SOME EARLY HYDRATION PROCESSES IN CEMENT PASTE AS MONITORED BY LIQUID PHASE COMPOSITION MEASUREMENTS
Abstract
A number of cement pastes were prepared in order to study the hydration processes taking place within the first 24 hours. In some cases controlled dosages of reagent gypsum, potassium sulfate, or both were added to ground clinker for use in formulating the cement pastes; others were prepared from commercial portland cements. The water:cement ratio used was ordinarily 0.50. Most of the experimental work involved expressing and analyzing portions of the liquid phase present in the pastes at closely-spaced time intervals within the first day. In addition, the hydration was monitored by x-ray diffractometry, temperature evolution measurements, and non-evaporable water determinations. Correlations were sought between concentration changes in the liquid phase and the processes underlying setting, temperature evolution, and changes in the solid phase compositions. In these measurements it was confirmed that setting is not associated with any sudden changes in the solution chemistry of the liquid phase. The only intensified chemical process detected at or shortly after setting is enhanced precipitation of Ca(OH)(,2). However, small but well-crystallized microcrystals of Ca(OH)(,2) were found to be present in expressed pore solutions at very early ages, far in advance of setting. The major finding in this work concerns the effect of solid gypsum on the alkalinity of the paste pore solution. It has been established that early concentrations of dissolved sulfates are higher than expected from the known solubility product of gypsum, and reflect the content of alkali sulfate provided by the cement. These levels of concentration are maintained as long as solid gypsum remains present; when the gypsum is exhausted by continued ettringite production the sulfate concentration rapidly declines. When this occurs the hydroxyl ion concentration increases in parallel with the decline of the sulfate concentration, in effect converting dissolved alkali sulfate into alkali hydroxide. The time at which this transformation begins can be extended by additional gypsum.
Degree
Ph.D.
Subject Area
Materials science
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