| China possesses rich mineral resources. Since 2009, more than one billion tons of mine tailings have been generated and accumulated annually due to the exploitation of mineral resources. Settling dams have traditionally been used for surface storage of tailings traditionally, but this method has failure potential. As the inert aggregate, the tailings are backfilled into underground pits is a promising disposal method. However, the relation between structure and properties of cemented tailings backfill (CTB) remains unclear; the effects of sulfate concentration on pore structure and unconfined compressive strength (UCS) are not well understood; the damage pattern of CTB under sulfate attack needs further investigation; the influence of tailings fineness on rheological property, flowability, development of microstructure, as well as mechanical performance need further study. In view of these situations above, the mechanical performance and damage pattern under sulfate attack have been studied; the effect of tailings fineness on rheological, UCS, and pore structure evolution have been investigated; the model of total porosity-UCS has been improved, and the model of tailings fineness-UCS has been established. Based on the results of lab-scale experiments, key parameters affecting the properties of CTB were analyzed from macro and micro aspects.Based on the experimental results, the main conclusions of this pater are listed as follows:(1) Based on the obtained results of lab-scale experiment, the damage pattern of consolidation mass under sulfate attack has been investigated; the relationship between compressive strength and porosity has been analyzedA binder (H) was prepared in which the ratio of 32.5 Portland (PC), ground slag, and freshly prepared alkaline activator was 67.5/30/2.5. Method of testing cements-determination of strength was applied to assess its binding performance. The compressive strength of mortars in which H is used is higher than those of 32.5 PC. It can be concluded that 32.5 PC can be replaced by H for cementing tailings in backfill. A particle size distribution test showed that the portion ranged from 3 to 64 um could reach 85.61%, which indicated that binder H was well graded; Ten kinds of sulfate concentrations (FeSO4·7H2O) in tap water were prepared to blend binder H and ground silica. Based on the testing results of compressive strength and dry density, it can be concluded that 5000 ppm is the optimal sulfate concentration for CTB; according to the stress-strain curve measured under the conditions of UCS, the constitutive model of backfill damage was constructed, the damage equation was derived and the parameters of this model were determined; the obtained results of damage experiments under sulfate attack demonstrates that the threshold damage value decreases with increasing of sulfate concentration; results of Mercury intrusion porosimetry (MIP) tests show that there is no strict negative correlation between total porosity and UCS. However, when the pore radii is higher than 10 urn, a significant negative correlation was found between UCS and porosity with high coefficient of determination.(2) The influence of tailings fineness on rheological properties of fresh CTB has been studied; the correlation equation between flowability and slump has been establishedTailings T and W were used to conduct sedimentation tests. The results showed that the pulp density of T reached 55.84%,88.59%, and 83.78%, whereas that of W reached 52.30%,79.80%, and 81.77% after 2 hours respectively when initial pulp densities were 45%,65%, and 75%. The sedimentation velocity and final pulp density of finer tailings are both lower than those of coarse tailings. High pulp density of fresh cemented paste backfill belongs to Bingham fluid based on the rheological experiments; then, the parameters such as yield stress and viscosity were fitted by the Herschel-Bulkley model, which revealed the fact that yield stress increased with the increase of tailings fineness; a linear equation between flowability and slump was constructed and confirmed the minimum value of flowability for cemented paste backfill was 18.87 cm. Besides, the effects of pulp density, cement/tailings ratio and tailings fineness on flowability were investigated. It turns out that flowability values decrease with the enhancement of pulp density and fines proportion, meanwhile, the increasing of binder content can slightly decrease flowability values.(3) The strength-total porosity model has been improved; the tailings fineness-strength model has been establishedIn order to study the effects of binder content, pulp density, and gradation on mechanical strength, binder H was used to bind T and W in different ratios. With the increasing of pulp density and binder content, UCS can be promoted. Besides, this growding trend in UCS could also be found with increasing in the coefficient of uniformity (Cu) and decreasing coefficient of curvature (Cc). An increase in tailings fineness decreased the dry density while increased the moisture content, compacted packing density and UCS. The UCS reached the highest value when the tailings fineness was 27.6%; the effects of tailings fineness on pore structure were studied in depth. The total water porosity (φw) determined by oven drying was always slightly higher than the mercury porosity (φHg). Total porosity and the critical pore diameter (dor) decrease with increasing of curing age. An increase in tailings fineness will lead to a higher φHg and lower der. With an increase in tailings fineness, the porosity at radii of>10μm decreases while the porosity at radii of<10μm increases at all curing ages. In this paper, the tailings fineness-UCS model and total porosity-UCS model are constructed and evaluated separately.(4) The differences between sulfate and sulfate-free cemented paste backfill have been investigated from micro perspective; the relationship between UCS and porosity has been further studiedUCS of CTB with sulfate content at 5000ppm exceeds CTB without sulfate. Results of XRD tests indicate that the hydration products do not differ significantly whether adding the sulfate on CTB or not; however, parts of calcium hydroxide (CH) have converted to gypsum with the presence of sulfate according to the TG tests; results of MIP tests showed that sulfate CTB at radii of>10μm exhibited finer porosity than that of sulfate-free CTB, while the situation reversed at radii of<10μm.(5) Two types of weight matrix are constructed and applied; Analytical Hierarchy Process (AHP) was used for determining the optimal mix proportion schemeAHP was adopted for determining the optimal mix proportion scheme of CTB. Two kinds of weight matrix were applied, due to the difference between self-support and roof-support for CTB. Finally, two types of best mix proportion schemes were obtained after evaluation. |
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