H2O2 (3939.
Zhao, Y.; Jow, J.; Cai, X.; Lai, S. (2015) Fly ash-based geopolymer
foam technology for thermal insulation and fire protection applications.
World of Coal Ash (WOCA) Conference.
, 101101.
Feng, J.; Zhang, R.; Gong, L.; Li, Y.; Cao, W.; Cheng, X. (2015)
Development of porous fly ash-based geopolymer with low thermal
conductivity. Mater. Des. 65, 529-533. https://doi.org/10.1016/j.matdes.2014.09.024.
, 102102.
Zhu, M.; Ji, R.; Li, Z.; Wang, H.; Liu, L.L.; Zhang, Z. (2016)
Preparation of glass ceramic foams for thermal insulation applications
from coal fly ash and waste glass. Construc. Build. Mater. 112, 398-405. https://doi.org/10.1016/j.conbuildmat.2016.02.183.
) and the reduction of pore diameters generated by Ca-stearate (7373.
Cui, Y.; Wang, D.; Zhao, J.; Li, D.; Ng, S.; Rui, Y. (2018) Effect of
calcium stearate based foam stabilizer on pore characteristics and
thermal conductivity of geopolymer foam material. J. Build. Eng. 20, 21-29. https://doi.org/10.1016/j.jobe.2018.06.002.
, 103-105103.
Jaya, N.A.; Yun-Ming, L.; Cheng-Yong, H.; Abdullah, M.M.A.B.; Hussin,
K. (2020) Correlation between pore structure, compressive strength and
thermal conductivity of porous metakaolin geopolymer. Construc. Build. Mater. 247, 118641. https://doi.org/10.1016/j.conbuildmat.2020.118641.
104.
Sumirat, I.; Ando, Y.; Shimamura, S. (2006) Theoretical consideration
of the effect of porosity on thermal conductivity of porous materials. J. Porous Mater. 13, 439-443. https://doi.org/10.1007/s10934-006-8043-0.
105.
Skibinski, J.; Cwieka, K.; Ibrahim, S.H.; Wejrzanowski, T. (2019)
Influence of pore size variation on thermal conductivity of open-porous
foams. Materials. 12 [12] , 2017. https://doi.org/10.3390/ma12122017.
) in the composition. The effect of fly ash and calcium stearate on thermal conductivity is seen in Figure 6b. The amount of fly ash was found to influence thermal conductivity, initially decreasing and later boosting it. Figure 6c demonstrates that when the alkali concentration increases, the thermal
conductivity values drop; however, the expanded perlite additive
enhances insulation. The thermal insulation qualities of the structure
increased because expanded perlite enhances the total porosity of the
structure (6565.
Sengul, O.; Azizi, S.; Karaosmanoglu, F.; Tasdemir, M.A. (2011) Effect
of expanded perlite on the mechanical properties and thermal
conductivity of lightweight concrete. Energy Build. 43 [2-3] , 671-676. https://doi.org/10.1016/j.enbuild.2010.11.008.
, 106106.
Demirboǧa, R.; Gül, R. (2003) The effects of expanded perlite
aggregate, silica fume and fly ash on the thermal conductivity of
lightweight concrete. Cem. Concr. Res. 33 [5] 723-727. https://doi.org/10.1016/S0008-8846(02)01032-3.
). The thermal conductivity value is thought to increase when the alkali content in the geopolymer density values rises (107107.
Karaaslan, C.; Yener, E. (2021) The effect of alkaline activator
components on the properties of fly ash added pumice based geopolymer. J. Inst. Sci. Technol. 11 [2] , 1255-1269. https://doi.org/10.21597/jist.840872.
, 108108.
Jaya, N.A.; Yun-Ming, L.; Abdullah, M.M.A.B.; Cheng-Yong, H.; Hussin,
K. (2018) Effect of sodium hydroxide molarity on physical, mechanical
and thermal conductivity of metakaolin geopolymers. IOP Conf. Ser. Mater. Sci. Eng. 343, 012015. https://doi.org/10.1088/1757-899X/343/1/012015.
