BITS Faculty Publications
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Item Use of alkali-silica reactive sedimentary rock powder as a resource to produce high strength geopolymer binder(Elsevier, 2017-11) Lahoti, MukundThis paper reports on an innovative way to utilize alkali-silica reactive (ASR) rocks as a natural resource to produce high strength geopolymer binder. Excavation of the Jurong rock caverns in Singapore has produced large quantities of sedimentary rocks. These rocks, however, cannot be used for ordinary Portland cement concrete production due to their ASR reactivity. An alternative way to beneficially utilize these rocks is to produce geopolymer binder. The excavated rocks were classified based on petrography into four types and then converted to powder form in a sequence of steps. The rock powders were used to synthesize geopolymers by replacing metakaolin in different replacement ratios. Results showed that geopolymer binder with 67 wt% rock powder and only 33 wt% metakaolin can achieve a high compressive strength of 80 MPa. Incorporation of sedimentary rock powder enhanced the compressive strengths by 15–30% as compared to pure metakaolin geopolymers. Microstructure analysis revealed that the enhancement in compressive strengths were primarily due to the densification of binder by filling the voids in matrix with rock powders.Item Viability of bacterial spores and crack healing in bacteria-containing geopolymer(Elsevier, 2018-04) Lahoti, MukundGeopolymer is an emerging alternative green binder to Portland cement. Geopolymer is often more brittle and thus it is highly desirable to impart self-healing into geopolymer. Unlike Portland cement concrete, self-healing through hydration of cement, and leaching and carbonation of hydration products are not feasible in geopolymer. Microbially induced carbonate precipitation (MICP)-enabled sealing is therefore a potential way to engage self-healing in geopolymer. Using Sporosarcina pasteurii as a model MICP bacterium, this paper investigated viability of bacterial spores in a metakaolin-based geopolymer and crack healing in bacteria-containing geopolymer. Spores of S. pasteurii were added into geopolymer mix directly without encapsulation or immobilization. Results showed that bacterial spores did not leak out from the geopolymer matrix and the spores remained viable in the metakaolin-based geopolymer. Cracks in bacteria-containing geopolymer were sealed with CaCO3 after conditioning in precipitation medium for 3 days, which suggests bacterial spores remain viable. The microstructure of metakaolin-based geopolymer is controlled by Si/Al, Na/Al, and H2O/Na2O molar ratio and less depend on age, which allows direct addition of bacteria into geopolymer mix without encapsulation or immobilization to engage MCP-induced self-healing in geopolymer.Item Effects of Si/Al molar ratio on strength endurance and volume stability of metakaolin geopolymers subject to elevated temperature(Elsevier, 2018-04) Lahoti, MukundGood structural performance in a fire scenario necessitates that the structural material possesses chemical stability, deformation resistance and strength endurance. Excellent chemical stability for geopolymers has been reported in literature at a microscale. However, their performance at macroscale has not yet been systematically explored and the underlying mechanisms remain unexplained. In current study, effect of variation in Si/Al molar ratio on the meso- and macro-scale thermal stability of metakaolin geopolymers has been comprehensively investigated to discover the underlying mechanisms governing the performance. Results show that all the geopolymer samples experienced reduction in compressive strengths after exposure to high temperature up to 900 °C. Although, the geopolymer mixes exhibited good chemical stability at microscale, they possessed poor volume stability at mesoscale with very high thermal shrinkage. It was observed that thermal shrinkage induced crack formation dominates the residual strength for geopolymer mixes with Si/Al molar ratio ≤ 1.50, while densification of matrix is the governing factor of the residual strength for geopolymer mixes with Si/Al molar ratio > 1.50. Re-crystallization of nepheline at high temperature adversely affect the strength by inducing expansion and cracking of the geopolymer matrix. Geopolymer sample with Si/Al ratio 1.75 retained highest strength (6 MPa) because viscous sintering of geopolymer mixes with high Si/Al ratio at temperature beyond 600 °C enables localized healing of micro-cracks and densification of matrix which favored compressive strength gain after exposure to 900 °C. At an even higher Si/Al of 2.0, foaming of unreacted silica upon heating can lead to expansion and cracking of the sample which reduce the strength. It was observed that due to high degree of cracking damage and low residual strength retention, it is essential to improve the macro-scale stability of metakaolin geopolymers for structural fire resistance applications.Item Investigating the potential reactivity of fly ash for geopolymerization(Elsevier, 2019-11) Lahoti, MukundThis study investigated potential reactivity of fly ash for geopolymerization. A combined dissolution of fly ash in NaOH in conjunction with HCl extraction of fly ash-NaOH residue is proposed in this study to more accurately estimate the total reactive content and reactive Si/Al ratio of fly ash. Results show that the total reactive content of fly ash used in the current study is about 68.80%, similar to the amorphous content (66.28%) obtained using quantitative XRD and XRF. The values of reactive Si/Al ratios are between 2.65 and 2.98 depending on the duration of dissolution. The reactive Si/Al values differ significantly from the total Si/Al ratio (1.63) calculated using XRF results and also differ from the vitreous Si/Al ratio (3.75) calculated using combined XRF and XRD results. The results suggest that total or vitreous Si/Al ratio should not be used for mix proportioning of fly ash geopolymers as is generally used for metakaolin-based geopolymers. Instead, the reactive Si/Al ratio determined based on the current study should be used for fly ash geopolymer mix design.Item A critical review of geopolymer properties for structural fire-resistance applications(Elsevier, 2019-10) Lahoti, MukundProtection of structures from fire is of extreme importance. Geopolymer is a novel material that has wide-ranging applications, and this review article focuses on assessing the potential of geopolymers towards enhancing the structural fire resistance by critically reviewing its properties subjected to elevated temperature exposure. The properties of geopolymers are categorized into three scales, namely, micro-scale, meso-scale and macro-scale, and are discussed at length. It is noted that geopolymers are chemically stable and do not undergo breakdown of chemical structure in contrary to OPC hydration products. Thermal deformations occurring in geopolymers, which cause macro-cracking, are discussed. Compressive strength of geopolymers is observed to be affected by microstructural changes (including crack formation, pore structure changes, densification, sintering, and melting) and phase composition changes (such as growth or destruction of crystals and transformations in geopolymer paste). Geopolymer-based binders show inherently superior fire resistance as compared to Portland cement-based binders. However, it requires careful mix design, to achieve substantial chemical stability, low volume changes, strength endurance, and spalling resistance. Factors such as choice of precursor, use of aggregates, total alkali content in geopolymer, water content, etc. are critical and should be controlled. The influence of these factors is discussed at length in this article. The current applications of geopolymers for heat and fire resistance have also been briefly presented.Item Tailoring sodium-based fly ash geopolymers with variegated thermal performance(Elsevier, 2020-03) Lahoti, MukundSodium-based fly ash geopolymers show great fire resistance potential and commercial advantage for structural applications. Hence, in current research, tailoring of sodium-based geopolymer mix design without changing the fly ash source has been studied. It was found that a wide variety of residual compressive strength ranging from significant reduction (~80%) to maintaining significant enhancement (~150%) after being exposed to 900 °C was observed. The contributory mechanisms were discovered by investigating their chemical stability, pore structures, volume stability, and strength endurance prior to and after exposure to high-temperature using different microstructure characterization techniques including XRD, FTIR, MIP, dilatometry, and SEM. Crack formation due to moisture migration, pore shrinkage, and re-crystallization of nepheline adversely affected compressive strength. Matrix densification due to shrinkage of pore and stronger inter-particle bonding due to viscous sintering, favored compressive strength gain. This work discusses at length these competing mechanisms influencing the residual compressive strength.