镁渣对超硫酸盐水泥水化与抗碳化性能的影响OA

Introduction The low alkalinity makes super-sulfate cement(SSC)vulnerable to being carbonated by CO2.In this paper,we investigated two magnesium slags,i.e.,MSⅠ(the proportion of β-C2S is higher)and MSⅡ(the proportion of γ-C2S is higher),and analyzed the impact of CO2 concentration(i.e.,3%and 20%,in volume)on the SSC from the perspectives of hydration and carbonation.The results show that the addition of MSⅠ(with the higher proportion of β-C2S)to SSC has a negative impact on the hydration and carbonation resistance due to β-C2S with a stronger hydration activity,suppressing the hydration of SSC itself and lowering the hydration degree of SSC,reducing the amount of hydrates and further deteriorating the strength and structure development of SSC.The addition of MSⅡ(with the higher proportion of γ-C2S)has the hydration reaction,and maintains the relative content of hydrates after carbonation and compressive strength increase.This is due to the fact that γ-C2S significantly enhances the carbonization resistance of SSC. Methods The binder consisted of blast furnace slag(BFS),desulfurization gypsum(DGP),red mud(RM),magnesium slag(MSⅠ(with the higher proportion of β-C2S)and MSⅡ(with the higher proportion of γ-C2S),and calcium hydroxide.The BFS substitution levels(i.e.,10%and 20%)by MSⅠ and MSⅡ magnesium slag were investigated.The mineral compositions of used magnesium slags were analyzed by X-ray diffraction(XRD).The prepared pastes were cast in the cubic molds with dimensions of 40 mm×40 mm×40 mm.After de-molding,the hardened samples were sealed and cured at 25℃.After being cured for 28 d,the carbonation test was conducted at different CO2 concentrations(i.e.,3%and 20%)and a relative humidity of 70%.At the carbonation age of 7,14 d,and 28 d,the compressive strength,phase composition,and carbonation depth of SCC with and without carbonation were determined by strength test,X-ray diffraction(XRD),thermogravimetric analysis(TGA)and phenolphthalein color test.The microscopic morphologies of SSC before and after carbonation were also analyzed by scanning electron microscopy(SEM).Besides,the early-hydration performance of fresh SSC pastes with MSⅠ and MSⅡ was evaluated via setting time and isothermal calorimetry tests. Results and discussion In the hydration stage,the MSⅠ significantly inhibits the hydration rate of SSC.The MSⅠadversely affects the setting time and delays the reaction of SSC.This is because β-C2S can inhibit the hydration of BFS and reduce the hydration degree of SSC,leading to reduced compressive strength,amounts of hydrates(i.e.,ettringite and Calcium aluminum silicate hydrates),and increased carbonation depth after carbonation.The porous and loose microstructures via the SEM images also validate an adverse effect due to the MSⅠ(with the higher proportion of β-C2S).Also,the addition of MSⅡ does not disrupt the hydration process of SSC and ensure the integrity of hydration phase matrix structure.Meanwhile,a high carbonization activity allows it to react with CO2 to generate more CaCO3 and a dense matrix structure,which enhances the carbonation resistance and compressive strength of SSC. Conclusions Different types of magnesium slag had different effects on the setting time,reaction rate,and amounts of SSC hydrates.Specifically,the MSⅠ with a higher proportion of β-C2S and a small amount of Ca(OH)2 could extend the setting time of SSC,decelerate the hydration rate of the matrix,and markedly reduce the quantity of hydrates.In contrast,the MSⅡ with a high γ-C2S proportion had a negligible impact on the hydration process of SSC.Furthermore,the XRD patterns and TG results indicated that compared with MSⅡ,the incorporation of MSΙ hindered the dissolution of gypsum and reduced the generation amount of hydration products,thus decreasing the compressive strength. Under carbonation environments of different concentrations,the carbonation of calcium silicate in both types of magnesium slags enhanced the compressive strength of magnesium-doped slag-based SSC after carbonation to different extents.Compared with the control group,after 28-d carbonation,SSC doped with MSⅠ exhibited a carbonation depth,which was nearly twice greater than that of the control group(16.3 mm),while the content of hydration products post-carbonation was lower than that of the control group.Conversely,MSⅡ,which was abundant in γ-C2S,demonstrated a superior protective effect on SSC during the carbonation process.After 28-d carbonation,the carbonation depth was only 6 mm,approximately 20%less than that of the control group.In addition,the content of hydration products post-carbonation remained consistent with the control group,and the strength after carbonation increased significantly. For the compressive strength of carbonated SSC,the incorporation of slag containing various types of calcium and magnesium silicates could enhance the anti-carbonation performance of magnesium-doped slag SSC to different extents.However,compared to the MSⅡ containing γ-C2S with a lower hydration activity and the MSⅠ with β-C2S with a higher hydration activity,the reaction rate of SSC could be reduced.Before the carbonation,only a loose structure with a limited amount of C-A-S-H gel appeared.Conversely,the MSⅡ exhibited a higher proportion of γ-C2S,which could have a low hydration activity and a high carbonation reactivity.This characteristic minimally affected the reaction rate of SSC,thereby ensuring that SSC doped with the MSⅡ demonstrated an excellent anti-carbonation performance in a carbonation environment.After the carbonation,there was a significant increase in matrix strength,and the microstructure became notably denser.