The large areas of crystalline calcium hydroxide [Ca(OH)2] formed during the hydration of tricalcium silicate (Ca3SiO5) correspond to low-porosity regions in the hydrated paste. During the early stage of hydration, areas between Ca(OH)2 crystals which consist of Ca3SiO5 particles bonded together by calcium silicate hydrate represent the high-porosity portion of the paste. Because of the presence of Ca(OH)2, fracture in the hardened paste during this period propagates preferentially through the areas bonded by the calcium silicate hydrate phase and around the Ca(OH)8, crystals. Calcium hydroxide also acts as a crack arrester. The influence of Ca(OH)2 on fracture diminishes with increased hydration.