Potential of Saudi Arabian bauxite to produce low-carbon cement

Abstract

To produce calcium aluminate cement (CAC), bauxites are usually fused with lime/limestone at high temperature (1600 °C). At this temperature, the bauxite´s hydrates of alumina break down - dehydroxylation - and combine with calcium forming monocalcium aluminate (CA), the principal active phase in CAC.

A previous study evidenced that the Saudi bauxite begins dehydroxylation at low temperature (300 °C). This paper investigates whether low temperature can produce a cement, to reduce the carbon footprint of cement production. Cements are sintered by fusing the bauxite with calcium sources (limestone and quicklime) at temperatures from 600 to 1200 °C.

The results evidenced that limestone fusion is the most efficient method, as it renders hydraulic phases at 800 °C (C₁₂A₇) and 1000 °C (haüyne). The early release of Ca²⁺ from the limestone acts as a flux, lowering the breakdown point of the bauxite´s components. C₁₂A₇ (mayenite) which can speed up hydration and setting, appears widely in the limestone-bauxite cements, beginning at 800 °C and remaining stable up to 1200 °C.

The bauxite´s gypsum released sulphur, affording the sintering of calcium-sulfoaluminate (haüyne) at 1000 °C. Therefore, the bauxite can produce sulfoaluminate cement, a green cement which can reduce carbon emissions and fight climate change.

The bauxite´s high silica content and the breakdown of its kaolinite polymorph nacrite, facilitate the production of hydraulic calcium silicate clinkers (belite, andradite, gehlenite, wollastonite and prehnite) which afford strength on hydration.

The fluxing action of iron, aluminium and sulphur, significant in the bauxite, lowered the clinkering temperature.