The effect of local processed kaolin from the southern part of Thailand, with fineness of about 9,800 cm(2)/gm, on strength and resistance to chloride permeability, was investigated. Between 0 to 40% cement replacement in mortar and 0-30% in concrete were studied for use as substitute for imported silica fume in high strength concrete with the target strength of 660-860 kg/cm(2), and as repair material. Metakaolin increased microstructure denseness. Cement replacement of up to 30% by metakaolin increased the compressive strength of mortar. At 40% the increase dropped slightly, but the strength was still higher than the control mix. The charge in the void system affected permeability development more than it did compressive strength. In the rapid chloride permeability determinations, the charge passed in metakaolin mortar decreased from moderate to very low level as ages or percentage replacement increased, while the control mixes provided high charges passed at all ages. The reduced penetration depth of chloride front indicated the substantial improvement of this material. For concrete, the strength improvement especially during the first 3 days was observed. The increase in compressive and flexural strength were in the range of 13-18%, and 1-16% respectively. An optimum percentage replacement of 20% was found for strength improvement. Significant microstructure improvement was revealed through the very high level of chloride ingress resistance, compared to the medium level of high strength concrete. The potential as a low cost, locally produced, supplement material for repair material and high strength and durable concrete was high.
