Rice husk is one of major agro-waste material in Sri Lanka. Rice Husk Ash (RHA) is produced by burning Rice husk as fuel in brick kiln. Rice husk ash produced by burning at high temperature contains pozzolanic constituents. In Sri Lanka, cement is extensively used to manufacture masonry blocks, although the cost of cement is high. Because of having highly pozzolanic constituents in RHA, it can be used as a building material, instead of cement, to produce cement sand masonry blocks. It was frequently reported that compressive strength of RHA based blocks increases with increasing the RHA content up to 5% and further addition of RHA causes to decrease in compressive strength. In this study, an attempt is made to increase the utilization of RHA by adding Ca 2+ to the mixture. Solid masonry blocks, having the size of 360 mm x 100 mm x 170 mm, were cast with the mix proportion of 1:6 Cement-Sand. Blocks were manufactured in two series. In the first series, RHA was used as addition with respect to weight of cement. In this series, four different RHA contents (i.e., 0%, 5%, 10%, and 15%) were used with constant lime content (10%). In the second series, RHA was used as partial replacement for cement with four different RHA contents (i.e., 5%, 10%, 15% and 20%) with constant lime content (10%).The blocks were tested for 7, 14 and 28 Day compressive strength. With the presence of lime (10%), the optimum 28 Day compressive strength was found at the level of 10 % RHA. When RHA was used as addition, the optimum 28 Day average compressive strength of block was found as 4.937 N/mm 2. When RHA was used as partial replacement for cement, 28 Day average compressive strength of block was found as 3.467 N/mm 2. Thermal performances of the RHA lime based blocks were also investigated. It was found that thermal conductivity of RHA lime based block was lower compared with that of the conventional block. The RHA lime based blocks showed better structural and thermal performances. Increased use of RHA will reduce the unit cost of masonry block while improving sustainability.

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