Regular Article
Surface Reaction of Blast Furnace Slag under Hydrothermal Conditions
2014 Volume 54 Issue 3 Pages 548-552
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2014 Volume 54 Issue 3 Pages 548-552
Hydrothermal treatment has various possibilities for adding value to blast furnace (BF) slag by forming hydroxide or hydrate crystals and introducing pores into the reaction product. To optimize the hydrothermal reaction process, understanding of the reaction behavior of BF slag during hydrothermal treatment is indispensable. In the present work, the mechanism of the hydrothermal reaction of BF slag was investigated by focusing on the reaction at the slag surface. The surface reaction behavior was reproduced using slag plate samples, which adjusted the effective amount of hot water participating in the reaction. The changes in the surface morphology of BF slag and synthesized CaO–SiO2–Al2O3(–MgO) slags during hydrothermal treatment at 250°C were examined. The reaction mechanism of BF slag as well as the effect of MgO on the reaction were discussed from the observed phenomena.
Blast furnace (BF) slag is a major by-product in iron- and steel-making processes, with almost 25 billion tons produced annually in Japan.1) Although most of it is recycled as raw materials in cements, roadbeds and concrete aggregates, new processes that produce more valuable materials from BF slag are required. Because BF slag consists of major components of ceramic materials such as SiO2, CaO, Al2O3 and MgO, sintering to obtain solidified compacts or melting to produce fibrous materials, which are generally used to produce ceramics, have been regarded as possible treatments for BF slag. However, when energy consumption and CO2 emission are considered, such high temperature treatments may not be feasible as sustainable recycling processes.
An alternative is the hydrothermal reaction, which is advantageous because its energy consumption is lowered by its ability to occur at relatively low temperature. In hydrothermal reactions, materials are exposed to reactive water in highly pressurized water vapor and/or high temperature aqueous solution. Hydrothermal reactions occur at a temperature range from 120 to 350°C, which could be controlled using waste heat exhausted from the iron- and steel-making processes. Therefore, researchers have treated BF slag hydrothermally to add value.2,3,4,5,6,7,8,9,10,11) Preparation of functional materials for applications such as heat storage and building has been attempted.2,3,4) Furthermore, the introduction of pores into the hydrothermal products were attempted to add value to BF slag.9,10,11)
Calcium silicate materials are well-known building materials with excellent properties in terms of lightness and heat insulation. The materials are produced by the hydrothermal reaction of cement with quartz sand below 200°C. The materials contain acicular crystals of tobermorite (5CaO·6SiO2· 5H2O) or fibrous crystals of xonotlite (Ca6(Si6O17)(OH)). Their formation improves the physical properties of the product because they act as a binder. Al2O3 is known to suppress the hydrothermal reaction of calcium silicate materials,12,13) so the Al2O3 content of the raw materials is generally controlled below 10 mass%. Thus, BF slag has not been used to produce calcium silicate materials because of its high Al2O3 content of 15–20 mass%. However, recent research3,4,6,8) has revealed that BF slag can be hydrothermally solidified despite containing Al2O3. As a result, functional building materials may be produced by hydrothermal treatment of BF slag. Furthermore, MgO in the slag accelerates the hydrothermal reaction because it promotes dissolution of water into slag powder and crystal formation.8)
During the hydrothermal reaction of slag powder, outer layer of slag reacts to form the hydrothermally reacted phases filling the gap between powders but most of internal part of slag keeps remaining.8) The reaction at the slag surface is thus considered to dominate the hydrothermal reaction of slags. In the present work, the mechanism of the hydrothermal reaction of BF slag is investigated, focusing on the reaction at the slag surface. Hydrothermal treatment of powdery BF slag and synthesized CaO–SiO2–Al2O3(–MgO) slags is conducted at 250°C, and the phases generated are identified. Then, the reaction behavior at the slag surface is reproduced by hydrothermal treatment of slag plates, taking into account of the effective amount of hot water participating in the reaction at the slag surface. The reaction mechanism of BF slag during hydrothermal treatment and effect of MgO are considered.
