*Article* **Characterization and Hydration Mechanism of Ammonia Soda Residue and Portland Cement Composite Cementitious Material**

**Dong Xu 1,2,3, Pingfeng Fu 3,4, Wen Ni 2,3,4,\*, Qunhui Wang 1,2 and Keqing Li 3,4**


**Abstract:** The use of ammonia soda residue (ASR) to prepare building materials is an effective way to dispose of ASR on a large scale, but this process suffers from a lack of data and theoretical basis. In this paper, a composite cementitious material was prepared using ASR and cement, and the hydration mechanism of cementitious materials with 5%, 10%, and 20% ASR was studied. The XRD and SEM results showed that the main hydration products of ASR-cement composite cementitious materials were an amorphous C-S-H gel, hexagonal plate-like Ca(OH)2 (CH), and regular hexagonal plate-like Friedel's salt (FS). The addition of ASR increased the heat of hydration of the cementitious material, which increased upon increasing the ASR content. The addition of ASR also reduced the cumulative pore volume of the hardened paste, which displayed the optimal pore structure when the ASR content was 5%. In addition, ASR shortened the setting time compared with the cement group, and the final setting times of the pastes with 5%, 10%, and 20% ASR were 30 min, 45 min, and 70 min shorter, respectively. When the ASR content did not exceed 10%, the 3-day compressive strength of the mortar was significantly improved, but the 28-day compressive strength was worse. Finally, the hydration mechanism and potential applications of the cementitious material are discussed. The results of this paper promote the use of ASR in building materials to reduce CO2 emissions in the cement industry.

**Keywords:** ammonia soda residue; hydration mechanism; cementitious material; resource utilization; CO2 emissions
