Gibbsite containing alumina samples have been heated up to 1200degreesC in air. The decomposition of gibbsite and/or the departure of hydroxyls are found to influence the mechanical behaviour of porous alumina in the presence of water to a large extent. Powdered products of gibbsite decomposition first lead to a decrease in the Young's modulus. Sintering then occurs, dominated by the hydroxyl content, to reestablish the initial values. Two mechanical models are presented. In the first one, cohesion of granular solids results from the density of H-bonds inside the solids. In the second one, surface energy effects across grain boundaries dominate the mechanical behaviour. Application to various granular solids is discussed.