Sulfate Soil Stabilization

About

Inventors at Worcester Polytechnic Institute have developed a novel method to improve the strength and bearing capacity of sulfate-rich soils, which have historically been challenging to stabilize with calcium-based stabilizers, by mixing them with geopolymers. Weak and soft soils tend to have low strength making them unsuitable for use in building or pavement foundations. Conventional calcium-based stabilizers work well in improving the strength of the soils, but the calcium can react with the SiO42- in the soil that is rich in sulfate with the presence of water to form ettringite crystals that can expand up to 250%, causing deformation and cracks, which introduce damage to the upper structures. Calciumfree geopolymers, on the other hand, have no such negative side effects on the soil, making them an improved replacement for calcium-based stabilizers. The mixing process is as follows: • First, silicon- and aluminum-rich raw materials (e.g., metakoalin and/or fly ash) are mixed with alkali solutions (e.g., NaOH and/or Na2SiO3) to form the geopolymer • Then, the geopolymer paste is mixed with a sulfate-rich soil, where extra water is added as needed to reach predetermined optimum water content • Finally, the mixture is compacted to predetermined maximum dry density The soils stabilized with geopolymers have significantly increased mechanical properties (i.e., unconfined compressive strength, Young’s Modulus and failure strain) when compared to untreated samples, and have comparable properties when compared to the soils stabilized with calcium-based stabilizers, confirming their viability as a suitable stabilizer for sulfate-rich soils. More importantly, the expansion of the soils stabilized with geopolymer is negligible and much less than that of lime-stabilized soils when exposed to water. Though this method was created primarily to stabilize sulfate-rich soils, it can be used in conjunction with any type of soil that needs an increase in strength and bearing capacity. Features • Simple three step method • Calcium-free geopolymer made from metakaolin or the other raw materials mixed with NaOH and Na2SiO3 won’t react with sulfate in soils, thus maintaining mechanical integrity over time Benefits • Increased strength and bearing capacity of sulfate-rich soils without excessive heaving • Structures and pavements are less likely to fail due to ettringite induced heave • Cost-effectiveness and improved sustainability