1. INTRODUCTION
⌅In
a global scenario, the building construction sector’s sustainable
development has been seriously tracked. In fact, Portland cement (PC) is
the most widely used material worldwide after water (11. Gagg, C.R. (2014) Cement and concrete as an engineering material: An historic appraisal and case study analysis. Eng. Fail. Anal. 40, 114-140. https://doi.org/10.1016/j.engfailanal.2014.02.004.
), and is composed primarily of non-renewable raw materials like limestone and clay (22.
Mikulčić, H.; Klemeš, J.J.; Vujanović, M.; Urbaniec, K.; Duić, N.
(2016) Reducing greenhouse gasses emissions by fostering the deployment
of alternative raw materials and energy sources in the cleaner cement
manufacturing process. J. Clean. Prod. 136 [B] 119-132. https://doi.org/10.1016/j.jclepro.2016.04.145.
). The production of this material means that cement industries are responsible for almost 8% of the world’s CO2 emissions (22.
Mikulčić, H.; Klemeš, J.J.; Vujanović, M.; Urbaniec, K.; Duić, N.
(2016) Reducing greenhouse gasses emissions by fostering the deployment
of alternative raw materials and energy sources in the cleaner cement
manufacturing process. J. Clean. Prod. 136 [B] 119-132. https://doi.org/10.1016/j.jclepro.2016.04.145.
, 33.
Rahman, A.; Rasul, M.G.; Khan, M.M.K.; Sharma, S. (2014) Recent
development on the uses of alternative fuels in cement manufacturing
process. Fuel. 145, 84-99. https://doi.org/10.1016/j.fuel.2014.12.029.
).
Therefore, more sustainable cementitious materials than PC cement are
required to minimize this environmental issue. This is the reason why
several researchers have studied alkali-activated and geopolymeric
binders (44. Zhang, Z.H.; Zhu, H.J.; Zhou, C.H.; Wang, H. (2015) Geopolymer from kaolin in China: An overview. Appl. Clay. Sci. 119 [1], 31-41. https://doi.org/10.1016/j.clay.2015.04.023.
, 55.
Mejía-Arcila, J.; Valencia-Saavedra, W.; Mejía de Gutiérrez, R. (2020)
Eco-efficient alkaline activated binders for manufacturing blocks and
pedestrian pavers with low carbon footprint: Mechanical properties and
LCA assessment. Mater. Construcc. 70 [340], e232. https://doi.org/10.3989/mc.2020.17419.
).
Geopolymers
are aluminosilicate binders with a low calcium content that yield a
tridimensional-molecular structure in a high alkaline environment (66.
Palomo, A.; Krivenko, P.; Garcia-Lodeiro, I.; Kavalerova, E.; Maltseva,
O.; Fernández- Jiménez A. (2014) A review on alkaline activation: new
analytical perspectives. Mater. Construcc. 64 [315], e022. https://doi.org/10.3989/mc.2014.00314.
, 77. Provis, J.L.; Palomo, A.; Shi, C. (2015) Advances in understanding alkali-activated materials. Cem. Concr. Res. 78 [A], 110-125. https://doi.org/10.1016/j.cemconres.2015.04.013.
). One of the most widely used manufactured precursors to produce geopolymer is metakaolin (MK) (88.
Shi, C.; Jiménez, A.F.; Palomo, A. (2011) New cements for the 21st
century: The pursuit of an alternative to Portland cement. Cem. Concr. Res. 41, 750-763. https://doi.org/10.1016/j.cemconres.2011.03.016.
),
although several agricultural and industrial waste types have been
used; i.e. fly ash, rice husk ash, sugar cane bagasse ash, catalytic
cracking catalyst residue, etc. (9-149. Vásquez, A.; Cárdenas, V.; Robayo, R.A.; Mejía de Gutiérrez, R. (2015) Geopolymer based on concrete demolition waste. Adv. Powder Technol. 27 [4], 1173-1179. https://doi.org/10.1016/j.apt.2016.03.029.
10.
Part, W.K.; Ramli, M.; Cheah, C.B. (2015) An overview on the influence
of various factors on the properties of geopolymer concrete derived from
industrial by-products. Constr. Build. Mater. 77, 370-395. https://doi.org/10.1016/j.conbuildmat.2014.12.065.
11.
Xie, T.; Ozbakkaloglu, T. (2015) Behavior of low-calcium fly and bottom
ash-based geopolymer concrete cured at ambient temperature. Ceram. Int. 41 [4], 5945-5958. https://doi.org/10.1016/j.ceramint.2015.01.031.
12.
Noor-ul-Amin; Faisal, M.; Muhammad, K.; Gul, S. (2015) Synthesis and
characterization of geopolymer from bagasse bottom ash, waste of sugar
industries and naturally available china clay. J. Clean. Prod. 129, 491-495. https://doi.org/10.1016/j.jclepro.2016.04.024.
13.
Castaldelli, V.N.; Moraes, J.C.B.; Akasaki, J.L.; Melges, J.L.P.;
Monzó, J.; Borrachero, M.V.; Soriano, L.; Payá, J.; Tashima, M.M. (2016)
Study of the binary system fly ash/sugarcane bagasse ash (FA/SCBA) in
SiO2/K2O alkali-activated binders. Fuel. 174, 307-316. https://doi.org/10.1016/j.fuel.2016.02.020.
14.
Rodríguez, E.D.; Bernal, S.A.; Provis, J.L.; Gehman, J.D.; Monzó, J.M.;
Payá, J.; Borrachero M.V. (2013) Geopolymers based on spent catalyst
residue from a fluid catalytic cracking (FCC) process. Fuel. 109, 493-502. https://doi.org/10.1016/j.fuel.2013.02.053.
