1. INTRODUCTION
⌅In today’s world, concrete is the next most widely used material after water (11.
Gagg CR. 2014. Cement and concrete as an engineering material: An
historic appraisal and case study analysis. Eng. Fail. Anal. 40:114–40.
).
The increasing demand for concrete is owed to infrastructure
development and urbanization around the globe. The high consumption of
concrete tends to increase the demand for Ordinary Portland Cement (OPC)
as it is utilized as a binder in concrete production. The manufacturing
of OPC uses a significant amount of natural resources, as well as emits
a huge quantity of carbon dioxide (CO2) into the atmosphere (22.
Naqi A, Jang JG. 2019. Recent progress in green cement technology
utilizing low-carbon emission fuels and raw materials: a review.
Sustain. 11(2):537. https://doi.org/10.3390/SU11020537.
).
It is impossible to overstate the increasing effects of the production
of OPC on the environment. To address this problem, a geopolymer binder
is recommended as an alternate to OPC for use in the concrete industry.
In 1978, Geopolymer was first introduced by Prof. Davidovits as an
alternative to the conventional binder (33. Davidovits J. 2017. Geopolymers: Ceramic-like inorganic polymers. J. Ceram. Sci. Technol. 8(3):335–50. https://doi.org/10.4416/JCST2017-00038.
). Geopolymer is a three-dimensional aluminosilicate binder that can reduce CO2 emissions by 22 % to 72 % and it costs nearly the same as OPC concrete (44.
Amran M, Debbarma S, Ozbakkaloglu T. 2021. Fly ash-based eco-friendly
geopolymer concrete: A critical review of the long-term durability
properties. Constr. Build. Mater. 270:121857. https://doi.org/10.1016/j.conbuildmat.2020.121857.
).
The geopolymer is prepared by a reaction mechanism of a combination of
aluminosilicate-rich source materials with a high alkali activator
solution to form a 3D structure of tetrahedral bonds of silicate and
aluminates (55. Cong P, Cheng Y. 2021. Advances in geopolymer materials: A comprehensive review. J. Traffic. Transp. Eng. 8(3):283–314. https://doi.org/10.1016/j.jtte.2021.03.004.
).
Aluminosilicate-rich materials such as Fly Ash (FA), Rice Husk Ash
(RHA), Ground Granulated Blast Furnace Slag (GGBS), and Metakaolin (MK)
can be used as geopolymer binder (66.
Castel A, Foster SJ, Ng T, Sanjayan JG, Gilbert RI. 2016. Creep and
drying shrinkage of a blended slag and low calcium fly ash geopolymer
Concrete. Mater. Struct. Constr. 49(5):1619–28. https://doi.org/10.1617/s11527-015-0599-1.
, 77.
Kong DLY, Sanjayan JG. 2010. Effect of elevated temperatures on
geopolymer paste, mortar and concrete. Cem. Concr. Res. 40(2):334–339. https://doi.org/10.1016/J.CEMCONRES.2009.10.017.
).
The sodium/potassium hydroxide and sodium/potassium silicate are
commonly used alkali activator solutions with geopolymer binder (88.
Assi LN, Carter K, Deaver E, Ziehl P. 2020. Review of availability of
source materials for geopolymer/sustainable concrete. J. Clean. Prod.
263:121477. https://doi.org/10.1016/j.jclepro.2020.121477.
).
The use of FA as an aluminosilicate source material in the geopolymer
was suggested by several studies due to the high concentration of silica
(Si) and alumina (Al) concentrations, its high abundance, and the ready
availability of this material (9-129.
Phoo-Ngernkham T, Chindaprasirt P, Sata V, Pangdaeng S, Sinsiri T.
2013. Properties of high calcium fly ash geopolymer pastes with Portland
cement as an additive. Int. J. Miner. Metall. Mater. 20(2):214–220. https://doi.org/10.1007/s12613-013-0715-6.
10.
Nath SK, Maitra S, Mukherjee S, Kumar S. 2016. Microstructural and
morphological evolution of fly ash based geopolymers. Constr. Build.
Mater. 111:758–765. https://doi.org/10.1016/J.CONBUILDMAT.2016.02.106.
11.
Hadi MNS, 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.
12.
Wazien AZW, Abdullah MMAB, Abd Razak R, Rozainy MAZMR, Tahir MFM. 2016.
Strength and density of geopolymer mortar cured at ambient temperature
for use as repair material. IOP Conf. Ser. Mater. Sci. Eng.
133(1):012042. https://doi.org/10.1088/1757-899X/133/1/012042.
). Fly ash is a by-product of the burning of coal in thermal power plants (1313.
Yao ZT, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, et al. 2015. A
comprehensive review on the applications of coal fly ash. Earth-Science
Reviews Elsevier. 105–21.
). Additionally, using FA in the building process lessens the load on landfills, which lowers land contamination. (1414.
De Rossi A, Ribeiro MJ, Labrincha JA, Novais RM, Hotza D, Moreira RFPM.
2019. Effect of the particle size range of construction and demolition
waste on the fresh and hardened-state properties of fly ash-based
geopolymer mortars with total replacement of sand. Process. Saf.
Environ. Prot. 129:130–137. https://doi.org/10.1016/j.psep.2019.06.026.
).
In the geopolymerisation process, the utilization of FA has issues with
delayed setting time and low gain in strength at an early age due to
their pozzolanic nature. The pozzolanic material (FA) has high silica
and alumina content with low cementitious properties which slow down the
chemical reaction delays the early setting time and gains in strength.
To get around that problem, either heat curing or the use of a
high-calcium (Ca) mineral is necessary to shorten the first setting time
and speed up the geopolymerization process to obtain early strength. (1515.
Adam AA, Horianto. 2014. The effect of temperature and duration of
curing on the strength of fly ash based geopolymer mortar. Procedia.
Eng. 95:410–414. https://doi.org/10.1016/j.proeng.2014.12.199.
, 1616.
Hardjito D, Wallah SE, Sumajouw DMJ, Rangan BV. 2004. On the
development of fly ash-based geopolymer concrete. ACI Mater. J.
101(6):467–72.
). It was observed that the Ca compounds
play a vital role in accelerating the chemical reaction and improving
the mechanical properties of the Geopolymer Mortar (GPM) (1717.
