Negative Effect of Tannic Acid on the Strength of Cement-Stabilized Soil

Organic matter in the soil makes a deep mixing project challenging since these organics interfere with cementitious reactions. It reduces the strength of cement-stabilized soil and/or delays the strength development, thereby requiring more cement to achieve the target strength of the project. A research program is ongoing to investigate the influence of organic matter on the strength of cement-stabilized organic soil and develop a framework for quantitative prediction of the strength of cement-stabilized organic soil. In our previous research, the effect of six organic compounds on the strength of cement-water slurry was tested, and tannic acid showed the strongest negative effect on the strength. This paper presents the finding of a laboratory study into the influence of organic matter on the strength of cement-stabilized fabricated clay with tannic acid. A fabricated lean clay (CL) and tannic acid at various concentrations were mixed with water and then blended with a cement-water slurry at various mix designs. The mixture was placed into a plastic mold and cured in plastic containers of water in the humidity-controlled room at room temperature for 3, 7, 14, and 28 days. After curing, unconfined compressive strength (UCS) test or laboratory miniature vane shear test was performed on cured samples. The test results show that the strength of cement-stabilized soil with tannic acid increases with curing time and the amount of cement added to the mixture, while it decreases with the increasing concentration of tannic acid. In this study, tannic acid proved its detrimental effect on strength development in cement-stabilized soil as well as cement-water slurry. In other words, tannic acid reduces the strength, but the negative effect of tannic acid is reduced by adding more cement or applying a longer curing time. Additionally, a threshold to overcome the negative effect of tannic acid was observed, and it was controlled by not only the amount of cement but also curing time and tannic acid concentration.