Geo-engineering Properties of Expansive Soil Treated with Geopolymer and Conventional Stabilizers

Abstract

Expansive soils occur worldwide and are also abundant in the Indian subcontinent. The shrink-swell mainly influences lightly loaded structures like road and railway pavements, developing surface heave and crack, and everlasting sealing problems during the impact of loading. Of the various techniques used for improving the properties of the expansive soils, the calcium-based additives have been effectively applied to improve the strength and control the volume change behavior of expansive soils both efficiently and economically. However, despite the benefits and multi-faceted utilization of lime and cement, high carbon emission and high-energy requirements associated with these binders represent environmental constraints. Hence, research is being focused on developing substitute chemical additives to cut the use of conventional cement. Consequently, waste products in industries have received the top most attention due to their low carbon footprints and alternative waste utilization advantages. Geopolymer produced from aluminosilicate-rich industrial wastes has been found suitable as an alternate binding material to conventional binders like cement and lime. Considering this in the present research, the geo-engineering properties of a locally available expansive soil treated with slag-geopolymer, cement, and lime are evaluated through laboratory tests. The whole work is divided into three phases. In the 1st phase, the compaction characteristics and unconfined compressive strength (UCS) of a large combination of soil and additive are used. For geopolymer stabilization, the proportions of slag in the mixture were varied as 0%, 5%, 10%, 15%, and 20% by weight of the dry soil, with the concentration of NaOH solution varying from 0 M, 0.5 M, 1 M, 2 M, 4 M, 6 M, and 8 M. Thus, altogether 35 soil-geopolymer combinations were tested. Similarly, for lime and cement treatments, the mixture's proportions of lime or cement were varied as 0, 1%, 2%, 4%, 8%, 12%, and 15% of the dry soil mass. The UCS value was used to measure the degree of reactivity of various components of the geopolymer in the treated expansive soil, and the results were evaluated at curing periods of 7 and 30 days. Based on the 30-day UCS value, three mixed proportions of geopolymer, cement, and lime were selected to evaluate their comprehensive geo-engineering properties. In the 2nd phase of the investigation, the effects of stabilizer content and curing period were explored for consistency limits, compaction parameters, hydraulic conductivity, swell-shrink properties, California bearing ratio value, strength properties, and durability performance of expansive soil. Finally, in 3rd phase of the investigation, the effects of delay time on geo-engineering properties are evaluated. A detailed examination of the geo-engineering properties of expansive soil treated with slag-geopolymer, cement, and lime, concerning strength, durability, and influence of delay period, is made. The underneath conclusions are drawn from this research: • The plasticity properties of expansive soil are reduced with an increase in the additive content. The plasticity properties such as LL and PI of geopolymer added soil are related to slag content, alkali solution concentration, and delay period. In the case of lime and cement, the reduction is due to these additives' non-plastic character and the flocculation effects. It is mainly due to the depression of the thickness of adsorbed layer of water and the microstructural changes of the clay particles. The virgin clay classified as CH type is changed to CI, CL, MI, and non-plastic with different proportions of additives. • Soil treated with different stabilizers exhibits different compaction characteristics. The MDU increased with the stabilizer content for cement and geopolymer treated soil. However, MDU reduces with an increase in lime content for lime-treated soil. The OMC soil-geopolymer mixes decrease with increased geopolymer content, while it increases for lime and cement-treated soils. With an increase in the delay time, the mechanical characteristics such as dry density and UCS of the stabilized soil are reduced. It is due to the formation of strong flocs, which leads to the loose packing of stabilized material with the increase in void ratio. It is identified that with the addition of chemicals, the grain size distribution (GSD) curve of the treated soils shifted towards a larger grain size. Overall, geopolymer stabilized material attains higher compacted density and mechanical strength than conventional stabilizers. • Slag-based geopolymer improves the strength and stiffness of the treated soil, but the ductility of the virgin soil reduces. This is more prominent as either the amount of slag, the concentration of NaOH activator, or the curing time increases. There is no optimum dose for slag content or NaOH concentration in geopolymer-treated soil mixtures. The UCS of soil-slag mixes increases with the amount of slag and strength of NaOH solution. Similarly, no optimum dose of cement or lime was observed in the present investigation, where a maximum of 15% of cement/lime is used. Geopolymer and cement-treated soil achieve higher early strength than lime-treated soil. At 30 days of curing, geopolymer stabilized soil (S20M8) developed a strong structural bond and had a peak strength of 5.11 MPa. • Chemical additives significantly control the swelling behavior of the treated expansive soil. At low additive contents, lime significantly decreases the free swell index value and swell pressure of the treated soil. However, as the additive content increased, all three stabilizers developed strong resistance toward swell pressure. The permeability coefficient reduces, whereas the California bearing value significantly improves with the addition of chemicals and the curing period. • The durability of geopolymer-treated soil under slake durability test (SDT) is superior to conventional stabilizers. The geopolymer treated soil retained its integrity even after four cycles of slacking. Based on the slaked durability indices, soil treated with 20% geopolymer, 15% cement, and 15% lime are categorized as a medium, low, and very low durable materials. Freeze-thaw (F-T) cycles have a significant influence on stabilized soil. The strength and mass loss are more at initial F-T cycles. However, stability is attained after the sixth cycle. The loss in strength of geopolymer, cement, and lime stabilized soil after 12 F-T cycles are 48%, 54%, and 60%, respectively. Water immersion and modified durability tests (MDT) demonstrate that the geopolymer treated soil has substantial resistance, followed by cement and lime treated soil. • The evolution of the mineralogy and microstructure results in the strength gain. It is assumed that the cementitious products formed by the alkaline activation of slag particles serve as nucleation sites on the clay surface. These reaction products bind clay plates to form clay clusters, strengthening the soil structure and thus enhancing its strength. Both hydration and geopolymeric compounds were detected in the geopolymer treated soil, whereas only calcium-based hydration products were found in both lime and cement treated soil. Therefore, geopolymer treated soil achieves higher strength gain. The massive and dense structure with the formation of rich hydration and geopolymeric compounds was identified from the SEM images of geopolymer-treated soil. In contrast, micro-pores were observed in the lime-stabilized soil due to the development of lesser reactive products. No such micro-pores were noticed in the cement, and geopolymer treated soil. Geopolymer treated clay has a dense and compacted microstructure compared to conventional stabilizers. It develops high early strength, higher ultimate strength, and better durability response compared to cement and lime-treated soil. Hence, geopolymer can be considered an effective cementing material to stabilize expansive soil than conventional stabilizers.