Nath P, Sarker PK, Rangan VB. 2015. Early age properties of low-calcium
fly ash geopolymer concrete suitable for ambient curing. Procedia. Eng.
125:601–607. https://doi.org/10.1016/j.proeng.2015.11.077.
).
The existence of OPC in the geopolymer matrix forms the additional Ca
hydrates gel which as a result enhances the Compressive Strength (CS).
The presence of dicalcium and tricalcium silicate in OPC helps to form
Ca hydrates which help to improve the strength, and durability
performance of geopolymers (1818.
Nath P, Sarker PK. 2015. Use of OPC to improve setting and early
strength properties of low calcium fly ash geopolymer concrete cured at
room temperature. Cem. Concr. Compos. 55:205–214. http://doi.org/10.1016/j.cemconcomp.2014.08.008.
). Shinde, et al. (1919. Shinde BH, Kadam KN. 2016. Properties of fly ash based geopolymer mortar with ambient curing. Int. J. Eng. Res. 8(1).
)
claimed that low calcium FA-based GPM and observed that with the
inclusion of a small amount of OPC content, the CS of mortar rises
significantly. Lodeiro et al. (2020.
Garcia-Lodeiro I, Palomo A, Fernández-Jiménez A, MacPhee DE. 2011.
Compatibility studies between N-A-S-H and C-A-S-H gels. Study in the
ternary diagram Na2O-CaO-Al2O3-SiO 2-H2O. Cem. Concr. Res.
41(9):923–931. https://doi.org/10.1016/j.cemconres.2011.05.006.
, 2121.
García-Lodeiro I, Fernández-Jiménez A, Palomo A. 2013. Variation in
hybrid cements over time. Alkaline activation of fly ash-portland cement
blends. Cem. Concr. Res. 52:112–122. https://doi.org/10.1016/j.cemconres.2013.03.022.
)
reported that the addition of Ca compounds in alkali-activated and
silica aluminate-rich materials formed the C-A-S-H/N-A-S-H gel which
helps to improve the properties. Similarly, Mehta and Siddique (2222.
Mehta A, Siddique R. 2017. Properties of low-calcium fly ash based
geopolymer concrete incorporating OPC as partial replacement of fly ash.
Constr. Build. Mater. 150:792–807. https://doi.org/10.1016/j.conbuildmat.2017.06.067.
)
also conclude that the substitution of 20% OPC in the place of FA
decreases the permeation qualities such as sorptivity, porosity, and
Water Absorption (WA). The results were further validated by
microstructure analysis, which revealed a considerable enhancement in
microstructure with the addition of OPC. However, OPC contains a good
amount of Ca content which can overcome the limitations of activation of
geopolymer reaction at ambient conditions but on the other hand
utilization of OPC can give rise to CO2 emissions into the
environment. Therefore, it is necessary to alter the utilization of OPC
with a geopolymer system by considering the other Ca-rich minerals which
can reduce the setting time of the FA-based geopolymer system and
enhance the geopolymer reactions at early stages of curing.
In
addition to this, to maintain sustainability in the construction
industry the utilization of Recycled Concrete (RC) is an important
factor to concern. The utilization of this RC is also beneficial to
reducing CO2 emissions, which conserves the environment from
global warming problems. The RC consists of different particles of
coarse and fine recycled aggregates. Fine particles (<4.75 mm) make
for about half of the recycled aggregates produced from RC (2323.
Fan CC, Huang R, Hwang H, Chao SJ. 2016. Properties of concrete
incorporating fine recycled aggregates from crushed concrete wastes.
Constr. Build. Mater. 112:708–715. https://doi.org/10.1016/j.conbuildmat.2016.02.154.
, 2424.
Diliberto C, Lecomte A, Aissaoui C, Mechling JM, Izoret L. 2021. The
incorporation of fine recycled concrete aggregates as a main constituent
of cement. Mater. Struct. Constr. 54(5). https://doi.org/10.1617/s11527-021-01796-6.
).
The hydrated cement paste, ettringite, and unhydrated cement are
present in the adherent mortar of the fine particles of RC. (2525.
Ashiquzzaman M, Hossen S. 2013. Cementing property evaluation of
recycled fine aggregate. Int. Ref. J. Eng. Sci. 2(5):63–8. Retrieved
from https://www.irjes.com/Papers/vol2-issue5/Version%20%201/I256368.pdf.
). The un-hydrated cement (25 to 30%) was present in the adhered mortar of fine particles of RC (2626.
Lv Z, Chen H. 2012. Modeling of self-healing efficiency for cracks due
to unhydrated cement nuclei in hardened cement paste. Procedia. Eng.
27:281–290. https://doi.org/10.1016/j.proeng.2011.12.454.
).
The anhydrous phase of adhered mortar plays an important function in
the hydration of cement in concrete and leads to the formation of
hydration products by producing more nucleation gel (2727.
Bordy A, Younsi A, Aggoun S, Fiorio B. 2017. Cement substitution by a
recycled cement paste fine: Role of the residual anhydrous clinker.
Constr. Build. Mater. 132:1–8. https://doi.org/10.1016/j.conbuildmat.2016.11.080.
).
Moreover, according to certain research, if RC fine particles are
further milled to increase their fineness and converted into a powder
which is termed Recycled Fine Powder (RFP) which can be used as an
activator with the primary binder to catalyze the geopolymer reaction
content for mortar and concrete production (2828.
Ahmari S, Ren X, Toufigh V, Zhang L. 2012. Production of geopolymeric
binder from blended waste concrete powder and fly ash. Constr. Build.
Mater. 35:718–729. https://doi.org/10.1016/j.conbuildmat.2012.04.044.
, 2929.
Ren P, Li B, Yu JG, Ling TC. 2020. Utilization of recycled concrete
fines and powders to produce alkali-activated slag concrete blocks. J.
Clean. Prod. 267:122115. https://doi.org/10.1016/j.jclepro.2020.122115.
). Bordy et al. (2727.
Bordy A, Younsi A, Aggoun S, Fiorio B. 2017. Cement substitution by a
recycled cement paste fine: Role of the residual anhydrous clinker.
