This experimental study investigates the strength characteristics of cemented super-fine unclassified tailings backfill (CSUTB) and its failure mechanism. Physical and chemical properties of tailings from the Zhongguan Iron Mine (ZGIM) were tested. A series of uniaxial compressive strength (UCS) tests was conducted to determine the relationships between UCS of CSUTB and three factors of cement-tailings ratio (CTR), solid content and curing time. Gray relational analysis (GRA) method was then used to study the sensitivity of UCS to these three factors. Results indicate that ZGIM unclassified tailings is a kind of alkaline super-fine tailings with high activity and quality. UCS of CSUTB increases exponentially with the increase of CTR and solid content, and increases linearly with curing time. The curing time is the most important factor for the UCS of CSUTB, followed by CTR and solid content. The stress-strain curves obtained from UCS tests show the failure process of CSUTB, including four stages of initial deformation, linear elastic, yield deformation and complete failure. It is the result of damage evolution.

This paper investigated the effect of blast furnace slags (BFS) characteristics on the properties achievement after being alkali activated. The physical and chemical characteristics of BFS were determined by X-ray fluorescence (XRF), X-ray Diffraction (XRD) and laser granulometry. Multi-technical characterizations using calorimetry, XRD, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetry (TG-DTG), scanning electron microscope (SEM), nitrogen sorption and uniaxial compressive strength (UCS) were applied to give an in-depth understanding of the relationship between the reaction products, microstructure and BFS characteristics. The test results show that the microstructure and mechanical properties of alkali activated blast furnace slags (BFS) highly depend on the characteristics of BFS. Although the higher content of basic oxide could accelerate the hydration process and result in higher mechanical properties, a poor thermal stabilization was observed. On the other hand, with a higher content of Fe, the hydration process in alkali activated BFS2 lasts for a longer time, contributing to a delayed compressive strength achievement.

Blast furnace slag was used as the main raw material to prepare the alkali activated slag (AAS), a low-carbon binder, for cemented paste backfill (CPB). The optimum parameters for preparing the AAS binders using an orthogonal experiment were obtained. Under the optimum conditions (NaOH content was 3 wt. %, Ordinary Portland cement (OPC) content was 7 wt. %, and gypsum dosage was 4 wt. %), the 28 days compressive strength of the binder was 29.55 MPa. The flow ability of the fresh CPB slurry decreased with solid content due to the increased yield stress, while the flow ability increased when rising the binder dosage. A predictive model for the compressive strength of CPB samples was reached through multivariate analysis and the R² values were higher than 0.9. Sensitivity analysis showed that the solid content is the most important parameter which influences on the development of the CPB strength with a correlation coefficient of 0.826. From the Toxicity Characteristic Leaching Procedure (TCLP) tests, the leaching concentrations of Pb and Cd were below the threshold. As a result, the AAS has potential application as an alternative binder and cemented paste backfill.

This work is a systematic study of the strength and ultrasonic properties of cemented paste backfill incorporating a foaming agent, known as foam-cemented paste backfill (FCPB). Based on determining the optimal admixture contents (foaming stabilizer, thickening agent, and foaming agent), a series of uniaxial compressive strength (UCS) tests were conducted to determine the relationship between the UCS of FCPB and four influencing factors, i.e., cement–tailings ratio (CTR), solid content (SC), curing time (T), and foaming agent content (FC). To analyze the sensitivity of UCS to these four factors, grey relational analysis (GRA) was introduced. Moreover, UCS results were correlated with the corresponding ultrasonic pulse velocity (UPV) parameters. The results indicate that the optimal contents of foaming stabilizer, thickening agent and foaming agent are 0.5%, 0.6%, and 1–3%, respectively. The UCS of FCPB exponentially increases with CTR and SC, while it logarithmically and linearly increases with T and FC, respectively. CTR has the most significant influence, followed by T, SC, and FC. There exists an evidently linear relationship between UPV and UCS of FCPB regardless of CTR, SC, T and FC. These results contribute to understanding the properties of hardened FCPB and to sound designs in practice.

In order to quantitatively study the influence of tailings fine content on the properties of cemented paste backfill (CPB) and further understand the mechanism of tailings fine content acting, the concept of packing density was introduced in this study. The packing density of each tailings sample was measured by the wet packing method after the samples with various fine contents were prepared. Moreover, CPBs with different tailings fine contents were tested by the mini slump test, rheological test, uniaxial compressive strength (UCS) test, and mercury intrusion porosimetry test. The results demonstrated that the flow spread and UCS both increase first and then decrease with the increase of tailings fine content, while the yield stress shows an opposite trend. The fine content of tailings affects the flowability of fresh CPB mainly through the packing density. When the fine content is high, the influence of the specific surface area of tailings cannot be ignored. The packing density is an important factor affecting the strength of CPB, and there is an obvious linear relationship between the packing density and UCS. The pore structure of CPB samples with different tailing fine contents is significantly different, and the macroscopic packing density changes the strength of CPB by affecting the microscopic pores.