The chemical composition of water-cooled BF slag used in this work is listed in Table 1. Slags with a composition of 50% CaO–40% SiO2–10 mass% Al2O3 with and without 5 mass% MgO were prepared by melting reagent grade quartz, CaCO3, Al2O3 and MgO powders in a Pt-20%Rh crucible at 1600°C for 3 h in air, which was followed by quenching on a copper block. All slags were confirmed to be glassy by X-ray diffraction (XRD) analysis.
| CaO | SiO2 | Al2O3 | MgO | MnO | T.Fe | S |
|---|---|---|---|---|---|---|
| 41.5 | 35.8 | 15.3 | 5.48 | 0.28 | 1.58 | 0.79 |
BF slag and pre-melted slags were ground, sieved at 63 μm. To conduct the hydrothermal treatment at “curing” condition, slag powder was then mixed with purified water to give a mass ratio of slag/water of 10:3. The mixture was charged into a Teflon crucible (I. D. = 12 mm, H = 15 mm) with a 1-mm hole at its rid. The crucible and 2.5 mL of purified water were placed in a Teflon container (I. D. = 26 mm, H = 40 mm) in an autoclave, as shown in Fig. 1. To conduct the hydrothermal treatment at “soaking” condition, 3 grams of slag powder were immersed in 10 ml purified water charged in a Teflon container in an autoclave. Hydrothermal treatment was conducted by heating the autoclave at 250°C for 4–16 h. After cooling to room temperature, the sample was dried in an oven at 80°C for 30 minutes. Samples were analyzed by XRD analysis and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDX) observation.
Schematic diagram of the autoclave used for the hydrothermal treatment of slag powder.
During the hydrothermal reaction of slag powder, incorporation of water into the slag as well as dissolution and precipitation of slag components into/from the hot water present in the gaps between slag particles occur. To reproduce the reaction at the slag surface, two slag plates were fixed at a certain distance to imitate the gap between powder particles during the hydrothermal reaction of slag and then treated hydrothermally.
Glassy plates of BF slag and 50% CaO–40% SiO2–10 mass% Al2O3 (–5 mass% MgO) slags were prepared as follows. Water-cooled BF slag particles were melted in a graphite crucible at 1520°C in an argon atmosphere. The slag melt was poured into a copper mold pre-heated at 600°C and then pressed into a plate with a thickness of about 2 mm with a copper block. CaO–SiO2–Al2O3(–MgO) slags were melted in a Pt-20%Rh crucible at 1600°C in an air, poured out and then shaped into a plate using two copper blocks. Pre-heating of a copper mold was necessary in the case of BF slag to avoid its crystallization.
Slag plates were mechanically polished with emery paper (#2000). A Teflon film with a thickness of 20, 50 or 100 μm was placed between the sample slag plate and cover slag to maintain the space between them, and then the plates were fixed with Teflon-coated copper wire (Fig. 2). Each sample was immersed in 10 mL purified water charged in a Teflon container in an autoclave, and then treated hydrothermally by heating at 250°C for 4–16 h. The plate sample was subjected to XRD analysis and SEM observation without any processing, to determine the surface composition and structure after the reaction.
Schematic diagram of the slag plate samples prepared for subsequent hydrothermal treatment.
When BF slag powder was treated at curing condition at 250°C, it started to agglomerate after 4 h and solidified to form a compact after 8 h (hereafter, compact indicates the solidified powder in this paper). Figure 3 shows the XRD patterns obtained for BF slag powder after hydrothermal treatment for different periods. Crystal formations were found after reaction for 8 h. Formations of hibschite (Ca3Al2Si2O8(OH)4) and tobermorite were identified, and the intensity of peaks from hibschite increased as the reaction time lengthened. Because solidification was observed at the same time as crystal formation, crystallization is considered to be the dominant step inducing hydrothermal solidification. We previously investigated hydrothermal treatment of synthesized CaO–SiO2–Al2O3 and CaO–SiO2–Al2O3–MgO slags with various compositions at 350°C using the hydrothermal hot pressing method, and observed similar solidification behavior.8) In contrast, BF slag powder was not solidified at soaking condition even after 24 h, and formation of a crystalline phase was not observed by XRD analysis. Solidification of BF slag during hydrothermal reaction obviously occurs more readily at curing condition than at soaking condition, which is similar to the findings of Tae and Morita.5)
XRD patterns of BF slag powder (<63 μm in size) before and after hydrothermal treatment at 250°C for various periods.