Constr. Build. Mater. 132:1–8. https://doi.org/10.1016/j.conbuildmat.2016.11.080.
)
have conducted an experimental investigation on mortars made with OPC
replaced by an RFP and the result shows that the substitution of RFP
showed good mechanical properties and resistance against carbonation.
The mineral composition of the RFP shows quartz, mullite, calcite
ettringite, with un-hydrated cement particles of OPC. Ren et al. (2929.
Ren P, Li B, Yu JG, Ling TC. 2020. Utilization of recycled concrete
fines and powders to produce alkali-activated slag concrete blocks. J.
Clean. Prod. 267:122115. https://doi.org/10.1016/j.jclepro.2020.122115.
)
demonstrated that the use of RFP in the alkali-activated slag system
and it has been found that the substitution of RFP in an
alkali-activated slag system enhances the mechanical properties and
decreases the WAand porosity value which shows RFP has good reactivity
and filling effect.
The literature studies revealed that extensive work has been done on the utilization of OPC as a rich mineral in FA-based GPM for activation at an early stage of curing to improve the strength and durability properties. However, limited information is available on the usage of RFP in FA-based geopolymer mortar. The current study focused on the utilization of RFP in place of OPC as a calcium source in the FA-based geopolymer to achieve sustainability in the construction sector. In this study, OPC was replaced with FA up to 20% in FA-based geopolymer mortar, and further, RFP was substituted with OPC at replacement levels of 5%, 10%, 15%, and 20%. The fresh and hardened properties of GPM were examined for both ambient and heat-curing conditions.
2. EXPERIMENTAL PROGRAM
⌅2.1. Material
⌅2.1.1. Binders
⌅The FA (Class-F) was used as a main binder in the experimental work confirming IS3812:2003 (3030.
IS 3812. 2013. Specifications for pulverized fuel ash, Part-1: For use
as pozzolana in cement, cement mortar and concrete. Bur. Indian Stand.
New Delhi, India.1–12.
). The OPC was firstly partially
substituted with FA as a Ca source to enhance the properties of GPM at
an early stage of curing. Secondly, RFP was blended with ternary
mixtures of FA and OPC at different replacement levels. The RFP was
prepared using Recycle Fine Aggregates (RFA) collected from the Concrete
Laboratory of the Author`s Institute. It was prepared by milling RFA
particles up to 75 μ size using a ball mill apparatus. The particle
distribution of FA, OPC, and RFP was determined using the Laser
diffraction technique as shown in Figure 1. The physical and chemical compositions of FA, OPC, and RFP are listed in Table 1.
Natural Fine Aggregate (NFA) was collected locally with a specific
gravity of 2.53, a fineness modulus of 2.34, and a WA rate of 1.5%.
| Properties | FA | OPC | RFP |
|---|---|---|---|
| Fineness, cm2/gm | 4125 | - | 3500 |
| Specific Gravity | 2.1 | 3.15 | 2.5 |
| SiO2 | 56.50% | 20.10% | 60.0% |
| Al2O3 | 22.50% | 6.80% | 11.0% |
| Fe2O3 | 11.0% | 4.30% | 4.0% |
| CaO | 3.20% | 61.30% | 15.0% |
| MgO | 1.25% | 3.50% | 2.5% |
| Na2O | 0.15% | 0.60% | 1.85% |
| K2O | 2.2% | 1.25% | 2.50% |
| SO3 | 0.25% | 1.28% | 1.25% |
| P2O5 | 0.60% | 0.08% | 0.15% |
| TiO2 | 2.2% | 0.55% | 0.65% |
| Loss of ignition | 1.20% | 1.20% | 1.80% |
Characterization of FA, OPC, and RFP
⌅For FA, OPC, and RFP, X-Ray Diffraction (XRD) patterns were observed from Malvern Panalytical’s XRD equipment are shown in Figure 2 (a). The figure illustrates that the FA contains two main substances quartz and mullite. The peak of the quartz is coming on 25-30 (2ⱺ). The RFP contains a higher content of quartz with few peaks of calcite in the form of both hydrated and anhydrous Ca content. According to Table 1, both FA and RFP have a large quantity of silica, about 56.50% and 58% respectively. On the other hand, OPC contains a peak in the range of 25-30 (2ⱺ) and contains a high proportion of allite, bellite, and calcite which shows a high amount of Ca compound in comparison to FA and RFP.
Furthermore, the Fourier-transform infrared (FTIR) spectroscopy was done using an ATIMATTSON FTIR-TM series spectrophotometer. In FTIR analysis, infrared radiation is transmitted through a sample, with some of the radiation being absorbed, and transmitted radiation is then recorded at various wavenumbers (cm-1). For FA, OPC, and RFP, the FTIR spectra typically span the range of 400 - 3500 cm-1. Figure 2 (b) shows the existence of Si and Al structure bonds of FA, OPC, and RFP, and the alumino-silicate functional group was presented in the range of 800 and 1100 cm−1 wavenumber. The asymmetric T-O-Si / Si-O-T stretching (T showing either Si or Al) of FA, OPC, and RFP were shown at 1045.57, 882.06, and 1038.88 cm−1 wavenumbers, respectively. Additionally, the peaks observed within the spectral range spanning from 676.80 to 780.11 cm-1 represent the bending vibrations associated with Si-O-T bonds.
Lime reactivity
⌅Fly
Ash is a pozzolanic material derived from industrial waste, rich in
aluminosilicates, while RFP is attained from construction and demolition
waste. The reactivity test for pozzolanic materials is conducted
according to IS 1727:1967 (3131. IS 1727. 1967. Methods of test for pozzolanic materials. Bur. Indian Stand. New Delhi.
)
standards. The CS reported for mortar cubes made with FA and RFP was
3.84 MPa and 4.5 MPa, respectively which shows the reactivity of their
components.
