Structure and properties of gels

Abstract

A brief review is presented of the structure and properties of inorganic gels. The mechanism of formation of gels and precipitates involves the aggregation of polymers or particles. If bonds form irreversibly, fractal aggregates grow and eventually fill space, resulting in a continuous elastic gel network; if the bonding is reversible, then the aggregates reorganize and become more compact, leading to precipitates. The existence of capillary pores in cement paste indicates that C-S-H is a precipitate, rather than a gel. Chemical reactions continue after gelation, leading to increasing rigidity and, in some cases, to spontaneous shrinkage (called syneresis). During drying, capillary pressure on the order of 10 to 100 MPa develops and produces huge shrinkage of compliant gels. As the gel shrinks, its modulus increases by several orders of magnitude; shrinkage stops when the network is stiff enough to resist the capillary forces. For gels, the modulus increases in proportion to the volume fraction of solids to a power between ∼3 and 4; consequently, the evolution of the modulus during drying and the final shrinkage can be estimated. Warping and cracking do not depend on the magnitude of the capillary pressure, but only on the gradient in pressure within the body. Slow drying reduces the gradients and prevents damage. The stresses and strains during drying can be calculated in terms of the permeability, physical dimensions, and viscoelastic properties of the gel. A variety of techniques have been developed to measure the latter properties, the simplest of which is a beam-bending measurement. When this method is applied to a rod of cement paste, the permeability and modulus are obtained in a matter of minutes.