College of Civil Engineering, Chongqing University, Chongqing 400045, China.
Key Laboratory of New Technology for Construction of Cities in Mountain Area (Chongqing University), Ministry of Education, Chongqing 400045, China.
Int J Environ Res Public Health. 2020 Feb 8;17(3):1077. doi: 10.3390/ijerph17031077.
The solidification/stabilization (S/S) method is the usual technique for the remediation of soils polluted by heavy metal in recent years. However, freeze-thaw cycles, an important physical process producing weathering of materials, will affect the long-term stability of engineering characteristics in solidified contaminated soil. In addition, it is still questionable whether using large dosages of binders can enhance the engineering properties of solidified/stabilized contaminated soils. In this study, the three most commonly used binders (i.e., cement, quicklime, and fly ash), alone and mixed in different ratios, were thus added to lead-contaminated soil in various dosages, making a series of cured lead-contaminated soils with different dosages of binders. Afterward, unconfined compression strength tests, direct shear tests, and permeability tests were employed on the resulting samples to find the unconfined compressive strength (UCS), secant modulus ( E 50 ), internal friction angle ( φ ), cohesion ( c ), and permeability coefficient ( k ) of each solidified/stabilized lead-contaminated soil after 0, 3, 7, and 14 days of freeze-thaw cycles. This procedure was aimed at evaluating the influence of freeze-thaw cycle and binder dosage on engineering properties of solidified/stabilized lead-contaminated soils. Results of our experiments showed that cement/quicklime/fly ash could remediate lead-contaminated soils. However, it did not mean that the more the dosage of binder, the better the curing effect. There was a critical dosage. Excessive cementation of contaminated soils caused by too much binder would result in loss of strength and an increase in permeability. Furthermore, it was found that UCS, E 50 , φ , c , and k values generally decreased with the increase in freeze-thaw cycle time-a deterioration effect on the engineering characteristics of solidified lead-contaminated soils. Avoiding excessive cementation, 2.5% cement or quicklime was favorable for the value of E 50 while a 2.5% fly ash additive was beneficial for the k value. It is also suggested that if the freeze-thaw cycle continues beyond the period supported by excessive cementation, such a cycle will rapidly destroy the original structure of the soil and create large cracks, leading to an increase in permeability. The results also showed that the contaminated soils with a larger dosage of binders exhibited more significant deterioration during freeze-thaw cycles.
固化/稳定化（S/S）方法是近年来修复重金属污染土壤的常用技术。然而，冻融循环作为一种重要的物理风化过程，会影响固化污染土壤的长期工程特性稳定性。此外，使用大量的固化剂是否能提高固化/稳定化污染土壤的工程性能仍然存在疑问。在这项研究中，单独添加了三种最常用的固化剂（即水泥、生石灰和粉煤灰），并以不同的比例混合，添加到不同剂量的含铅污染土壤中，制备了一系列不同剂量固化剂的固化/稳定化含铅污染土壤。随后，对制备的固化/稳定化含铅污染土样进行无侧限抗压强度试验、直剪试验和渗透试验，得到各固化/稳定化含铅污染土样在 0、3、7、14 天冻融循环后的无侧限抗压强度（UCS）、割线模量（E50）、内摩擦角（φ）、黏聚力（c）和渗透系数（k）。本研究旨在评估冻融循环和固化剂剂量对固化/稳定化含铅污染土工程特性的影响。实验结果表明，水泥/生石灰/粉煤灰可以修复含铅污染土壤。然而，这并不意味着固化剂的剂量越多，固化效果越好，存在一个临界剂量。过多的固化剂会导致污染土壤过度胶结，从而导致强度损失和渗透性增加。此外，还发现 UCS、E50、φ、c 和 k 值随冻融循环时间的增加而降低，即固化铅污染土的工程特性恶化。避免过度胶结，2.5%的水泥或生石灰有利于 E50 值，而 2.5%的粉煤灰添加剂有利于 k 值。研究还表明，如果冻融循环持续时间超过过度胶结所允许的时间，这种循环将迅速破坏土壤的原有结构并产生大的裂缝，导致渗透性增加。结果还表明，固化剂剂量较大的污染土壤在冻融循环过程中表现出更显著的劣化。
