Alkali-bauxite fusion method of synthesizing ambient-cured volcanic ash geopolymer cement mortars

•VA-GPC = volcanic ash-based geopolymer cement (VA-GPC), faBX = sodium aluminate admixture prepared as NaOH-bauxite fusion reagent.•Ambient-cured plain VA-GPC shows weak reactivity, long setting and weak strength.•Synthesized ambient-cured 10% faBX/VA-GPC gave normal setting and good strength.•Synthesized ambient-cured faBX/VA-GPC gave typical hardening cement properties.•Geopolymeric reactivity of faBX increased with rise in Na2O content. The plain volcanic ash-based geopolymer cement (VA-GPC) system typically exhibits weak strength development under ambient temperature curing, thereby limiting its potential application in engineering construction. This study aimed at synthesizing an improved VA-GPC binder using a highly reactive sodium aluminate material as an admixture for enhancing geopolymerization. Bauxite was fused with sodium hydroxide alkali at 750 °C to prepare the sodium aluminate admixture (faBX). The faBX admixture composition was varied to different amounts of sodium oxide comprising 20, 30, 40, 50 and 60 % Na2O by weight of bauxite. A combined solution of sodium silicate and sodium hydroxide was used as the alkaline activator reagent. In the study, the VA-GPC binder was synthesized by blending volcanic ash with 0 or 10 wt% faBX admixture. Mortar mixtures were then made by mixing the synthesized VA-GPC binder with fine aggregate, followed by curing at ambient temperature. Setting time, workability, compressive strength, drying shrinkage, pore volume and water absorption properties of the geopolymer specimens, were evaluated. Analytical studies were done using XRD, FTIR and SEM/EDS. It was found that the 28-day compressive strength results gave values of up to 26.9 MPa, representing 1500 % strength increase over that of the control. The initial and final setting times were significantly shortened as Na2O content of faBX increased. Microanalysis results showed that an amorphous geopolymeric gel was produced, consisting mainly of poly(silicate-siloxo) and poly(ferro-silico-aluminate) phases. Evidently, incorporation of faBX in VA-GPC enables the binder system to develop suitable normal or rapid hardening geopolymer cement properties under ambient curing conditions.