Degree of hydration in cement paste and C3A-sodium carbonate-water systems (original) (raw)

Journal of Thermal Analysis and Calorimetry, 2007

Abstract

ABSTRACT This research provides a fundamental understanding of the early stage hydration of Portland cement paste, tricalcium aluminate (C3A) paste at water to cement ratio of 0.5 and C3A suspension at water to cement ratio of 5.0 modified by 2 or 4 mass% of sodium carbonate. A high conversion of unreacted clinker minerals to gel-like hydration products in the cement-Na2CO3 pastes takes place rapidly between 1st to 24th h. Contrary the Ca(OH)2 formation within the same time interval is retarded in the excess of CO 32− ions due to intensive rise and growth of CaCO3 crystals in hydrated cement. 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 a decrease in strength and an increase in porosity between hardened Portland cement paste without sodium carbonate and those modified by Na2CO3 are observed. C3A hydrates very quickly with sodium carbonate between 1st and 24th h forming hydration products rich in bound water and characterized also by complex salts of (x)C3A·(y)CO2·(zH2O type, whereas C3A-H2O system offers C3AH6 as the main hydration product. Higher content of the formed calcium aluminate hydrates in C3A-Na2CO3-H2O system also contributes to early strength increase of Portland cement paste.

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