Stabilization of bentonite and kaolinite clays using recycled gypsum and liquid sodium silicate

Meehan, Christopher L.Sustainable soil stabilization of clays utilizing chemical agents relies primarily on chemical reactions between additives and soil materials to attain the desired geotechnical properties such as strength, compressibility, and durability. In this regard, the use of chemicals for ground stabilization is one of the most favorable soil improvement techniques to improve weak engineering properties of soils by combining unbound materials through fabricated cementation products. A variety of soil stabilizers are available for ground stabilization and are categorized as “traditional” (Portland cement, fly ash, hydrated lime) and “non-traditional” (liquid alkali activators, sodium silicate, polymers, enzymes). The production of traditional additives (such as Portland cement or hydrated lime) emits large amounts of greenhouse gases (CO2) into the atmosphere worldwide. As a separate problem, an excessive amount of waste materials are produced from the construction and demolition of civil engineering projects around the word, and the disposal cost of the associated waste materials is high. As a result, more recently, the use of nontraditional additives (such as sodium silicate) and recycled materials (such as gypsum) in earthwork projects has become attractive as a replacement for traditional stabilization agents due to their economic and environmental benefits for society. ☐ Blending soil and alkaline solutions fabricates new cementation materials named geopolymers, achieving a sustainable improvement in the engineering properties of soils, which produces similar mechanical performance relative to traditional stabilizers such as Portland cement. Geopolymers can be synthesized using a variety of sources including industrial waste as well as fine materials such as natural clays. ☐ This research investigates the use of two nontraditional stabilizers, recycled gypsum produced from wall plasters (sometimes referred to as “sheetrock” in the United States), and a sodium silicate solution, to enhance the strength of two types of clay soils, Bentonite and Kaolinite. Three different stabilizer combinations are assessed during this study: (1) “gypsum only”, (2) “sodium silicate only”, and (3) a 50/50 combination of “gypsum and sodium silicate”. For both of the clay minerals that were stabilized, as well as the three stabilizer combinations that are denoted above, four levels of additive stabilization were explored, at 3%, 6%, 9% and 12%. After stabilization, specimens were subjected to various curing intervals, including 0, 3, 7, 14, 28 and 56 days of curing, and unconfined compressive strength (UCS) testing was conducted to determine the strength development with curing time for each of the stabilized soil mixtures. The change in the pH values of the additive-soil mixtures at different curing periods was monitored. Additional microstructural characterization tests including x-ray diffraction (XRD), field emission scanning microscopy (FESEM), energy-dispersive X-ray analysis (EDAX), fourier transform infrared spectrometry (FTIR), and the nitrogenbased Brunauer-Emmet-Teller (N2-BET) test were all used to explore and assess changes in the soil microstructure as soil stabilization progressed with curing time. ☐ The UCS test results demonstrate that the use of powdered recycled gypsum, a sodium silicate solution, and their combination all considerably increased the strength of both stabilized clay soils. Strength increases measured for gypsum stabilized bentonite and kaolinite were 4 and 2.5 times greater than the strengths measured for the corresponding untreated clays, respectively, at all stabilizer mix ratios and curing times that were assessed. Similarly, strength increases measured for sodium silicate stabilized kaolinite and bentonite were 3.5 and 3.5times greater than the strengths measured for the corresponding untreated clays, respectively. Strength increases measured for gypsum and sodium silicate (50/50) stabilized kaolinite and bentonite were 3.5and 2.5 times greater than the strengths measured for the corresponding untreated clays, respectively. It should be noted that these strength multipliers are the lower bound of the observed strength gain, and that many of the tested specimens exhibited significantly higher strengths at various stabilizer concentrations and curing times. ☐ The required optimum additive content of stabilizers depended upon the type of soils, and was different for different curing times. In this study, the optimum stabilizer contents were determined based upon the stabilizer mix ratio that yielded the largest gain in strength in the treated specimens after 56 days of curing. The optimum additive contents for bentonite stabilized with gypsum, sodium silicate, and a 50/50 mixture of gypsum and sodium silicate were 3, 12, and 6, respectively. The optimum additive contents for kaolinite stabilized with gypsum, sodium silicate, and a 50/50 mixture of gypsum and sodium silicate were 12, 6, and 6, respectively. As shown, in general, the kaolinite clay needed a higher content of recycled gypsum relative to the bentonite clay, whereas the sodium silicate stabilized kaolinite required a lower content of sodium silicate relative to the bentonite. The combination of recycled gypsum and sodium silicate was found to have benefits regarding the improvement of engineering properties of both soils, with the same amount of admixture (6%) yielding the greatest strength gain for both soils. The observed chemical reactions for all of the soil stabilization processes were time-dependent, especially for the bentonite treated with the combination of recycled gypsum and sodium silicate. ☐ The XRD tests show the formation of new cementation products via the appearance of new diffraction peaks, along with a reduction of the intensities of the peaks corresponding to the aluminum silicate minerals for both of the tested clays. The FESEM tests showed the transformation/modification of the soil microstructure and clay particle surfaces for both of the clays that were tested, and for the three stabilizer combinations that were utilized. Moreover, new crystalline gel (geopolymer) phases of cementation were observed. Alteration of the chemical composition of both treated soils was validated using energy-dispersive X-ray analysis (EDAX). The modifications of the functional groups of both clay minerals were confirmed utilizing Fourier transform infrared spectrometry (FTIR). In general, the nitrogen-based Brunauer-Emmet-Teller (N2-BET) tests showed a decrease in the surface area of both stabilized clays in the longterm for the different stabilizers that were assessed, as cementation products were created and the pore space between the specimens was filled. At some of the intermediate curing times, increases in surface area of the treated specimen were observed; this behavior is attributed to dissolution of the base materials prior to formation of stabilizing cementitious compounds. These N2-BET surface area results are generally consistent with the UCS test results as the strength reported for intermediate curing times is sometimes lower than the initial strengths that were measured. ☐ From the results of this study, it is believed that the combination of recycled gypsum and sodium silicate improves the soil strength properties significantly, offering positive benefit for long-term soil stabilization. The potential for beneficial reuse of waste gypsum can reduce the quantity of this material that ends up in landfills, and the replacement of traditional Portland cement and lime stabilizers with the combination of gypsum and sodium silicate could serve to decrease the emission of greenhouses gases that are associated with the production of these more traditional soil stabilizers.University of Delaware, Department of Civil and Environmental EngineeringM.C.E