高钙矾石含量固化土重金属固化稳定机制和约束固化重金属污染土桩承载特性研究

Polluted soil with high concentration of heavy metals is mixed with stabilizer including high content of expansible component and is filled in an engineering plastic pipe to form a confined stabilized soil pile (CSSP). Combination of the pipe confinement and the stabilizer expansion produces an unique effect that: high content of expansible hydrate (ettringite, AFt) could trap heavy metals more efficiently; the expansion under the confinement could reduce voids in the stabilized soil（SS）significantly, increase SS strength, and enforce pre-stress on the pipe; the pipe confinement enhances the bearing capacity of the pile by several times. In order to investigate the environment suitable for AFt and calcium silicate hydrate (CSH) harmonious producing, the behavior of interaction of AFt and CSH with different heavy metals, and the mechanism and favorable condition for trapping heavy metals, pore water in SS is chemically analyzed; change of the haydrates and micro-structure in SS are analyzed by scanning electron microscopy, energy dispersive analysis of X-rays, and X-ray diffraction; and both leaching toxicity test and compressive strength test are also conducted for the SS with high compactness and high ratio of AFt/CSH. Based on these results, parameters could be provided for making confined stabilized soil which enables the three functions of trapping heavy metals, reducing the void, and producing the pre-stressing to develop harmoniously. Also, axial compress test of confined SS column, model test and numerical simulation analysis of CSSP are conducted to investigate the load transfer mechanism, failure mode, and bearing behavior of CSSP under the effect of such factors including mechanical property and compactness of SS, restraining stiffness and pre-stressing force of the confinement pipe.

将含高量膨胀组分的固化剂与高浓度重金属污染土拌合填充于工程塑料筒内，构成约束固化土桩(CSSP)。约束体和膨胀性固化剂结合可产生独特效果：膨胀水化物(钙矾石，AFt)可更高效率俘获重金属、约束下的膨胀可消减固化土孔隙提高其强度并使约束体产生预应力，约束体的环向约束可成倍提高固化土桩承载力。用扫描电镜、X射线能量色散谱仪等测固化土中水化物和微结构变化，做固化土孔隙水化学分析、毒性淋滤试验和强度试验；研究在高密实度、高AFt/CSH(水化硅酸钙)比的固化土中，AFt、CSH协调发展的水化环境，AFt、CSH与重金属离子的相互作用以及俘获重金属的机理和适宜反应条件；为制备重金属俘获、消减固化土孔隙和建立预应力三功能协调发展的约束固化土提供参数。做约束固化土柱轴压试验、CSSP模型试验和数值模拟，探讨固化土力学性能和密实度、约束体刚度及预应力对CSSP的荷载传递机理、破坏模式和承载特性的影响规律。

试验表明：约束固化土（CSSC）将约束体和膨胀性固化剂结合，可加大固化剂中生成AFt的膨胀组分比例、并提高孔隙填充效率，提高固化土强度；给固化土施加预压应力；使固化土在竖向压力下处于三向受压状态；高AFt含量可提高重金属稳定能力。对液相指数IL为0.25–1.16的土样，CSSC极限承载力为同强度无约束固化土（USSC）的3.95–4.04倍。在CSSC荷载位移曲线的线性段1，主要由保持结构整体性的固化土承受荷载，其终点变形与USSC的极限变形相当；在斜率较小的近似线性段2，随着固化土结构逐步解体荷载转嫁到约束体，终点处应变约为10%，主要是固化土压密变形；极限承载力由约束体抗拉失效决定。. 用所生成的水化物AFt/CSH不同的6种固化剂分别与浓度各为0⁓3%的铅、锌、铜、三价铬、镉、六价铬等8种重金属污染土制成固化土。试验表明：除K2CrO4外，各重金属对固化土强度有不利影响；不同重金属对CSH或AFt的形成的影响不同，Zn (NO3)2对CSH的形成和作用有不良作用，对AFt则无明显影响；Pb(NO3)2对固化土强度的影响与之相反；CSH或AFt对不同重金属的稳定能力各不相同；淋滤浓度与固化土强度无直接关系。例如：无AFt的固化Pb(NO3)2污染土在该系列中强度最高，但其淋滤浓度也最高；随着AFt增加固化土强度降低，其淋滤浓度也相应降低。AFt对大部分重金属的稳定作用高于CSH。. 取两个强度等级的固化土强度发展速率不同但最终强度相近的各三个基准固化剂，取4种膨胀速率不同的膨胀组分，将各膨胀组分以不同比例分别掺入到各基准固化剂中，并制成固化土。发现：固化土强度在0.44~1.07 MPa间生成的AFt的膨胀使固化土产生显著的直接强度增量，且可容纳的膨胀量较高；在同膨胀量下产生的最终固化土强度增量大；AFt增强固化土强度的机理为：AFt膨胀提高土体密实度，使CSH以高效率胶结土体，间接地带来固化土强度增量；当固化土形成适当强的结构时，在其约束下AFt膨胀填充固化土中孔隙，提高固化土密实度，并与CSH一起形成网络骨架，从而对固化土强度提供显著直接强度增量。