Shrinkage mechanism of red clay based on changes in the thickness of bound water film
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摘要:
结合水的含量及存在形式对红黏土的物理力学性质具有重要影响, 而红黏土宏观收缩会引起土粒、孔隙、结合水微观结构变化, 进而可能产生土体浅表层开裂引发渗透、失稳等工程地质问题。采用热重分析试验、BET测试试验、电镜扫描和Zeta电位试验对原状红黏土收缩过程中结合水的变化特征进行了研究, 建立了基于结合水膜均匀分布下的球状和薄片状黏土颗粒结构模型, 并推导出结合水膜厚度的计算公式。研究结果表明, 原状红黏土中的水分大部分以结合水的形式存在, 红黏土的收缩过程一直贯穿着弱结合水的损失, Zeta电位和比表面积不断减小, 结合水膜厚度也不断减小。研究结果揭示了红黏土失水收缩的内在机理, 可为解决环境工程地质问题提供理论支持。
Abstract:Objective The content and existence form of bound water have an important influence on the physical and mechanical properties of red clay, and macroscopic shrinkage of red clay leads to microstructure changes of soil particles, pores and bound water, which in turn may cause soil surface cracking, triggering infiltration, destabilization, and other engineering geological problems.
Methods Thermogravimetric analysis tests, BET tests, scanning electron microscopy (SEM) and zeta potential tests were conducted to study the variation characteristics of bound water during the shrinkage of undisturbed red clay. The structural model of spherical and lamellar clay particles based on the uniform distribution of bound water film was established, and the formula for the calculation of bound water film thickness was derived.
Results The results showed that most of the water in undisturbed red clay exists in the form of bound water and the loss of weakly-bounded water continues throughout the shrinkage process of red clay. During the process, the zeta potential, specific surface area and the thickness of the bound water film decreased continuously.
Conclusion The results reveal the intrinsic mechanism of water loss and shrinkage of red clay, which can provide theoretical support for solving environmental engineering geological problems.
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Key words:
- bound water /
- red clay /
- shrinkage test /
- thermogravimetric analysis /
- microscopic test
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图 2 3种不同含水率的原状红黏土收缩曲线
Figure 2. Shrinkage curves of red clay of 3 different water contents
图 3 红黏土热重曲线
TG曲线代表土样质量随温度变化的损失率；DSC曲线代表土样质量损失随温度的变化速率
Figure 3. Thermogravimetric curve of red clay
图 4 红黏土5 000倍下的SEM图片
a. 初始阶段; b. 正常收缩阶段; c. 残余收缩阶段; d. 零收缩阶段
Figure 4. SEM images of red clay at 5 000 times
图 5 红黏土不同收缩阶段等效孔径
Figure 5. Equivalent pore size of red clay at different stages of shrinkage
图 9 红黏土颗粒简化模型
h. 球状结构红黏土的水薄厚度; t. 薄片状结构红黏土的水薄厚度；a′, b′, c′分别为薄片状结构的高、长、宽
Figure 9. Simplified model of red clay particles
表 1 基本物理指标
Table 1. Basic physical indicators
天然含水率/
%相对密度 液限/
%塑限/
%干密度/
(g·cm-3)孔隙比 68.9 2.39 80.7 50.9 1.14 1.11 
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表 2 红黏土矿物成分
Table 2. Mineral composition of red clay
矿物成分 埃洛石 高岭石 绿泥石 伊利石 氧化铁 氧化铝 其他 wB/% 60.4 3.9 6.0 7.0 11.9 6.8 3.0 注：其他主要包括结晶比较弱或者非晶矿物等物质
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表 3 各收缩阶段蒸发水类型
Table 3. Type of evaporated water at each stage of shrinkage
试样 质量含水率
wB/%质量绝对含水率
wB/%收缩阶段 自由水质量分数wB/% 弱结合水质量分数wB/% 强结合水质量分数wB/% S1 64.9 64.0 初始阶段 1.17 — — 正常收缩阶段 — 21.90 — 残余收缩阶段 — 9.88 — 零收缩阶段 — 15.53 5.92 S2 69.5 69.1 初始阶段 1.07 — — 正常收缩阶段 — 27.35 — 残余收缩阶段 — 8.67 — 零收缩阶段 — 19.75 —
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表 4 不同收缩阶段红黏土结合水膜厚度
Table 4. Thickness of the bonded water film of red clay at different stages of shrinkage
收缩阶段 吸附水含水量wB/% 比表面积
Sg/
(m2·g-1)结合水密度
ρw/
(g·cm-3)结合水膜的厚度
h/nm初始阶段 64.9 20.202 1.783 18.018 正常收缩阶段 46.6 19.524 1.787 13.356 残余收缩阶段 35.7 18.128 1.791 10.996 零收缩阶段 24.0 16.885 1.793 7.927
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