Ceramics-Silikáty 45, (1) 16 - 23 (2001)

This research provides a fundamental understanding of the early stage hydration of Portland cement paste modified by 2 and 4 wt.% of sodium carbonate. An excess of CO₃²⁻ ions retards the Ca(OH)₂ development and enables an intensive rise and growth of CaCO₃ crystals in hydrated cement. This process lasts very intensively from 6 to 24 hours. A high conversion of unreacted clinker minerals to hydration products in the cement - Na₂CO₃ pastes takes place rapidly between 1 and 24 hours. Later the conversion of clinker minerals to the hydrate phase is reduced and higher contents of calcite and vaterite relative to that of Ca(OH)₂ in comparison with those found in the Portland cement paste are observed. As a consequence of this, differences in strength, dynamic modulus of elasticity and porosity between hardened Portland cement paste and those modified by Na₂CO₃ are found. The decrease in two-year compressive strength and elasticity modulus of cement - Na₂CO₃ pastes relative to Portland cement pastes is caused by the loss in binding capability due to preferential CaCO₃ formation at the early stage of cement hydration and consequent growth of CaCO₃ crystals due to gradual carbonation, particularly in air with 60 % of relative humidity. In this respect the higher volume of non-affected products of the hydration process by the action of CO₂, the better compressive strength and elasticity modulus the cement paste becomes.