Super-critical carbonation of glass-fibre reinforced cement. Part 2: Microstructural observations

Abstract

The deterioration of glass-fibre reinforced cement/concrete (grc) arises to a substantial extent from the alkalinity and calcium hydroxide content of the matrix. Carbonation of the matrix significantly lowers both factors, but under normal circumstances the reaction proceeds too slowly to be of practical use in improving durability. If carbonation is effected using super-critical carbon dioxide the reaction can be completed within hours rather than years, rendering it potentially attractive as a treatment for enhancing the durability of grc. The efficacy of such treatment is dependent on the moisture content of the samples prior to treatment expressed in terms of a degree of drying (DOD), as established in Part 1 of this paper. Grc samples thus treated, were examined using petrography, scanning electron microscopy and mercury intrusion porosimetry before and after a period of accelerated ageing. Treatment transforms the matrix into a groundmass of amorphous material intimately mixed with microcrystalline calcium carbonate. Dark inclusions appear to be remnants of unhydrated cement particles. The mechanical property enhancements detailed in Part 1 are attributed to the precipitation of calcium carbonate reducing porosity. The nature of the carbonation reaction was found to be dependent on DOD. In samples indicated by differential thermal analysis to be completely carbonated, some uncarbonated unhydrated cement and uncarbonated amorphous groundmass remained in isolated patches.