Manufacture of glass ceramics derived from

blast furnace slag mixed with other solid

materials: an overview

Feng LIU

School of Metallurgical and Ecological Engineering,

University of Science and Technology Beijing. Beijing

100083, China.

E-mail: liufengustb@hotmail.com

Abstract

Conversion of blast furnace slag (BFS) into glass-

ceramic materials is another utilization method except

for applying BFS for road construction and cement

industry. With the aim of improving the value of BFS as

well as protecting environment and saving natural

resources, researchers from all over the word have

conducted extensive studies. This paper presents relevant

research results and some other aspects related to BFS-

glass ceramics. In the first part, the origin of slag-

glass ceramic was introduced. The second part described

the fundamentals of glass ceramic production involving

nucleation and crystallization. It is noteworthy that the

crystallization mechanism together with the role of

nucleation agents was given special attention in this

part. It can be concluded that slag-glass ceramic

products are attractive as floor coverings, wall facings

and decorative materials due to their excellent

mechanical strength and resistance to abrasion and

chemical corrosion.

Key words: Blast Furnace Slag; Glass Ceramic;

Nucleation Agents

1. Introduction

For conventional blast furnace process, blast furnace

slag (BFS) is an important by-product during the

extraction of iron from iron ore. Generally speaking,

usually 300 kg slag is generated when one ton hot metal

is produced [1]. It can be easily computed that if one

billion ton hot metal flows out of blast furnace per year

in the world, about 300 million ton BFS will be obtained.

Besides, the amount of BFS might continue to increase due

to the decline of the quality and iron grade of iron ore

[2]. However, in the strictest sense, there is a big

difference between BFS and other solid wastes such as

incinerator ash and steel slag. Incinerator ash including

fly ash and bottom ash contains some heavy metals and

organic pollutants and thus is defined as “hazardous

solid waste” [3]. In contrast, BFS is a type of by-

product, and can be directly used for cement industry and

road construction without too much processing.

Conversely, steel slag need to be pre-treated for further

application due to large content of zinc and other

components which are disadvantage for construction use.

Although BFS has already been fully used for cement

industry, road construction and brick production for

building, it is expected to improve the value of BFS by

converting it into more advanced products with high

marketing price.

Table 1 shows the chemical compositions of BFS and

several typical solid wastes.

Table 1. Composition (wt%) of BFS and Several Typical Solid Wastes

BFS Fly Ash Bottom Ash Steel Slag

SiO2 35.8 27.1 45.4 12.12

CaO 43.5 23.4 15.3 48.19

Al2O3 11.8 11.1 19.3 2.58

MgO 6.2 2.0 3.1 6.44

Fe2O3 0.3 2.4 9.7 8.52

TiO2 0.8 2.3 - -

Cr2O3 - - - -

Na2O 0.1 2.8 1.0 -

K2O - 3.1 - -

From the data of Table 1, these oxides can be classified

into four groups [4]:

(1)glass-forming oxides or network-forming oxides: SiO2

forms the basic three-dimensional framework;

(2)modifier: alkaline or alkaline-earth oxides can break

the Si-O and thus lower the softening point of glass

and increase the tendency of glass towards

crystallization.

(3)intermediate oxides: Al2O3 is such an oxide that the Al

can partially substitutes the role of Si so as to take

part in the formation of network.

(4)nucleating agents: minor constitutes such as Fe2O3,

TiO2 and Cr2O3 are capable of promoting the formation

of numerous finely nuclei and thus are necessary for

the production of glass ceramics which requires high

mechanical strength.

Therefore, from the point of view of chemical

composition, BFS which belongs to SCAM (SiO2-CaO-Al2O3-

MgO) system is a good candidate for glass production due

to its substantial SiO2 content. As a matter of fact,

since the successful case in 1950s when the first true

glass-ceramic product known as Fotoceram was created by

Stookey at Corning Glass Works [5], the concept has been

expended to other various composition systems and

applications, and slag-glass ceramic is just such a good

example. As mentioned above, BFS, steel slag and

incinerator ash are suitable to serve as starting

materials for fabricating slag-glass ceramics. Among

these raw materials, BFS was the first to be used and now

also has a relatively large utilization proportion

compared to other solid wastes [6]. To produce and

commercialize slag-glass ceramic products, several

attempts have been made. Typical examples are the British

Iron and Steel Research Association and the former Soviet

Union. It is of interest that they developed slag-glass

ceramics almost at the same time which is 1960s. Besides,

the researchers in the former Soviet Union usually called

the slag-glass ceramics as slag sital compared to the

name of slagceram in UK. The theoretical fundamentals and

properties of slag-glass ceramics will be discussed in

detail in the following parts. Anyway, the slag-glass

ceramics with excellent mechanical strength and low cost

will have a bright future in the aspects of construction,

industry and some other fields.

