在高放核废料处置库中,作为缓冲回填材料主料,膨润土一方面受到地下水中可溶盐的侵蚀作用,另一方面受核废料衰变产生的热能影响,会发生干燥收缩。以高庙子膨润土为研究对象,以NaCl、CaCl2(浓度为0.5、1.0、2.0 mol/L)盐溶液侵蚀为研究条件,开展恒温干燥收缩试验,分析盐溶液对压实膨润土干燥收缩进程的影响。试验结果表明:盐溶液浓度越高,试样初始饱和含水率越低,残余含水率越高,样品水分蒸发速率越低,且Ca2+对水分蒸发的影响要大于Na+;干燥过程中,压实膨润土表面裂隙呈现出裂隙张开、裂隙维持、裂隙闭合及裂隙稳定4个阶段,盐溶液可以抑制土体开裂,浓度越高,样品表面裂隙率越小。盐溶液作用使膨润土的微结构从分散的黏粒形态向团聚结构转变,从而导致收缩应变减小、收缩各向异性弱化,随着盐溶液浓度和阳离子化合价增大,单位含水率降低诱发的体积收缩应变εvf增大。因此,研究盐溶液作用下膨润土干燥收缩特性,对于合理评估膨润土的工程服役性能具有重要意义。
Abstract
As a major buffering backfill material for high-level radioactive waste disposal repository, bentonite is affected by soluble salts in groundwater and thermal energy generated by nuclear waste decay, leading to drying shrinkage. Studying the drying shrinkage characteristics of bentonite under the influence of salt solutions is crucial to rationally evaluate its engineering performance. Constant-temperature drying shrinkage tests were carried out on bentonite from Gaomiaozi under the exposure of NaCl and CaCl2 solutions with concentrations of 0.5, 1.0 and 2.0 mol/L, respectively. The influence of salt solutions on the drying shrinkage process of compacted bentonite was analyzed. Results show that the higher the salt solution concentration, the lower the initial saturated water content and the higher the residual water content of the sample, indicating a lower water evaporation rate. Meanwhile, the effect of Ca2+ on water evaporation is greater than that of Na+. During the drying process, the surface fissures of compacted bentonite exhibit four stages, namely fissure opening, fissure maintenance, fissure closure, and fissure stability. Salt solutions could inhibit the cracking of soil mass, and the higher the concentration, the smaller the surface crack ratio of the sample. Salt solutions can change the microstructure of bentonite from a dispersed viscous particle form to an aggregated structure, thereby reducing the shrinkage strain and weakening the anisotropy of the shrinkage. As the concentration and cation valence of salt solutions increase, the volume shrinkage strain εvf induced by the decrease of unit water content increases.
关键词
压实膨润土 /
盐溶液 /
水分蒸发 /
干燥收缩 /
裂隙
Key words
compacted bentonite /
salt solution /
water evaporation /
drying shrinkage /
fissures
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