).
The inclusion of these residues in geopolymeric matrices is an
essential advantage for geopolymer production because they emit less CO2 (1515.
Mellado, A.; Catalán, C.; Bouzón, N.; Borrachero, M.V.; Monzó, J.M.;
Payá, J. (2014) Carbon footprint of geopolymeric mortar: Study of the
contribution of the alkaline activating solution and assessment of an
alternative route. RSC Adv. 4, 23846-23852. https://doi.org/10.1039/c4ra03375b.
).
Generally
speaking, the composition of these residues is siliceous or
aluminosiliceous, which is fundamental for the geopolymeric reaction (1616. Ma, C-K.; Awang, A.Z.; Omar, W. (2018) Structural and material performance of geopolymer concrete: A review. Constr. Build. Mater. 186, 90-102. https://doi.org/10.1016/j.conbuildmat.2018.07.111.
).
The literature has specifically pointed out the incorporation of sewage
sludge ash (SSA) into the geopolymer based on its physicochemical
characteristics (1717. Payá, J.; Monzó, J.; Borrachero, M.V.; Soriano, L. (2019) Sewage sludge ash. New Trends Eco-efficient Recycl. Concr. 121-152. https://doi.org/10.1016/B978-0-08-102480-5.00005-1.
),
apart from its environmental-friendly immobilization needed. Such
residue, SSA, comes from incinerating sewage sludge generated during
wastewater treatment. Its global generation is estimated at 1.7 million
tonnes annually, with an increasing trend (1818. Vouk, D.; Nakic, D.; Stirmer, N.; Cheeseman, C.R. (2017) Use of sewage sludge ash in cementitious materials. Rev. Adv. Mater. Sci. 49, 158-170. Retrieved from https://www.ipme.ru/e-journals/RAMS/no_24917/05_24917_nakic.pdf.
). This residue is mainly composed of SiO2, CaO, Al2O3, Fe2O3, P2O3 and SO3, with an average content of 34.0%, 15.8%, 12.8%, 11.4%, 10.8% and 5.2%, respectively (1818. Vouk, D.; Nakic, D.; Stirmer, N.; Cheeseman, C.R. (2017) Use of sewage sludge ash in cementitious materials. Rev. Adv. Mater. Sci. 49, 158-170. Retrieved from https://www.ipme.ru/e-journals/RAMS/no_24917/05_24917_nakic.pdf.
, 1919.
Smol, M.; Kulczycka, J.; Henclik, A.; Gorazda, K.; Wzorek, Z. (2015)
The possible use of sewage sludge ash (SSA) in the construction industry
as a way towards a circular economy. J. Clean. Prod. 95, 45-54. http://doi.org/10.1016/j.jclepro.2015.02.051.
). Mineral phases as quartz (SiO2), calcium phosphate (Ca(PO4)2), calcite (CaCO3), and hematite (Fe2O3) are the most common ones found in the SSA composition (1818. Vouk, D.; Nakic, D.; Stirmer, N.; Cheeseman, C.R. (2017) Use of sewage sludge ash in cementitious materials. Rev. Adv. Mater. Sci. 49, 158-170. Retrieved from https://www.ipme.ru/e-journals/RAMS/no_24917/05_24917_nakic.pdf.
, 2020.
Cyr, M.; Coutand, M.; Clastres, P. (2007) Technological and
environmental behavior of sewage sludge ash (SSA) in cement-based
materials. Cem. Concr. Res. 37 [8], 1278-1289. https://doi.org/10.1016/j.cemconres.2007.04.003.
).
According to the reports found in the literature, the amorphous content
of SSA is largely varied, being found values in a range of 35%-75% (2121.
Lynn, C.J.; Dhir, R.K.; Ghataora, G.S.; West, R.P. (2015) Sewage sludge
ash characteristics and potential for use in concrete. Constr. Build. Mater. 98, 767-779. https://doi.org/10.1016/j.conbuildmat.2015.08.122.
). The specific gravity, BET specific surface area, and Blaine fineness of SSA vary in a range of 1.8-2.9, 2500-23100 m2/kg, 500-3900 m2/kg, respectively (2121.
Lynn, C.J.; Dhir, R.K.; Ghataora, G.S.; West, R.P. (2015) Sewage sludge
ash characteristics and potential for use in concrete. Constr. Build. Mater. 98, 767-779. https://doi.org/10.1016/j.conbuildmat.2015.08.122.
). The average bulk density of SSA is 805 kg/m3, which is a low value due to its porous particles (2121.
Lynn, C.J.; Dhir, R.K.; Ghataora, G.S.; West, R.P. (2015) Sewage sludge
ash characteristics and potential for use in concrete. Constr. Build. Mater. 98, 767-779. https://doi.org/10.1016/j.conbuildmat.2015.08.122.
).
Given these physicochemical characteristics, SSA has been evaluated as
raw material to produce blended PC, mortars, bricks, ceramics and glass (1919.
Smol, M.; Kulczycka, J.; Henclik, A.; Gorazda, K.; Wzorek, Z. (2015)
The possible use of sewage sludge ash (SSA) in the construction industry
as a way towards a circular economy. J. Clean. Prod. 95, 45-54. http://doi.org/10.1016/j.jclepro.2015.02.051.
, 22-2822.
Baeza-Brotons, F.; Garcés, P.; Payá, J.; Saval, J.M. (2014) Portland
cement systems with addition of sewage sludge ash. Application in
concretes for the manufacture of blocks. J. Clean. Prod. 82, 112-124. https://doi.org/10.1016/j.jclepro.2014.06.072.
23.