Figures 4(a) and 4(b) show the fracture surface and cross section, respectively, of the slag compact after hydrothermal treatment for 16 h. The inside of the slag compact is dense. The magnifications in Figs. 4(c) and 4(d) show fibrous and spherical crystals in the reaction area. Combined with the phases identified in Fig. 3, EDX analysis suggested the fibrous and spherical crystals were tobermorite and hibschite, respectively. The crystals filled the gaps between original particles and contributed to the solidification of the material.
Fracture surface (a, c) and cross section (c, d) of BF slag after hydrothermal treatment at 250°C for 16 h.
To identify the reaction mechanism during hydrothermal treatment, the change in microstructure of the surface of BF slag was investigated using plate samples. Figure 5 shows the surface of the slag plate around the edge of the cover slag after hydrothermal reaction for 12 h when the gap between two slag plates were kept at 100 μm. The slag surface below the cover slag was filled with bloom-like crystals of calcium-silicate hydrate, which were identified to be tobermorite in the latter part of this section. In contrast, only small cubic crystals were observed in the free surface region where the slag was stripped by hot water. These crystals were identified as Ca(OH)2 by EDX analysis. We speculate that such different reaction tendencies in different regions are observed for the following reasons. For the reaction in the free surface region, components eluted from the slag into the hot water, and were transported away from the slag/hot water interface. This results in reduced formation of hydrate or hydroxide crystals on the surface as seen in Fig. 5. Ca(OH)2 forms because of the relatively poor solubility of Ca2+ ions in water at high temperature.14) In contrast, below the cover slag, slag components eluted into the hot water were trapped in the gap between the slag plates. Then, successive elution from the slag caused supersaturation of the components in the hot water, resulting in formation of calcium silicate hydrate. Similar surface morphology was observed when the gap between the plates was either 20 or 50 μm. Note that the gap distance between remaining slag particles was found to be several 10 s μm after the hydrothermal reaction of slag powder as seen in Fig. 4(b) and was comparable to the controlled gap between the plates here. Accordingly, the surface reaction of the slag powder during hydrothermal treatment was reproduced well by maintaining an appropriate gap between the slag plates. Hereafter, the reaction behavior was investigated by observing the slag surface below the cover slag with a gap of 100 μm.
Surface morphology of a BF slag plate around the edge of the cover slag after hydrothermal treatment at 250°C for 12 h (gap distance = 100 μm).
The surface morphologies of the BF slag plate after hydrothermal treatment for different periods and corresponding XRD patterns are shown in Figs. 6 and 7, respectively. The formation of hydrothermally reacted phases was observed after 4 h. The outermost phases on the slag surface initially possessed a ‘kelp-like’ structure and changed to a ‘bloom-like’ structure after 8 h. The size of the crystals increased as the reaction period extended. Spheroidal crystals grew directly on the slag surface during the reaction. From the XRD analysis, formations of tobermorite and hibschite were confirmed. Combining with the EDX analysis of locations I (composition: 46.3% CaO–43.2% SiO2–9.5% Al2O3–1.0 mass% MgO) and II (composition: 46.4% CaO–33.0% SiO2–18.9% Al2O3–1.7 mass% MgO) in the microstructure of the product after reaction for 12 h (Fig. 6), the bloom-like and spheroidal crystals were identified as tobermorite (substitution of silicon sites by aluminum) and hibschite, respectively. Cross sections of the BF slag plate after reaction for 4–16 h are presented in Fig. 8. The structure formed after hydrothermal reaction of BF slag is composed of three phases, with outermost acicular crystals (tobermorite), spheroidal crystals (hibschite) and a dark layer, which was thought to be a hydrate layer formed by the incorporation of water into glassy slag, as found in our previous work.8) The size of each phase increased simultaneously as the reaction time extended, although the acicular crystals grew the most. Figure 9 shows the elemental distribution around the reacting region after hydrothermal treatment for 16 h. Calcium and silicon existed in high concentration in the acicular crystals and low in the hydrate layer, suggesting their selective elution from the slag and subsequent formation of hydrate or hydroxide crystals. In contrast, a high concentration of magnesium was identified in the hydrate layer. The reaction mechanism during hydrothermal treatment of BF slag will be discussed in Section 3.3.
Surface morphologies of BF slag plates below cover slag after hydrothermal treatment at 250°C.
