Mechanical and swelling properties, as well as micro-mechanism of sulfate-bearing soil stabilized by magnesium oxide and cement
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摘要: 硫酸盐（渍）土易引发溶陷、膨胀、腐蚀地基等岩土灾害，而水泥固化会导致膨胀性矿物——钙矾石的生成，造成硫酸盐土体膨胀、强度损失和耐久性降低。为了避免上述问题，采用氧化镁（MgO）部分替代水泥（即MgO复合水泥），用于固化处理硫酸盐土（即石膏土）。通过开展垂直膨胀、无侧限抗压强度（UCS）、X射线衍射（XRD）、扫描电镜（SEM）和核磁共振（NMR）试验，研究MgO与水泥配比对固化土膨胀特性和力学性能的影响规律，并揭示MgO复合水泥（MgO-水泥）固化硫酸盐土的微观机理。试验结果表明：随着MgO与水泥配比增加，固化土的总膨胀率先减小后增大，而UCS总体呈现为先增大后减小。在微观机理方面，加入适量MgO可减少钙矾石生成；而加入过量MgO后，水化硅酸镁（MSH）的形成抑制了水化硅酸钙（CSH）形成，削弱了CSH的影响。综上，MgO∶水泥（MgO∶C）=0.5∶9.5是固化硫酸盐土较优的MgO与水泥配比。Abstract: The sulfate-bearing (saline) soil may easily cause geotechnical disasters, such as subsidence, expansion and foundation corrosion. However, stabilizing the sulfate-bearing soil by cement can lead to the formation of expansive mineral-ettringite, resulting in soil swelling, strength loss and poor durability. To avoid the aforementioned problems, the magnesium oxide (MgO) is used to partially replace cement (MgO combined with cement) in the stabilization treatment of the sulfate-bearing soil (gypseous soil). By conducting the tests of vertical swelling, unconfined compressive strength (UCS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR), the effects of the ratio of MgO to cement on the swelling and mechanical properties of the stabilized soil are explored. Furthermore, it reveals the micro-mechanism of the sulfate-bearing soil stabilized with MgO and cement (MgO-cement). The results show that as the ratio of MgO to cement increases, the total swelling percentage of the stabilized soil first decreases and then increases, while the UCS has an opposite trend, first increasing and then decreasing. In terms of the micro-mechanism, the addition of an appropriate amount of MgO can reduce the formation of ettringite. However, when an excess of MgO is added, the formation of magnesium silicate hydrate (MSH) inhibits the formation of calcium silicate hydrate (CSH), thereby weakening the effects of CSH. In summary, MgO: cement= 0.5∶9.5 is considered to be an optimal ratio of MgO to cement for stabilizing the sulfate-bearing soils.
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图 3 MgO-水泥固化土的垂直膨胀率
Figure 3. Vertical swelling rates of soils stabilized by MgO-cement
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图 5 不同条件下固化土应力-应变曲线
Figure 5. Stress-strain curves of stabilized soils under different conditions
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图 7 高岭土和MgO-水泥固化土浸泡后的XRD图谱
Figure 7. XRD patterns of kaolin and soils stabilized by MgO-cement after soaking
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图 8 浸泡后MgO-水泥固化土SEM图
Figure 8. SEM images of soils stabilized by MgO-cement after soaking
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图 9 浸泡后MgO-水泥固化土核磁共振T2谱
Figure 9. NMR T2 spectra of soils stabilized by MgO-cement after soaking
表 1 主要材料氧化物组分及百分含量（%按重量计算）
Table 1 Oxide compositions of main materials (% by weight)
组成 SiO2 CaO Al2O3 MgO Fe2O3 SO3 K2O 其他 烧失量 高岭土 53.90 — 43.24 — 0.89 0.08 0.19 1.7 0 石膏 8.32 58.39 2.61 5.21 0.72 23.70 0.55 0.5 0 水泥 20.66 59.39 5.60 3.87 3.23 4.99 0.10 1.27 0.89 MgO 10.23 5.62 6.34 76.72 — 0.55 — 0.54 0 
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