偏高岭土对大掺量石灰石粉水泥抗氯离子渗透性能的影响

Effect of metakaolin on the resistance to chloride ion permeability of cement with high-volume of limestone powder

石灰石粉替代部分水泥熟料能有效降低水泥的碳排放,但大掺量石灰石粉会降低水泥基材料抗氯离子渗透性能。偏高岭土的掺入可显著改善石灰石粉水泥体系的抗氯离子渗透性能,然而改善机理暂不明确。为了揭示偏高岭土对大掺量石灰石粉水泥抗氯离子渗透性能的改善作用,应用热力学模拟方法预测多组分胶凝体系水化物相组成(体积比),并基于模拟结果设计大掺量石灰石粉的复合胶凝材料组成。采用吸水率、电通量法表征混凝土试件的抗氯离子渗透性能,结合基体孔隙结构、物相组成分析抗氯离子渗透性的改善原因。研究结果表明:热力学模拟能够较好地预测偏高岭土-石灰石粉-水泥复合胶凝体系的水化物相组成。加入偏高岭土后,尽管由于稀释效应使得基体总孔隙率有所增大,但火山灰反应增加了水化硅酸钙凝胶数量,细化了孔径分布;且增加的水化硅酸钙凝胶增强了对碱离子的吸附作用,显著降低孔溶液中离子浓度,从而极大地降低混凝土的电通量,使得水泥石的抗氯离子渗透性能得到显著提升。在石灰石粉掺量为25%的条件下,掺入10%偏高岭土后试件的24 h吸水率和6 h电通量分别下降了约67.2%和90.3%。掺加偏高岭土后,通过细化孔径分布、增强C-S-H凝胶的吸附作用等多个方面改善了石灰石粉水泥基材料抗氯离子渗透性能。

The substitution of cement clinker by limestone powder can effectively reduce carbon dioxide emission;however,the incorporation of high-volume limestone powder might lead to a decrease in the chloride penetration resistance of cementitious materials.The combined use of metakaolin together and high-volume limestone powder can significantly improve chloride penetration resistance.Nevertheless,the mechanism underlying this improvement is still unclear.Toelucidate the role of metakaolin in enhancing the chloride penetration resistance in cementitious materials blended with high-volume limestone powder,the phase assemblage(volume fraction)of the metakaolin-limestone-cement system was predicted by thermodynamic modelling.Based on these modelling results,the mix proportion of this ternary blend system was designed.The water absorption and electric flux tests were conducted to gauge the chloride penetration resistance of concrete specimens,and the reasons for the improvement were analyzed by considering both pore structure test and phase assemblage examination.Results indicate that the thermodynamic modelling can accurately predict the phase assemblage of the metakaolin-limestone-cement system.Although the addition of metakaolin increased the porosity of the matrix due to the dilution effect,the increased amounts of C-S-H gel derived from pozzolanic reaction resulted in a refined pore size distribution.Moreover,the increased amounts of C-S-H gel also enhance the adsorption of alkali ions,considerably reducing the ion concentration in the pore solution.Consequently,the electrical flux was reduced,and the chloride penetration resistance of concrete was improved.In the ternary system with limestone powder replacement level of 25%(by mass),with the presence of 10%metakaolin,the 24 h water absorption and 6 h electric flux of the specimen were decreased by 67.2%and 90.3%respectively.By adding metakaolin,the chloride ion penetration resistance of limestone powder cement-based material was enhanced by refining the pore size distribution and increasing the adsorption of ions in pore solution by C-S-H gel.