The electron microscope has been used to study changes in particle size, shape, and arrangement that occur during the hydration of clay in soil—cement. Samples of kaolinite, of a mixture of silica flour and montmorillonite, and of a natural silty clay were mixed with sufficient portland cement to produce soft—cement. Specimens were cured at constant moisture content for periods up to 32 weeks after compaction at optimum moisture content for the mixture. At the end of the curing period surfaces of fresh fractures were replicated, and the replicas were studied using the electron microscope.

All three mixtures showed similar behavior. Initially the fabric is one of separate portland cement grains distributed throughout the clay soil. As hydration of the cement proceeds, cement hydrate gel forms along the edges of groups of clay particles. Reaction between the soil and the cement is observed early in the hydration period. As hydration continues the soil grains are more and more broken down and the cement gel diffuses more extensively throughout the mass. Eventually, the breakdown of the soil minerals and the formation of hydration products reaches a point whore the soil and cement can no longer be distinguished as separate phases.