磷酸镁水泥加固低液限粉土的pH和电导率响应与孔隙特征研究

Study on pH,Conductivity Response and Pore Characteristics of Low Liquid Limit Silt Reinforced by Magnesium Phosphate Cement

为了探究磷酸镁水泥(Magnesium phosphate cement,MPC)对黄河淤积低液限粉土的加固问题,本试验研究不同MPC占干土的掺量、镁磷比(m(M/P))及硼酸与氧化镁比(m(BA/M))对粉土初始pH、电导率的响应;探究选定m(M/P)、m(BA/M)下不同龄期和不同MPC掺量固化土的无侧限抗压强度、pH、电导率与水化产物的变化规律,并基于CT扫描技术重构了三种不同MPC掺量下固化土的孔隙网络模型,结合XRD物相分析,揭示其固化机理。结果表明:随着MPC掺量的增加,m(M/P)较小时固化土的初始pH逐渐减小,而m(M/P)较大时pH先增大后减小。MPC掺量和m(M/P)一定时,随着m(BA/M)的增加,pH在m(BA/M)为某一值(2%)时降为最低,随后逐渐增大。考虑到低液限粉土的低用水量与促进后期水化需求,选择偏酸性配比(m(M/P)=1,m(BA/M)=2%)与低于15%MPC掺量较为合适。pH随着龄期和MPC掺量的增加而增加,电导率变化规律则与pH相反,二者呈负指数的关系,且能定性地表征水化情况。力学试验结果表明MPC能使粉土强度大幅提升,龄期与MPC掺量的增加均使固化土的强度增长。XRD与CT扫描结果表明,MPC固化机制为各龄期均出现的鸟粪石和MgO胶结在土粒之间形成交互结构,且随着MPC的掺量增加,土颗粒间的粘结更加紧密,形成的土骨架提升了宏观力学强度,同时扩大孔隙空间,增加了孔隙率,增强了孔隙间的连通性,导致渗透性增大,因此工程应用时可通过改善孔隙结构进一步提升固化性能。

To explore the reinforcement of magnesium phosphate cement(MPC)on low liquid limit silt deposited in the Yellow River,this experiment studied the response of different MPC contents to dry soil,magnesium-phosphorus ratio m(M/P)and boric acid-magnesium oxide ratio m(BA/M)to the initial pH and conductivity of silt.The unconfined compressive strength,pH,conductivity,and hydration products of solidified soil with different ages and MPC contents were selected under defined m(M/P)and m(BA/M).Based on CT scanning technology,the pore network models of solidified soil with three kinds of MPC content were reconstructed,and the solidification mechanism was revealed by XRD phase analysis.The results show that with the increase of MPC content,the initial pH of the solidified soil decreases gradually when m(M/P)is small,while the pH increases first and then decreases when m(M/P)is significant.When the MPC content and m(M/P)were constant,with the increase of m(BA/M),the pH decreased to the lowest at a certain value(2%)and then gradually increased.Considering the low water consumption of low liquid limit silt and the need to promote hydration in the later stage,the acidic ratio(m(M/P)=1,m(BA/M)=2%)and less than 15%MPC content is recommanded.The pH increases with age and MPC content,and the conductivity decreases with pH increasing,these two have a negative exponential relationship and can qualitatively characterize hydration.The mechanical test results show that MPC can significantly improve the strength of silt,and the strength of solidified silt will increase as age and MPC content increase.The results of XRD and CT scanning show that the MPC solidification mechanism is S-Struvite and MgO cementation appearing and the interactive structure forming both between soil particles at all ages.With MPC content increasing,the bonding between soil particles is closer,and the formed soil skeleton improves the macroscopic mechanical strength.At the same time,it also expands the pore space,increases the porosity,enhances the connectivity between the pores,and increases permeability.Therefore,the solidification performance can be further improved in engineering applications by improving the pore structure.