Quantifying microstructural variations in cement pastes: Implications on drying shrinkage

The objective of this research was to better understand the interrelationship between processing/composition, microstructure, and properties/performance in cement pastes. The most challenging aspect of this process is the investigation of microstructure. The primary hydration product in cement pastes, calcium silicate hydrate (C-S-H), has historically had an elusive chemistry and microstructure. It is generally accepted that both the microstructure and stoichiometry of this phase are variable and highly responsive to environmental changes. This sensitivity makes examination of C-S-H problematic.; A method of examining the microstructure of cement pastes must meet two criteria. First, the method must be reproducible and sensitive to minute changes occurring in C-S-H. Second, it is desirable that the method be quantitative so as to facilitate correlation with processing variables and properties. With both of these criteria in mind, the nitrogen sorption method of measuring surface area and porosity was chosen for this investigation.; Nitrogen sorption results have historically not been reproducible. This dilemma was approached first, and the effects of sample preparation were examined. Next, the surface area was manipulated using temperature and chemical admixtures. Finally, the effects of these variables on total, reversible, and irreversible drying shrinkage were examined.; Results indicate that there are at least two types of C-S-H. One type has a high density (HD) while the other is less dense (LD). Nitrogen sorption measures only LD surfaces. Processing and chemical admixtures influence the ratio of the two types of C-S-H (LD/HD). Drying shrinkage results indicate that the greater the proportion of LD (as seen by higher nitrogen surface area), the greater the irreversible shrinkage.