2.1.2 Alkali activator
⌅The alkali activators used in the experimental work in the form of Sodium Hydroxide (SH) and Sodium Silicate (SS) in combination. The 12M solution of SH was prepared using 98% pure pellets in the experimental work. The pellets were mixed in water to make an SH solution prior to 24 h of casting. Further commercially available solution with a weight ratio of Si/Na of 3.02 was used with chemical compositions of 27.2 % SiO2,9% Na2O, and 63.8% H2O. The properties of SH and SS are shown in Table 2.
| Property | NH | NS |
|---|---|---|
| Molecular formula | NaOH | Na2SiO3 |
| pH value | 13-14 | 13-14 |
| Na2O content (wt %) | - | 9 |
| SiO2 content (wt %) | - | 27.2 |
| H2O content (wt %) | - | 63.8 |
2.2. Mixture design and curing condition
⌅The mixed proportion of GPM is presented in Table 3. The alkali/binder ratio (0.45), SS/SH ratio (22.
Naqi A, Jang JG. 2019. Recent progress in green cement technology
utilizing low-carbon emission fuels and raw materials: a review.
Sustain. 11(2):537. https://doi.org/10.3390/SU11020537.
),
SH molarity (12M), and water/solid ratio (0.35) were fixed based on a
trial-and-error approach. The FA was used as a key binder whereas, OPC
and RFP were substituted with FA at different percentages. Firstly, 20%
FA was replaced with OPC in the second mixture, after that OPC was
replaced with RFP for other mixtures at 5%,10%,15%, and 20% replacement
levels while maintaining the FA at 80% in all the GPM mixtures.
| Mixture Notation | Mixture description | Mixture Ingredients | |||||
|---|---|---|---|---|---|---|---|
| FA | OPC | RFP | NFA | SS | SH | ||
| C0R0 | 100% FA+ 0% OPC + 0% RFP | 500 | - | - | 1500 | 150.4 | 75.22 |
| C20R0 | 80% FA + 20% OPC + 0% RFP | 400 | 151.42 | - | 1500 | 150.4 | 75.22 |
| C15R5 | 80% FA +15% OPC + 5% RFP | 400 | 113.59 | 29.65 | 1500 | 150.4 | 75.22 |
| C10R10 | 80% FA + 10% OPC +10% RFP | 400 | 69.48 | 54.54 | 1500 | 150.4 | 75.22 |
| C5R15 | 80% FA + 5% OPC + 15% RFP | 400 | 37.72 | 89.06 | 1500 | 150.4 | 75.22 |
| C0R20 | 80% FA+ 0% OPC + 20% RFP | 400 | - | 118.93 | 1500 | 150.4 | 75.22 |
*NFA - Natural Fine Aggregates
For the preparation of geopolymer mortar, NFA and blended binder were mixed dry for 3 minutes in a pan mixer. After that alkali solutions were added to the dry mixture and mixed for more than 3 to 5 minutes. Further, a fixed water content of 57.77 kg/m3 for all mixtures was added to obtain the necessary workability of the geopolymer mixture. After that, the freshly mixed mortar was dispensed into the mould and sealed to resist moisture loss. Further, for ambient curing, the GPM samples were placed at room temperature, and heat curing was done at 60ºC in the oven for 24 hours.
3. EXPERIMENTAL WORK
⌅3.1. Slump-flow and setting time
⌅The slump-flow and setting time of GPM were observed in their fresh state. The slump-flow was performed by IS 4031- Part 7 (3232.
IS 4031-7. 1988. Methods of physical tests for hydraulic cement:
Determination of compressive strength of masonry cement. Bureau of
Indian Standards, New Delhi.
). The slump should be 110
± 5 percent as per standards. Further, the initial-final setting time
of GPM was examined in accordance with IS: 4031, Part 5 [30].
3.2. Compressive strength test
⌅the 70.6 mm cube specimens were used to examine CS in accordance with IS 4031, Part 7 (3232.
IS 4031-7. 1988. Methods of physical tests for hydraulic cement:
Determination of compressive strength of masonry cement. Bureau of
Indian Standards, New Delhi.
) at, 7, and 28 days of of
both ambient and heat curing. The 2000Kn load capacity compression
testing machine was used and an average of three specimens were taken to
record the CS of GPM for each curing.
3.3. WA and porosity test
⌅the 70.6 mm cube specimens were used to evaluate the WAand porosity of GPM mixtures using ASTM C642 -13 (3333. ASTM C642-13. ASTM International. 2013. Standard test method for density, absorption and voids in hardened concrete.
) at 28 days of each ambient and heat curing. The WAand porosity in terms of the percentage of each mixture.
3.4. Dry shrinkage test
⌅The dry shrinkage was observed in accordance with IS 4031: Part 10 (3434. Annu B ASTM Stand. 2001. Standard test method for drying shrinkage of mortar containing hydraulic cement. 2(4):11–3.
) using 25 mm x 25 mm x 285 mm prisms. Measurements were obtained at 7, 14, 28, and 56-day intervals throughout two months.
3.5. Acid attack
⌅For conducting the chemical attack test 5% sulphuric acid (H2SO4) was used for the current experimental work. The acid resistance was performed as per ASTM C-267 (3535.
Van Deventer JSJ, Provis JL, Duxson P, Brice DG. 2010. Chemical
research and climate change as drivers in the commercial adoption of
alkali activated materials. Waste and Biomass Valorization.
1(1):145–155. https://doi.org/10.1007/s12649-010-9015-9.
).
The mortar cubes were first weighed before being placed into an acidic
solution in a container. After being placed at 28 days in the acid
water, the cubes were removed, cleaned, weighed, and dried for another
24 hours at 100 ± 10 °C. The specimens were then taken out of the oven,
weighed, brushed, and re-weighed to determine the mass loss due to acid
degradation. In a 28-day soaking period, the acid water was replaced
after a 7-day interval.
3.6. Microstructure analysis
⌅The microstructure of GPM samples was examined using SIGMA 500VP Field Emission Scanning Electron Microscopy (SEM). The fine powder obtained by grinding the 28-day geopolymer paste sample to decrease its particle size was subjected to SEM examination with a 1000X scale at 10 and 15 kV.
