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Aluminum hydroxide (gibbsite) powder was ground in air by a double rotating cylinders mill (UF mill) under two different grinding conditions: one is grinding by rotating both cylinders inversely, called Case-1, and the other grinding by rotating the inner cylinder only, called Case-2. The ground powder was mixed together with cement powders with water to form a hardened body to investigate the effect of grinding on hardened body properties. The impact energy density of balls in Case-1 is higher than that in Case-2. The reactivity of product ground in Case-1 is lower than that in Case-2, when they are compared at the same degree of amorphization for the gibbsite sample. This leads to the formation of calcium aluminate hydrate with dense phase in the hardened body prepared in the Case-1. The compressive strength of hardened body prepared in Case-1 is higher than that in Case-2. The grinding gibbsite sample in Case-1 is suitable for the product to form the hardened body with a high compressive strength.

Milling of gibbsite by an agitation ball mill brought about mechanochemical dehydration and amorphization. A remarkable increase in hydration reactivity with Ca(OH)2 is demonstrated by using a conduction calorimeter. An integral heat of hydration from mechanically activated gibbsite is about 300 times larger than that from the intact gibbsite. We found high correlation between the integral heat of hydration and the 3h compressive strength of conventional Portland cement paste containing milled gibbsite with or without Ca(OH)2. Acquired high activity of milled gibbsite was sustained for at least 2 months under 50%RH. These properties warrant applicability of mechanically activated gibbsite to rapid hardening agents for Portland cement.