## 1 Introduction
In this study, high calcium desulfurization ash (HCDA) is used as the calcium raw material in the cementitious material and contains unreacted calcium hydroxide, which can be partially used as an alkali activator to composite stimulate the potential activity of the substrate. Therefore, the utilization of HCDA with granulated blast furnace slag (GBFS) to develop desulfurization ash-slag-based paste backfill (DSPB) materials can effectively reduce costs and lead to the resource utilization of solid waste. Fresh paste backfill (PB) is usually prepared in a backfill plant at the surface and then transported by gravity or pumping through pipelines into underground excavated goat areas [36; 37]. The strength of backfill material is an important indicator of its ability to effectively support the structure of the goat area, while the flow characteristics are a key factor in determining whether the backfill material can be successfully pumped to the working face through pipeline transportation and be constructed properly [38]. The flowability of PB is mainly related to its yield stress and viscosity (i.e., rheological properties) [39]. As a non-Newtonian fluid, the complex rheological properties of freshly mixed DSPB slurry directly affect its transportation in the pipeline. Therefore, it is extremely important to study the rheological properties of DSPB, which is of great significance for the safe and effective implementation of backfill operations [40]. In response to the above problems, many scholars have made remarkable progress in the study of rheological properties of PB materials. [PERSON] et al. [41] used a slump test to determine the rheological properties of cement paste backfill (CPB) and to study the effect of their fine particle content on the rheological and mechanical properties of the paste. [PERSON] et al. [42] investigated the effect of superplasticizer on rheological properties of superfine-taining CPB by shear rheology tests. The above studies have shown that slump and rheological tests are important methods for studying the rheological properties of slurries. Common methods for controlling the rheological properties of slurries include adjusting the water-solid ratio, the proportion of material components, and the external environment (e.g., temperature, humidity, and pH, etc. ), and adding admixtures [43]. DSPB, as a high-solid-content-dispersed solid-liquid flow system, is susceptible to particle precipitation, and the bias seepage phenomenon when the content of water is too high, on the contrary, will lead to a lack of fluidity. Therefore, in order to achieve better backfill results, the most effective means of improving the rheological properties was tested by rheological characterization methods. While existing studies have used solid wastes such as desulfurization ash as mine backfill cementitious materials, their application in paste backfill is still limited due to insufficient understanding of the effect of multi-mixture coupling on rheological properties and their changes. Current backfill rheology studies mainly focus on the rheological model and parameters of slurry fitting and macroscopic fluidity, and lack an analysis of the mechanism of system rheological property-microstructure changes. Therefore, unlike traditional methods for rheological characterization of backfill pastes, this study investigated the effects of HCDA, polycarboxylate superplasticizer (PCE), and sodium silicate (SS) content on the rheological properties of DSPB. The flow curves were fitted by a shear rheology test, and its characteristics were analyzed by rheological parameters. In addition, the workability of DSPB under different influencing factors was analyzed by measuring the setting time and conducting mini-slump tests. The effect of free water transition within the slurry system on the rheological properties of DSPB was investigated by low-field nuclear magnetic resonance (LF-NMR). Finally, the effect mechanisms of the differences were revealed by microanalysis.