Ammonia-soda residue and metallurgical slags from iron and steel industries as cementitious materials for clinker-free concretes

Abstract

Ammonia-soda residue (ASR) is a solid waste released during the ammonia-soda process, which lacks proper disposal. In this paper, ASR was combined with various metallurgical slags from the iron and steel industries to prepare a clinker-free concrete. As a new building material without cement, it can replace traditional concretes, thereby reducing CO2 emissions during the clinker production. Specifically, ASR, ground granulated blast-furnace slag (GGBS), steel slag (SS), and flue gas desulfurization gypsum (FGD) were used as cementitious materials, whereas waste iron ore tailings served as aggregates. According to the workability test results, ASR could reduce the fluidity of concrete and shorten the initial and final setting times. Furthermore, the compressive strength of a clinker-free concrete with cementitious material composed of ASR 6%, GGBS 62%, SS 20%, and FGD 12% could reach 36.29 and 66.31 MPa after 3 and 360 days, increasing by 32% and 27% compared with a control specimen, respectively. The XRD and SEM results revealed that the hydration products of a hardened paste prepared using ASR-GGBS–SS–FGD cementitious materials were mainly C-(A)-S-H gel, ettringite (AFt), and Friedel's salt (FS). In addition, the information about hydration heat and porosity of the obtained materials evidenced the ASR-accelerated hydration reaction of cementitious materials, reducing the 20–50 nm pore volume of the hardened pastes, however, increasing the >50 nm pore volume. The free-Cl- concentrations in the concrete with 6% ASR in the cementitious material were 2.69 × 10−3 and 1.24 × 10−3 g/g after 3 and 360 days, respectively. The results show that ASR clinker-free concretes with ASR contents less than 6% are suitable for preparation of reinforced concretes, which is also confirmed by the reinforcement corrosion tests. Finally, the hydration mechanisms and characteristics of ASR-GGBS–SS–FGD-based cementitious materials were carefully assessed. Silicates and aluminates used for obtaining hydration products were derived from GGBS under the condition that SS provided a sufficient alkaline environment for the hydration reaction, while chloride and carbonate in the ASR accelerated the hydration process. In this respect, the present work ensures a solid experimental basis for the synergistic utilization of multi-solid wastes.