The effects of using rice husk ash (RHA), as a cement replacement material, on the flowability, rheology, rate of flowability loss and compressive strength of cementitious materials were extensively studied in a controlled experimental program. The possibility of using RHA for producing high strength concrete (HSC) was also attempted. Various 0.5 w/c OPC mortar mixes containing different contents of RHA were therefore prepared and subjected to the mortar flow test at different elapsed periods from mixing. Cubical specimens were taken from these mixes, cured with different curing regimes (air, moist and sealed) and finally tested for compressive strength at age of 56 days. Concrete specimens made with different RHA contents and water cement ratios (0.4, 0.3 and 0.25) were also prepared and tested for compressive strength. It was found that the incorporation of RHA in OPC mixes has led to a notable reduction in the initial flowability, rate of flowability loss and parameters of rheology, and an increase in the compressive strength. RHA can be used for manufacturing HSC provided that w/c ratios of less than 0.30 are avoided.