Influence of Nucleation Seeding on the Hydration Mechanisms of Tricalcium Silicate and Cement

The fundamental chemical hydration process of portland cement and its main mineral component, tricalcium silicate, was studied by investigating the effects of various additives. A relatively small amount (1−4 wt %) of well-dispersed calcium silicate hydrate (C−S−H), a pure form of the main hydration product, significantly increases both the early hydration rate and the total amount of hydration during the early nucleation and growth period (the first ∼24 h), as measured by calorimetry. This is attributed to a seeding effect whereby the C−S−H additive provides new nucleation sites within the pore space away from the particle surfaces. This mechanism is verified by a digital simulation of the hydration process that reproduces key features of the hydration kinetics. The results provide strong evidence that the hydration process is autocatalytic such that the C−S−H gel product stimulates its own formation. The seeding effect of C−S−H also provides a new explanation of the hydration-accelerating effects of various forms of reactive silica because these additives form C−S−H by reacting with aqueous calcium ions released by cement dissolution. Experiments involving sucrose, a hydration retarder, confirm that sucrose interferes with the normal nucleation process on the particle surface.