Chen, Z.; Poon, C.S. (2017) Comparative studies on the effects of
sewage sludge ash and fly ash on cement hydration and properties of
cement mortars. Constr. Build. Mater. 154, 791-803. https://doi.org/10.1016/j.conbuildmat.2017.08.003.
24.
Monzó, J.; Payá, J.; Borrachero, M.V.; Girbés, I. (2003) Reuse of
sewage sludge ashes (SSA) in cement mixtures: the effect of SSA on the
workability of cement mortars. Waste Manag. 23 [4], 373-381. https://doi.org/10.1016/S0956-053X(03)00034-5.
25.
Pérez-Carríon M.T.; Baeza Brotons, F.; Garcés, P.; Galao Malo, O.; Payá
Bernabeu, J. (2013) Potencial use of sewage sludge ash as a fine
aggregate replacement in precast concrete blocks. Dyna-Colombia. 80 [179], 142-150.
26.
Tarrago, M.; Garcia-Valles, M.; Aly, M.H.; Martínez, S. (2017)
Valorization of sludge from a wastewater treatment plant by
glass-ceramic production. Ceram. Int. 43 [1], 930-937. https://doi.org/10.1016/j.ceramint.2016.10.083.
27.
Yusuf, R.O.; Noor, Z.Z.; Din, M.F.M.; Abba, A.H. (2012) Use of sewage
sludge ash (SSA) in the production of cement and concrete - a review. Int. J. Glob. Environ. 12, 214. https://doi.org/10.1504/IJGENVI.2012.049382.
28.
Pérez-Carrión, M.; Baeza-Brotons, F.; Payá, J.; Saval, J.M.; Zornoza,
E.; Borrachero, M.V.; Garcés, P. (2014) Potential use of sewage sludge
ash (SSA) as a cement replacement in precast concrete blocks. Mater. Construcc. 64 [313], e002. https://doi.org/10.3989/mc.2014.06312.
).
Geopolymer systems seem to be another sustainable alternative for the
immobilization of SSA. However, such an SSA application has not yet been
properly explored, being found a few studies focused on this field (29-3129.
Yamaguchi, N.; Ikeda, K. (2010) Preparation of geopolymeric materials
from sewage sludge slag with special emphasis to the matrix
compositions. J. Ceram. Soc. Japan. 118 [1374], 107-112. https://doi.org/10.2109/jcersj2.118.107.
30.
Istuque, D.B.; Reig, L.; Moraes, J.C.B.; Akasaki, J.L.; Borrachero,
M.V.; Soriano, L.; Payá, J.; Malmonge, J.A.; Tashima, M.M. (2016)
Behaviour of metakaolin-based geopolymers incorporating sewage sludge
ash (SSA). Mater. Lett. 180, 192-195. https://doi.org/10.1016/j.matlet.2016.05.137.
31.
Istuque, D.B.; Soriano, L.; Akasaki, J.L.; Melges, J.L.P.; Borrachero,
M.V.; Monzó, J.; Payá, J.; Tashima, M. (2019) Effect of sewage sludge
ash on mechanical and microstructural properties of geopolymers based on
metakaolin. Constr. Build. Mater. 203, 95-103. https://doi.org/10.1016/j.conbuildmat.2019.01.093.
).
Yamaguchi and Ikeda (2929.
Yamaguchi, N.; Ikeda, K. (2010) Preparation of geopolymeric materials
from sewage sludge slag with special emphasis to the matrix
compositions. J. Ceram. Soc. Japan. 118 [1374], 107-112. https://doi.org/10.2109/jcersj2.118.107.
)
reported a binary geopolymer produced by fly ash and SSA. At room
temperature, this system had setting problems, and its compressive
strength was inadequate at high temperatures (80°C). However, the work
by Istuque et al. (3030.
Istuque, D.B.; Reig, L.; Moraes, J.C.B.; Akasaki, J.L.; Borrachero,
M.V.; Soriano, L.; Payá, J.; Malmonge, J.A.; Tashima, M.M. (2016)
Behaviour of metakaolin-based geopolymers incorporating sewage sludge
ash (SSA). Mater. Lett. 180, 192-195. https://doi.org/10.1016/j.matlet.2016.05.137.
)
on another binary system reported promising results for a geopolymer
produced by SSA and MK (SSA/MK-based geopolymer), with 10 wt.% SSA and
90 wt.% MK cured at room temperature (25°C) for 7 days. According to
these authors, the SSA/MK-based geopolymer presented a similar
compressive strength (≈28 MPa) to the geopolymer reference, which
composed only MK as a precursor. In another study, Istuque et al. (3131.
Istuque, D.B.; Soriano, L.; Akasaki, J.L.; Melges, J.L.P.; Borrachero,
M.V.; Monzó, J.; Payá, J.; Tashima, M. (2019) Effect of sewage sludge
ash on mechanical and microstructural properties of geopolymers based on
metakaolin. Constr. Build. Mater. 203, 95-103. https://doi.org/10.1016/j.conbuildmat.2019.01.093.
)
demonstrated how the SSA/MK-based geopolymer (10 wt.% SSA and 90 wt.%
MK) was activated by an activating solution with a NaOH concentration of
8 mol.kg-1 and an SiO2/Na2O molar
ratio of 1.6, which reached compressive strength to about 50 MPa after
14 curing days at 25°C. Nevertheless, their study reported research only
into the mechanical development of the SSA/MK-based geopolymer for up
to 180 curing days with low SSA content in the geopolymer composition
(10 wt.%). It would be interesting to increase the SSA content by
replacing MK because this last component is a synthetic material that
requires the use of non-renewable raw material and a considerable power
supply (3232. Khatib, J.M.; Baalbaki, O.; ElKordi, A.A. (2018) Metakaolin. In: Waste Supplem. Cement. Mater. Concr.: Charact. Prop. Applicat. 493-511. https://doi.org/10.1016/B978-0-08-102156-9.00015-8.
