The goal of internal curing is to maintain saturated conditions within a hydrating cement paste in order to avoid self-desiccation and the accompanying autogenous stresses and strains that may lead to early age cracking. The effectiveness of internal curing, using saturated lightweight aggregates (LWA) or superabsorbent polymers (SAP), for example, has been evaluated based on the measurement of a wide variety of performance properties, including internal relative humidity, autogenous shrinkage, restrained shrinkage and creep, degree of hydration, and compressive strength development [1-4]. All of these measurements have indicated that the internal reservoirs (SAP or LWA) can be an effective means for supplying the extra curing water needed to compensate for the chemical shrinkage occurring in the cement paste during hydration. Still, it is desirable to make an even more direct observation of the water movement from the internal reservoirs to the surrounding cement paste during hydration.
X-ray absorption is one non-destructive technique that can be used to probe the dynamic microstructure of porous materials such as cement pastes and mortars [5-8]. In the past, x-ray transmission measurements have been used to investigate water movement during early age curing/drying of small cement paste and mortar specimens as a function of water-to-cement ratio (w/c), cement particle size distribution, curing conditions, and the addition of a shrinkage-reducing admixture [5-7]. Lura et al. [8] have also applied the x-ray transmission technique to investigating water movement from a single saturated LWA to cement paste during hydration, successfully comparing the estimated water movement to the measured (and modeled) chemical shrinkage of the cement paste under equivalent hydration conditions. However, due to the resolution limit of about 100 μm of the x-ray absorption equipment employed in that study and its inability to operate in a tomography mode, only the “bulk” movement of water could be estimated. X-ray microtomography [9, 10] offers the possibility to perform similar measurements on an entire three-dimensional specimen and isolate water movement from individual “pores” in the LWA to the surrounding cement paste during hydration. Such an experiment has been summarized recently [11]; here, a more detailed quantitative analysis of the water movement based on image processing and analysis of the four-dimensional image set will be presented.
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