Coupling effect of curing temperature and relative humidity on the unconfined compressive strength of xanthan gum-treated sand

Xanthan gum, as an environment-friendly biopolymer, holds great promise to replace conventional binders for engineering construction. Yet, the impact of curing temperature and relative humidity (RH) on the strength of xanthan gum-treated sand remains uncertain. In this study, xanthan gum-treated sand specimens were cured under different temperatures (20°C-70°C) and RH (10 %-80 %), respectively, until an equilibrium moisture content was reached. Through a series of Unconfined Compressive Strength (UCS) tests, an optimal curing humidity of 40 % was identified for all the testing temperatures. It was also found the UCS at 10 % RH is always slightly higher than that at 70 % RH, and the UCS increases linearly with the curing temperature under the same RH. The mechanisms of the findings were further studied by X-ray Computed Tomography (CT) scan tests and Scanning Electron Microscopy (SEM) tests. The strengthening of xanthan gum-treated sand was attributed to an internal hydrogel effect and an external hydrogel-particle network effect. In particular, the hydrogel breakage at low RH and the small hydrogel volume at high RH contribute negatively to the network effect and thus relatively small UCS at low/high RH. Meanwhile, elevating the curing temperature could trigger the conformation transformation of xanthan gum hydrogels and thicken the shells of hydrogels, both of which contribute positively to the hydrogel and network effects and thus provide a higher UCS.