The effect of limestone powder additions on strength and microstructure of fly ash blended cements

摘要

Limestone powder additions influence the hydration products in an OPC system. The calcium carbonate present in the limestone powder interacts with the calcium aluminate hydrates (AFm), forming calcium monocarboaluminate hydrate instead of calcium monosulphoaluminate hydrate, thereby stabilizing the ettringite (AFt) and increasing the amount of bound water. This could in turn lead to lower porosity and subsequently higher strength. The idea was that the effect of limestone powder might be more pronounced in fly ash blended cement than in OPC. This would be due to the higher amount of calcium aluminate hydrates generated in the fly ash containing system, compared to the limited amounts generated by OPC. A systematic study of several mixes containing different combination of ASTM class F fly ash and limestone powder, replacing 35% percent of the OPC has been carried out. The aim was to relate microstructural properties such as the degree of reaction, nature of hydration phases and capillary porosity to compressive strength. In order to understand the effect of limestone powder in this system, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and quantitative microstructural analysis were performed on paste samples using scanning electron microscopy (SEM) with image analysis (IA). These experimental results were then compared with the output of a thermodynamic model. The model predicts the hydration assemblage at equilibrium for a given composition of raw materials. The results of the model can be used to investigate the sensitivity of the system for certain parameters e.g. the degree of hydration of the FA, the limestone or gypsum content. The compressive strength results showed a strength decrease when OPC is replaced with limestone powder alone, whereas a strength increase occurred when 5% of fly ash was replaced with limestone powder. The increase in compressive strength appears to correspond to the changes in the AFm and AFt hydration phases and the subsequent increase in total volume of hydration products. The effect of limestone powder additions was greater in the case of the fly ash blended cement than for the OPC and the effect increases with increasing degree of reaction of the fly ash. Thus the initial hypothesis was confirmed.