C3A polymorphs related to industrial clinker alkalies content

The C(3)A crystalline structure in Portland clinkers is basically determined by the incorporation of alkali-oxides and is also function of the cooling rate, both actions have a combined effect on the changes in reactivity of clinkers. Industrial clinkers have been found to contain cubic or orthorhombic forms of aluminate, alone or in combination; the monoclinic modification has not been observed. The aim of this research is to evaluate the different polymorphs of C(3)A present on industrial clinkers produced with different amounts of sodium and potassium oxide in the raw mix. The optical microscopy was useful to distinguish the different kinds of C(3)A based on their crystal features. The SEM (scanning electron microscopy)-EDS analysis was used on the chemical characterization of the C(3)A. An accurate quantification of the phases, including proportions of C(3)A polymorphs, could be achieved using X-ray diffraction with Rietveld refinement. The quantification was confirmed by chemical selective dissolution applied in samples for which the refinement indicated almost only one type of C(3)A polymorph. Finally, correlations between alkalies contents, Na2O and CaO for white cement clinker and K2O and CaO for grey clinker suggest partial substitution of CaO and incorporation of Na2O or K2O in the vacant site of the cubic C3A leading to the formation of the orthorhombic phase. (C) 2004 Elsevier Ltd. All rights reserved.