).
3.4 SEM analysis
⌅ Figure 7 presents the microstructure properties of the synthesized fly ash-based
geopolymers as evaluated by SEM. The microstructure of foam geopolymers
generated with different concentration levels of alkaline activator is
shown in Figures 7a-7b.
Increasing concentrations of alkaline activator create more pores and
reduce the pore sizes in the areas examined at equal magnifications. The
viscosity of the geopolymer mixture increases as the alkalinity rises.
The increased external pressure around the pores inhibits their growth
and development (3434.
Yatsenko, E.A.; Goltsman, B.M.; Smoliy, V.A.; Kosarev A.S. (2016)
Investigation of a porous structure formation mechanism of a foamed slag
glass based on the glycerol foaming mixture. Res. J. Phar, Bio. Chem. Sci. 7 [5] , 1073-1081.
, 109109.
Kadoi, K.; Nakae, H. (2011) Relationship between foam stabilization and
physical properties of particles on aluminum foam production. Mater. Trans. 52 [10] , 1912-1919. https://doi.org/10.2320/matertrans.F-M2011817.
). As a result, the pores formed in geopolymer compositions with lower alkalinity are smaller. In Figure 7c-7d, the pore size of the foam geopolymer grows as the amount of H2O2 increases. As the amount of dissociated O2 increased in tandem with the amount of H2O2, the pores enlarged and more porosity occurred (4949. Vaou, V.; Panias, D. (2010) Thermal insulating foamy geopolymers from perlite. Minerals Eng. 23 [14] , 1146-1151. https://doi.org/10.1016/j.mineng.2010.07.015.
). The pore size distribution in geopolymer foams at different foaming temperatures is shown in Figures 7e and 7f.
As the foaming temperature increased, the number of large pores
reduced, and the distribution of pores became more uniform. The
fundamental reason for this is that as the temperature goes up, the
geopolymerization process accelerates, and the gas generated as a result
of the decomposition of H2O2 in the composition remains trapped in the structure as it begins to harden and is unable to join with other pores and expand (110110.
Gu, G.; Xu, F.; Huang, X.; Ruan, S.; Peng, C.; Lin, J. (2020) Foamed
geopolymer: The relationship between rheological properties of
geopolymer paste and pore-formation mechanism. J. Clean Prod. 277, 123238. https://doi.org/10.1016/j.jclepro.2020.123238.
). Figure 7g
presents scanning electron microscopy images of the geopolymer
containing expanded perlite, which appears to have high porosity.
Expanded perlite, which has high porosity, is exceptionally light, and
it is inevitable that it causes a severe decrease in density depending
on the amount added to the final product (6565.
Sengul, O.; Azizi, S.; Karaosmanoglu, F.; Tasdemir, M.A. (2011) Effect
of expanded perlite on the mechanical properties and thermal
conductivity of lightweight concrete. Energy Build. 43 [2-3] , 671-676. https://doi.org/10.1016/j.enbuild.2010.11.008.
).
Excess expanded perlite content, which has low strength, in the mixture
causes deterioration in the mechanical properties of the final product,
as it will cause the formation of porous and low strength areas.
3.5 FTIR analysis
⌅The
influence of curing temperature, alkali concentration and
aluminosilicate raw material combinations on the structural alterations
and reaction products of fly ash-based foam geopolymers was investigated
using FTIR experiments. For the FTIR study, a Bruker Vertex 70V type
spectrometer device (400-4000 cm-1) was used. The results of
FTIR spectroscopy are represented graphically as transmittance vs.
wavenumber. The spectra lines of foam geopolymer samples are shown in Fig. 7.
The H-OH bending and -OH symmetric and asymmetric stretching functional
groups are apparent in the broad and strong peaks at 3600-3000 and 1650
for foam geopolymer samples. The water and potential -OH groups in the
oligomeric solute species are responsible for these bands. The
significant peaks detected in FTIR analyses of geopolymers between 1000
and 600 cm-1 are known to be connected with asymmetric vibrations of T-O-Si bonds (T = Si or Al) (111111.
