Elaboration of compressed earth blocks based on phosphogypsum and phosphate mining by-products

Abstract

The present paper aims to propose new recycling solutions to maximize the use of phosphogypsum (PG) in construction applications. The performance of several formulations for the manufacture of cement-stabilized compressed earth bricks (CEBs) made of raw PG mixed with phosphate mining waste rocks (Calcareous material (CM) and red clay (RC)) was investigated. The main roles of CM and RC are respectively to neutralize the acidity generated by the PG and improve the consistency of CEBs. This research work started at a laboratory scale to optimize and find the best formulation that will satisfy national and international standards. The optimized mixtures were scaled up through pilot experiments using two brick shapes (solid and hollowed). The impact of the brick shape elaborated in the pilot scale was also examined. The mineralogical, environmental, and geotechnical characteristics of the raw materials were also studied. The toxicity characteristic leaching test (TCLP) was performed on the raw PG and also on the ground bricks to investigate the risk of contamination. The most optimized laboratory mixture involved 40% PG and only 8% of ordinary Portland cement. This mixture allowed to reach an unconfined compressive strength after 28 days around 8.1 and 7.7 MPa at laboratory and pilot scale respectively. The thermal conductivity of the developed CEBs was interesting; between 0.484 and 0.506 W m−1 K−1. The values of bulk density and water absorption coefficient by capillarity are in accordance with the requirements indicated by the standards. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses highlighted the formation of tobermorite (5CaO·6SiO2,2H2O) and ettringite (Ca6Al2(SO4)3(OH)12,26H2O). The TCLP test of PG used and optimized bricks in this study showed no contaminant release. The obtained results are very promising; they showed that the elaborated CEBs can be adapted to compressed earth blocks applications.