Abstract
This paper investigates effects of a partial replacement of cement with metakaolin (MK) on the durability of hollow sandcrete blocks. Sandcrete hollow blocks of size 225 mm x 225 mm x 450 mm were produced from a mix of ratio 1:6 (binder : sand), and cured in water for 28 days. The cement was replaced with metakaolin at 0% to 35% levels at 5% intervals. Effect of replacing cement with MK in the sandcrete blocks on their dry density, total water absorption and porosity, wet and dry compressive strength were investigated. The average dry density of metakaolin hollow sandcrete blocks produced at various replacement levels after 28 days is 1892 Kg/m3, which falls within the minimum specification of 1500 Kg/m3 recommended for first grade (type “A” - load bearing block) sandcrete blocks by Nigeria Industrial Standard NIS 87: 2000. The inclusion of MK must have transformed the weaker and more porous metakaolin sandcrete blocks fabric into a far denser, more homogeneous and more impermeable component. Sandcrete hollow blocks samples made with 10% metakaolin had lowest water absorption of 6.25% which produces blocks with dense and compacted unit; this is as result of metakaolin ability to fill the void within the block fabrics attributed to metakaolin fineness. There was a sudden increase in water absorption from 15% to 35% replacement level caused by increase in metakaolin content in the mix. This might be due to the formation of a denser hydration phase. The porosity values of all metakaolin replacement with cement sandcrete blocks was less than 30% and are considered to be of low porosity. The ratio of dry and wet compressive ranges between 1.3 and 2.0 with the control block having 1.7. The findings shows that the higher and broader the ratio between mean dry compressive strength and wet compressive strength, the lower can the degree of inter granular bonding are expected to be. The results obtained indicated that replacement with 10% metakaolin satisfies Nigeria Industrial Standard requirements and British Standard Specification (6.25%) for water absorption. Metakaolin sandcrete blocks produced at 10% cement replacement with metakaolin is adequate for use as load bearing and non-load bearing structures in areas where they may be subjected to moisture ingress.

Key words: Metakaolin, block water absorbtion, porosity value, mean compressive strength, cement partial replacement.