Usability of limestone and mineral additives as partial replacement for tailings and binder in cemented paste backfill

Billions of tons of tailings are produced every year in the mining sector, which has an extremely important place for national economies. As well as extracting valuable minerals, these huge quantities of tailings, which could potentially contribute to the economy, need to be well managed. Currently, the most common method used to dispose of tailings in an efficient and sustainable manner is Cemented paste backfill (CPB). However, the problems associated with tailings disposal and the economic and ecological concerns associated with cement production have made it imperative to find cost-effective and eco-friendly alternatives to the materials used in CPB. Within the scope of the thesis, the potential for the use of different mineral admixtures has been investigated in order to reduce the gradation-induced performance problems of CCCs, to reduce cement costs and to limit the amount of use considering the potential use of tailings for different purposes. For this reason, crushed calcareous sand (CS) as a waste substitute and blast furnace slag or fly ash (YFC or UK) as a cement substitute were planned to improve the short/long term performance of the CPBs and reduce cement related costs. Mechanical (UCS, stress-strain and modulus of elasticity), ultrasonic (UPV, ultrasonic impact velocity) and microstructural (MIP, XRD, TG/DTG and SEM) characterization tests were experimentally investigated to evaluate the quality and performance of the CPBs. The samples were prepared at short/long curing times (3-180 days), with constant solid (72 wt.%) and cement (5 wt.%) contents. Five different ratios of sand content (tailings/sand: 100/0, 90/10, 80/20, 70/30, 60/40 and 50/50) as process tailings (aggregate) substitute and five different ratios of mineral additive content (PC/YFC-UK: 100/0, 90/10, 80/20, 70/30, 60/40 and 50/50) as cement substitute were selected. Furthermore, the tests/analyses were also repeated by using different ratios (KK-YFC: 10% and 30% and KK/UK: 10% and 30%) of KK and YFC and UK in the CCC mixtures. The results showed that the use of KK as a tailings substitute improved the strength of the CPBs up to 2.74 MPa. This indicates the KK improves the skeletal structure of the CPBs, promoting the formation of a more compact structure and reducing the gradation problems caused by fine tailings. Also, due to KKs high CaO content (54 wt%), it contributes to the hydration reactions of the cement and prevents acid/leakage formation that may occur during long curing times. As a result of the tests, it was observed that 30% of sand based CPBs (KK30) achieved the best strength values. When the compressive strength results of the CPBs were examined, a gradual increase was observed until the 90-day curing period, while significant decreases occurred in the strength of all backfill samples beyond 90 days. But, especially in the long term, the strength losses of KK and YFC reinforced CPBs were considerably less compared to the other samples. This is interpreted as the high CaO content of YFC (YFC: 32 wt%), as well as KK, promotes hydration reactions and prevents acidification of the backfill in the long term. Considering the microstructural tests, it is seen that mineral admixtures (especially KK) used in optimum ratios improve the gradation of the CPBs and contribute important chemical/mineralogical components for cement hydration. Moreover, excellent consistent correlations were obtained between UCS and UPV results depending on the different types of mineral admixtures. When the relationship between UPV and UCS was evaluated, regression analysis was found to be successful in predicting the UCS results for all CPB samples. In conclusion, this thesis will demonstrate the contribution of materials to be used as alternatives to tailings and their relationship to CPB technology, enabling an operationally cost-effective, easy and feasible backfill design for underground metal mines.