Influence of Pozzolanic Material in the Conversion and Corrosion Behaviour of Calcium Aluminate Cement

Abstract

Calcium aluminate cement (CAC) is used as a protective lining to combat attack by microorganisms and their metabolic products on reinforced concrete sewer pipes. CAC corrosion behaviour is related to the conversion process of the aluminate hydrates. Full conversion can result to a 5-8 times increase in corrosion rate. Hence, understanding and controlling the rate and the processes that affect conversion is important in the effective use of CAC as a mitigation strategy against the attack of microorganisms. This study was focussed on the role of the pozzolanic material, i.e. fly ash, in suppressing the conversion of CAC. CAC mortars were prepared with constant water-cement ratio of 0.4 and with fly ash contents of 0%, 5%, and 25% weight. To assess the effectiveness of the fly ash, CAC mortars were cured at high temperatures (80-100oC) in water bath to accelerate conversion. Thermal analysis (TGA), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) with integrated EDS and EBSD system were used to monitor the physical and mineralogical transformation of CAC during the conversion and leaching tests. Our study showed that pozzolanic materials have negative and positive impacts in CAC. Based on thickness loss, addition of fly ash improved the corrosion resistance of CAC. This can be attributed to the increased formation of the stratlingite (C2ASH8) that was found to be resistant to both organic and mineral acids. However, analysis based on weight loss and dissolved Al metal showed that addition of fly ash suppresses early corrosion but later corrosion is increased, which is attributed to greater formation of C3AH6 and greater porosity.