4. RESULTS AND DISCUSSION
⌅4.1. Fresh properties of GPM
⌅4.1.1. Slump-flow value
⌅The slump-flow results of all GPM mixtures are shown in Figure 3. The attained results show that the addition of OPC and RFP makes the mixture stiffer than the C0R0. For mixture C20R0 with 20% OPC, the slump-flow decreases marginally in contrast to the control mixture(C0R0). Further, 5% substitution of RFP with OPC i.e., C15R5 mixture, the slump value declined by 5% relative to the C0R0. Similarly for mixture C10R10, the slump value was reduced by 7% comparative to the C0R0. Furthermore, for mixtures C5R15 and C0R20, the slump was reduced by less than 10% relative to the C0R0l. Importantly, substituting RFP up to 20% had a limited influence on the flow properties related to the C0R0. The reduction in flow or stiffness was attributed to the irregular shape of RFP particles being replaced by the rounded spherical shape of FA particles. Furthermore, for all geopolymer mixes even after a reduction in the slump-flow values all GPM is in satisfactory limits as per IS 4031-Part 7 standard.
4.1.2. Setting time
⌅In
practical application, the setting times of GPM are significant because
they determine the time available for transportation, placement, and
compaction. The setting times of all GPMs are presented in Figure 4.
The initial-final setting times of the geopolymer paste of the C0R0 are
454 and 1400 minutes respectively at room temperature. The geopolymer
paste initial-final time was considerably reduced with the substitution
of OPC and RFP in the GPM mixtures. For mixture C20R0 i.e., with the
substitution of 20% OPC, the initial setting time decreased
significantly to 50 minutes from 454 minutes and the final setting time
decreased to 130 minutes from 1400 minutes. Further, for mixture C15R5,
i.e., a blend of 15% OPC and 5% RFP, the initial-final setting time also
decreases as compared to the C0R0. Nath and Sarker (1818.
Nath P, Sarker PK. 2015. Use of OPC to improve setting and early
strength properties of low calcium fly ash geopolymer concrete cured at
room temperature. Cem. Concr. Compos. 55:205–214. http://doi.org/10.1016/j.cemconcomp.2014.08.008.
)
also observed that the presence of OPC in FA-based GPM reduces the
initial-final setting time significantly. However, in contrast to C20R0,
the initial-final setting time increased by 10 and 20 minutes
respectively. Similarly, for mixtures C10R10, C5R15, C15R5, and C0R20
the initial-final significantly decreased as related to the C0R0 but
increased with the rise in the content of RFP. However, an increase in
RFP content does not affect both setting times significantly, and still,
the initial-final setting time is restricted to 217 and 420 min
respectively which is significantly lesser than the C0R0 setting time
values. The presence of Ca in OPC and RFP plays a key role in
accelerating the setting process of geopolymer mortar. The rate at which
geopolymerization reactions increase prominently influences the setting
time of a geopolymer mixture. Moreover, OPC releases more heat during
the hydration process in comparison to RFP due to the higher content of
Ca, which as a result accelerates the geopolymerization reactions,
leading to faster setting times as compared to RFP. The presence of
appropriate Ca content in both OPC and RFP resulted in a considerable
drop in the setting times of geopolymer paste. The inclusive results of
GPM show that the inclusion of OPC and RFP individually or in
combination up to 20% helps to reduce the setting time influentially.
4.2. Hardened properties of geopolymer mortar
⌅4.2.1. Compressive Strength
⌅(a) Effect of inclusion of OPC and RFP on the compressive strength of GPM at ambient curing
⌅The CS results of GPM mixtures at different stages of ambient curing are given in Figure 5.
The results of GPM mixtures illustrate that the inclusion of both OPC
and RFP enhanced the CS. For example, with a 20% substitution of OPC
i.e., for the C20R0 mixture, the CS enhanced significantly by 987% and
322% at 3 and 7 days of curing relative to the C0R0. Similarly, the CS
increased by 57% at 28 days of curing comparative to the C0R0. Mehta and
Siddique (2222.
Mehta A, Siddique R. 2017. Properties of low-calcium fly ash based
geopolymer concrete incorporating OPC as partial replacement of fly ash.
Constr. Build. Mater. 150:792–807. https://doi.org/10.1016/j.conbuildmat.2017.06.067.
)
also revealed that the addition of OPC in geopolymers increases the
strength significantly due to the development of extra nucleation sites
in the form of Ca hydrates along with the gepolymerization of FA and
activators. The GPM with 0% OPC i.e. in mixture C0R0 only geopolymer
matrix is generated, Figure 6 (a),
whereas the substitution of OPC in GPM produces supplementary C-A-S-H
gel generated with geopolymer matrix as shown in the SEM image in Figure 6 (b).
In a previous study, the microstructural analysis also claimed that the
presence of OPC in geopolymer forms the C-A-S-H gel in the concrete
matrix (3636.
Pangdaeng S, Phoo-ngernkham T, Sata V, Chindaprasirt P. 2014. Influence
of curing conditions on properties of high calcium fly ash geopolymer
containing Portland cement as additive. Mater. Des. 53:269–274. https://doi.org/10.1016/j.matdes.2013.07.018.
). Further, with the substitution of 5% RFP with OPC i.e. (15% OPC + 5%
RFP) C15R5, the strength enhanced by 1190% at 3 days, 382% at 7 days,
and 76% at 28 days of ambient curing respectively in contrast to the
C0R0. In the mixture with 10% OPC and 10% RFP i.e., C10R10, the CS was
enhanced by 1383% and 531% at 3 and 7 days of curing respectively, and
88% at 28 days of curing relative to the C0R0. Moreover, the CS of C15R5
and C10R10 is higher than the C20R0 mixture, which may be due to the
high content of Ca in OPC with a low content of Si and Al in comparison
to FA and RFP. The decrease in Si and Al content hinders the geopolymer
reaction which tends to reduce the CS. Conversely, crystalline silica
present in RFP works as a filler in the concrete matrix which helps to
enhance the CS of GPM.
In
the substitution of RFP with OPC in the GPM mixture, the strength
starts decreasing but is still higher than the C0R0. For mixtures C5R15
and C0R20, the CS increased in comparison to the C0R0 but decreased
relative to C10R10. However, the decreasing trend of CS with an increase
in the content of RFP is due to a lower content of Ca in RFP than in
OPC. Therefore, the increasing trend observed in the mixture C20R0 is
going to reverse in the mixture C5R15. For mixture C5R15, the CS
escalates by 670% at 3 days of curing 197%, and 33% at 7 and 28 days of
curing respectively relative to the C0R0. Similarly, with 20% RFP and
0%, OPC i.e., for the C0R20 mixture, the CS shows an increasing trend
concerning the C0R0. The overall observed values of CS show an
increasing trend with the inclusion of both OPC and RFP for all
geopolymer mixtures and C10R10 shows maximum CS as related to other
mixtures. Figure 6 (c),
shows the formation of C-A-S-H gel and needle shape particles in the
form of N-A-S-H gel along with geopolymer matrix which enhances the CS.
