Synergistic effect of zeolite and biochar on geotechnical and fertility properties of vegetation concrete prepared by sandy soil

The mechanical strength and fertility retention capacity of vegetation concrete (VC) are insufficient when prepared using sandy soil. Biochar (BC) has been used to improve the fertility retention capacity of VC; however, it negatively affects mechanical strength and is prone to aging. In this study, a novel method using the synergistic effects of zeolite (ZL) and BC was proposed. Four different ZL contents (0%, 1%, 2%, and 4% of the planting soil weight) were investigated to determine the influence of ZL on the physical, mechanical, chemical, and biological properties of sandy VC. The results reveal that the ZL not only alleviated the negative effect of the BC but also enhanced its positive effects. In particular, by adding ZL, the porosity, permeability, and nutrient (ammonium nitrogen and nitrate nitrogen) leaching loss rates of the sandy VC decreased, whereas the cohesion, internal friction angle, matric suction, dissolved organic carbon content, and microbial biomass increased. In addition, the secondary hydration reaction between the ZL and calcium hydroxide enhanced the mechanical properties of the VC, and the adsorption of aged broken small BC particles on the ZL prolonged the fertility retention of the BC. The secondary hydration reaction of ZL occurs first, and then the remaining ZL absorbs the small BC particles. Considering the effects and cost of ZL, the recommended amount of ZL to be mixed with sandy VC is 2–4 wt%.