CEMENT PASTES OF LOW WATER TO SOLID RATIO - AN INVESTIGATION OF THE POROSITY CHARACTERISTICS UNDER THE INFLUENCE OF A SUPERPLASTICIZER AND SILICA FUME

Porosity was investigated in cement pastes with a water to solid ratio of 0.28 and cured at normal temperature. Samples with and without superplasticizer admixture were prepared using an ordinary portland cement in which 0-5-10 and 15 per cent of weight was replaced by condensed silica fume (CSF). Solvent exchange treatment and vacuum drying of samples were carried out before the use of the mercury intrusion technique. Capillary water suction (CWS) and the pore sizes distribution (PSD) were determined after various hydration times ranging from 1 day to 3 years. PSD of all CSF-containing composites showed a majority of pores with radii below 15 nm even after 1 day of hydration. In some pastes after 250 days of water curing a re-increase of median pore size was observed. Changes of the CWS and PSD parameters indicated the superplasticizer action on the formation of pore structure of cement stones. This action was also confirmed by measurements of the temperature of hydrating pastes.