Synthesis of Slag-Waste Glass Binary Geopolymer and Its Application as a Sustainable Stabilizing Material

Abstract

Concrete is the second most consumed material after water. A higher dependency on cement concrete tremendously enhanced the demand and production of Portland cement. Despite its higher demand, the production process of cement has severe environmental issues, including depletion of natural resources, higher energy consumption, and emission of greenhouse gases. Geopolymer is a green binder proposed as an alternative to conventional binders. Any industrial by-product possessing amorphous aluminosilicate components could be used to produce geopolymers through alkali-activation. It has gathered significant attention from researchers due to its better performance and eco-friendly nature. While significant research has been carried out to synthesize geopolymer binders from industrial-based aluminosilicates, recyclable aluminosilicates are also being used as precursors for synthesizing geopolymer binders. Waste glass is one of the recyclable materials with high silica component used for preparing geopolymer binders in combination with alumina-based materials. Hence, in the current study, laboratory investigation was made to synthesize slag-waste glass geopolymer and assess its suitability as a sustainable stabilizer for geotechnical applications. In the first phase of laboratory investigation, the geopolymer binder was synthesized using slag and glass powder (GP) as precursors and sodium hydroxide (NaOH) as an alkali activator. The solubility of silicon, aluminum, and calcium ions from the precursors in the NaOH solution was examined. The fresh and hardened properties of the geopolymer were assessed at different GP contents (0%, 10%, 20%, 30%, and 40%), NaOH concentrations (2 M, 4 M, 6 M, 8 M, and 10 M) and liquid alkali activator-to-solid binder (L/S) ratios (0.25, 0.30, 0.35, 0.40, and 0.45). Mineralogical and microstructural studies were made to substantiate the changes observed in fresh and hardened properties. The effect of synthesis parameters on the compressive strength (CS) was statistically analyzed by developing a statistical model. The CS of slag-GP geopolymer at various temperatures (-15 °C to 90 °C) and curing durations (1 day to 90 days), as well as the effect of initial-temperature curing under sealed and humid conditions, were explored. The strength development in submerged and autoclave curing conditions was also assessed. Further, the durability of geopolymer mortar against high-temperature exposure (HTE) of 200 °C to 1000 °C, wet-dry (W-D) and freeze-thaw (F-T) cycles, water slaking, surface abrasion, alkali-silica reaction (ASR) expansion, and chemical attacks were determined and compared with cement mortar. In the second phase of the investigation, pond ash (PA) was stabilized with slag-GP geopolymer, cement, and lime. The effect of different additive contents (3%, 6%, 9%, 12%, and 15%) on the hydro-mechanical properties of the stabilized PA was assessed. The hydro-mechanical properties include compaction characteristics, unconfined compressive strength (UCS), indirect tensile strength (ITS), California bearing ratio (CBR), hydraulic conductivity, and compressibility index. The durability of the 28-day cured stabilized PA against W-D and F- T cycles, water slaking, water immersion, capillary action, and dispersion was studied. The effect of delayed compaction on the engineering properties of the stabilized PA was also examined. Mineralogical and microstructural analysis were assessed and correlated with strength and durability properties. In addition, leachable concentrations of different metallic ions from stabilized PA were examined. The experimental results indicated that the solubility of the precursors depends on the alkali concentration and reactivity behavior of the precursors. Slag has a higher solubility in NaOH solution compared to GP. The normal consistency of slag-GP geopolymer was increased with an increase in GP content and NaOH concentration. However, the setting period and workability of the geopolymer paste were reduced with increased GP content and NaOH concentration. The final setting period was shortened by 56 min, and the flow diameter was reduced by 26.57% when GP content increased from 0% to 40% at 6 M NaOH concentration. Based on 28-day CS, an optimum NaOH concentration of 8 M and an optimum L/S ratio of 0.35 were observed for slag-based geopolymer. With GP inclusion, the NaOH requirement was reduced by up to 50% due to the availability of alkali cations in GP. At higher GP contents (>20%), the rise of Si/Al, Na/Al, and Si/Ca molar ratios beyond their ideal range causes a decline in the CS of slag-GP geopolymer. However, the resistance to soundness and drying shrinkage improved as GP content increased. The key reaction phases observed in slag-GP geopolymers are sodium alumina silicate hydrate (N-A-S-H), calcium alumina silicate hydrate (C-A-S-H), sodium alumina silicate (N-A-S), and calcium silicate hydrate (C-S-H). A non-linear statistical model developed with GP content, alkali concentration, and L/S ratio as input variables predicts the CS with an R-square of 0.9072. At ambient temperature (30 °C) and prolonged curing (90 days), higher strength was observed at 10% GP content. However, a higher dose of GP is beneficial for specimens cured at elevated temperatures (45 °C to 60 °C). Little improvement in strength with the curing period was noticed for the specimens cured at temperatures below the freezing point of water. Under humid curing, slag-GP geopolymers attained dense microstructure, whereas micro-cracks were observed in the specimens cured under dry environments. Submerged curing in alkali solutions achieved better strength compared to curing in normal and saline water. Autoclave curing showed rapid strength gain due to the advancement in the geopolymer mechanism. Moreover, the proposed analytical models predict the CS well at different GP contents, curing temperatures, and curing durations. The slag-GP binary geopolymer at 10% GP content has better durability against harsh environments, such as HTE, W-T and F-T cycles, water slaking, surface abrasion, and chemical attacks, than slag-based geopolymer and cement. The GP-rich geopolymer (20% to 40% GP) showed better structural integrity after exposure to higher temperatures (> 700 °C) due to the filling of micro-pores with molten glass particles. However, ASR expansion was found to increase marginally with GP contents. In addition, laboratory studies show that the PA can be efficiently stabilized with synthesized slag-GP geopolymer. The PA stabilized with slag-GP geopolymers achieved higher strength properties such as UCS, ITS, and CBR, as well as lower hydraulic conductivity and compressibility index compared to cement and lime-stabilized PA. Further, a denser and more compact microstructure of geopolymer-stabilized specimens achieved excellent durability than cement and lime stabilized specimens. Based on strength and durability properties, it is confirmed that PA stabilized with 6% geopolymer can be effectively used as a cementitious base in flexible pavements as per IRC 37-2018 and cementitious subbase in rigid pavements as per IRC 58-2015. With delayed compaction, the dry density and strength properties were reduced, whereas the hydraulic conductivity and compressibility index increased. At 15% geopolymer content, the compacted dry density was reduced by 11.92%, 18.74%, and 22.62% after 6 h, 24 h, and 72 h delay periods, respectively. Similarly, the UCS was reduced by 18.64%, 54.86%, and 67.35%, respectively. Whereas the hydraulic conductivity and compressibility index values increased by 3.61, 6.59, and 9.61 times, respectively, and by 15.87%, 33.89%, and 55.43%, respectively. The formation and deposition of cementitious products and agglomeration during delay periods lead to poor compaction and deteriorate the mechanical performance. The leachable concentration of iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), zinc (Zn), nickel (Ni), and copper (Cu) from the stabilized PA was within the threshold limits of WHO and IS 10500-2012 water quality standards. However, the traces of lead (Pb), mercury (Hg), and arsenic (As) were higher than the permissible limits. The geopolymer-stabilized specimens achieved higher metallic ion encapsulation than cement and lime-stabilized specimens.