Application of Iron Tailings-Based Composite Supplementary Cementitious Materials (SCMs) in Green Concrete

Abstract

How to treat the iron tailings of mining solid waste with high value is an urgent problem on a global scale. In recent years, the application of iron tailings in the building materials industry has attracted the attention of many scholars. The conversion of iron tailings into green building materials helps achieve carbon neutrality and high-value utilization of solid waste, and promotes sustainable development. Although iron tailings have been extensively studied as supplementary cementitious materials, the performance of concrete is not ideal due to its low activity. In this study, the hybrid supplementary cementitious materials system was prepared by iron tailings, phosphorus slag, and steel slag, and the effects of supplementary cementitious materials type, iron tailings content, iron tailings grinding time, and supplementary cementitious materials content on concrete performance were studied. The compressive properties, iron tailings properties, pore structure, interfacial transition zone, and element distribution of hydration products of concrete were tested by compressive strength tests, X-ray Diffractometer (XRD), X-ray Photoelectron Spectroscopy (XPS), Mercury Intrusion Porosimetry (MIP), Backscattering Electron Tests (BSE), and Energy Dispersive Spectrometer (EDS). The results show that further grinding improves the iron tailings activity. There is a synergistic mechanism between steel slag and phosphorus slag in the composite supplementary cementitious materials, which overcomes the low activity defect of iron tailings and produces concrete with a compressive strength exceeding 40 MPa. The composite supplementary cementitious materials can optimize the interfacial transition zone of the concrete interface and reduce the calcium–silicon ratio of the hydration products. However, it will deteriorate the pore structure of the concrete matrix, cause part of the concrete matrix to be damaged and lead to a loss of compressive strength, and the loss is acceptable. This work broadens the methods of comprehensive utilization of iron tailings and also provides a reference for a more detailed understanding of the properties of iron tailings-based concrete.

Mechanical Properties and Hydration Mechanism of Iron Tailings–Cement-Based Supplementary Cementitious Materials

“…The calcium hydroxide content was higher in IOT pastes compared to cement. On the one hand, this indicates that the SCM including IOTs promoted the hydration of cement; on the other hand, the hydration of the calcium-silica phases in SS and GBFS was also a source of calcium hydroxide [21].…”

“…products [21,22,58]. The synergistic effect between IOTs and SS significantly improved the 28 d compressive strength of IOT mortars.…”

“…An increase in the strength of SS and the IOT cement system was similarly observed. It was concluded that IOTs facilitated the reaction of the silicate in SS, while the dissolved components from IOTs continued to react with the calcium hydroxide derived from the hydration of SS to form more hydration products [21,22,58]. The synergistic effect between IOTs and SS significantly improved the 28 d compressive strength of IOT mortars.…”

“…This shows that most of the FA in sample D12 ruptured and decomposed in the system in which it was exposed, releasing internally reactive silica and alumina, which then participated in the hydration reaction to produce C-(A)-S-H gels and Ca(OH) 2 . Reducing the SS:SL ratio while keeping the IOTs unchanged can increase the alkalinity of the hydration system, which in turn stimulates the participation of C 2 S and C 3 S in the steel slag in the hydration process to produce C-S-H gel and Ca(OH) 2 [21,22]. This is consistent with the fact that the percentage weight loss of ettringite, C-(A)-S-H gels, and Ca(OH) 2 is greater than that of D16 in the TG analyses.…”

“…In order to solve the above problems, Zhang et al proposed the idea of using IOTs to prepare composite SCMs, which consist of IOTs and other solid waste materials. It is worth mentioning that the composite SCM also provides a recycling channel for other solid wastes [21,22].…”