Controlled low-strength materials containing waste precipitates from mineral processing

The aim of this study was to investigate the novel utilisation of waste precipitates derived from the neutralisation of effluents from mineral processing in controlled low-strength materials (CLSM); maximising the amount of waste used while satisfying performance requirements for classification as CLSM. Mechanical, physical, micro-structural and leaching properties of laboratory-scaled CLSM specimens were evaluated. Waste precipitates chosen were an ochreous minewater sludge and a jarosite residue from zinc extraction; the former containing lower amounts of hazardous components than the latter. The results showed that formulations with 10 wt% of the wastes examined could be beneficially re-formed as CLSM with unconfined compressive strength not greater than 2 MPa at 28 days, the upper limit for excavatability. The SEM investigation showed a porous microstructure with visible cement and pozzolanic hydration products. Owing to high water contents required for flowability, porosity (34-45%) and hydraulic conductivity (10−6-10−7 m/s) were correspondingly high at 28 days, but similar to conventional CLSM in other studies. Diffusion-leaching tests revealed that heavy metals least immobilised were Ba, Cr, Pb and Zn. Pb and Zn were significantly leached from CLSM with jarosite residue, suggesting that effective lining or encapsulation will be required in field applications. Leaching of heavy metals from CLSM with the ochreous waste was at reasonable levels, suggesting this material may be suitable for sub-surface applications.