Mechanisms of high-durability performance of plain and blended cements

Durability of concrete has recently become a problem of major concern for construction industry in the Middle East and throughout the world. Concrete structures in the Middle East have shown premature deterioration within a short span of 10-15 years, the main causes for which are corrosion of reinforcement sulfate attack. Results of comprehensive study on concrete durability at King Fahd University of Petroleum and Minerals indicate that higher C{dollar}sb3{dollar}A cements show better corrosion resistance than low C{dollar}sb3{dollar}A cements and that certain blended cements perform manifold better than plain cements in terms of resistance to corrosion and alkali-silica reactivity (ASR).; In this research, an attempt has been made to study the mechanisms controlling the performance of plain and blended cements against corrosion of reinforcement and ASR. The effect of various compositional and environmental factors, such as C{dollar}sb3{dollar}A and alkali contents, concomitant presence of sulfates, temperature and mode of occurrence of chlorides, on chloride binding and pore solution alkalinity has been studied in plain cements, in an effort to evaluate their influence on mechanism of corrosion resistance. Mechanisms controlling resistance to corrosion and ASR of blended cements, formulated using fly ash, blast furnace slag and microsilica have been studied through their effect on chemical environment and physical characteristics of hardened concrete.; Results show that high C{dollar}sb3{dollar}A cements have more chloride binding capacity than low C{dollar}sb3{dollar}A cements, explaining their higher corrosion resistance. Increase in curing temperature and alkali content of cement and concomitant presence of sulfates decrease chloride binding capacity in plain cements. Plain cement bind more primary chlorides than secondary external chlorides. Fly ash and slag have little effect on chemical environment of concrete compared to microsilica, which drastically increases aggressivity of the chemical environment. The blending materials bring about significant improvement in the physical structure of concrete through pore refinement, decrease in permeability, chloride and oxygen diffusion and increase in electrical resistivity. These significant improvements in the physical characteristics due to blending materials are responsible for the improved corrosion resistance of the blended cements. Also, the blending materials significantly reduce OH{dollar}sp-{dollar} concentration in pore solution. The reduction in the OH{dollar}sp-{dollar} concentration and possibly the improved physical structure of blended cements are responsible for their increased resistance against ASR.