Regulation On Microstructure Of Silicate Phases In Belite-barium Calcium Sulphoaluminate Cement Clinker And Modification Of The Cement Property

Ordinary Portland cement has many drawbacks, for example, high energy consumption,requirement for a large amount of high quality limestone resources, vast amount of CO2emissions, poor stability and durability of hardened cement paste and so on.Belite-C2.75B1.25A3$cement is one of the cement systems which are proved to beenvironmentally friendly and excellently performed in the long term. But this kind of cementhas the fault at insufficient early mechanical property.This paper is aimed at improving theearly age compressive strength of the cement with incorporation of minor components andadjustment of calcination system by modifying microstructure of silicate phases, belite andalite, through stabilizing their high temperature and highly active crystal forms or produce theoriginal lattice distortion. The following results can be reached as:(1) For C2S single mineral system doped with trace components, polymorphs of C2S weremainly β-C2S and γ-C2S. Ability of ions to stabilize β-C2S can be listed as SO3, SrO, Na2O,K2O, BaO, undoped and MgO. In addition, BaO was able to stablize αL’-C2S to a certaincontent while Na2O and K2O can make α-C2S and α’-C2S exist in very slight content.Thecomposite doping can decrease pulverization rate significantly and stablize high temperaturepolymorphs of C2S. For C3S single mineral system doped with trace components, polymorphsof C3S were mainly T-C3S and M-C3S. MgO can stabilize M3-C3S obviously and even a smallamount of R-C3S can exist with appropriate content of MgO. At the mixing amount of1.5%-3%BaO, M-C3S can be obtained. With K2O, C3S existed primarily as T-C3S. When thedosage of Na2O was in the range of0.5%-1.5%, C3S was M polymorph. C3S with1%SrO cantransformed into M-C3S. In the desiged cement system containing both interstitial phases andC2.75B1.25A3$mineral, belite presented as α-C2S. When C3A/C4AF=2:5, C3S exsisted as thethe good mechanical M1form.(2) The appropriate dosage of SrO was in the range of3-5%for the innovative cementclinker. Compressive strength of cement paste at1d,3d,7d and28d compressive strength canreach8.4MPa,35.8MPa,71.3MPa and103.7MPa.1%MgO was beneficial to improvementin early strength of the cement. Compressive strength for1d,3d,7d and28d were separately14.7MPa,23.9MPa,34.4MPa and75.5MPa. Considering the excellent ability of the innovative cement in absorption more MgO, more MgO was added into the clinker. It wasconcluded that, with7%MgO, the cement had the best compressive strength and the valueswere33.6MPa,59.7MPa,89.1MPa and95.1MPa for1d,3d,7d and28d compressive strength.At the mixing amount of2%for K2O, early compressive strength of the cement was best,respectively14.9MPa,48.6MPa,75.6MPa and106.5MPa at1d,3d,7d and28d. The bestdoping content of Na2O was0.5%in the clinker and1d,3d,7d and28d compressive strengthwere respectively13.9MPa,40.7MPa,76.6MPa and108.9MPa.(3) Doped with trace components, belite mineral in belite-C2.75B1.25A3$cement mainlyexisted in α ’-C2S; alite doped with SrO was primarily M3with trace content of M1and M2,and with MgO were coexistence of M1and M3without M2. By hydration heat release curve,it can be seen that the following hydration rate order, listed as5%SrO,7%MgO,1%MgO andundoped sample and0.5%Na2O,2%K2O and the undoped sample for alkalies.The maximumrate of hydration with SrO sample was2.5times as great as sample with1%MgO andundoped sample. For alkalies, the sequence was0.5%Na2O,2%K2O and the undoped sample.(4) The effect of calcination process on the microstructure of cement clinker and themechanical property of the cement has also been studied. Results showed that compressivestrength of cement paste doped with2%K2O at the curing age of7d and14d under the rapidcalcination condition (The sample was heated at the rate of30oC/min from ambienttemperature to510oC,20oC/min from510oC to810oC and10oC/min from810oC to1380oC)were both10MPa higher than the corresponding sample with the ordinary calcinationprocess (The sample was heated at the rate of5oC/min from ambient temperature to1380oC),which was attributed to the increasing amount of alite; the polymorph of alite was M1whilebelite existed mainly as α-C2S and slight β-C2S. Comparing with the rapid calcinationcondition, samples under liquid nitrogen cooled system did not have an obvious effect on theformation of silicate phases and the stability of high hydraulic activity crystal forms of silicatephases.