Puertas, F.; Torres-Carrasco, M. (2014) Use of glass waste as an
activator in the preparation of alkali-activated slag. Mechanical
strength and paste characterisation. Cem. Concr. Res. 57, 95-104. https://doi.org/10.1016/j.cemconres.2013.12.005.
). The peak at 970-800 cm-1 is linked to Al-O-Si bond bending vibrations, while the peak at 420 cm-1 is linked to Si-O-Si bending vibrations (112112.
Tchakouté, H.K.; Rüscher, C.H.; Kong, S.; Kamseu, E.; Leonelli, C.
(2016) Geopolymer binders from metakaolin using sodium waterglass from
waste glass and rice husk ash as alternative activators: A comparative
study. Construc. Build. Mater. 114, 276-89. https://doi.org/10.1016/j.conbuildmat.2016.03.184.
). The slight changes in the wavenumber 2200-1800 cm-1 detected in the geopolymer compositions with added organic surfactant reveal the symmetrical and asymmetrical vibrations of CH2 due to the organic surfactant component (113113.
Monich, P.R.; Romero, A.R.; Höllen, D.; Bernardo, E. (2018) Porous
glass-ceramics from alkali activation and sinter-crystallization of
mixtures of waste glass and residues from plasma processing of municipal
solid waste. J. Clean. Prod. 188, 871-878. https://doi.org/10.1016/j.jclepro.2018.03.167.
). In addition, carbonyl stretching peaks (COO-) from the chemical structure of Ca-stearate were detected at 1600 cm-1. The peak for the Si-O-Ca vibration can be seen at 750 cm-1 (114114.
Husain, S.; Permitaria, A.; Haryanti, N.H.; Suryajaya, S. (2019) Effect
calcination temperature on formed of calcium silicate from rice husk
ash and snail shell. J. Neutrino. J. Fisika dan Apl. 11 [2] , 45-51.
).
3.6 XRD analysis
⌅ Figure 9 shows the results of X-ray diffraction investigations of geopolymer
foam samples. The examination results for all fly ash-based samples
revealed the presence of quartz phase. The effects of varying the amount
of fly ash, alkaline concentration, and the curing temperature were all
noticed. The characteristic hump (2Θ = 20-40°) of amorphous gels was
detected due to the creation of a geopolymer structure generated during
the dissolution of the fly ash glassy phase (6262. Zhang, X.; Bai, C.; Qiao, Y.; Wang, X.; Jia, D.; Li, H.; Colombo, P. (2021) Porous geopolymer composites: A review. Comp. Part A: App. Sci. Manufac. 150, 106629.
, 115115. Abdulkareem, O.A. (2017) Effects of high activator content on fly ash-based geopolymers exposed to elevated temperatures. J. Mater. Appl. 6 [1] , 1-12.
).
Because the regions beneath the characteristic hump curves were
comparable with width and quartz peak positions, it was assumed that all
samples would react similarly.
4. CONCLUSIONS
⌅A guideline study was conducted using the experimental design program to investigate the impacts of six distinct variables on compressive strength, density, and thermal conductivity including fly ash-metakaolin ratio, alkalinity, curing temperature, calcium stearate, H2O2, and expanded perlite. The study findings were statistically assessed, and the three most effective variables on density and strength were chosen, with ternary graphs made as a guide. The variation of thermal conductivity values was also analyzed with the help of contour plots. In this case, H2O2 and curing temperature were identified as the variables that had a combined effect on the strength and density values. Pearlite and alkaline concentration, respectively, were the third and fourth variables that influenced the strength and density values. The data for the technical needs stated in TS13655 were collected for non-load-bearing wall elements, and the working ranges that may be utilized as a guide were identified. Working with less than 1.75% H2O2, a curing temperature greater than 70 °C, an alkali concentration greater than 8 M, and expanded perlite less than 2.75% will match the predicted values when the operating ranges for both density and strength are examined together.