Research on properties evolution of ultrafine fly ash and cement composite

Abstract

Compared with fly ash, addition of ultrafine fly ash manufactured by mechanical grinding will significantly improve the performances of the cement-based materials. In this paper, firstly the cement hydration is simulated based on the modified shrinking core model with the concept of multi components. Then it is combined with the fly ash pozzolanic reaction via the contents of capillary water and calcium hydroxide (CH) in the blended cement paste. The effects of the heterogeneous nucleation, fineness of fly ash and local w/c augmentation are quantitatively analyzed and incorporated into the hydration models. The integrated models are validated by the contents of CH and non-evaporable water (NEW) in the (ultrafine) fly ash and cement composite The coupled evolution of cement hydration, pozzolanic reaction and porosity is presented using the integrated hydration models. Compared with the fly ash and cement composite (FAC), it is found that the extent of the pozzolanic reaction significantly increases in the ultrafine fly ash and cement composite (UFAC) while the increment of cement hydration extent due to nucleation effect is relatively small. Furthermore, the contributions of gel content by different effects are quantitatively calculated and compared between UFAC and FAC. Finally, an empirical formula is established to predict the compressive strength of the hardening UFAC. The filling effect of the ultrafine fly ash, gel content and cumulative pore volume are adopted to evaluate the development of the compressive strength. The results indicate that the physical filling effect of ultrafine grains reflected on the compressive strength of UFAC will gradually increase during the first seven days of curing and then keeps constant. With the increase of cement replacement ratio (0 ~ 0.5), the physical filling effect brought by UFA will be greatly enhanced. The analyses presented in this paper will give a better understanding on the hydration process and strength development for the FAC and UFAC.