Properties of Alkali Activated Materials made with bauxite refining residue (red mud-RM) and blends of RM with fly ash (FA)

Abstract

A bauxite refining residue (RM), both alone and blended with FA at 30% and 50%, is activated with alkalis (Na2SiO3 and Na2SiO3/NaOH) to produce geopolymer cements. Alkali-activated materials (AAMs)/geopolymers are a more sustainable alternative to Portland cement (PC) products because they largely consist of waste. They can save energy (consumed for the calcination of raw materials and the grinding of PC clinker) and reduce carbon emissions (emitted on combustion of fossil fuels and decarbonisation of the raw materials for PC clinker production). They can provide an alternative route to transform waste into construction materials. They can also reduce landfill use, lowering soil and water contamination, and lessen the exploitation of non-renewable resources such as the shale, limestone and clay use for PC production. The RM geopolymers achieved significant strengths. Even the materials made with RM alone reached significant strengths up to ~7MPa at 90 d. Replacing RM with FA increased strength. The compressive strength of the 30% FA materials at 28 d.: ~7 to 11MPa, increased to ~9 - 12MPa with 50% FA substitution, reaching over 15MPa at 90 days. Using a blend of activators at high silica ratio (Na2SiO3/6M NaOH=2.5) provides the greatest strengths and the maximum durability under accelerated weathering. The Na2SiO3 activator alone delivers low strengths and reduces the resistance to weather, probably due to an excess of unreacted Si preventing polymerisation. The setting times are within workable limits. Initial setting times range from 340 to 660 minutes, and the pastes with the highest FA content are the slowest to set. The geopolymers made with RM alone form scarce cementing hydrates, hence the RM acted as a filler during polymerisation. However, the RM-FA geopolymers show abundant hydrates including low crystallinity gels and needle-shaped phases-probably AFm. The gels contain significant Na that hints to the presence of N-A-S-H.