膨润土加砂混合物干燥收缩特征及缩裂机制研究

Study on desiccation–shrinkage characteristic and shrinkage cracking mechanism of bentonite and sand mixtures

膨润土加砂混合物作为核废料储存库缓冲材料,在库内高温高吸力条件下易干燥收缩,对工程安全运营有着重要影响,研究其收缩特性具有实际意义。以砂-膨润土混合物的饱和压实样为研究对象,采用自制恒温箱控制其收缩进程,分析含砂量及干密度对混合物收缩特性的影响。研究结果表明:干燥―收缩过程水分蒸发可分为减速阶段与残余阶段。减速阶段内水分迁出受控于干密度,而最终残余含水率主要受膨润土含量的影响。增加初始干密度和降低膨润土含量可使混合物变形协调,减少收缩变形的各向异性。膨润土含量M和初始干密度ρ是影响干燥―收缩过程的重要物理量,与Cornelis收缩模型中的拟合参数及最终孔隙比e0具有很好的相关性。同等干密度条件下,随膨润土含量降低,压实膨-砂混合样的微观结构形态趋于致密化的趋势,从而有效地抑制了干燥收缩过程中裂隙发育与演化程度。

As a buffer material for nuclear waste storage,bentonite-sand mixture tends to shrink under high temperature and suction conditions,which affects the safety operation of engineering projects.Therefore,it is of great practical significance to study the shrinkage characteristics of bentonite-sand mixture.This paper investigates the influence of sand content and compactness on desiccation shrinkage characteristic of saturated compacted sample of bentonite-sand mixtures placed at custom designed thermostatic chamber.It is shown that water evaporation in the process of desiccation can be divided into deceleration and residual stages.In the former stage,the water migration is dominated by the dry density,while the final residual water content is mainly affected by bentonite content.An increase in the initial dry density and decrease in the bentonite content can coordinate the deformation of the mixture and hence reduce the anisotropy of shrinkage deformation.The bentonite content M and the initial dry densityρare the key physical parameters affecting the drying-shrinkage process and have a close correlation to the Cornelis model parameters and the final pore ratio e0.At a given dry density,the microstructures of compacted samples tend to densify as M decreases,thereby effectively inhibiting the development and evolution of cracks during the drying-shrinkage process.