Hydration of mechanically activated mixtures of Portland cement and fly ash

Fly ash (FA) can be used in cement mixtures with certain limitations. For experimental purposes, mixtures of Portland cement (PC) and FA were used. The content of FA in the mixtures was either 30% or 50%. The mechanical activation was performed in a vibrating mill with two rings, type AM 95413, of different diameters. Due to the vibration motion, the working elements (which are significantly heavier than the sample) instigate simultaneously the grinding process and mechanical activation of the samples. The volume of the mill container was 2 dm(3), the mass of sample 200 g and electromotor power 0.3 kW. The temperature of the mill container was 80 degrees C, influenced by the vibrating process of the rings. The tested samples were characterized using XRD analysis (confirming changes at the structural level due to mechanical activation), differential thermal analysis (a shifting of the peak position for the decarbonisation process to lower temperatures after mechanical activation was observed) and thermogravimetry (confirming an increase of the total mass loss of the activated samples, which further indicates increased of reactivity of the mechanically activated mixtures). The experimental results of the compressive strength of the activated and non-activated mixtures and the changes of their specific surfaces area proved that the mixtures PC+FA had been mechanically activated during the grinding process. The highest increase of compressive strength was achieved in the early period of setting, which indicates an improvement in the early hydration of the mixture.