Effectiveness of industrial wastes-based geopolymers in improving the durability of stabilized/solidified heavy metal polluted soil

The cement-based stabilization/solidification (S/S) method is widely used in modifying soils polluted by heavy metals (HMs), although it may face technical, economic, and environmental limitations. Therefore, the present work was designed to investigate the effectiveness of a type of geopolymer based on the steel slag (SGP) and its combination with microparticles of zeolite (SGPZ), compared to cement (as a traditional additive), in enhancing the stability of S/S products. In so doing, different percentages (0 to 250 mg/g-soil) of SGP, SGPZ, and sole cement were separately added to the S/S samples containing different concentrations of lead (including 5000, 10000, 20000 and 40000 mg/kg-soil). After adequate curing (up to 28 days), a set of macro and micro scale experiments were performed to assess the long-term performance of the amended soil samples using a laboratory accelerated aging procedure that simulated 25, 50, 75 and 100 years of exposure to the acid rain and wet and dry (W-D) cycles in the field. It was found that, while low amounts of cement (PC) would greatly reduce the initial bioavailability of pollution in the pore fluid of soil, increasing the contact time of the PC-treated specimens upon harsh conditions, especially in the presence of high level of Pb, would dramatically diminish the efficiency of the precipitation mechanism as well as the degree of encapsulation process which play a significant role in increasing the ability of S/S sample to release the toxic ions stabilized/solidified previously. At simulated 100 years, the toxicity characteristic leaching procedure leached Pb from the PC-treated sample with 250 mg/g-soil binder would exceed the permitted threshold of pollution leaching (≥ 5 mg/L) by 508%, indicating that meeting the S/S regulation limits requires a large consumption of cement. The study showed that, unlike treatment conditions with the same level of PC, the use of novel cement-free S/S binders (especially SGPZ) would significantly limit the negative influences of the environmental changes on HM remobilization risks. In addition, the mechanical characteristics of those series of samples were sometimes up to 1.4 times higher than that of the soil modified with cement alone. Based on the X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images, this enhancement can be mainly due to i) reduction in the adverse HM-binder interactions, ii) intensification in the level of hydration reactions, iii) formation of secondary complex hydrated phases (e.g., Hydrotalcite: Mg6Al2CO3(OH)16.4H2O), and iv) creation of a three-dimensional network of solidification in the system containing geopolymer, wrapping the matrix of S/S products against the structure disintegration upon contact to the aggressive environments. Therefore, under the destroying impacts of acid attack and W-D scenario, adding 25% SGPZ composite could pass the S/S regulation limits. In general, based on the obtained results, the use of geopolymer (especially containing zeolite) is suggested as an effective and environmentally friendly alternative for sustainable soil improvement, even with the high contents of HM ions. Following the USEPA and UKAE standards to achieve the safe S/S performance, the optimal dosage of GP binder was determined to be approximately 6 mg/g-soil per 1 g/kg of lead in the sample.