Development Of High-Slag Cement And Study On The Activation Mechanism Of Its Early-strength

Granulated blast furnace slag has a potential hydraulicity. But, the early strength of the slag cement decreases greatly if slag content exceeds 30% of the cement. It has far-reaching meaning on a strategy of sustainable development for the cement industry to activate the potential hydraulicity and to produce high-slag cement (HSC).In this study, a compound admixture was developed, with which the early strength of high-slag cement was enhanced, and the cement with slag content being as high as 70% of the cement and the performance being better than that of PS 42.5 grade Portland-blastfurnace slag cement was made. Then, the hardening process and hydrating mechanism of the HSC were studied by means of XRD,SEM,DTA,velocity testing of the hydrating heat releasing and the liquid phase analyzing. The results show that the concentrations of ions such as Na+, OH- and SO42- in liquid phase during the initial time of the HSC are higher than that of Portland cement, which accelerates the decomposition and hydration of the slag. Meanwhile, since calcium sulphoaluminate clinker is added into the HSC, the early strength of this cement is higher than that of other slag cement without this compound admixture being added.The effect of admixture component on early hydration process of the HSC was studied also. Results show that the positive ions such as Na+ ions in liquid phase only have the role of balancing the surface electrovalent of the slag grain during the alkali-activity process. However, the role of OH- ions is not only the decomposition of the slag, but also increasing the product of the ionic concentration in liquid phase, shortening the tempting time of C3S hydration, which accelerats the decomposition of slag. On the other hand, the solubility and soluble velocity of different gypsum are different, this leads the ionic concentrations of SO42- in liquid phase of the HSC at early stage to be different, so the shape, configuration and formation position of ettringite are different, this is why the early strength of HSC is discrepant when the different gypsum is added. Because the growth process of ettringite could be adjusted reasonably by adding the sodium silicate into the HSC properly when NaOH-Ca(OH)2-CaSO4 activation is used, the early strength of the paste is enhanced. Whereas, if excessive sodium silicate is added into the HSC, the ettringite growth will be baffled and the dimension of ettringite will be lessened, so the early strength of the paste will be decreased. It is calcium sulphoaluminate clinker that enhance the early strength of the HSC greatly, but the cement will be quick-setting if excessive calcium sulphoaluminate clinker is added. By adding 0.5% of Na2SiF6 into the HSC, the setting time of the HSC can be normal.