Published online by Cambridge University Press: 21 February 2011
Mortar bars of tricalcium silicate or ordinary portland cement with small aggregate particles have been sectioned and examined in the transmission electron microscope (TEM). The presence of calcium silicate hydrate (CSH) gel at the interface is seen by TEM in many cases in intimate contact with the aggregate. In some cases calcium hydroxide (CH) is observed near the interface but is seldom in contact with the aggregate. Microanalysis in the TEM shows that in the case of reactive silica aggregate a gel of relatively low Ca:Si ratio is found at the interface. Aggregates of different degrees of reactivity and mortars of different ages are compared. It is suggested that interface fractures may occur beyond the coating of CSH rather than at the true interface in the case of the type of particles we have studied.