The effect of water-cement ratio on the microstructure of set Portland cement

Abstract

Set Portland cement consists of a ‘gel’ phase of calcium hydrosilicates with smaller amounts of crystalline and amorphous calcium hydroxide and crystalline calcium aluminate hydrate salts together with unhydrated cement clinker minerals. Electron-optical studies of single stage carbon replicas of fracture surfaces of set Portland cement pastes show that the calcium hydrosilicates formed at low water-cement ratios, e.g. 0.25, are of a fine plate-like nature with a few fibres in the pores; an increase in water-cement ratio brings about an increase in the porosity and an increase in the fibres formed in the pores. The compressive strength falls with an increase in water-cement ratio because of the increase in porosity.