);
moreover, it is desirable to increase the content of SSA in the
mixtures because immobilization will be a key issue in the SSA
sustainable management.
Therefore, this work aimed to evaluate the compressive strength development of the SSA/MK-based geopolymer with different SSA contents (0, 10, 20 and 30 wt.%) cured at room temperature (25°C) from 3 to 720 curing days. The mineralogy of geopolymeric pastes was investigated by X-ray diffraction (XRD), as well as the microstructure by scanning electron microscopy (SEM).
2. EXPERIMENTAL PROCEDURES
⌅2.1. Materials
⌅2.1.1. Sewage sludge ash production
⌅Dried-granular sewage sludge, collected at a wastewater treatment plant (Serviço Municipal Autônomo de Água e Esgoto - SEMAE) in São José do Rio Preto city, SP, Brazil, was incinerated by a process called uncontrolled autocombustion in a cylindrical oven (200-liter volume). About 20 kg of dry sewage sludge were placed inside the oven. Only 1 minute of gas fire was necessary to start the combustion process. A thermocouple was placed in the center of the oven to record combustion temperature. After 15 h, the combustion process ended and about 8.6 kg of SSA was obtained (43% of the dry sewage sludge mass). Figure 1 shows the maximum average temperature of 774°C, which was generally reached after 3 h of combustion. The ash (SSA) from the uncontrolled combustion was ground for 50 minutes at an SSA/ball weight ratio of 0.10. The milled SSA had a mean particle diameter of 20.28 µm, with d(0.1), d(0.5) and d(0.9) being 1.58 µm, 11.17 µm and 52.45 µm, respectively. As can be seen in Table 1, SSA was mainly composed of SiO2 (38.28%) and Al2O3 (35.47%). In accordance with the XRD pattern of SSA, which was showed in Figure 5, quartz (SiO2), anhydrite (CaSO4) and hematite (Fe2O3) were the main secondary phases. The insoluble residue content of SSA was 27.20%, that since the sum of SiO2 (38.28%), Al2O3 (20.72%) and Fe2O3 (11.27%) percentages from the SSA chemical composition was 70.17%, and considering that the crystalline phase dissolution during the insoluble residue test is very low, could be inferred that at least significant percentage of the SiO2 was amorphous, as well as a considerable amount of Al2O3. The presence of amorphous phases in the SSA was pointed out by the slight deviation of the baseline between 18-32° 2θ in the XRD pattern of SSA (See section 3.2.2.).
| Oxides (%) | SiO2 | Al2O3 | Fe2O3 | P2O3 | CaO | SO3 | TiO2 | MgO | K2O | Na2O | Others | LOI |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| MK | 58.39 | 35.47 | 2.71 | - | 0.01 | - | 1.51 | 0.3 | 1.44 | - | 0.07 | 0.10 |
| SSA | 38.28 | 20.72 | 11.27 | 7.28 | 5.51 | 4.18 | 3.73 | 1.91 | 0.73 | 0.70 | 1.97 | 3.72 |
2.1.2. Other materials
⌅MK was supplied by Metacaulim do Brasil™ to produce geopolymeric pastes and mortars, whose chemical composition is presented in Table 1. NaOH pellets (98% of purity) and sodium silicate solution (waterglass, 61.4% H2O, 29.7% SiO2 e 8.9% Na2O) were used to prepare the activating solution. Water was employed to adjust the water/binder ratio (binder = MK+SSA) of the geopolymeric pastes and mortars. Siliceous sand from Castilho city - SP, Brazil, was obtained to produce the geopolymeric mortars with a fineness modulus of 2.05 and a specific gravity of 2.67 g/cm3.
2.2. Producing geopolymeric pastes and mortars
⌅This study was divided into two steps (see Figure 2).
In Step 1, the optimum NaOH concentration was achieved for the mortars
with 10 wt.% of SSA and 90 wt.% of MK cured at 3 and 7 days at room
temperature (25ºC). Activating solutions with different NaOH
concentrations and SiO2/Na2O molar ratio (ε) of 8 mol.kg-1 (ε=1.6), 10 mol.kg-1 (ε=1.3) and 12 mol.kg-1
(ε=1.0) were prepared. The waterglass/binder mass ratio was set at 0.78
for all the mortars assessed in Step 1. A previous study (3131.
Istuque, D.B.; Soriano, L.; Akasaki, J.L.; Melges, J.L.P.; Borrachero,
M.V.; Monzó, J.; Payá, J.; Tashima, M. (2019) Effect of sewage sludge
ash on mechanical and microstructural properties of geopolymers based on
metakaolin. Constr. Build. Mater. 203, 95-103. https://doi.org/10.1016/j.conbuildmat.2019.01.093.
), which evaluated the influence of the SiO2/Na2O molar ratio for the mortars containing 8 mol.kg-1 of NaOH, established that a SiO2/Na2O ratio of 1.6 offered the best compressive strength results.
With the optimum NaOH concentration, the influence of SSA content (10, 20, 30 wt.% replacing MK) on the long-term compressive strength of the MK-based geopolymeric mortars was assessed in Step 2. Compressive strength tests were carried out at 3, 7, 28, 90 and 720 curing days at 25°C (climatic chamber, relative humidity of 95%). X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were conducted to corroborate the mechanical results.
