SOIL IMPROVEMENT USING CALCIUM PHOSPHATE COMPOUNDS AS A NOVEL SUSTAINABLE METHOD: A REVIEW
Keywords:Soil improvement, Calcium Phosphate Compounds (CPCs), Morphology, pH dependency, Ground improvement
Many new soil reinforcement techniques have recently emerged, the most popular of which are microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP). They are environmentally friendly and more sustainable than conventional methods for soil stabilization, however during carbonate (e.g., calcite) precipitation, ammonia (NH3) and ammonium (NH4+) emissions are released into air and groundwater, which are hazardous. There are techniques for removing NH4+ from the soil, however, the ammonia problem remains to be addressed. By using calcium phosphate compounds (CPCs), ammonium emissions can be eliminated by more than 90%. The precipitation of calcium phosphate occurs when the calcium and phosphorus sources interact at increasing pH of the medium. Deposition type of CPCs depends on pH of environment and Ca/P ratio of solution. The most common precipitation methods are: 1) mixing calcium and phosphorus sources directly; and 2) mixing urea and acid urease or acidic bacteria with Ca and P sources. Deposition takes place in between sand particles enhancing their contact and, therefore, strengthens the soil. Given the relatively low popularity and lack of research on CPCs for soil improvement, this review discusses soil improvement methods using CPCs, their prospects, and their limitations. In addition, it will also show differences in products when using different methods of obtaining CPCs and merits of using CPCs.