## 1 Introduction  
China is the world's largest steel-producing country. According to official statistics/reports [1], China's crude steel production reached 10.65 billion tons in 2020. The steel slag (SS) is a type of solid waste residue discharged during the steel smelting process. For every 1 ton of steel produced, 0.15-0.20 tons of SS will be generated. Currently, the utilization rate of SS in China is less than 30% [2]. A large amount of SS stockpiles occupy several land resources and lead to severe environmental pollution. Therefore, it is crucial to maximize the comprehensive utilization of SS.  
Numerous studies [2; 3; 4] have revealed that calcium silicate in SS enhances its potential cementing activity. Similarly, the application of SS in the field of cementitious materials is the most effective approach for achieving large-scale utilization. The main challenges [5; 6] are the coarseness of SS particles, their poor grindability, and the presence of inert components (such as RO equivalent) leading to a much lower hydration activity than cement and a negative impact on the properties of cementitious composites. In contrast, red mud (RM) and electrolytic manganese residue (EMR) exhibit better grindability and relatively small particle sizes. The cementitious materials prepared by composite with SS can increase mortar's density, whereas the soluble alkali in RM and sulfate substances rich in EMR can be used to excite the activity of SS. A few scholars have discovered [5,7,8] that the activity of both composite excitations of SS is better than that of individual excitation. This method not only maximizes the applications of multiple solid waste resources to prepare cementitious materials but also consumes less energy, is less costly, more feasible, and better coordinated with cement. [PERSON] [9] et al. found that the hydration products of RM-SS-EMR composite cementitious materials were stable and delayed the appearance of exothermic peaks while significantly reducing the total amount of exotherm. Meanwhile, [PERSON] [10], [PERSON] [11], and other researchers revealed that the composite use of alkali excitation and sulfate excitation contributes to more stable hydration products of SS-based cementitious materials, which are primarily hydrated calcium silicate (C-S-H gel) with low alkalinity along with less calcium hydroxide (Ca(OH)\({}_{2}\)) and calcium alumina (AFt) contents. These gels also contain incompletely reacted SO\({}_{4}\)\({}^{-}\) inside, which can resist the erosion of foreign ions. However, there are few studies on preparing composite mortar using SS, RM, and EMR composite. The synergistic gelling mechanism of their multicomponent solid waste is not clear yet. Hence, there is still much room for research.  
The effects of synergistic interactions among SS, RM, and EMR multisolid waste on the workability, mechanical properties, and hydration characteristics of mortar specimens were investigated in this paper. Meanwhile, different analytical test methods, such as the heat of hydration assessment, X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), and mercury-in-pressure (MIP) testing were used to characterize the exothermic heat of hydration, hydration products, and microstructure of SS-RM-EMR-based composite mortar. These methods were applied to better understand the synergistic cementation among SS, RM, and EMR multiple solid wastes and prepare excellent-performance SS-RM-EMR-based composite cementitious. The theoretical basis for preparing superior SS-RM-EMR-based composite cementing materials is presented, further enriching the theoretical knowledge of the synergistic cementing mechanism of multicomponent solid waste. Meanwhile, it can effectively reduce the environmental pollution of SS, RM, and EMR, thus achieving significant economic and environmental benefits.