Porosity of a Fast-Setting Mortar with Crystallization Admixture and Effect of a SA-PA Modification

Air permeability measurements according to the Hagen–Poiseuille equation, Scanning Electron Microscopy (SEM) and mercury intrusion porosimetry (MIP) tests were conducted on samples of cementitious mortar at different curing times to study the correlation between the increased crystallization and their microstructure. The mortar samples were prepared with a commercial fast-setting premix containing calcium silicates and quartz. The average permeability coefficient (K) was 2.96 × 10−15 m2 after 3 days and decreased to 3.07 × 10−17 m2 after about one month. The continuous C-S-H nucleation in the capillary pores of the cement mortar changes their shape and improves the mortar’s impermeability. The SEM images showed the development of crystals that refine the pore size distribution of the cement paste, with more of the smallest pores, and fewer of the largest, as demonstrated by the MIP measurements. Adding a superabsorbent polyacrylate (SA-PA) in the amount of 0.5% wt of dry powder, without adding any extra water, makes a mortar less fluid but not faster-setting. Twenty-four hours after mixing and casting, it is still plastic and, with time, the pore size distribution differs from that of standard mortar. Over time in air, permeability remains high, but in water it could be low due to swelling of SA-PA residues.