Pozzolanic and hydraulic activity of bauxite for binder production

Abstract

Bauxite is mainly used to produce aluminium and calcium aluminate cements. Both applications carry high environmental impacts. However, the carbon emissions generated from mining bauxite (3–5 kg CO2e/t) are negligible when compared those generated for aluminium or cement production (620–700 kg CO2e/t for CEM II). Hence, an alternative use for bauxite would lower environmental impacts worldwide. This paper measures the pozzolanic and hydraulic capacity of raw and thermally activated bauxite with a view to use it, as a binder, in construction.

The results indicate that the bauxite is pozzolanic and, when fired over 900 °C it becomes slightly hydraulic. An amorphous reactive phase begins to appear at 300 °C, but the lime binding potential is enhanced at 550 °C because most of the aluminium is in amorphous form. Some active phases (except for ϒ- Al2O3) could not be detected with XRD due to their amorphousness. However, their pozzolanicity was evidenced with direct and indirect measurements.

The bauxite's pozzolanic and mechanical indices are high, comparable or superior to other pozzolanic and cementing materials. Gibbsite, boehmite and kaolinite are the main phases in the bauxite. Their layered atomic structures with high specific surface areas (SSA) and active surface hydroxyls that enhance adsorption, and hence nucleation, precipitation and dissolution, provide a high pozzolanic activity. The high kaolinite content, and the presence of the highly-reactive, kaolinite-polymorph nacrite, enhance reactivity, and the presence of sulphates also contributes to the initial activity.

The SSA of the bauxite particles, is superior to CEM II and other pozzolanic and cementing materials, and heating, even at low temperature (300 °C), drastically increases the SSA of the particles (by∼60%). The methods that determine initial pozzolanic activity rate the 550 °C bauxite as the most active, while the mechanical methods rate the 700–800 °C bauxites as the most reactive. This is because the high SSA of the 550 °C bauxite enhances lime combination in the earlier stages of the pozzolanic reaction, while the 700–800 °C bauxite has the highest amorphous/active alumina content which control late pozzolanic activity. As evidenced with XRD, at 700–800 °C, boehmite has disappeared, transformed into amorphous transition aluminas, and kaolinite has completely turned into reactive metakaolin.