Conclusions

Experimental and computer simulation studies have been applied to examining the drying/hydration behavior of thin cement paste and mortar specimens at sub-millimeter resolution. In general, the studies indicate that:

• 1) thin cement paste samples of "uniform" microstructure dry out relatively uniformly throughout their thicknesses as opposed to exhibiting the sharp drying front encountered in other porous materials,
• 2) in composite layered specimens, movement of water from "coarser" pore layers to the "finer" ones is observed regardless of whether the coarser pores are due to a locally higher w/c or to the use of a coarser cement,
• 3) no clear differential movement of water could be observed between cement paste and mortar layers of the same nominal w/c,
• 4) the NIST CEMHYD3D cement hydration model has been extended to include drying (and carbonation) as well as hydration and in general the agreement between experimental and computer model results is quite good, and
• 5) the computer simulations particularly indicate that both moisture content and degree of hydration are affected by drying and large differences in achieved degree of hydration were observed between the layers in the two-layer composites examined in this study.

These results have numerous implications for design and performance of cement-based materials, particularly the curing of high-performance concrete and the application of repair materials to existing structures.