Influence of cement composition on concrete durability

Cement composition significantly affects the durability performance of concrete. Pore fluid studies and x-ray diffractograms show that C₃A partially removes dissolvable corrosion-inducing chlorides from the pore solution by forming Friedel's salt. Accelerated corrosion tests and exposure site data on concrete specimens show the practical significance of chloride binding by the C₃A phase of cement. Corrosion initiation time, time-to-cracking of cover concrete, and chloride threshold values were found to increase systematically, whereas metal loss from reinforcement decreased as the C₃A content of the cement increased. The data also show that, for given C₃A and chloride contents, more chlorides are bound if they are present initially in the mix at the time of making concrete (called internal or primary chlorides), compared to chlorides that penetrate into concrete from external sources later during the service life of a structure (called external or secondary chlorides). Also, with increasing levels of chloride addition to concrete, the quantity of bound chloride increases, and the rate of chloride binding decreases. Although cement alkalies have an inhibiting effect on chloride binding, they also significantly and concomitantly increase the OH^- concentration of the pore solution resulting in a lowering of the Cl^-/OH^- ratio, which is a measure of corrosion risk. The latter effect dominates and overshadows the inhibiting effect of cement alkalies on chloride bindings, thereby reducing the corrosion risk. Cement alkalies also cause concrete durability problems due to alkali-silica reaction in conjunction with aggregates containing reactive siliceous constituents. In addition to the C₃A phase, cements with high C₃S/C₂S ratios also are prone to sulfate attack of a softening type caused by gypsum formation. Tests show that such an attack may also occur in low C₃A cements in which the composition is characterized by high C₃S and low C₂S contents to achieve high early age strengths. C₃S and C₃S phases of cement influence temperature rise and thermal cracking in concrete, whereas gypsum content influences shrinkage cracking. Blended cements formulated by partial replacement of ordinary Type I high C₃A portland cement by 10 percent silica fume, 20 percent fly ash, or 70 percent blast furnace slag concomitantly show a significantly superior performance in terms of corrosion-initiation time and resistance to sulfate attack. These cements also show lower chloride permeability, only one-tenth to one-half of the allowable expansion due to alkali-silica reaction, and lesser risk of cracking due to heat of hydration gradients. Composition of modern cements usually is characterized by markedly higher C₃S/C₂S ratios than in older cements. Concretes made with these cements and specified in terms of 28-day strength only usually would satisfy these specification at higher water-cement ratios than concretes made with older cements. In general, this would lead to a more permeable, and hence less durable, concrete.