The dosage of each geopolymeric mortar in Steps 1 and 2 are shown in Table 2. The nomenclature for the mortars from Step 1 was adopted: MKx y(ε), where x represents the percentage of SSA, y represents the NaOH concentration, and ε represents the SiO2/Na2O molar ratio. The nomenclature for the mortars from Step 2 was simplified to MKx,
considering that all of them present the same values of NaOH
concentration and ε according to the given results from Step 1. In both
Steps 1 and 2, all the geopolymeric mortars were prepared by maintaining
the water/binder and sand/binder mass ratios at 0.6 and 2.5,
respectively (3131.
Istuque, D.B.; Soriano, L.; Akasaki, J.L.; Melges, J.L.P.; Borrachero,
M.V.; Monzó, J.; Payá, J.; Tashima, M. (2019) Effect of sewage sludge
ash on mechanical and microstructural properties of geopolymers based on
metakaolin. Constr. Build. Mater. 203, 95-103. https://doi.org/10.1016/j.conbuildmat.2019.01.093.
).
Precursors (MK and SSA) were homogeneously mixed before adding the
activating solution, which was cooled to room temperature (25°C). After
obtaining a homogeneous geopolymeric paste, sand was gradually added.
The whole mechanical mixing process took 5 minutes. The geopolymeric
mortars were molded in prismatic molds (4×4×16 cm3) and left
for 1 minute on the vibration table (35 Hz) to release any incorporated
air. The geopolymeric mortars were demolded after 24 h and stored in the
climatic chamber until testing ages were reached.
| Binder | Activating solution | water/binder | ||||
|---|---|---|---|---|---|---|
| MK (wt.%) | SSA (wt.%) | [NaOH] | ε | |||
| Step 1 | MK10 8 | 90 | 10 | 8 mol.kg-1 | 1.6 | 0.6 |
| MK10 10 | 10 mol.kg-1 | 1.3 | ||||
| MK10 12 | 12 mol.kg-1 | 1.0 | ||||
| Step 2 | MK0 | 100 | - | [NaOH]-ε from Step 1 | ||
| MK10 | 90 | 10 | ||||
| MK20 | 80 | 20 | ||||
| MK30 | 70 | 30 | ||||
2.3 Compressive strength tests
⌅Compressive strengths were measured by an EMIC Universal machine according to UNE 196-1:2018 (3333. UNE-EN 196-1 (2018), Methods of Testing Cement - Part 1: Determination of Strength. https://www.en-standard.eu/une-en-196-1-2018-methods-of-testing-cement-part-1-determination-of-strength/.
). A device to apply the load in a 4×4 cm2
area on two opposite sample faces was used in these tests. Compressive
strength was calculated as the average of at least five values.
2.4. Characterization of geopolymeric pastes
⌅The characterization of the geopolymeric pastes (MK + SSA + activating solution) based on the XRD and SEM analyses were carried out for the selected geopolymeric pastes assessed in Step 2 (see Figure 2). The mix proportion was the same as that used to prepare mortars without adding sand. A Shimadzu XRD-6000 system was employed to obtain the XRD patterns within a 2θ range of 5-60° in an angle step of 0.02° and a step time of 1.20 s/step. Cu-Kα radiation and an Ni filter were used at a voltage of 30 kV and a current intensity of 40 mA. The SEM images were taken by a ZEISS microscopic (model EVO LS15) from the fractured surface pastes covered with gold.
3. RESULTS AND DISCUSSION
⌅3.1. The results obtained from Step 1: selection of the optimum NaOH concentration
⌅3.1.1. Compressive strength
⌅In Step 1, three different NaOH concentrations (8 mol.kg-1 (ε=1.6), 10 mol.kg-1 (ε=1.3), 12 mol.kg-1 (ε=1.0)) in the activating solution were evaluated in the geopolymeric mortars with 10 wt.% SSA and 90 wt.% MK. The compressive strengths of those geopolymeric mortars cured for 3 and 7 days at room temperature are depicted in Figure 3.
The
increasing NaOH concentration lowered the compressive strength of the
SSA/MK-based geopolymeric mortars for both curing times (3 and 7 days).
The mortars prepared using the 8 mol.kg-1 NaOH and ε=1.6
(MK10 8(1.6)) activating solution yielded the highest compressive
strengths (30.9 and 31.9 MPa at 3 and 7 days, respectively) compared to
the other geopolymeric mortars. The literature reports that a NaOH
concentration of approximately 8 M is often used to produce MK-based
geopolymers (3434.
Fernández-Jiménez, A.; Cristelo, N.; Miranda, T.; Palomo, Á. (2017)
Sustainable alkali activated materials: Precursor and activator derived
from industrial wastes. J. Clean. Prod. 162, 1200-1209. https://doi.org/10.1016/j.jclepro.2017.06.151.
, 3535.
Ozer, I.; Soyer-Uzun, S. (2015) Relations between the structural
characteristics and compressive strength in metakaolin based geopolymers
with different molar Si/Al ratios. Ceram. Int. 41 [8], 10192-10198. https://doi.org/10.1016/j.ceramint.2015.04.125.
). Kuenzel et al. (3636.
Kuenzel, C.; Neville, T.P.; Donatello, S.; Vandeperre, L.; Boccaccini,
A.R.; Cheeseman, C.R. (2013) Influence of metakaolin characteristics on
the mechanical properties of geopolymers. Appl. Clay Sci. 83-84, 308-314. https://doi.org/10.1016/j.clay.2013.08.023.
)
assumed that the NaOH concentration around 8 M is enough for a proper
dissolution of the reactive phases of metakaolinite. Moreover, it is
well-known that as higher the NaOH concentration, faster is the MK
geopolymerization, though a higher geopolymerization rate could lead to a
lower polycondensation degree of the geopolymer, affecting the
compressive strength development (3737. Singh, N.B.; Middendorf, B. (2020) Geopolymers as an alternative to Portland cement: An overview. Constr. Build. Mater. 237, 117455. https://doi.org/10.1016/j.conbuildmat.2019.117455.
