Biopolymer Stabilised Earthen Construction Materials

Abstract

Earthen structures (i.e. structural units manufactured from soil) are often regarded as sustainable forms of construction due to their characteristically low carbon footprint. Unstabilised earthen construction materials have low embodied energy, excellent hygroscopic properties and recycling potential. However, in this form, the material is susceptible to deterioration against water ingress and most modern earthen construction materials rely on cement to improve their durability properties. Using cement leads to compromises in hygroscopic properties and recyclability potential. In this situation, it is imperative to look for alternatives to cement, which can address these issues without compromising on the desired engineering properties of these materials. This thesis explores the use of biopolymers, namely guar and xanthan gum, as stabilisers for earthen construction materials. As an initial step, an experimental campaign was undertaken to understand biopolymer stabilisation and optimise their use to stabilise earthen construction materials. The results from this campaign reveal that biopolymer stabilised soils derive their strength through a combination of soil suction and hydrogel formation. The intrinsic chemical properties of the biopolymer affect the nature of hydrogel formation and in turn strength. In a subsequent campaign of experimental work, hydraulic and mechanical properties of these biopolymer stabilised soils were determined. The hydraulic properties of the biopolymer stabilised soils indicate that for the range of water contents, the suction values of biopolymer stabilised soils are higher than unamended soils. The soil water retention curves suggest that both biopolymers have increased the air entry value of the soil while affecting the void size distribution. Shear strength parameters of biopolymer stabilised soils were obtained through constant water triaxial tests, and it was noted that both biopolymers have a significant and yet different effect on soil cohesion and internal friction angle. With time, guar gum stabilised soils derive strength through the frictional component of the soil strength, while xanthan gum stabilised soil strength has a noticeable contribution from soil cohesion. Macrostructural analysis in the form of X-RCT scans indicate that both biopolymers form soil agglomerations and increase overall porosity. The void size distribution curves obtained from XRCT scanning complement the findings of the suction tests. As a final study, the performance of biopolymer stabilised earthen construction materials was assessed as a building material. Durability performance of these materials against water ingress was evaluated, and it was noted both biopolymers provide satisfactory stabilisation to improve the erosional resistance of the material. In conclusion, unlike cement, biopolymer stabilised earthen materials do not compromise on hygroscopic properties and have better mechanical performance than unamended earthen construction materials. Finally, recyclability tests suggest that apart from improving the strength, durability and hygroscopic properties of the material, biopolymer stabilised earthen construction materials have a better potential for recycling without any environmental concerns.