The effects of air drying on the strength of sand-lignosulfonate-water mixes

Abstract

The purpose of this research was to investigate the effects of drying on the strength gain of masonry sand stabilized with a co-product from wood pulping called calcium lignosulfonate. Lignin is an amorphous polymer found in plant cell walls. It provides protection against disease and allows the transport of water and nutrients. Adhesive properties of lignin generated interest in adding its modifications to soils as means to prevent erosion from wind and vehicle traffic on unpaved roads. Lignin has the potential to become a more sustainable alternative to traditional stabilizers because its source is renewable and abundant, and its toxicity is negligible. Extensive testing has recently been completed to quantify the stress-strain relationships and Mohr-Coulomb strength parameters of sand- calcium lignosulfonate-water (S-CaL-W) mixes at early age (Bartley, 2011). The experimental program consisted of performing Standard Proctor Tests to determine maximum densities and optimum moisture contents for mixes having different gravimetric lignin contents and direct shear tests on selected sample configurations. Based on these results, it was decided to conduct shear strength testing of the samples containing 4%, 6% and 9% of calcium lignosulfonate after they had been exposed to air drying. To this end, responses of the selected sample configurations to drying at 71° F and 27% relative humidity were measured to determine the target water contents for shear strength testing. Drying curves were obtained by plotting the measured water content or water to CaL ratio versus the elapsed time. Drying times for shear strength were chosen based on how long it took the moisture contents to decrease by specified levels. The available results of direct shear tests show that drying significantly increases both the cohesion and the friction angle of the S-CaL-W mixes with respect to the early age cohesion and friction angle. In addition to the direct shear test program a laboratory compaction test was conducted on CaL and water only, thus providing the maximum dry density of CaL and the corresponding optimum water to CaL ratio. It is also noted that relative humidity was discovered to be the limiting factor in the strength gain of S-CaL-W mixes. The reasons behind its sensitivity to water are due to the presence of HPLC sugars within the calcium lignosulfonate structure. These sugars hold the water through the chemical interaction of the sugars with hydrogen ions and water molecules.