Hydration of αL′- and β-dicalcium silicates with identical concentration of phosphorus oxide

The hydration behavior of specimens with identical chemical compositions (Ca(1.955)square(0.045)) (Si0.91P0.09)O-4 but different in phase compositions was examined, One composition was 30% alpha'(L) and 70% beta, whereas the other composition was 92% alpha'(L), and 8% beta. During the early stage of the hydration process of about 3 days, the specimen rich in the beta-phase hewed a much higher hydraulic activity than that rich in the alpha'(L)-phase. Because these powder specimens showed almost the same specific surface areas and particle size distributions, the higher reactivity with water was ascribed to the more favorable defect state on the crystal surface, The beta-phase showed polysynthetic twinning on (100)(beta). The intersections of the twin boundaries and the crystal surface would behave like active centers of the reaction with water. The beta-phase, therefore, showed a much higher cumulative heat for the 3-day curing than the alpha'(L)-phase, the crystal fragments of which were nearly uniform with occasional line defects. After hydration for 15 days, both specimens showed almost the same cumulative heat of about 120 J/g. The heat evolution continued for 34 days, with the eventual cumulative heat of 143 +/- 3 J/g. (C) 1999 Elsevier Science Ltd. All rights reserved.