Synthesis of Pure C_3S with Mineralizers CaSO_4 and CaF_2

【Author】

Yongmin Zhou;Tom Robl;Kevin Henke;

【Abstract】

<正>The phase C_3S is very important for both OPC and CSA/I clinkers.To study the characteristics and performances of C_3S,it is necessary to synthesize the pure C_3S in lab.Mineralizers CaF_2 and CaSO_4 were used to lower the sintering temperature for the synthesis of pure C_3S.High purity C_3S was synthesized at different temperature 1350℃,1400℃and 1450℃by using pure chemicals. The assumed purity is>95%for sample PC_3S-1350;for PC_3S-1400 and PC3S-1450,purity is >97%.The free limes in all 3 samples were tested and proved less than 1%;the chemical compositions shows that only trace SO_3 in the samples PC_3S-1400 and PC_3S-1450,which indicates that almost all CaSO_4 were decomposed into CaO to form C_3S with part of SiO_2.All samples were characterized by XRD and SEM.Powder XRD results indicate the presence of monoclinic Ml and/or M3 C_3S polymorph(s) in all three samples;SEM shows the microstructures are different for different samples sintered at different temperature.Formation process of pure C_3S with mineralizers was investigated by DTA mainly,it is important different from the solid reaction between C_2S and CaO as usually. Originality The mineralizers,especially CaF_2 and/or CaSO_4 are usually used for lowering the temperature for sintering cement products including pure mineral phases of cement clinker.For the pure phases of cement clinker,the mineralizers will decrease the purity of the main phase due to the unnecessary minor phases formed.To decrease the minor phases as possible,the additive of the mineralizers should be as low as possible.The high purity C_3S has been prepared with mineralizers CaSO_4 and CaF_2,the sintering temperature has been lower to 1350~1450℃,the corresponding addition of CaSO_4.0.5H_2O and CaF_2 are 3.539%and 0.635%.The results of XRF shows that the residual quantity of SO_3 in the C_3S is <0.01%when the temperature is 1400℃and 1450℃,it reveals that only trace minor phases caused by CaSO_4,and almost all mineral phases consist of SO_3 were turned into gas phases and released the only possible impurities caused by mineralizers should be CaF_2.The DTA analysis reveals the formation process of C_3S with mineralizers CaSO_4.0.5H_2O and CaF_2 is two stage process,the first stage is formation of C_3S by CaO from Ca(OH)_2 and SiO_2,the second stage is formation of C_3S by CaO decomposed from CaSO_4 and C_2S the role of the mineralizers CaSO_4 and CaF_2 is formation of liquid phase to lower the sintering temperature for C_3S at the first stage;at the second stage,the liquid phase will decomposed into C_2S,CaSO_4 and CaF_2,the CaSO_4 the decompose into CaO,then C_3S can be formed by C_2S and CaO.The result of two stage formation process of C_3S is first originality. Based on the two stage formation process of C_3S,the formulations for pure C_3S with mineralizers CaSO_4 and CaF_2 can be calculated accurately,it is second originality.

【Keywords】

C_3S;;Preparation;;Formation process;;Mineralizers

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