Rheological and Physical Performances of Mortars Manufactured with Plain and Ultrafine Fly Ashes

摘要

The paper focuses on the rheological, elasto-mechanical and physical properties of mortars manufactured replacing Portland-based cements with low calcium (class F according to ASTM C618 or type V according EN 197-1 and EN 450-1) fly ash (FA) or ultrafine fly ash (UFFA) from coal-fired electric generating plants. Five different cements (CEM I, CEM Ⅱ/A-LL, CEM Ⅲ/A, CEM Ⅲ/B and CEM IV according to EN 197-1) were used. The replacement of cement with FA or UFFA was achieved up to 50% vs cement mass. Experimental results show that mechanical strength of UFFA-based mortars is significantly higher than that of mixtures manufactured with unmilled FA, both at early and long ages. This behavior could be ascribed to the larger specific surface area of UFFA, compared to that of FA, and hence both to the improvement of quality of transition zone and the higher pozzolanic activity. On the other hand, the grinding process of fly ash produces significant changes in the shape and size of particles, strongly reducing the superplasticizing effect of UFFA respect to that of FA at equal water content. Furthermore, replacement cements with fly ash or ultrafine fly ash does not appreciably reduce the drying shrinkage of mixtures.