Hybrid geopolymer paste from high calcium fly ash and glass wool: Mechanical, microstructure, and sulfuric acid and magnesium sulfate resistance characteristics

Abstract

Geopolymer is a sustainable binder composed of source materials containing aluminosilicates under strongly alkaline conditions promising for use in place of cement in building materials due to its low carbon footprint. Although high calcium fly ash can be used as a source material for geopolymer binder, it has rarely been studied with other wastes especially, glass wool waste. Therefore, this study presents the use of glass wool (GWF) from the insulating materials waste and high calcium fly ash (HCFA) as binary precursors to produce geopolymer paste. HCFA was replaced by GWF at a ratio of 10–40% by weight. The sodium silicate-to-sodium hydroxide (NS/NH) ratio of 1.0 and a liquid-to-binder ratio of 0.6 were used. The geopolymer pastes were cured at room temperature before mechanical, microstructural, and durability property analyses. The results showed that the setting time and slump flow increased with increasing GWF content. The incorporation of GWF had little effect on the early strength of the geopolymer paste, but the 28-day compressive strength was significantly enhanced up to ∼67% (30% replacement) higher than that of the HCFA geopolymer paste. Likewise, the flexural strength increased with increasing GWF replacement content. The total porosity of the hybrid geopolymer paste was lower than that of the HCFA one resulting from the reaction and filling effect of the geopolymer matrix. The XRD, FTIR, and SEM-EDX analyses showed the presence of geopolymerization products with calcium silicate hydrate regardless of the geopolymer mixture composition. This suggested that GWF could react with alkaline activators to form geopolymer products even though some unreactive GWF was observable when GWF replaced at 40% by weight. Furthermore, an increase in GWF content led to better resistance to sulfuric acid and magnesium sulfate attack, which was observed within 180 days. All of these results suggested that GWF was an effective secondary precursor for improving the fresh and hardened properties of HCFA-based geopolymer paste. In addition, the residue acts as a reinforcing material in the matrix paste. Consequently, improved resistance to acid corrosion and sulfate expansion were significant.