C-S-H, the poorly crystalline calcium silicate hydrate formed in cement paste and aqueous suspension, is characterized by extensive disorder and structural variations at the nanometer scale. An analysis of new and published solubility data for C-S-H formed by different preparation methods and with a broad range of compositions illustrates a previously unrecognized family of solubility curves in the CaO-SiO 2 -H 2 O system at room temperature. As demonstrated by 29 Si magic-angle spinning (MAS) NMR data and by charge balance calculations, the observed differences in solubility arise from systematic variations in Ca/Si ratio, silicate structure, and Ca-OH content. Based on this evidence, the family of solubility curves are interpreted to represent a spectrum of metastable C-S-H phases whose structures range from purely tobermorite-like to largely jennite-like. These findings give an improved understanding of the structure of these phases and reconcile some of the discrepancies in the literature regarding the structure of C-S-H at high Ca/Si ratios.

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.

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Cement-based materials have complex multi-component, multiscale structures that first form through chemical reaction and then continue to change with time. As with most classes of materials, the porosity of cement paste strongly influences its properties, including strength, shrinkage, creep, permeability and diffusion. Pores in cement paste range in size from nanometers to millimeters, and numerous investigations and models have been reported in the literature. This paper reviews some key concepts and models related to our understanding of the pore system and surface area. A major reason for the complexity of cement-based materials is that the principal reaction product, calcium silicate hydrate (C-S-H), forms with a significant volume fraction of internal, nanometer-scale pores. This gel pore system contains water that is also adsorbed to the solid surfaces, blurring the distinction between the solid phase and pores. The gel pore system changes not only with the chemical composition and extent of reaction, but also with changes in relative humidity, temperature, and applied load. Pores can be characterized by their surface area, size, volume fraction, saturation, and connectivity, but precise quantitative models are still not available. A useful approach for characterizing the structure of cement paste is to document the influence of time and external factors on structural changes. Scientific progress will be facilitated by the development of models that accurately describe the structure and use that structure to predict properties. This is particularly important because the composition and chemistry of commercial concretes is changing more rapidly than laboratory experimentation can document long-term properties such as durability. Some of the possible models are discussed.