An Integrated Approach for Understanding Hydration Behaviour in Portland cement Systems Containing Silica Fume

One of the key behaviours that predict the performance of portand cement system containing silica fume is hydration. Hydration affects the microstructure of concrete, which in turn alters its engineering properties such as strength, elastic moduli, toughness and other durability related properties like porosity and permeability. Therefore in order to predict such properties, fundamental study of the hydration process of portland cement system containing silica fume is very important. The main aim of the present investigation is to gain insight on the effects of silica fume addition on the hydration process and microstructural development of cement paste. This was done by adopting an integrated approach involving combination of theoretical models and experimental investigation, and developing mathematical equations to predict hydration behavior of the system. Quantitative studies on cement hydration is conducted using thermo-gravimetric analysis and other techniques, and qualitative studies is done using X-ray diffraction techniques. The non-evaporable water content and the calcium hydroxide content of paste obtained from experiments coupled with the use of suitable theoretical models is used to determine the degree of hydration, degree of pozzolanic reaction and calculated porosity for paste containing silica fume at different dosages of silica fume. (0%, 4%, 8% and 12%). Attempts were made to further understand the trends obtained using curve-fitting techniques and other theoretical models. Results from the study indicated that the addition of silica fume at all replacement levels depletes Ca (OH) 2 produced from cement hydration and forms C-S-H gel and this reaction starts at early curing periods. In addition, the non-evaporable water content and the fraction of silica fume that completely reacted decreases, with increase in its replacement level and helped in determining the degree of cement hydration and pozzolanic reaction of silica fume particles. A significant portion of unreacted silica fume remains in the paste and is involved in the pore filling process. The porosity values for paste containing silica fume decreased with increase in its replacement levels due to both its pozzolanic reactivity and pore-filling ability. The compressive strength data showed that mortars containing 4% and 8% replacement level of silica fume have higher strength than the control mortar. However, SF-12% mortars registered lower strength than the control. The trends obtained are reasoned to be due to variations in the amounts of primary and secondary C-S-H gel. Further, mathematical equations are developed through curve-fitting of experimental data, and analysis of these equations revealed that the effect of variation observable with control and SF-4% paste, and SF-8% and SF-12% are merely due to the dosage of silica fume while the effect of variation observable with SF-4% and SF-8% is due to the dosage of silica fume and other mechanisms associated with its dosage.