Ahmari et al. (28)28.
Ahmari S, Ren X, Toufigh V, Zhang L. 2012. Production of geopolymeric
binder from blended waste concrete powder and fly ash. Constr. Build.
Mater. 35:718–729. https://doi.org/10.1016/j.conbuildmat.2012.04.044.
also show the formation of needle-shaped particles along with a
geopolymer matrix with the presence of RFP. Furthermore, the rise in CS
up to 7 days is noticeable as compared to 28 days at ambient curing
conditions (2828.
Ahmari S, Ren X, Toufigh V, Zhang L. 2012. Production of geopolymeric
binder from blended waste concrete powder and fly ash. Constr. Build.
Mater. 35:718–729. https://doi.org/10.1016/j.conbuildmat.2012.04.044.
).
(b) Effect of inclusion of OPC and RFP on the compressive strength of GPM at heat curing
⌅The CS results of all mixtures for heat curing are shown in Figure 7. A considerable change was found in the strength result of the geopolymer mixtures with the substitution of both OPC and RFP. For example, 20% replacement of OPC with FA in the mixture C20R0 shows a significant increase of 288% at 3 days and 154% at 7 days in CS and 48% at 28 days of curing in comparison to the C0R0. This enhancement in the CS was observed due to the provided heat, good reactivity of OPC, and additional Ca silicate gel produced with the geopolymer matrix in comparison to the C0R0 as shown in Figures 8 (a) and (b). Furthermore, the addition of RFP content with OPC in the mixture C15R5 i.e. (15% OPC + 5% RFP), C10R10 i.e. (10% OPC + 10% RFP), and C5R15 i.e. (5% OPC + 15% OPC) show a substantial increase in the CS results at all days of curing relative to the C0R0. For example, in mixture C15R5, the increase in CS was observed to be 321%, 170%, and 55%, respectively, on the 3, 7, and 28 days of curing. However, the maximum increase in CS was observed for mixture C10R10 (10% OPC + 10% RFP) i.e., 347%, 170%, and 66% for the same curing ages respectively. Moreover, the increment in the CS in the mixture C10R10 was clearly understood from the SEM image Figure 8 (c) which shows the proper reaction of FA particles with supplementary N-A-S-H and C-A-S-H gel formation. However, the mixture C5R15 and C0R20 having 15% and 20% RFP was substituted with FA and also showed an increase in CS as compared to the C0R0 but showed lower strength than C10R10 as discussed in the above section at ambient curing condition. From the overall result, it was concluded that the inclusion of 20% RFP alone in the FA-based GPM can achieve good CS at an initial stage of heat curing conditions.
4.2.2. WA and Porosity
⌅(a) Effect of inclusion of OPC and RFP on WA and porosity at ambient curing
⌅The observed values of WA of all mixtures with varying replacement of OPC and RFP at 28 days of ambient curing are shown in Figure 9.
Results obtained illustrate that the substitution of both OPC and RFP
content in the mixtures reduces the WA value as related to the C0R0. For
example, mixture C20R0 having 20% substitution of OPC in the mixture
reduces the WA by 21% relative to the C0R0. The decline in the WA values
is due to the densification of the microstructure of the geopolymer
mixture with the inclusion of OPC (2222.
Mehta A, Siddique R. 2017. Properties of low-calcium fly ash based
geopolymer concrete incorporating OPC as partial replacement of fly ash.
Constr. Build. Mater. 150:792–807. https://doi.org/10.1016/j.conbuildmat.2017.06.067.
). Pangdaeng et al. (3636.
Pangdaeng S, Phoo-ngernkham T, Sata V, Chindaprasirt P. 2014. Influence
of curing conditions on properties of high calcium fly ash geopolymer
containing Portland cement as additive. Mater. Des. 53:269–274. https://doi.org/10.1016/j.matdes.2013.07.018.
)
claimed that the substitution of OPC filled the pores in the concrete
matrix which tends to decrease the WA of high Ca FA geopolymers.
Moreover, in mixture C15R5 i.e. (15% OPC + 5% RFP) was observed that
with the substitution of RFP, a decrease of 27% of WA value was observed
in comparison to the C0R0. The maximum reduction of WA was found i.e.,
31% in mixture C10R10 having 10% RFP and 10% OPC substituted with the FA
in the mixture. However, WA values of mixtures C5R15 and C0R20 also
show a 16% and 9% reduction respectively as related to the C0R0. Sharma
et al. (3737.
Sharma A, Singh P, Kapoor K. 2022. Utilization of recycled fine powder
as an activator in fly ash based geopolymer mortar. Constr. Build.
Mater. 323:126581. https://doi.org/10.1016/J.CONBUILDMAT.2022.126581.
)
claimed that the presence of RFP in FA-based GPM reduces their WA due
to the formation of additional Ca hydrates along with the geopolymer
matrix. The overall results show that substitution of both OPC and RFP
up to 20% individually or in combination declines the WA values as
compared to the C0R0 and mixture C10R10 shows maximum reduction in WA as
compared to other GPM mixtures.
Figure 9 also shows the porosity result of GPM mixtures cured at ambient curing.
The result concludes that the substitution of OPC and RFP in the
blended mixture reduces the porosity value. For example, the mixture
C20R0 having 20% OPC substitution with the FA shows a 13% decrease in
the porosity value as compared to the C0R0. It results from the pore
refinement of the voids in geopolymer mixtures when OPC is added. The
previous study also shows the same trend of a reduction in porosity with
the addition of OPC in GPM (3838.
Kaya M, Köksal F. 2021. Effect of cement additive on physical and
mechanical properties of high calcium fly ash geopolymer mortars.
Struct. Concr. 22(S1):E452–465. https://doi.org/10.1002/suco.202000235.
).
