## 5 Conclusions  
In this study, the effects of four different activator types (namely Q, DA, GM, and SH) on the compressive strength behavior were analyzed emphatically. Various UCS tests were carried out on laboratory-prepared CTB samples containing different activators. A quadratic polynomial stepwise regression equation was established to determine the optimal proportion of the activator dosage. Then, the Matlab software was used to predict the UCS value using a combination of range analysis and polynomial stepwise regression. The following conclusions can be drawn:  
1. The compressive strength of 14-day and 28-day cured CTB samples stimulated by DA reached 1.259 and 2.429 MPa, which were 2.03 and 2.23 times that of the OPC 32.5 R cement sample, respectively. Thus, the samples met the requirements for safe and high-efficiency mining.
2. SH is the most significant activator that affects the strength gain of CTB samples. The order of the influence of different activator types for both 14-day and 28-day CTB samples was as follows: SH > GM > DA > Q and SH > GM > Q > DA, respectively. When the SH content gradually increased to 1%, the strength gain of 14-day and 28-day cured CTB samples showed a gradually increasing trend. DA affected the early strength of CTB samples, but it had no obvious effect on long-term strength gains.
3. The growth law of CTB's strength gain at different activator dosages followed the function \(y=ax^{3}-bx^{2}+cx-d\). The minimum strength growth factor was 1.45, and the maximum reached 2.03. Thus, the DA formula was significantly better than the cement activator.
4. The constructed neural network model can grasp the nonlinear mapping relationship between CTB strength and different activator dosages, and it clearly demonstrated a certain reliability. The optimal decision-making results for the DA formula were as follows: the number of the hidden layer nodes of 5, Q 4.5% or 5%, DA 19%, GM 2.5%, and SH 0.7%.  
The DA formula via the orthogonal design experimental scheme could meet the requirements for the 28-day cured CTB strength, hence protecting the surface morphology, solving the technical problems of riverbed and aquifer mining, and improving the mining recovery rate. However, the following deficiencies were also noted. The whole tailings containing a high fine content and poor size distribution led to difficulties in the experimental study. Consequently, this paper was limited to the backfill strength with a cement-to-sand ratio of 1:8 and a slurry density of 73%. Meanwhile, further analyses of other activator materials available are necessary to maximize the use of solid wastes and adhere to solid waste recycling and green mining roads.  
[PERSON] and [PERSON] analyzed the experimental data and initiated the writing of the paper; [PERSON] and [PERSON] modified the manuscript and corrected the English writing.  
This research was generously funded by China Postdoctoral Science Foundation (grant number: 2018M631341), the Open fund of the Key Laboratory of Ministry of Education for Efficient Mining and Safety of Metal Mines (grant number: USTBMSLAB201804) and the Fundamental Research Funds for the Central Universities (grant number: FRF-TP-17-075A1).  
**Acknowledgments:** The authors would like to sincerely appreciate the detailed and constructive reviews and suggestions from two anonymous reviewers, which greatly improved this paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.