Hydration mechanism and microstructure characteristics of modified magnesium slag alkali-activated coal-fired slag based cementitious materials

Abstract

As the second most important solid waste produced by coal-fired power plants, the improper management of coal-fired slag has the potential to result in environmental pollution. It is therefore imperative that high-value utilization pathways for coal-fired slag should be developed. In this study, modified magnesium slag (MMS), produced by a magnesium smelter, was selected as the alkali activator. The activated silica-aluminum solid wastes, namely coal-fired slag (CFS) and mineral powder (MP), were employed as pozzolanic materials in the preparation of alkali-activated cementitious materials. The alkali-activated cementitious materials prepared with 50 wt% MMS, 40 wt% CFS and 10 wt% MP exhibited favorable mechanical properties, with a compressive strength of 32.804 MPa in the paste sample cured for 28 d. Then, the activated silica-aluminum solid waste consisting of CFS-MP generated a significant amount of C-S(A)-H gels, AFt, and other products, which were observed to occupy the pore structure of the specimen. In addition, the secondary hydration reaction of CFS-MP occurs in high alkalinity environments, resulting in the formation of a mutually stimulated and promoted reaction system between CFS-MP and MMS, this will subsequently accelerate the hydrolysis reaction of MMS. It is important to emphasize that the amount of MMS in alkali-activated cementitious materials must be strictly regulated to avert the potential issue of incomplete depolymerization-repolymerization of active silica-aluminum solid waste containing CFS-MP. This in turn could have a deleterious impact on the late strength of the cementitious materials. The aim of this work is to improve the joint disposal of MMS, CFS and MP and thereby provide a scientific basis for the development of environmentally friendly and low-carbon modified magnesium slag alkali-activated coal-fired slag based cementitious materials for mine backfilling.

1. Introduction

Portland cement is the most widely used cementing material utilized in mine filling [1, 2], and its production process necessitates the consumption of substantial quantities of limestone and clay raw materials, while simultaneously releasing considerable quantities of greenhouse gases into the atmosphere. The production of 1 t Portland cement is estimated to release approximately 1 t of CO2 into the atmosphere [3, 4]. The carbon released during the production process, which involves two grinding and one burning stages, is estimated to account for 5% −8% of the total global carbon emissions. This represents a significant contributor to the overall carbon footprint and has the potential to exacerbate climate change and its associated environmental challenges, including extreme weather patterns. It is not conducive to the development of energy conservation and emission reduction in China [5]. At the same time, Portland cement is the most widely used cementing agent in the field of mine filling, and its cost is about 75% of the total filling cost, which has become a key factor restricting the development of mining filling technology [6]. Therefore, researchers commenced the search for low-cost filling cementitious materials as alternatives to Portland cement in order to achieve long-term resource utilization [7, 8].

The term “coal-fired slag” is used to describe the residue discharged from the bottom of the boiler after the high-temperature combustion of coal in a variety of contexts, including coal-fired power plants, industrial boilers and domestic boilers. The residue is mainly composed of coarse particles and pulverized fine ash [9, 10]. As the main component of non-renewable energy, coal burning is responsible for the production of up to 600×10^8 t fly ash and coal-fired slag each year. The proportion of coal-fired slag among these is estimated to be between 10% and 20% of the total fly ash, which represents the second most prevalent form of solid waste in coal-fired power plants after fly ash [11].