However, in the mixture, C15R5, i.e. (5% RFP + 15% OPC) the reduction
in the porosity value was observed as 16% as compared to the C0R0. For
mixture C10R10 i.e. (OPC 10% + RFP 10%) shows the maximum decrease in
porosity value of 20% with respect to the C0R0. Ren et al. (3939.
Ren P, Li B, Yu JG, Ling TC. 2020. Utilization of recycled concrete
fines and powders to produce alkali activated slag concrete blocks. J.
Clean. Prod. 267:122115. https://doi.org/10.1016/j.jclepro.2020.122115.
)
claimed that partial substitution of RFP with FA decreases the porosity
values due to pore refinement of alkali-activated concrete blocks. A
significant decrease in the porosity value was observed due to the
enhancement in the microstructure and void filling of the geopolymer
mixtures. However, further increasing the content of RFP in mixture
C5R15 i.e. (5% OPC + 15% RFP) and C0R20 (0% OPC + 20% RFP) there is a
slight increase in the porosity value at 28 days relative to C10R10 but
related to the C0R0 it is very low. It shows that the substitution of
OPC with RFP may increase the porosity due to the lower reactivity of
RFP as compared to OPC.
(b) Effect of inclusion of OPC and RFP on WA and porosity at heat curing
⌅The WA results of GPM mixtures at heat curing are given in Figure 10. It was observed that the WA value of heat-curing GPM mixtures is less as compared to ambient cured mixtures. The result depicts that with 20% OPC in the mixture C20R0 i.e. (20% OPC + 0% RFP), the WA value was reduced by 5.63% relative to the C0R0. It illustrates that the presence of OPC in the GPM mixture improves their microstructure, Figure 8 (b). However, the accumulation of RFP with OPC in the mixture C15R5 showed a reduction of 22% in WA value as compared to the C0R0. The maximum reduction in the heat curing was observed in the mixture C10R10 i.e. (10% OPC + 10% RFP) which is 29 % as related to the C0R0. However, mixture C5R15 and C0R20 having 15% and 20% RFP with FA shows an insignificant increase in the WA value in comparison to C10R10 but as compared to the C0R0 the WA value decreased by 25% and 5% respectively.
Similarly, the porosity value of heat-curing mixtures is shown in Figure 10. The result concludes that the substitution of OPC and RFP helps to decrease the porosity value. For example, the mixture C20R0 shows a 7% decrement in porosity value as related to the C0R0. However, the substitution of OPC and RFP in the GPM mixture shows a significant decrement in the porosity value up to a 10% replacement level. For example, the mixture C15R5 and C10R10 show a 12% and 18 % decline in the porosity value as related to the C0R0. Furthermore, replacing RFP as a place for OPC shows the same trend as ambient curing conditions. For example, in mixtures C5R15 and C0R20, the decrement in the porosity value was 10% and 7 % as compared to the C0R0.
4.2.3. Correlation between compressive strength, water absorption, and porosity
⌅The relationship between CS with WA and porosity at ambient cured mixtures is shown in Figure 11 (a). For mixture, C10R10, the highest CS of the GPM mixture was found to be 28.4 MPa after ambient curing. The result of CS shows similar trends like WA and porosity for all the GPM mixtures. The GPM mixture having the lowest WA and porosity value shows the highest CS i.e., C10R10. In other words, CS shows an inverse relation with WA and porosity for all mortar mixtures in both curing conditions. Similarly, the relations between water absorption, porosity, and CS at heat-cured GPM mixtures are shown in Figure 11 (b). The heat-cured result shows similar trends as the ambient-cured result in terms of WA and porosity. It was observed that the maximum CS of 32.8 MPa was found for mixture C10R10 with the lowest WA and porosity value.
Figure 12 shows the linear relation between porosity and WA for GPM mixtures at 28 days of both curing. The correlation analysis after correlating the WA with porosity for ambient curing conditions is expressed by the quadratic equation shown in Figure 12 (a) with R2 = 0.99. It shows that WA and porosity change with the presence of OPC and RFP in the same trend. The overall results show that the WA and porosity decrease with the inclusion of 10% OPC and RFP in combination and can further rise with the higher content of RFP. Similarly, the correlation analysis after correlating the WA with porosity for heat curing condition is expressed quadratic equation shown in Figure 12 (b) with R2 = 0.94 with the same trend of ambient curing condition.
4.2.4. Acid attack
⌅ Figure 13 shows the change in the mass loss of geopolymer mixtures cured in both
curing (ambient and heat) at 28 days of the soaking period. The outcome
reveals that the mass loss of GPM mixtures was marginally affected at 28
days of soaking. For example, the C0R0 was shown a slight decrement in
mass loss i.e., 0.56% at 28 days of sulphuric acid exposure. The calcium
sulfates and calcium aluminum sulfates generally cause the
deterioration which is generated through a sulphuric acid reaction with
Ca present in the binder. However, FA (Class-F) used in the present
study has low Ca content which restricts the production of the sulfate
products, therefore it shows a negligible effect on FA-based geopolymer
mortar. Furthermore, mixture C20R0 shows a maximum decrement in mass
loss was 1.61% in ambient curing conditions. Mass loss due to the
presence of OPC was mostly driven by the interaction of the Ca hydroxide
with the acid. However, the result demonstrates that the inclusion of
RFP in the mixture as a replacement for OPC reduces the mass loss
percentage. For example, the mass loss of mixture C15R5, C10R10, C5R15,
and C0R20 was observed at 1.61 %, 1.45 %, 1.07 %, and 0.80 %
respectively at ambient curing. The depolymerization of the
aluminosilicate network of the geopolymer matrix caused by acid assault
occurs in the exchange of alkali cations by H3O+ or H+ ions of acid, trailed by reduction of siliceous phases. This condition,
in turn, causes a considerable loss of mass during an acid attack (4040.
Afridi S, Sikandar MA, Waseem M, Nasir H, Naseer A. 2019. Chemical
durability of superabsorbent polymer (SAP) based geopolymer mortars
(GPMs). Constr. Build. Mater. 217:530–542. https://doi.org/10.1016/j.conbuildmat.2019.05.101.
).
