The effect of fly ash and silica fume on mechanical properties and durability of coral aggregate concrete

This report presents the results of experimental work on the mechanical properties of coral aggregate concrete (CAC) containing fly ash (FA) and silica fume (SF). The aim of this study was to investigate the effects of FA and SF on the fresh and aged mechanical properties of CAC. Apart from the workability of the fresh concrete, the compressive strength, splitting tensile strength, capillary water sorptivity, chloride penetration, and micro-hardness of the interfacial transition zone (ITZ) were measured. CAC with only ordinary Portland cement as the binder was used as the control mixture. The results indicated that the cohesiveness and water retention of CAC were improved by the addition of FA and SF under the condition of equal slump. CAC showed lower compressive but higher splitting tensile strength, water sorptivity, and chloride penetration compared to normal concrete because of its porous and rough surface and the high water absorption of the coral aggregate itself. FA and SF have different effects on improving the mechanical properties, durability, and structure of the ITZ of CAC. FA had a negative effect on strength at early ages, but significantly enhanced the later-age strength, while SF had a positive effect at all ages. Similar results were obtained in the chloride penetration test. However, it was found that, besides improving the compressive strength and the splitting tensile strength, the combination of FA and SF had a positive effect in reducing the water sorptivity and chloride permeability, as well as improving the micro-hardness in the ITZ, of CAC. The above properties of CAC containing 25% FA and 5% SF as partial replacements for cement are superior to those of CAC containing 5% SF alone.