石英砂掺量对膨润土–砂混合物泥浆样干缩开裂的控制机制

Influence mechanism of quartz sand content on drying shrinkage and crack of paste-like bentonite-sand mixtures as buffer/backfill materials

高放废物地质处置工程中,膨润土–砂混合物作为缓冲回填材料的干缩开裂特征对工程屏障的安全性有重要影响。以混合型缓冲回填材料为研究对象,分别制备掺砂率为0%～50%膨润土–砂的浆状试样,通过室内恒温干燥试验,研究混合物的干缩开裂特征。结果表明:当掺砂率小于30%时,混合物的收缩曲线与径向应变曲线均基本重合,石英砂悬浮在膨润土中,混合物的干缩开裂特性由膨润土决定;当掺砂率大于30%时,混合物中石英砂逐渐相互接触,增加了颗粒间的摩擦力且有大孔隙形成,进而能够抑制混合物的干缩开裂。石英砂颗粒的相互接触显著提高了混合物进气值,略微提高了缩限,限制了混合物的干燥收缩。大孔隙的形成导致毛细水作用力的降低;颗粒间摩擦力的增加,增强了混合物抵抗断裂的能力,进而抑制了混合物干燥裂隙的发展。最终确定了抑制膨润土–砂混合物干缩开裂的最低掺砂率为30%。

The shrinkage and crack characteristics of bentonite-sand mixtures as buffer/backfill materials play an important role in the security of high-level radioactive waste(HLW) geological disposal.In this investigation,the paste-like bentonite-sand mixtures with sand ratios of 0% to 50% in dry mass are prepared and subjected to indoor thermostatic drying in order to study their volumetric shrinkage and desiccated crack behaviors.The tests results indicate that when the sand ratio is less than 30%,the soil shrinkage characteristic curves(SSCC) and radial shrinkage strain curves are almost identical,respectively,indicating that the shrinkage and crack characteristics of the mixtures are dominated by bentonite because sand particles are suspended in bentonite powders without interaction.When the sand ratio is larger than 30%,the sand particles gradually contact to increase friction,and larger pores are formed by sand mixing,a phenomenon which restrains the shrinkage and crack development of the paste-like mixtures.The amount of air entry remarkably increases,and the shrinkage limit slightly increases because of the increased contact of sand particles,indicating the restraint of shrinkage of bentonite.Larger pores weaken the capillary force,and a higher fraction reinforces the cap ability of the mixtures to resist fractures,a phenomenon which directly restrains the extension of drying cracks.The minimum sand ratio to restrain shrinkage and desiccated crack propagations of the paste-like bentonite-sand mixtures is 30%.