).
The findings herein agree with such understanding. Hence, the
dissolution of the amorphous phase of the system SSA/MK at 8 mol.kg-1
NaOH lead to a proper geopolymerization ratio, which results in higher
compressive strength development. At the highest NaOH concentration (12
mol.kg-1), the compressive strength loss (3.8MPa), after 7
curing days, could be related to somewhat zeolite formation from the
geopolymer matrix, which can occur for MK-based geopolymer systems even
at room temperature (3838.
Granizo, N.; Palomo, A.; Fernandez-Jiménez, A. (2014) Effect of
temperature and alkaline concentration on metakaolin leaching kinetics. Ceram. Int. 40 [7], 8975-8985. https://doi.org/10.1016/j.ceramint.2014.02.071.
).
The following evaluations proposed in Step 2 were made with the
geopolymeric mortars and pastes using the activating solution of 8
mol.kg-1 NaOH (ε=1.6).
3.2. The results obtained from Step 2: influence on SSA percentage
⌅All the evaluations in Step 2 were carried out with the geopolymeric pastes and mortars using an activating solution of 8 mol.kg-1 NaOH (ε=1.6), varying the content of SSA in 10 wt.% (MK10), 20 wt.% (MK20), and 30 wt.% (MK30).
3.2.1. Compressive strength
⌅The compressive strength of geopolymer mortars MK0, MK10, MK20 and MK30 at 3, 7, 28, 90 and 720 days were measured and are depicted in Figure 4. The geopolymeric mortar containing no SSA (MK0) content was set as a reference.
The reference geopolymeric mortar (MK0) presented a higher compressive strength for all the curing times (3 to 720 days), except for sample MK10, whose compressive strength was similar at 720 days. The compressive strength of the MK0 mortar was 66.9±2.9 MPa, while that recorded for the MK10 mortar was 60.7±4.3 MPa, both recorded at 720 days. According to the Tukey test, the difference between the average compressive strength of these mortars (MK0 and MK10) was not significant at the 0.05 level.
Furthermore, the increasing SSA
content in the geopolymeric mortars decreased the compressive strength
for the first and last curing ages. Although SSA had displayed a
reactive and synergic behaviour in Portland cement systems in accordance
with previous works (2222.
Baeza-Brotons, F.; Garcés, P.; Payá, J.; Saval, J.M. (2014) Portland
cement systems with addition of sewage sludge ash. Application in
concretes for the manufacture of blocks. J. Clean. Prod. 82, 112-124. https://doi.org/10.1016/j.jclepro.2014.06.072.
,2828.
Pérez-Carrión, M.; Baeza-Brotons, F.; Payá, J.; Saval, J.M.; Zornoza,
E.; Borrachero, M.V.; Garcés, P. (2014) Potential use of sewage sludge
ash (SSA) as a cement replacement in precast concrete blocks. Mater. Construcc. 64 [313], e002. https://doi.org/10.3989/mc.2014.06312.
), it showed different behaviour in the alkali-activation system. According to Cheng et al. (3939. Cheng, H.; Lin, K.L.; Cui, R.; Hwang, C.L.; Chang, Y.M.; Cheng, T.W. (2015) The effects of SiO2/Na2O molar ratio on the characteristics of alkali-activated waste catalyst-metakaolin based geopolymers. Constr. Build. Mater. 95, 710-720. https://doi.org/10.1016/j.conbuildmat.2015.07.028.
),
MK-based geopolymer develops high compressive strength, mainly because
of the large surface area and high reactivity of MK, which lead to
greater geopolymerization due to increasing dissolved aluminosilicate
phases. Zhu et al. (4040.
Zhu, H.; Liang. G.; Zhang, Z.; Wu, Q.; Du, J. (2019) Partial
replacement of metakaolin with thermally treated rice husk ash in
metakaolin-based geopolymer. Constr. Build. Mater. 221, 527-538. https://doi.org/10.1016/j.conbuildmat.2019.06.112.
)
evaluated the replacement of MK by other material as reactive as MK, in
thermal-treated geopolymer production, that was RHA. According to the
authors, a replacement content of MK by RHA in 20% offered a significant
compressive strength enhancement around 62.5% and 21.7% at 7 and 28
curing days at 50°C, respectively. In such a study, the compressive
strength enhancements were attributed to the enrichment of the content
and nature of the gel due to the dissolution of the silica from RHA. In
this current work, increasing MK replacement with SSA, that presented a
lower content of amorphous phases containing alumina and, mainly, silica
compared to MK, reduces compressive strength likely due to a sum of a
dilution effect and a changing of the nature of the gel formed, which
could occur when the SiO2/Al2O3 and Na2O/Al2O3 ratios of the system are changed (4141. Sarkar, M.; Dana, K. (2021) Partial replacement of metakaolin with red ceramic waste in geopolymer. Ceram. Int. 47 [3], 3473-3483. https://doi.org/10.1016/j.ceramint.2020.09.191.
).
Such effects could explain the decrease of the compressive strength
higher than the MK replacement percentage in the mortars. However, at
intermediate curing times, the increasing SSA content had no significant
effect on compressive strength.
Although a partial MK replacement
with SSA lowered the geopolymerization rate, the reaction progressed
for an extended duration. The geopolymerization progress was clearly
identified by an increasing compressive strength gain by rising the SSA
content between 3 and 720 days. The compressive strength gain was 96.4%,
96.7% and 143.6% for geopolymeric mortars MK10, MK20 and MK30,
respectively, but was only 43.9% for MK0 for the same time frame. This
behaviour means that the SSA reaction rate was slower than for MK, and
the geopolymerization process for SSA/MK systems required a longer
curing time. Zhang et al. (4242.
