Synergistic effects of bauxite refinery residue with fly ash and slag on strength development and hydration in mortar

The growing demand for supplementary cementitious materials (SCMs) has prompted research into bauxite refinery residue (BRR), a by-product of alumina refining, as a potential cement substitute. In this work, the performance of BRR combined with fly ash (FA) and ground granulated blast-furnace slag (GGBS) as an SCM to replace Portland cement (PC) was evaluated. Mortar cubes incorporating 5–30% BRR were tested for compressive strength. The mixes achieving higher 28-day strengths were further analysed using X-ray diffraction to identify hydration products. The BRR demonstrated significant early reactivity, enhancing the initial strength. However, its contribution diminished over time, resulting in lower 28-day strengths at higher replacement levels. Despite this reduction, the high alkalinity and alumina content of BRR encouraged greater formation of the mono-carboaluminate (MCA) phase. Notably, the mix with 10% BRR and 20% GGBS attained 91.6% of the compressive strength of hydrated PC mortar, while the mix with 10% BRR and 15% FA reached 87.7% of that strength. These improvements were attributed to the development of stable MCA and calcium silicate hydrate gel. Overall, the results indicate that BRR, used alongside FA and GGBS, can improve strength properties, supporting the feasibility of higher cement replacement levels for sustainable construction.