Preferential precipitation of calcium hydroxide on alkali-resistant glass fibres

Abstract Carbon discs 12.5 mm diameter were coated with roughly equal areas of Portland cement (PC) and Alkali Resistant Glass (ARG); PC and Blastfurnace slag; PC and Fly Ash (FA): and with combinations of PC, ARG and BFS and PC, ARG and FA. The dry materials were separated by blank strips of carbon. The reaction was started by wetting the dry components with water. The discs were then stored at 100 percent relative humidity until they were examined in the SEM. Calcium Hydroxide (CH) released into solution by hydration of PC could thus precipitate on ARG or BFS or FA or on the blank areas of carbon. In all cases CH displayed a remarkable affinity for ARG in distinct preference to BFS and FA which are also known to attract deposits of CH. Further the morphology of CH deposited on glass suggested that any tendency of the fibre to move relative to the matrix would result in principal compressive stress perpendicular to the fibre axis. This would reduce the longitudinal strength. Embrittlement of Glass Reinforced Concrete with age is attributed to the affinity of CH with ARG and the morphology of CH precipitate.