Role of heat curing in concrete durability: Effects of lithium salts and chloride ingress on delayed ettringite formation.

摘要

Concretes are usually exposed to elevated temperatures at early ages during heat curing to accelerate early strength gain. But other factors can cause high concrete temperatures including the use of high cement contents, large pours, and hot weather conditions. It is generally accepted that there are some adverse effects when concrete is exposed to heat at early ages. Delayed ettringite formation (DEF) is a deterioration phenomenon attributed to high temperature curing. This investigation was conducted in two phases using concretes and mortars heat cured at temperatures in the range of 65 to 95°C.;Phase One investigation was conducted to determine the effect of high-temperature curing on the physical properties of concrete. Portland cement concretes of 0.35 and 0.45 water-cement (w/c) ratios, and ternary cement (5% silica fume, 25% slag) concretes of 0.35 water-cementitious (w/cm) ratio were cured at 23, 65, 80, and 95°C. The properties examined were the compressive strength, chloride permeability, bulk diffusion, salt scaling, rebar corrosion, pore structure, and microstructure. Results showed that there were drastic changes in concrete properties as the curing temperatures increased above 65°C. This has been termed the 'critical temperature effect'. Salt scaling of concretes was highly sensitive to elevated curing temperatures but the w/c was found to be more influential. Most adverse effects of heat curing were significantly mitigated by using ternary cement concrete except in the case of compressive strength.;Phase Two investigation examined the influence of other deterioration processes on the potential for DEF. The effects of lithium admixtures, alkali-silica reaction and freeze-thaw microcracks, and chloride ingress were investigated using concretes of 0.45 w/c and mortars of 0.47 w/c heat cured at 95°C. Studies included X-ray diffraction and differential thermal analysis. It was found that the use of lithium admixture in cementitious systems controls DEF. Also, externally applied chlorides have a pessimum effect on expansion due to DEF. This deviates from the commonly held view that chlorides mitigate sulphate attack in concretes. A mechanism describing the pessimum effect of chlorides on DEF expansion has been proposed.