Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.