Effect Of Different Silica Fumes On Hydration Of Calcium Aluminate Cement And Properties Of Castables

Previous researches have shown that the addition of silica fume has remarkable influence on the flowability and workability of calcium aluminate cement?CAC?-bonded castables.The insallation properties of CAC-bonded castables are closely dependent on the hydration behavior of CAC?hydration rate,hydration products,etc.?.However,there are few researches regarding the effect of different silica fume on the properties?flowability and green strength?of CAC-bonded castables in combination with investigating the impact of silica fume on hydration rate of CAC.Moreover,since silica fumes are generally of industrial by-products,the properties of silica fume from different producers or even the same manufacture vary greatly.Therefore,the effects of different silica fumes on the hydration rate of CAC would be different from each other,which would consequently influence the flowability and working time of CAC-bonded castables.In this work,three kinds of silica fumes A-FS,B-FS and C-FS?with pH values of 2.77,6.91 and 7.66,respectively?and CAC?Secar 71?were used as raw materials.The effect of silica fume with different pH value,particle size and particle size distribution on hydration rate of CAC at 10°C,20°C,30°C and 40°C were investigated using hydration exothermic method.The effects of different silica fume on the phase and microstructure of hydration products was investigated by stopping hydration process using freeze-vacuum methed,which will further examine the effect of different silica fumes on the hydration rate of CAC.The effect of the above three silica fumes on the hydration rate of CAC at different temperatures was investigated in this work.The hydration heat curve showed that there was abnormal setting behavior for pure CAC.The hydration rate of the pure CAC sample decreased with the increase of curing temperature in the range of 10°C and 30°C,and the hydration rate of the pure CAC sample at 40°C was higher than those at the other three temperatures.However,the hydration rate of CAC with three kinds of fume silicas accelerated with the increase of curing temperature?10-40°C?.The above results indicated that the nomalous setting behavior of CAC was changed by the present of silica fume,due to the promotion effect of silica fume on the hydration rate of CAC at 20°C and 30°C.The XRD and SEM results showed that silica fume could speed up the formation of hydration products and promoted the hydration rate of the CAC,especially,when the curing temperature was 40°C,the plate-like hydration products of C₂AH₈ and C₂AH₆ and the amorphous hydration product of AH₃ appeared after curing for merely2 h in the samples with silica fume,in contrast,only a tiny amount of C₂AH₆appeared in the pure CAC sample after curing for even 4 h.The hydration rate of CAC with silica fume is increased because silica fume with small particle sizes?1-500 nm?and large surface areas provides more effective nucleation area for hydration products of CAC.It was also demonstrated that the effect of silica fume on the hydration of CAC increased with the increase of the pH value of silica fume.This was because the silica fume with a relatively high pH value dissociated out more OH^-,which promoted the“hydroxylation”in the hydration process of cement,thereby promoting the formation of hydration products and accelerating the hydration rate of CAC.This work studied the effect of silica fume on the flowability,flow decay behaviour and green strength of CAC-bonded castables.The castables with acidic silica fume had the best flowability?205 mm?and the lowest green strength?5.1MPa?;while the castables with basic silica fume had the lowest flowability?175 mm?and the highest green strength?5.9 MPa?.The above results indicated that the lower pH value benefited the flowability and the slower flow decay of the castables,but leads to lower green strength.When the pH of the silica fumes was higher than 7,the H⁺dissociated by the silane alcohol group was quickly neutralized by OH^-in the system,which accelerated the hydration rate of cement and the system will be flocculated rapidly.Therefore,the castable with basic silica fume had the smallest flowability and fast flow decay.The large amount of hydration products produced at the same time will result in a higher green strength of the castable.When the pH value of the fume silica was lower than 7,a large amount of H⁺in the system will delay the“hydroxylation”in the hydration process,thereby delaying the hydration rate.Therefore,the flowability of the castable with acidic silica fume was the best and the flow decay was the slowest.The green strength of the castable could be explained by the reduction of hydration products due to the delay of Ca²⁺release in the cement.