Improvement in the Unconfined Compressive Strength of Sand Test Pieces Cemented with Calcium Phosphate Compound

As a novel chemical grout material, calcium phosphate compounds (CPC-Chem) have unique physical and chemical properties, such as adequate physical strength, self-setting property, pH dependence of precipitation, non-toxicity and recyclability. Novel grout using CPCs has been used for countermeasures for liquefaction in geotechnical engineering applications and it is an economical and environment-friendly technique that develops to form calcium carbonate precipitates in the soil, leading to an increase in soil strength. In this paper, we added calcium carbonate (CC) powders from commercial CC and scallop shells (SS) to a novel grout made from CPC-Chem to increase the ground strength improvement afforded by CPC-Chem. For the CPC-Chem, calcium acetate (CA) was used as a calcium solution and diammonium phosphate (DAP) and dipotassium phosphate (DPP) were used as phosphate solutions. Toyoura sand test pieces were cemented with CPC-Chem by addition of CC and SS powders and cured until 56 days of curing period. These specimens were analyzed with unconfined compressive strength (UCS) tests. Moreover, a series of laboratory experiments were conducted, including pH measurements, scanning electron microscope (SEM) observation in order to observe the microscopic structure, and wet density before and after curing etc. The results indicated that the UCS of the sand test pieces cemented by CPC with CC and SS powders was larger than that of the test pieces without added powders. In addition, the UCS value of sand test pieces cemented with CA: DPP only was drastically increased with time. Particularly, the UCS value of sand test pieces cemented with CA: DAP by addition of SS and CC powders achieved our target value of 100 kPa. Moreover, the results indicated that the density and the pH of the sand test pieces cemented by CPC with SS and CC powder were larger than that of the test pieces without added powders.