XRD patterns obtained for surface of BF slag plates below cover slag after hydrothermal treatment at 250°C for various periods.
Cross sections of the surface of BF slag plates below cover slag after hydrothermal treatment at 250°C.
Elemental distribution around the surface of BF slag below cover slag after hydrothermal treatment at 250°C for 16 h.
Figure 10 shows the XRD patterns of 50% CaO–40% SiO2–10 mass% Al2O3 slag powder with and without 5 mass% MgO before and after hydrothermal treatment for 16 h. Formation of tobermorite was identified in both slags, although its peak intensity was much larger in the one with MgO. In addition, solidification was more progressed in the slag with MgO, which is consistent with our results from hydrothermal hot pressing experiments.8)
XRD patterns of 50% SiO2–40% CaO–10% Al2O3 slag powder with and without 5 mass% MgO after hydrothermal treatment at 250°C for 16 h.
To determine the effect of MgO addition on the surface reaction, hydrothermal treatment of slag plates was conducted. Figure 11 shows cross sections around the interface of slag plates with and without MgO after hydrothermal reaction for 16 h. A hydrate layer with a thickness of 10 μm was clearly seen in the slag with MgO (Fig. 11(a)), whereas fibrous crystals attached to the slag matrix and hydrate layer were not observed in the slag with no addition of MgO (Fig. 11(b)). In addition, a high concentration of MgO (up to 12 mass%) was found in the hydrate layer. Therefore, MgO promoted the formation and growth of the hydrate layer.
Cross sections of the surface of 50% SiO2–40% CaO–10% Al2O3 with and without 5 mass% MgO below cover slag after hydrothermal treatment at 250°C for 16 h.
The hydrothermal reaction of BF slag is mainly caused by incorporation of water into the slag (hydration) and elution of components from the slag into hot water, followed by crystal formation. The hydration to form hydrate glass layer at slag surface was estimated in the previous works.6,8) In addition, the hydrothermal reactivity of slag was not explained only by slag basicity8) and was enhanced by the presence of MgO in the slag, which promoted the formation and growth of the hydrate layer. Considering the importance of the hydrate formation on the hydrothermal reactivity of glassy CaO–SiO2–Al2O3 slag, we assume that the total reaction step occurs as follows (Fig. 12): (1) A hydrate glass layer forms through the incorporation of water into the original slag, and at the same time, some components of the slag, especially calcium and silicon, elute into the hot water. (2) Subsequent elution of slag components causes their supersaturation in the hot water, resulting in the precipitation of hydroxide or hydrate crystals in the gaps between slag particles. Unlike crystalline materials, BF slag possesses glass properties, so diffusion in the glass network is considered to be the dominant process during elution. Thus, hydration may affect the reaction kinetics during the hydrothermal reaction by modification or dilution of the glass network caused by incorporation of –OH or H2O into the slag matrix although further study, especially the spectroscopy of the chemical structure of the reacted phases, is required to determine the proposed process.
Proposed reaction mechanism of hydrothermal treatment of BF slag.
To better understand the hydrothermal reaction behavior of BF slag, we investigated the reaction behavior at the slag surface at 250°C using slag plates. The following results were obtained:
(1) The surface reaction of the BF slag powder was reproduced well by the hydrothermal treatment of two BF slag plates separated by a distance of 50 or 100 μm.
(2) Observation of the surface of the BF slag plate after hydrothermal reaction identified formation of acicular crystals of tobermorite, spheroidal crystals of hibschite and a hydrate layer. In addition, the elution of calcium and silicon from the hydrate layer and condensation of magnesium in it were determined.
(3) Observation of the surface of synthesized CaO–SiO2–Al2O3 and CaO–SiO2–Al2O3–MgO slags after hydrothermal treatment confirmed that MgO contributed to the formation of a hydrate layer, and thereby enhanced the hydrothermal reaction of the slag.
(4) The reaction mechanism of hydrothermal treatment of BF slag is proposed to occur as follows: (1) A hydrate glass layer forms on the original slag and slag components elute into the hot water, and then (2) subsequent elution results in precipitation of hydroxide or hydrate crystals in the gaps between slag particles.
This research was partly supported by the Research Promotion Grant from the Iron and Steel Institute of Japan and Priority Assistance for the Formation of Worldwide Renowned Centers of Research - The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
