Effect of sea sand and recycled aggregate replacement on fly ash/slag-based geopolymer concrete

The purpose of this study is to investigate the impact of recycled aggregate (RA) and sea sand (SS) replacement on fly ash (FA) slag-based geopolymer concrete (GPC). An orthogonal array design is employed to obtain the optimum mix proportions, and geopolymer mixes are prepared using slag percentages of 10%, 20%, and 30% slag in FA/slag-based GPC. Sodium hydroxide (NaOH) solution is prepared at three concentrations (8 mol/L, 12 mol/L, and 16 mol/L). The mechanical properties of the geopolymer mixes are determined based on the tensile strength, compressive strength, flexural strength, and elastic modulus. GPC is prepared using water-binder ratios of 0.3, 0.4, and 0.5 at 0%, 25%, 50%, 75%, and 100% of RA replacement. The results showed that the variation in the RA replacement ratio had little effect on the strength and elastic modulus of sea sand geopolymer concrete (SS–GPC), but it had a significant effect on river sand geopolymer concrete (RS–GPC). The RA replacement ratio also showed a noticeable change in the damage surface of the specimens. In addition, SS hinders the hydration reaction of the geopolymer in the early stage and reduces the early strength of the GPC; however, in the later stages, the effect becomes insignificant.