Electron microscope studies of Portland cement microstructures during setting and hardening

The rates of hydration of the individual phases in Portland cement have been related both to the disappearance of these phases from the microstructure and to the appearance of their products of hydration. After 12-15 h hydration a shell of product is formed containing short rods of AFt, an infilling of C-S-H and ettringite rods 1-2 pm long from the hydration of C3A. Later hydration takes the form of Hadley grains, by dissolution of the core from inside the shell and precipitation of product either inside the shell or in between the shells. The network of shells forms the cohesive element in the system after 10-12 h, with the coherent inner product filling up the space progressively inside the shells but not being attached to them. The implication of these loose fillings for the tensile strength and permeability are discussed. The onset of rapid hydration of C4AF has been identified by quantitative X-ray diffraction with a new peak found by conduction calorimetry; in the microstructure, fibres of AFt phase, 5 pm or more in length, appear suddenly from the C4AF and these are a familiar feature of o .p .c. pastes after 2 or 3 days hydration. After about 5 days at 20 °C the local availability of water is reduced and the rate of hydration of all the phases is reduced.