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The effects of thermally (MK) or mechanically activated kaolin (AK) on the compressive strength of mortars and microstructure of pastes were investigated. Mortar mixtures, in which 10%, 20% and 30% of ordinary Portland cement (OPC) was replaced by either MK or AK, were prepared (w/b of 0.5) and ordinary (age 2, 28 or 90 days) and autoclave cured. Hydration products were determined by X-ray diffraction (XRD) and differential thermal analysis/thermal gravimetry (DTA/TG) analysis, while microstructure was investigated by mercury intrusion porosimetry (MIP). MK substitution increases the compressive strength of ordinary-cured mortars, as a result of the higher content of reactive silica that caused more pronounced pozzolanic reaction, as well as by effective refinement of their pore structure. Positive effects on the compressive strength could be achieved up to 30% substitution of OPC by MK. The OPC substitution by AK resulted in lower strengths of ordinary-cured mortars, compared to both MK mortars and reference. Higher specific surface area and finer particles of AK were insufficient to compensate, through filler effect, lower pozzolanic reaction and additional negative influence of the kaolinite presence. The highest compressive strength was obtained for mortar with 10% of AK (relative strength of 94%). In comparison to the reference, autoclaved MK and AK mortars, exhibited lower compressive strength, as a consequence of increasing the hydrogarnet formation, instead of tobermorite. The highest strength was achieved for mortar with 10% of AK. © 2016 Elsevier Ltd