Behaviour of Rice Husk Ash in Self Compacting High Performance Concrete The main objective of this thesis is to investigate the characteristics of rice husk ash RHA) and then its behaviour in self compacting high performance concrete (SCHPC) with respects to rheological properties hydration and microstructure development and alkali silica reaction in comparison with silica fume (SF). The main results show that the RHA is a macro mesoporous amorphous siliceous material with a very high silica content comparable with SF. The pore size distribution is the most important parameter of RHA besides amorphous silica content. This parameter affects pore volume specific surface area and thus the water demand and the pozzolanic reactivity of RHA and its behaviour in SCHPC. The incorporation of RHA decreases filling and passing abilities but significantly increases plastic viscosity and segregation resistance of SCHPC. Therefore RHA can be used as a viscosity modifying admixture for SCHPC. The incorporation of RHA increases the superplasticizer adsorption the superplasticizer saturation dosage yield stress and plastic viscosity of mortar. Fresh mortar formulated from SCHPC is a shear thickening material. The incorporation of RHA SF ecreases the shearthickening degree. The incorporation of RHA SF increases the degree of cement hydration. SF appears more effective at 3 days possibly due to the better nucleation site effect whereas RHA dominates at the later ages possibly due to the internal water curing effect. The incorporation of RHA SF increases the degree of C3S hydration particularly the C3S hydration rate from 3 to 14 days. The pozzolanic reaction takes place outside and inside RHA particles. The internal pozzolanic eaction products consolidate the pores inside RHA particles rather than contribute to the pore refinement in the cement matrix. In the presence of the high alkali concentration RHA particles act as microreactive aggregates and react with alkali hydroxide to generate the expansive alkali silica reaction products. Increasing the particle size and temperature increases the alkali silica reactivity of RHA. The mechanism for the successive pozzolanic and alkali silica reactions of RHA is theorized. Additionally a new simple mix design method is proposed for SCHPC containing various supplementary cementitious materials i.e. RHA SF fly ash and limestone powder.