Evaluation of sample drying methods to determine the apparent porosity and estimation of degree of hydration of portland cement pastes

Abstract

The hydration of Portland cement incorporates chemically bound water (CBW) into the structure of the formed hydrates and keeps water adsorbed on their surface, especially in calcium silicate hydrate (C-S-H). The evolution of hydration gradually reduces the porosity of the paste, keeping water remaining in the pores. Determining the volume of voids contained in the solid requires the removal of non-constitutive water from the hydrates without significant degradation of the microstructure. The free water withdrawal is performed by different methods, but some impact the adsorbed water (gel water) and also the chemically bound water. In this situation, partial degradation of hydrates interferes with the porosity of the paste and the estimation of the degree of hydration. This work determined the apparent porosity and compressive strength of Portland cement paste type CP V – ARI (high early strength cement) and CP II – Z (cement with pozzolan), molded with water/cement ratio of 0.35; 0.45 and 0.55 at the ages of 7, 28 and 56 days. The removal of free water contained in the pores of the pastes was performed by the following methods: oven drying, solvent exchange, and lyophilization (freeze-drying). The apparent porosity results were influenced by the drying methods, especially when the samples were submitted to a temperature of 105 °C. Oven drying results in apparent porosity significantly higher than those determined by solvent exchange or lyophilization. This method causes desaturation, desorption, and partial dehydration in the paste. The degradation of hydrated compounds can be in the order of 10%. The porosities obtained in the solvent exchange and lyophilization were similar, showing the potential of immersion in alcohol for drying samples. These methods promote the desaturation of the paste, keeping gel water and CBW in the hydrated compounds.