This paper presents an experimental study on the effect of chemical composition and physical properties of rice husk ash (RHA) on the strength of mortar. The aim of this investigation was to establish the optimal RHA replacement levels as blending component in cement. Four different types of RHA (A, B, C and D) were used of which RHA-C and D had the highest content of amorphous silica. Compressive and tensile strength and workability of the mixes were compared with control OPC samples. Results show that the early age (7days) strength of RHA blended mortar samples was lower than the compressive and tensile strength of the OPC control samples. However, the 28 day strength of samples with up to 20% RHA-C and RHA-D was higher than that of the control samples. The 90 day strength of all blended samples with up to 40% RHA was higher than that of the control samples. RHA-D performed best in the experiments with a compressive strength increase of 16% at 20% RHA replacement and an increase of compressive strength of 8.6% at 40% RHA replacement. The results at 90 days show that cement could be replaced with up to 50% RHA with only a small reduction in strength compared to OPC mortar. From the results it was found that the content of amorphous silica has the biggest influence on the strength of the mortar samples while the workability was higher for samples with finer grained RHA.