Careful experimental studies using novel sensors have demonstrated that cement particle size distribution, through its influence on the initial pore size distribution of fresh cement paste, has a large effect on the early age autogenous properties of sealed specimens at identical w/c ratios. The larger pores present in the coarser cement paste reduce the rate of relative humidity decay with increasing hydration, concurrently reducing the associated capillary stresses within the cement paste pore solution. In turn, both the autogenous shrinkage and eigenstress (and associated microcracking) are reduced. Thus, engineering the PSD of the cement may be one method for reducing or eliminating early age cracking (both macro and micro) of high performance concretes. Since most HPCs contain one or more supplementary cementitious materials, careful consideration must also be given to their PSD, as it is the overall pore size distribution that is critical in determining the development of autogenous properties within the hydrating cement paste. While coarse pores are relatively beneficial in minimizing early age cracking, they are detrimental to long term strength, so that care must be taken in producing a concrete with optimum performance both in the short term and in future years. However, avoiding early age cracking is critical for the long term durability of these high performance materials.
The NIST microstructural model has been successfully used in conjunction with the presented experiments, both to separate kinetics effects from true microstructure differences and to "quantify" the initial pore size distribution of the cement pastes. Thus, this study demonstrates the synergistic effects of using a dual experimental/computer modeling approach to elucidate the complex relationships between microstructure and properties in cement-based materials.