Hydration and microstructure of ultra high performance concrete incorporating rice husk ash

Rice husk ash (RHA) and silica fume (SF) have a similar chemical composition and a very high specific surface area, but RHA is not an ultra-fine material like SF. The high specific surface area of RHA originates from its internal porosity. For this reason RHA can be expected to behave differently from SF in terms of the hydration and the resulting microstructure of concrete. This still remains unclear in Ultra High Performance Concrete (UHPC). The objective of this research was to study the effect of RHA on the hydration and microstructure development of UHPC. The results are compared to those obtained with a control sample and a sample made with SF. The results show that the addition of RHA can increase the degree of cement hydration in UHPC at later ages. RHA can also refine the pore structure of UHPC and reduce the Ca(OH)2 content, but less significantly than SF. The thickness of the interface transition zone (ITZ) between sand particles and cement matrix of all samples is very small at the age of 28 days. The compressive strength of the sample made with RHA after 7 days was higher than that of the control sample and the sample made with SF. It is suggested that the porous structure of RHA and the uptake of water in this porous structure results in a kind of is attributable to the internal water curing of the RHA modified mixture.