Hydration of the cement paste with Na2CO3 addition

This research provides a fundamental understanding of the early stage hydra tion 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 i ntensive 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 miner als to the hydrate phase is reduced and higher contents of calcite and vate rite relative to that of Ca(OH)(2) in comparison with those found in the Po rtland cement paste ara observed. As a consequence of this, differences in strength, dynamic modulus of elasticity and porosity between hardened Portl and cement paste and those modified by Na2CO3 are found. The decrease in tw o-year compressive strength and elasticity modulus of cement - Na2CO3 paste s relative to Portland cement pastes is caused by the loss in binding capab ility dire to preferential CaCO3 formation at the early stage of cement hyd ration and consequent growth of CaCO3 crystals due to gradual, carbonation, particularly in ail with 60 % of relative humidity. In this respect the hi gher 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.