Pore structure of concrete with mineral admixtures and its effect on self-desiccation shrinkage

High-performance concrete (HPC) is known for the specificity of its pore system compared with that of conventional concrete. Indeed, its capillaries are characterized by very fine pores. Refinement of these pores mainly depends on the type, proportion, and fineness of the pozzolanic materials incorporated. The finest HPC pores, however, induce two opposite phenomena: on the one hand, better mechanical performances, which lead to considerable improvement in durability; and on the other hand, increased self-desiccation shrinkage caused by the high capillary depression in these finer pores. This study highlights both fundamental aspects resulting from the use of mineral admixtures in HPC. Concrete mixtures with two water-binder ratios (w/b) of 0.35 and 0.30, and three types of binders were used in this research. Shrinkage was measured on massive blocks that somewhat simulated a real concrete structure. The pore size distribution was measured at different ages by mercury intrusion porosimetry (MIP). The results obtained show that refinement of the capillary network and its evolution with time are closely related to mineral addition type and fineness. Moreover, refinement of the pores evolves significantly at a very early age depending on the type of binder used. This change in the pore structure of capillaries induces an increase in the magnitude of self-desiccation shrinkage during this lapse of time. Also, the lower the w/b of the mixtures, the higher the effect of refinement of the capillary network on selfdesiccation shrinkage development. It was also found that the incorporation of mineral admixtures considerably reduces the average pore radius while it increases the amount of pores smaller than 15 nm (5.9 × 10^-7 in.). In general, the smaller the average pore radius is, the higher the shrinkage will be.