Similarly, in the heat curing the mass loss of the geopolymer mixture was higher than the ambient cured mixture. For example, the C0R0 shows higher resistance in ambient curing conditions towards sulfuric acid relatively heat-cured mixture. In the heat curing the mass loss of C0R0s was 0.79% at 28 days of curing. Moreover, the observed mass loss of heat-cured mixtures was at 1.57%, 1.37%, 1.26%, and 1.57% for C15R5, C10R10, C5R15, and C0R20 respectively. The overall results show that the GPM mixtures show good resistance to acid attack at 28 days of the soaking period.
4.2.5. Dry shrinkage
⌅(a) Effect of inclusion of OPC and RFP on the drying shrinkage values at ambient curing
⌅The observed values of the dry shrinkage test of GPM mixtures cured at ambient temperature are shown in Figure 14.
The decrease in dry shrinkage was observed by substituting FA with OPC
at 20%, C20R0. The inclusion of OPC fastens the reaction and densifies
the microstructure which as a result reduces the evaporation of inner
water and aids in lessening the shrinkage. In a previous study, it was
stated that the addition of Ca products in GPM reduces dry shrinkage (4141.
Riahi Dehkordi E, Moodi F, GivKashi MR, Ramezanianpour AA. 2023.
Investigation of affecting factors on drying shrinkage and compressive
strength of slag geopolymer mortar mixture. Arab. J. Sci. Eng.
49:5679–5696. https://doi.org/10.1007/s13369-023-08373-9.
).
For example, in mixture C20R0 the reduction in drying shrinkage at 7
days was 15.29 %, for 14 days was 6.34 %, and for 28 and 56 days the dry
shrinkage reduction was 9.15 and 8.08 % respectively. Similarly,
mixture C15R5 having 5% RFP is replaced by OPC the drying shrinkage
value is decreased by 12.65% for 7 and 6.06% for 14 days. Further, the
decrease in drying shrinkage values was 8.37 % at 28 and 7.36 % at 56
days relative to the C0R0. The RFP in the blended GPM mixture shows an
insignificant decrement in the shrinkage value. For mixture C10R10
having 10%, RFP content is replaced with OPC the reduction in drying
shrinkage for 7 and 14 days was 9.79 % and 4.96 %. At 28 and 56 days of
curing, the decrease in the drying shrinkage values was near about 6%
each. However, in mixture C5R15 the drying shrinkage reduction was
observed at 7.37 %, 4.55 %, 4.57 %, and 5.05 for 7, 14, 28, and 56 days
respectively related to the C0R0. For mixture C0R20 having 20 % RFP was
replaced by fly ash the drying shrinkage value shows a 6.71 % decrement
at 7 days. Furthermore, for 14, and 28, 56 days, it shows near about 4 %
decrement as related to the C0R0. The substitution of RFP helps in pore
refinement which in effect reduces the inner water loss as a result
reduces the dry shrinkage (4242.
Yan S, Sagoe-Crentsil K. 2012. Properties of wastepaper sludge in
geopolymer mortars for masonry applications. J. Environ. Manage.
112:27–32. https://doi.org/10.1016/j.jenvman.2012.07.008.
).
The overall results show that the substitution of both OPC and RFP
shows a decline in drying shrinkage at all ages of ambient curing.
Further, for each GPM mixture the shrinkage values increased linearly up
to 28 days, and beyond 28 days, no significant variation in dry
shrinkage values was observed as shown in Figure 14.
(b) Effect of inclusion of OPC and RFP replacement on the drying shrinkage value at heat curing
⌅Similarly, the drying shrinkage result of all mixtures at heat curing is shown in Figure 15. The mixture C20R0 having 20% FA was replaced by OPC the decrease in the drying shrinkage value was 11% each at 7 and 14 days each as compared to the C0R0. Further, the drying shrinkage values decreased was 11.37% and 11.61% at 28 and 56 days respectively. Likewise, for the mixture, C15R5 having 5%, RFP was replaced by OPC the reduction in drying shrinkage was observed as near about 11% at all days of curing. For mixture C10R10 having 10% RFP with 10% OPC, the drying shrinkage value was observed as a 9% decrement at 7 and 14 days and 10% at 28 and 10.53 % at 56 days as related to the C0R0. Further mixtures C5R15 and C0R20 show similar trends of dry shrinkage values as of ambient cured mixtures. Further, the dry shrinkage value increased linearly up to 7 days of curing, and after that, there was no significant change in the shrinkage value up to 56 days of curing for each GPM mixture at heat curing conditions.
6. CONCLUSIONS
⌅The overall results show that RFP can be the finest replacement for OPC to enhance the geopolymer reaction in FA-based geopolymer mortars. The substitution of RFP up to 20% alone in GPM can reduce the setting time and enhance their properties.
-
The presence of OPC and RFP in the GPM mixtures marginally reduces the slump-flow value relative to the control geopolymer mixture however in acceptable limits with the addition of OPC and RFP for all mixtures as per requirement. Additionally, the initial-final setting time was reduced substantially with the inclusion of OPC and RFP in GPM mixtures.
-
Across each of the curing ages, the highest CS was achieved for the GPM mixture having 10% RFP and 10% OPC content in both curing conditions. Further, the utilization of 20% RFP alone also shows promising CS in comparison to the C0R0 which can overcome the consumption of OPC in geopolymer mortar. Furthermore, both OPC and RFP are beneficial to improve the CS at an early stage of ambient curing.
-
It was concluded that WA as well as porosity values decrease with the substitution of OPC and RFP individually or in combination in all GPMmixtures. The GPM mixture C10R10 shows the lowest WA and porosity value due to its denser microstructure in comparison to other GPM mixtures.
-
In comparison to ambient curing conditions, there is lesser dry shrinkage of GPM under heat curing conditions. Further, the dry shrinkage value as related to the C0R0 reduced with the inclusion of OPC and RFP in the GPM mixtures.
-
The GPM mixtures show good acid resistance in both curing conditions. The maximum percentage decrease in mass of GPM is 1.6% with 20% utilization of OPC in the GPM mixture.
-
It was concluded from SEM analysis with the inclusion of the OPC and RFP in the mixture the microstructure of the GPM mixtures is improved in comparison to the C0R0. Moreover, the supplementary N-A-S-H or C-A-S-H gel is produced with the inclusion of OPC and RFP along with the geopolymer matrix at 28 days of curing.