Formulating and optimising the compressive strength of controlled low-strength materials containing mine tailings by mixture design and response surface methods

Controlled low-strength materials (CLSM), like other cement-based backfill materials, are typically formulated by trial-and-error methods to yield the desired product characteristics. This paper presents the use of mixture design and response surface methods as tools to optimise formulations of CLSM to achieve desirable mechanical integrity with a minimum amount of statistically-sound experiments; while minimising the amount of cement and maximising the amount of by-products used. Statistical combinations of three-component mixtures were formulated to investigate the unconfined compressive strength (UCS) of CLSM comprising: Portland cement, fly ash and mine flotation tailings from a Ni–Cu ore. The data is analysed using the response surface method (using a mixture design of a constrained triangular surface) and ANOVA. Optimum formulations are simulated using a desirability function set at lower (1.0 MPa), target (2.0 MPa) and upper (3.0 MPa) UCS values after 28 days curing. All mix combinations had a constant spread diameter of 229 ± 10 mm, the standard workability for conventional CLSM. Results are compared to conventional CLSM incorporating silica sand in the place of the tailings. A significant quantity of tailings (up to 80 wt% solids) and low quantity of cement (up to 5 wt% solids) produced CLSM with UCS within the 2 MPa target value of excavatability. UCS of CLSM is a function of the individual component proportions, and the mixture design approach can be an important tool to help develop and optimise formulations of cement-based materials consisting of several components.