Zhang, Z.; Wang, H.; Zhu, Y.; Reid, A.; Provis, J.L.; Bullen, F. (2014)
Using fly ash to partially substitute metakaolin in geopolymer
synthesis. Appl. Clay Sci. 88-89, 194-201. https://doi.org/10.1016/j.clay.2013.12.025.
)
studied the influence of MK replacement by fly ash on the reaction
process in the MK-based geopolymers. According to the authors, the MK
replacement of 10% by fly ash increased the reaction extent due to the
lower dissolution of fly ash compared to the MK. The results obtained in
the study herein are endorsed by such a report.
3.2.2. XRD analyses
⌅The
XRD patterns of the geopolymeric pastes with an equivalent mix
proportion of mortars MK0, MK10 and MK30 were determined at both 90
curing days and 25°C. According to the XRD patterns in Figure 5, the crystalline phases of SSA were quartz (SiO2, PDFcard#331161), hematite (Fe2O3, PDFcard#130534) and anhydrite (CaSO4, PDFcard#371496), as mentioned previously in item 2.1.1, while those of MK were quartz (SiO2, PDFcard#331161), kaolinite (Al2Si2O5(OH)4, PDFcard#140164) and muscovite (KAl3Si3O10(OH)2,
PDFcard#210993). Regarding the XRD patterns of the geopolymeric pastes,
all the pastes presented the same crystalline phases identified in the
XRD pattern of the precursors because of the non reacted phases of MK
and SSA. As reported by Belmokhtar (4343.
Belmokhtar, N.; Ammari, M.; Brigui, J.; Allal, B.L. (2017) Comparison
of the microstructure and the compressive strength of two geopolymers
derived from Metakaolin and an industrial sludge. Constr. Build. Mater. 146, 621-629. https://doi.org/10.1016/j.conbuildmat.2017.04.127.
),
the structures of muscovite and quartz are not affected during
geopolymeric, which means that those phases do not react and,
consequently, do not offer any contribution to developing cementing gel.
This result was corroborated by the compressive strength results: the
samples containing SSA had lower compressive strengths because of the
low reactivity of ash. In addition, in the XRD patterns of all samples
containing SSA was not identified zeolite peaks, pointing out any
zeolite formation from the geopolymer matrix, which could lead to
compressive strength loss. Such a result endorses that the lower
compressive strength of the samples MK10 and MK30 are more related to a
dilution effect and a changing the nature of gel formed due to SSA
addition.
Diffuse
halo signals were noted at about 16° - 34° and 19° - 32° of the 2θ
angle in the XRD patterns of MK and SSA, respectively, which indicated
the presence of amorphous phases (4444.
Aboulayt, A.; Jaafri, R.; Samouh, H.; Cherki, El Idrissi, A.C.;
Roziere, E.; Moussa, R.; Loukili, A (2018) Stability of a new geopolymer
grout: Rheological and mechanical performances of metakaolin-fly ash
binary mixtures. Constr. Build. Mater. 181, 420-436. https://doi.org/10.1016/j.conbuildmat.2018.06.025.
)
in the composition of the precursors. These amorphous halo signals were
displaced to 18° - 38° in the XRD patterns of geopolymeric pastes MK0,
MK10 and MK30. This displacement denoted the occurrence of a
geopolymeric reaction, which yielded new amorphous phases as N-A-S-H gel
(3434.
Fernández-Jiménez, A.; Cristelo, N.; Miranda, T.; Palomo, Á. (2017)
Sustainable alkali activated materials: Precursor and activator derived
from industrial wastes. J. Clean. Prod. 162, 1200-1209. https://doi.org/10.1016/j.jclepro.2017.06.151.
, 4444.
Aboulayt, A.; Jaafri, R.; Samouh, H.; Cherki, El Idrissi, A.C.;
Roziere, E.; Moussa, R.; Loukili, A (2018) Stability of a new geopolymer
grout: Rheological and mechanical performances of metakaolin-fly ash
binary mixtures. Constr. Build. Mater. 181, 420-436. https://doi.org/10.1016/j.conbuildmat.2018.06.025.
, 4545.
Timakul, P.; Rattanaprasit, W.; Aungkavattana, P. (2016) Improving
compressive strength of fly ash-based geopolymer composites by basalt
fibers addition. Ceram. Int. 42 [5], 6288-6295. https://doi.org/10.1016/j.ceramint.2016.01.014.
).
When comparing the XRD pattern of pastes MK0 and the others containing
SSA (MK10 and MK30), no significant difference was found for pastes MK10
and MK30, except for the remaining crystalline phases from SSA.
3.2.3. SEM analyses
⌅The
microstructural analyses were carried out using SEM characterization on
pastes MK0, MK10 and MK30. Paste MK10 was chosen to be analysed by SEM
for its low SSA content, along with paste MK30 because it had the
highest SSA content. The micrograph images of pastes MK0 were taken as
the reference. The micrograph images of samples MK0, MK10 and MK30 at
both 90 curing days and 25°C are presented in Figure 6.
At a magnification between 700 and 10000x, geopolymeric pastes MK0,
MK10 and MK30 presented a dense microstructure, described in the
literature as the result of N-A-S-H type gel forming by a geopolymeric
reaction (46-4846.
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. Constr. Build. Mater. 114, 276-289. https://doi.org/10.1016/j.conbuildmat.2016.03.184.
47.
