Investigation into the use of cement stabilised gypsum waste as a backfill material

Conventional backfill for utility trenches comprises of granular crashed rock (type I subbase). which is made of high quality materials. The increasing cut of this material and need to reduce gypsum waste led to the investigation which was aimed at principally exploring the use of gypsum (both red gypsum and waste plaster-board for utility trench backfill. Backfills developed were to be cementitious and self levelling. Unconfined compressive strength (UCS), indirect tensile strength (ITS) and flowability test of different materials mixed with gypsum were conducted in the laboratory. UCS of ordinary Portland cement and red gypsum at a ratio of 1 and 5 was more than 2 MPa (28-day), which is the specified strength for cementitious backfill materials. To increase the flowability of the mix, a strength accelerator sodium Meta silicate (Na₂SiO_3,5H₂O) was added with more water in the mix, 1.5% Na₂SiO₃, 5H₂O proved to be adequate for obtaining an acceptable flowable backfill. Quarry fines in cement-gypsum mix also showed encouraging results. Cement, red gypsum and quarry fines mixed at a ratio of 1,4 and 15 also complied with the Specifications. The same mix with plaster-hoard gypsum in place of red gypsum and with slightly higher cement (1.3:4:15) also complied with the Specifications. The above materials could be ideal replacements of granular backfill materials.