A porosimetric study of calcium sulfoaluminate cement pastes cured at early ages

Calcium sulfoaluminate and Portland cement pastes, both prepared with a water/solid mass ratio equal to 0.5 and cured for time periods comprised between 2 h and 28 days, show completely different pore size distributions by mercury intrusion. Portland cement pastes aged at 12 h and 1 day exhibit a unimodal distribution of pore sizes related to a continuous network of capillary pores with a threshold pore radius decreasing from nearly 650 to 350 nm. After 7 and 28 days of curing, this parameter shifts to about 150 nm and a region having smaller pores appears (with a second threshold pore radius roughly comprised between 10 and 30 nm), made discontinuous by blockages of hydration products which occlude the interconnected pore system and isolate the interior space. For calcium sulfoaluminate cement pastes, a bimodal distribution is rapidly established, in which the regions with a lower porosity (threshold pore radii up to about 25 nm) are dominant, while the decrease of total porosity almost ceases at later ages. The porosimetric behaviour of calcium sulfoaluminate-based cement is related to its very fast hydration rate and to the lack of water needed to continue the hydration reactions.