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Study on the curing mechanism of cemented backfill materials prepared from sodium sulfate modified coal gasification slag

https://doi.org/10.1016/j.jobe.2022.105318Get rights and content

Highlights

  • The solidification mechanism of modified coal gasification slag in MCGS-CB was revealed.

  • The optimal addition amount of SS in MCGS-CB was determined.

  • The activation mechanism of SS on modified coal gasification slag is revealed.

  • The degradation mechanism of compressive strength of MCGS-CB under high SS was revealed.

Abstract

In order to promote the resource utilization of coal gasification slag (CGS) solid waste, the use of modified coal gasification slag (MCGS) to replace the cement in the cemented backfill materials is an effective way to improve the utilization rate of CGS resources, reduce the backfill cost and reduce the environmental pollution of the cement industry. In this paper, the dosage of sodium sulfate (SS) in the MCGS cemented backfill materials (MCGS-CB) system composed of MCGS–cement–aeolian sand is optimized, and the curing mechanism of MCGS-CB under the action of SS is revealed. The results show that the addition of MCGS delays the early hydration reaction process, accelerates the consumption of sulfate in the system, and shows the phenomenon of sulfate deficiency; The pozzolanic reaction of MCGS increased the ratio of Al/Ca, Si/Ca, S/Ca and Na/Ca of hydration products, changed the structure of hydration products, promoted the formation of hydration products such as C-S(A)-H, C-N-S(A)-H and AFt, increased the density of matrix, reduced the pore volume of harmful pores, and increased the compressive strength. Under the action of SS, the hydration reaction process of the early MCGS-CB was accelerated, the time of the sulfate depletion (SD) point was delayed, and the reaction time of the MCGS was advanced; The consumption of Al and Si ions drives the dissolution of the aluminosilicate glass body in the MCGS, thereby further promoting the formation of hydration products such as C-S(A)-H, C-N-S(A)-H and AFt, and increasing its matrix compactness. About 4% SS has a good excitation effect on MCGS, and excessive SS addition will cause microcracks in MCGS-CB, resulting in a reduction in strength. The results of this study can provide theoretical guidance for the ratio design and field application of sulfate-coal gasification slag-cement cementation system.

Introduction

Coal is the main energy resource in China, accounting for about 70% of the primary energy structure. In order to reduce the environmental pollution caused by the direct utilization of coal, in recent years, coal clean technology has been vigorously developed [1]. However, there are still some difficulties in realizing “zero emission” of coal cleaning technology. It is estimated that about 15% 20% of coal gasification slag (CGS) will be discharged during the coal conversion process, including coarse coal gasification slag discharged from the slag outlet of gasifier (about 60% 80%) and fine coal gasification slag produced by dust removal device (20% 40%) [2]. In 2016, there were about 700 gas furnaces in use in China, with an annual coal processing capacity of 250 million tons, and about 40 million tons of CGS solid waste discharged [3], [4]. The massive discharge of CGS not only causes waste of land resources, but also brings serious potential safety hazards to the local ecological environment and groundwater resources [5]. Therefore, many scholars have done a lot of research on the resource utilization of CGS, in order to reduce the environmental pressure caused by the storage of CGS.

Generally, the main chemical components of CGS are SiO2, Al2O3, Fe2O3 and CaO, the mineral phase is composed of crystalline phases such as silicate, aluminosilicate, calcium iron and iron oxide and glass phases such as calcium iron aluminosilicate [6]. At present, the research on CGS mainly focuses on the following aspects: (1) Building materials, including bricks, blocks, wall materials, cement and concrete, etc [7], [8], [9]; (2) Soil and water restoration, including soil improvement, water restoration, etc [4], [10], [11]; (3) Utilization of residual carbon, including extraction of residual carbon, circulating mixed combustion, etc [12]; (4) High value-added materials, including catalyst support, ceramic materials, silicon-based materials, etc [2], [6], [13]; Among them, the building materials industry has been widely studied as the focus of large-scale disposal of CGS. However, due to the high carbon content and low activity of CGS, the large-scale application of CGS in the building materials industry has not been realized so far. The treatment main method of CGS is still in-situ landfill, and its recovery rate has not yet reached a satisfactory level [10], [14], [15].

