Cemented paste backfill modification using different types of binders and insight into its mechanical, micro-structure, and flow properties

Abstract

Cemented paste backfill (CPB) is widely used in backfilling underground mine voids. It consists of fine grained mine tailings with a small dosage of binder (~ 3–10%). Cement is the common binder, which is currently being used in most of the mines in North Queensland, Australia. In an effort to control backfilling costs, investigations are being currently carried out for the use of some alternative binders, that is, pozzolanic binder such as blended cement (BC) containing slag or fly ash in CPB. A thorough study to quantify the effects of additives, such as blast furnace slag and fly ash with ordinary Portland cement, in certain percentages on CPB could save large sums of money for Australian mining companies, if the required cement percentage is reduced in binder for CPB. In an attempt to dispose more tailings underground, the mines prefer increasing the solid content to the maximum possible limit, as dense CPB needs less binder to achieve a target strength. On the other hand, a high solid content slurry has both increased yield stress and viscosity, which increase the likelihood of blocking the reticulation pipeline. In this research, the possibilities of partial replacement of cement binder in CPB were studied through detailed laboratory investigations on influence of binder types as well as dosages in both mechanical and flow properties. Also, five different curing methods for CPB samples were studied to propose proper method. UCS test on 25 identical samples from three different CPB mixes were conducted for the detailed statistical analysis to assess the variability of mechanical properties of CPB samples and to validate the proposed curing method. Furthermore, the possibility of using a relatively smaller cylinder (110 mm height and 110 mm diameter) in slump test is assessed via extensive slump test along with standard slump cone on different CPB mixes. Moreover, series of scanning electron microscopy (SEM) analyses were performed on cracked UCS sample to investigate the relationship between microstructure and long-term mechanical properties. Finally, possibility of using UCS and indirect tensile (IDT) strength to determine the shear strength parameters (c and φ) for CPB was assessed with proper validation based on both experimental test and numerical simulation. For all these studies and investigations conducted in this research, various laboratory experiments, such as uniaxial compressive strength (UCS) test, indirect tensile (IDT) strength test, slump test, yield stress test, triaxial test and SEM analysis, with analytical correlation techniques and numerical simulation were employed. Slag blended cement (60% Slag + 40% Portland cement) can be used as replacement of cement binder in preparation of CPB with less dosage to achieve similar strength and stiffness as like cement binder, even in short curing time. This significant reduction in Portland cement usage not only leads to the significant cost saving, but also may slightly contribute for a sustainable development. In addition, through the extensive laboratory experiments on rheological properties, it is found that the there is hardly any difference between the cement and slag blended cement binders on flow properties namely the yield stress and slump, while the binder dosage has an effect. The smaller cylindrical slump device appears to have good potential for slurries like mine tailings or dredged mud that have high water content for slump test as there was strong correlation between the two different slump test devices used. Also, It is found that there is strong inter-relationship among solid content, slump, yield stress, and bulk density of CPB. In addition, a simple and proper curing method for CPB samples for the laboratory tests was recommended. In addition, the mechanical properties of CPBs cured in this proposed method show less variability for UCS and Young's modulus with relatively small coefficient of variation of less than 10% and 25%, respectively and hence these small variabilities of mechanical properties demonstrates the consistency of sample casting, curing, and testing methods. Furthermore, it is found that there is a positive relationship between microstructure and mechanical properties of CPB. Finally, the estimated shear strength parameters of CPB using the UCS and IDT strengths show good agreement with experimental and numerical simulation. Throughout the thesis, sets of empirical models and correlations were proposed to predict the long term UCS, stiffness, yield stress, slump height, and cohesion. In summary, the great potential of partial replacement of existing cement binder using slag blended cement in preparation of CPB is identified through various laboratory tests. In addition to the mechanical properties, the microstructure and flow properties of modified CPB were extensively investigated. This research will not only help to reduce the cement consumption in CPB production but also provides an alternative way of recycling industrial waste, such as slag and fly ash in vast volumes.