HYBRID CEMENTS WIH VERY LOW OPC CONTENT

摘要

Hybrid cements based on the alkali activation of aluminosilicates blends with low Portland cement clinker contents (less than 30 %), combining the best features of two components, constitutes a promising field of research for the development of new binders. The present paper compares the behaviour of a cement blend containing 80 % fly ash + 20 % Portland cement clinker with (4 % Na2CO3 or 4 % Na2SO4) and without alkali activation. Reaction kinetics were determined by isothermal conduction calorimetry, composition and microstructure by XRD, NMR analyses. The findings showed that, when additioned with solid alkaline activators, blends with a very low (20 %) clinker content and high (80 %) percentages of fly ash can yield viable binders. Since the solid activators used in this study (Na2CO3 and Na2SO4) are not highly alkaline in themselves, their handling would not entail an occupational hazard. They nonetheless generated the chemical reactions necessary to enhance the alkalinity of the medium and favour fly ash reactivity. This, in turn, yielded a mix of cementitious gels whose thermodynamic behaviour would favour the formation of a (N,C)-A-S-H type gel. The carbonate or sulfate nature of the anion present in the activator affected the formation of secondary reaction products, leading to a predominance of calcite in one case and ettringite in the other. Two possible mechanisms are proposed when the reaction is activated by adding Na2SO4: i) the conversion of the gypsum initially formed into ettringite; ii) the evolution of phase U formed to ettringite