Similar to fly ash and blast furnace slag, CGS exhibits potential pozzolanic activity, which can be used as a supplementary cementitious material (SCM) instead of cement [16], [17]. However, CGS mostly exists in the form of coarse particles and its reaction activity is low. Mechanical grinding is the most commonly used method to reduce the fineness of SCM particles and improve their reaction activity [18], [19]. Research shows that mechanical grinding can reduce the fineness of CGS, increase the specific surface area of CGS particles, improve the dissolution rate of active substances such as SiO2 and Al2O3, and accelerate their hydration reaction rate [2], so as to effectively improve the compressive strength of cement based materials (CBM) [20]. In addition, chemical activation has always been one of the most commonly used activation methods of SCM because of its low cost and high efficiency [21], [22]. In the existing research, desulfurization gypsum, lime, high salinity wastewater and sodium sulfate (SS) are mainly used as chemical activators of CGS. Desulfurized gypsum is beneficial to improve the reactivity of fine coal gasification slag powder, promote the formation of hydrated products such as acicular ettringite and tabular monosulfate, and improve the initial strength of coal gasification slag-cement mortar [23], [24]; Lime dissolves in water to form Ca(OH)2 (CH), which improves the alkalinity of the hydration environment of CGS, helps the hydrolysis of active substances such as alumina and silica in CGS, and forms new hydration products through secondary hydration reaction with CH, so as to improve the compressive strength of the mixture [3], [25]. Sulfate and chloride ions in high salinity waste brine can also improve the reactivity of CGS, promote the formation of C-S(A)-H and other hydrated gels, thereby promoting structural densification and enhancing compressive strength [26].

The above research on CGS as SCM is mainly aimed at building materials, but most coal chemical plants are built attached to coal mines, which are located in remote areas, and high transportation costs making it difficult for CGS to be applied to building materials on a large scale. At present, mine backfill is widely used as an effective means to deal with coal mine associated solid waste (coal gangue, fly ash, coal bottom ash, etc.) [27]. Pozzolanic materials such as fly ash and coal bottom ash have been resource and applied to mine backfill, and the amount of cement has been greatly reduced (the cement cost accounts for about 75% of the total backfill cost), so as to save the cost of backfill materials and reduce the environmental pollution caused by cement production [28], [29]. Therefore, we carried out a preliminary feasibility study on the preparation of backfill materials from modified coal gasification slag (MCGS)-cement-aeolian sand [30], based on Krstulović-Dabić model analyzes the hydration dynamic characteristics of MCGS–cement–aeolian sand under the action of SS [31], but it does not clearly give the optimal dosage of activator and the curing mechanism of coal gasification slag cemented backfill material (MCGS-CB). In CBM, the amount of sulfate added is an important parameter index. An appropriate amount of sulfate can promote the formation of AFt, which is beneficial to increase the volume of hydration products to fill pores and reduce porosity, thereby improving compressive strength [32]; However, when sulfate levels are too high, a expansion effect occurs due to too much ettringite, resulting in deterioration of strength. Therefore, the optimal sulfate content is a key factor for the production of CBM with high strength and good durability. Especially when SCM is contained, it will change the demand for sulfate in the cementitious system [33], so most scholars have optimized the sulfate dosage in CBM [34]. MCGS-CB as a special CBM, there is no doubt that the addition of CGS will change the sulfate demand of backfill material. If we do not fully understand the sulfate demand mechanism of driving system, it may have an adverse impact on the strength development of MCGS-CB.

Considering the above issues, the novelty and core objectives of this paper are the optimization of the amount of SS in MCGS-CB, and revealing the curing mechanism of MCGS-CB. Using isothermal calorimetry, X-ray Diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and mercury intrusion porosimetry (MIP) and other characterization methods, the hydration properties, microstructure and mechanical properties of MCGS-CB were analyzed. On this basis, the optimal amount of SS was determined, and the solidification mechanism of MCGS-CB was revealed to promote the application of coal gasification slag in mine backfill.

Section snippets

Raw materials

Modified coal gasification slag cemented backfill material is made of cement, aeolian sand (EA), MCGS and water.

Compressive strength of MCGS-CB

Fig. 7(a) and (b) show the variation of compressive strength of MCGS-CB with SS and hydration age, respectively. It can be seen from the figure that the addition of SS has a significant effect on the compressive strength of MCGS-CB. In the control group, since the influence of sulfate is usually considered in the formulation design of OPC, the addition of an appropriate amount of SS will have a positive effect on the strength in the early stage, but it will have a negative effect on the

Conclusion

In this paper, the influence of SS on OPC–MCGS–EA Backfill material system is studied from the aspects of strength, hydration reaction and microstructure. The main goal is to determine the optimal dosage of SS and reveal the curing mechanism of MCGS-CB, and the possible influence mechanism of SS is discussed. The following conclusions are drawn:

(1) Compared with the control group without MCGS and SS, the addition of MCGS increased the compressive strength of MCGS-CB at all ages, and the MC0

CRediT authorship contribution statement

Pan Yang: Methodology, Writing – review & editing, Data curation. Yonglu Suo: Validation, Resources, Investigation. Lang Liu: Conceptualization, Supervision, Writing – original draft. Huisheng Qu: Data analysis, Formal analysis. Caixin Zhang: Review & editing. Shunchun Deng: Review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (Nos. 52074212, 51874229, 51674188, 51504182, 51974225, 51904224, 51904225, 51704229), Shaanxi Innovative Talents Cultivate Program-New-star Plan of Science and Technology (No. 2018KJXX-083), Natural Science Basic Research Plan of Shaanxi Province of China (No. 2015JQ5187, 2019JM-074), Scientific Research Program funded by the Shaanxi Provincial Education Department (Nos. 15JK1466, 19JK0543), China Postdoctoral

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