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The partial substitution of cement (both high C₃A and intermediate C₃A) with metakaolin (MK) is investigated in terms of resistance of MK mortar to sodium sulphate (Na₂SO₄) solution. Results on strength, porosity, pore size distribution, and calcium hydroxide (CH) contents are also reported. The sulphate expansion results demonstrate that the sulphate resistance is increased as the replacement level of cement with MK increases, up to at least 25% replacement. Refinement in pore structure and reduction in CH content increase with increase in MK content (for moist-cured mortar), and these two factors are considered to be the principal factors affecting improvement in sulphate resistance. Also, although after prolonged periods of exposure to Na₂SO₄ solution, there is (relative to water-cured specimens) significant strength loss of PC mortars and mortars with low levels of MK (5 and 10%); for mortars with high levels …