Wan, Q.; Rao, F.; Song, S.; García, R.E.; Estrella, R.M.; Patiño, C.L.;
Zhang, Y. (2017) Geopolymerization reaction, microstructure and
simulation of metakaolin-based geopolymers at extended Si/Al ratios. Cem. Concr. Compos. 79, 45-52. https://doi.org/10.1016/j.cemconcomp.2017.01.014.
48.
Moraes, J.C.B.; Tashima, M.M.; Akasaki, J.L.; Melges, J.L.P.; Monzó,
J.; Borrachero, M.V.; Soriano, L.; Payá, J. (2017) Effect of sugar cane
straw ash (SCSA) as solid precursor and the alkaline activator
composition on alkali-activated binders based on blast furnace slag
(BFS). Constr. Build. Mater. 144, 214-224. https://doi.org/10.1016/j.conbuildmat.2017.03.166.
). The XRD analysis did not reveal the presence of zeolitic phases by the chemical evolution of N-A-S-H gel (3030.
Istuque, D.B.; Reig, L.; Moraes, J.C.B.; Akasaki, J.L.; Borrachero,
M.V.; Soriano, L.; Payá, J.; Malmonge, J.A.; Tashima, M.M. (2016)
Behaviour of metakaolin-based geopolymers incorporating sewage sludge
ash (SSA). Mater. Lett. 180, 192-195. https://doi.org/10.1016/j.matlet.2016.05.137.
). The SEM micrographs confirmed the absence of crystalline zeolitic-type particles.
The non-formation of these zeolitic phases agrees with compressive strength increasing for long curing times (3030.
Istuque, D.B.; Reig, L.; Moraes, J.C.B.; Akasaki, J.L.; Borrachero,
M.V.; Soriano, L.; Payá, J.; Malmonge, J.A.; Tashima, M.M. (2016)
Behaviour of metakaolin-based geopolymers incorporating sewage sludge
ash (SSA). Mater. Lett. 180, 192-195. https://doi.org/10.1016/j.matlet.2016.05.137.
, 4949.
Król, M.; Rożek, P.; Chlebda, D.; Mozgawa, W. (2019) ATR/FT-IR studies
of zeolite formation during alkali-activation of metakaolin. Solid State Sci. 94, 114-119. https://doi.org/10.1016/j.solidstatesciences.2019.06.004.
). There was a significant difference in the microstructure organisation among pastes MK0 (Figure 6a and 6b), MK10 (Figure 6c and 6d) and MK30 (Figure 6e and 6f). Many cementing particles (N-A-S-H type cementing gel) were observed for MK0 (Figure 6b). When a small amount of MK was replaced with SSA (MK10), the same type of particles were observed, but in smaller numbers (Figure 6d).
These particles were bonded to the amorphous matrix (probably formed
with N-A-S-H gel and the unreacted SSA particles). Finally, for sample
MK30, fewer characteristic gel particles were observed in MK0 and the
bonding matrix among them was larger. The changes in the microstructure
of the cementing matrix of geopolymers due to the presence of SSA could
explain the differences observed in mechanical strength development.
In order to analyze the potential contribution of SSA in the geopolymeric reaction, SSA-activated pastes were prepared (maintaining the same proportions in the activating solution and the activator/SSA ratio than those for MK/SSA systems previously described. A 28-days (room temperature cured) SSA activated paste was prepared and low strength was reached (<10MPa): this value means that SSA had a limited reactivity in the activating conditions selected. However, there is a contribution to the development of binding gels. SEM micrographs of the SSA and the activated SSA samples were taken (Figure 7). Figure 7a shows the irregular morphology of SSA particles and figure 7b shows the presence of platelet-like particles attributed to decomposed clay compounds. Figure 7c shows the microstructure of the binding gel formed by geopolymeric reaction; Figure 7d shows a detailed microstructure of the N-A(F)-S-H gel (due to the presence of iron compounds in the SSA, the gel also contained this element) and a reacted plate-like SSA particle. The gel was formed by the agglutination of nanoparticles (less than 200 nm) and its composition (EDS) presented a SiO2/(Al2O3+Fe2O3) molar ratio of 3.6 and (Al2O3+Fe2O3)/Na2O molar ratio of 0.62.
4. CONCLUSIONS
⌅The aim of the paper was to evaluate the mechanical development of MK-based geopolymers containing SSA by microscopical and mineralogical analyses, and compressive strength development at long curing times. According to the results, the key conclusions were obtained:
The SSA/MK-based geopolymeric mortars activated with an activating solution of NaOH and Na2SiO3 at a NaOH concentration of 8 mol.kg-1 (ε=1.6) presented higher compressive strength.
The geopolymeric mortars containing 10 wt.% of SSA (MK10) had a compressive strength of 60.7±4.3 MPa for 720 curing days at 25°C, which was similar to the reference geopolymeric mortar (MK0), whose compressive strength was 66.9±2.9 MPa for the same curing conditions.
Although SSA had a delaying effect on the mechanical development of the MK-based geopolymer, the compressive strength gain between 3 and 720 curing days at 25°C was maximum (143.6%) in the sample with the highest SSA content (MK30) vs. 43.9% of the compressive strength gain for reference sample MK0.
Although the increasing SSA content diminished the compressive strength of the geopolymeric mortars, all the evaluated SSA contents (10, 20, 30 wt.%) had a compressive strength over 40 MPa for 720 curing days at 25°C.
The microstructure of the geopolymeric pastes containing SSA (10 and 30 wt.%) was morphologically changed by the SSA incorporation in the geopolymer matrix. It presented fewer N-A-S-H type gel with morphology similar to the one observed for the reference geopolymer paste with MK only. Such behaviour was considered key to explain the differences in compressive strength development.