击实状态下膨润土加砂混合物膨胀特性及影响因素

膨润土加砂混合物是进行核废料深埋处置的理想缓冲回填材料,其膨胀变形特性是评价处置库屏障体系的重要参数。以不同掺砂率下的高庙子钠基膨润土为研究对象,在室内分别开展了击实试验、有荷膨胀率试验、膨胀力试验,重点研究其膨胀变形,并通过微观电镜扫描分析其膨胀特性的影响因素。试验结果表明:随着掺砂率的增加,膨润土加砂混合物的最大干密度也会增大,且击实曲线从相对平缓、开阔逐渐向陡立和狭窄发展,即当掺砂率增加时,混合物干密度梯度变化值受含水率的影响显著;混合物的膨胀过程大致分为3个阶段,即快速膨胀阶段、减速膨胀阶段和缓慢膨胀阶段,有荷膨胀率δ_ep与时间的关系可采用双曲线关系拟合,而最大膨胀力与掺砂率的关系可采用指数关系拟合;基于不同初始状态下的微观结构,综合分析得出单位体积的蒙脱石含量和颗粒结构是影响其膨胀特性的根本因素。

Swelling Characteristics and Its Influencing Factors of Bentonite-Sand Mixture under Compacted Condition

The mixture of bentonite and sand is an ideal buffer material for deep landfill disposal of nuclear waste. The swelling deformation of bentonite-sand mixture is an important parameter in evaluating the barrier system of disposal repository engineering. In this investigation, the swelling deformation of sodium bentonite mixed with sand was examined through compaction tests, loaded expansion ratio tests, and expansive force tests. Furthermore, the influencing factors on expansive deformation were analyzed in terms of microstructure via scanning electron microscopy. Results show that with the increase of sand dosage, the maximum dry density of bentonite-sand mixture also increases; also the compaction curve gradually evolves from being relatively flat and open to vertical and narrow, which means that the variation of dry density gradient of the mixture is affected markedly by moisture content. The expansion process of the mixture, in terms of the time curve, can be divided into three stages: the rapidly expanding stage, the decelerated expanding stage, and slowly expanding stage. Moreover, the relation between loaded expansive rate δ and time can be fitted by hyperbola; the relation between maximum expanding force and sand dosage follows exponential distribution. The microstructure of samples under different compacted conditions obtained from scanning electron microscope demonstrated that montmorillonite content of unit volume and microstructure are the fundamental factors influencing the expansion of bentonite-sand mixture.