Proper curing of concrete is widely recognized as a necessity for assuring adequate field performance of concrete structures and has long been addressed in the American Concrete Institute (ACI) Manual of Concrete Practice. 1 Not only is it recognized as important to minimize evaporation of the concrete mixture water, but it is equally emphasized to provide a source of external (or internal) curing water to replace that consumed by chemical shrinkage during the hydration of the cement. A cement paste, mortar, or concrete cured under sealed conditions will self-desiccate, resulting in the creation of coarse capillary pores within the microstructure. For water-cement ratios (w/c) greater than about 0.42, there is sufficient water in the mixture such that complete hydration of the cement can be achieved theoretically without supplying additional water to the cement paste.2 However, even if complete hydration were achievable, the lack of additional curing water may still result in the creation of relatively large pores within the final microstructure. In this case, the addition of curing water would assure that all pores remain water-filled and eligible as locations for the precipitation and growth of hydration products during curing.
Proper curing of the newer high-performance concretes is both more critical and more difficult.2 Thus, high-performance concrete has been plagued by ubiquitous early-age cracking problems, and novel solutions, such as the use of internal curing,3−5 have been developed to avoid or at least reduce such cracking. Early-age cracking is a complex phenomenon, depending on thermally induced strains, autogenous stresses and strains due to self-desiccation, and development of mechanical properties, among others. 6 The microstructure of hydrating cement paste within concrete clearly has a large influence on this phenomenon. Thus, a more fundamental understanding of how this microstructure develops during the course of early hydration, as a function of both w/c and curing conditions, would assist in optimizing current curing strategies and also in formulating new and innovative ones.
This paper will apply three experimental techniques to characterize the hydration and microstructure of w/c=0.35 and w/c=0.435 cement pastes cured under saturated and sealed conditions at 20 ºC as a function of age. These techniques are loss on ignition (LOI) to estimate the degree of hydration, scanning electron microscopy (SEM) to evaluate the microstructure and the degree of hydration, 7-9 and low temperature calorimetry (LTC) to investigate the developing pore structure in the hydrating cement pastes. 10-14 Specifically, low temperature calorimetry will be used to indicate the connectivity (percolation) of various size pore networks in the hydrating specimens.10 Igarishi et al. have recently applied SEM analysis to investigate the effect of curing conditions on the development of coarse capillary pores in cement pastes cured under saturated and sealed conditions at temperatures of 20 ºC and 40 ºC for w/c=0.25, 0.40, and 0.60.15 They noted that it was extremely difficult to avoid the development of coarse capillary pores in the lowest w/c paste via conventional saturated curing, and also pointed out the critical role of the initial packing of cement particles in influencing the developed microstructure (pore structure).
Next: Research Significance Up: Main Previous: Main