| There are many factors which could have influence on the hydration of calcium aluminate cement in the refractory castable system. The characters of the cement and the additives in the castable system are the two important factors. However, there are few literatures which compare the hydration behavior of the same-grade commercial cements from different companies, or study the changes of hydration behavior of these cements when some dispersants are added.In this paper, four CA-70 type commercial calcium aluminate cements from home and abroad were chosen as the research object, which are CA70, Secar71, CA270 and FCA70. Heat evolution, electrical conductivity and setting time of the cement paste or mortar at different curing temperatures were tested, freeze vacuum drying method was utilized to terminate the cement hydration at the designated times and X-ray Diffraction and SEM were conducted to analyze the phases of the hydrated products. The hydration and setting behavior differences of the four cements with and without dispersant(STP, CA and FS10) were analyzed.The results illustrate that Ca O and Al2O3 are the two major chemical components of the four CACs, whose total content accounts for more than 98%. The main phases of the CACs are CA, CA2 and a-Al2O3, but their contents are different in each of the cements. Cement CA70 has less CA phase; CA2 content is lower in CA270 and FCA70; Cement CA270 and FCA70 both have a lot of a-Al2O3. The sequence of the cements by specific surface area is FCA70>CA70>Secar71>CA270. Comparatively, the hydration of CA70 and FCA70 is fast at curing temperature from 10°C to 40°C, while the curing temperature has a marked impact on the hydration of Secar71 and CA270. Three of the four cements, except CA70, show abnormal hydration phenomenon, that is, their hydration rate is faster at 10°C and 40°C, while slower at 20°C and 30°C.STP accelerates the hydration of CACs in most cases, and the more STP is added the faster will the hydration be. However, for Secar71 and CA270 hydration at 10°C, STP shows a retarding effect which is irregular. And the hydration rate changes of them related to STP content are almost the same. The addition of STP shortens the setting time of CAC mortars,which means STP is a accelerator for CAC.CA postpones the hydration of CACs. Unlike other cements, Secar71’s hydration rate changes irregularly with the increase of CA addition. When CA addition is a bit high, the hydration of all the CACs except FCA70 is heavily inhibited. Curing temperature has significant influence on CA’s retarding effect. Electrical conductivity test indicates that the dissolution of CAC in water is inhibited because of CA. The delay of CAC mortar proves that CA is a retarder.FS10 also puts the CACs’ hydration off, and its retarding effect performs more noticeable in Secar71 and CA270. The electrical conductivity curves of FCA70 suspension show that FS10 prolongs the induction period of CAC hydration. However, the setting time of CAC mortar is accelerated when FS10 exists.Vacuum freeze drying method could halt the cement hydration at specific times and remove the free water effectively, after which the phase and microstructure transformation of cement in hydration process could be analyzed conveniently. XRD patterns of specimen before and after exothermal peak testify the accelerating effect of STP, retarding effect of CA and influence of curing temperature. However, to get better records, the curing condition of cement in heat evolution test and vacuum freeze drying experiment should be as same as possible. |
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