2. Fundamentals

2.1 Methods of Manufacturing Glass-Ceramics

Glass-ceramics are polycrystalline materials produced by

controlled crystallization of suitable parent glasses. At

present, four methods are capable of accomplishing the

transformation from glassy phase to crystalline phase:

(1) Conventional method: this method is also described

as “two-step” method. The obtained glass article

derived from standard glass-forming techniques is

heated to two different temperatures to induce

nucleation and crystal growth respectively. The heat

treatment schedule is shown in Fig. 1(b).

Fig. 1. Crystallization of a glass to form a glass-ceramic. (a)

Temperature dependence of the nucleation and growth rate with weak

overlap and (b) Conventional method schedule

(2) Modified conventional method: this method is more

energy-saving compared with conventional method because

only one step is needed to realize crystallization.

That is to say, nucleation and crystal growth are

expected to occur simultaneously at a specified

temperature. Fig. 2(b) describes this sort of heat

treatment schedule. Except for the advantage of energy-

saving, this method is particularly suitable for the

situation where the rate curve of nucleation and that

of crystal growth have extensive overlap. This can be

easily understood from Fig. 2(a). Moreover, according

to the experimental results of Barbieri et al. [7] and

Tang et al. [8], it is reasonable to consider that

there already have existed numerous nuclei or

heterogeneous droplets embedded in the glass matrix

when the parent glass is formed. Thus, only one

temperature is needed for crystallization.

Fig. 2. Crystallization of a glass to form a glass-ceramic. (a)

Temperature dependence of the nucleation and growth rate with weak

overlap and (b) One-step heat treatment schedule

(3) Sinter-crystallization method: this method involves

concurrent sintering and crystallization when glass

frit was heated. In some senses, this method is

attractive for BFS-glass ceramics production. Because,

nowadays, water-quenching technique is widely used for

the aim of cooling molten slag [9]. If certain

additives are added into molten slag before quenching,

the resultant glass frit is a good candidate for

manufacturing glass-ceramics products.

(4) Petrurgic method [10]: although “one-step” method

can save much energy compared with “two-step” method,

reheating parent glass is still an indispensable

procedure. The concept of Petrurgic method aims at

directly precipitating crystals during the cooling path

of molten glass. It is worth noting that Petrurgic

method always generates coarse grains. Fig. 3 shows the

scheme of temperature change during cooling molten

glass.

All four methods above have their own individual

features. Particularly, “one-step” method, sinter-

crystallization method and Petrurgic method should be

paid special attention. Because, for the reuse and

recycle of by-products and solid wastes, one important

thing we should always keep in mind is to reduce cost due

to added thermal treatment processes and some expensive

additives.

Fig. 3. Crystallization of a glass to form a glass-ceramic. (a)

Temperature dependence of the nucleation and growth rate with weak

overlap and (b) Petrurgic method schedule

2.2 Mechanism of Crystallization

From the viewpoint of nucleation sites, mechanism of

crystallization is divided into surface crystallization

and bulk crystallization (also called internal

crystallization). It is well-known that in normal glass-

article production, surface nucleation which may decrease

the mechanical strength of glass and make the resulting

glass distorted is expected to be avoided during the

glass-forming process [11]. So, if extensive surface

nucleation is induced rather than bulk nucleation in the

course of heating parent glass, the obtained glass-

ceramic will possess poor properties. Of course, for some

special glass-ceramics, surface nucleation may be

necessary to produce desired performances [5].

The primary idea of producing glass-ceramics is to

generate numerous nuclei or heterogeneous droplets inside

the glass body during the nucleation stage followed by

the subsequent growth of metastable or main crystalline

phase on these nuclei. To realize the bulk nucleation,

nucleating agents (also called nucleating catalysts) as

minor constituents of glass batch are added. The kinds of

nucleating agents and their functions will be discussed

later.

For a given parent glass, heat treatment schedule is a

key factor to produce satisfied microstructure. Heat

treatment schedule mainly includes heating rate kh,

nucleation temperature TN, nucleation holding time tN,

crystal growth temperature TC, crystal growth holding time

tC and cooling rate kc. For “one-step” method, only kh, TC,

tC and kc are needed. For sinter-crystallization method,

sintering temperature TS and sintering hold temperature tS

replace TN and tN respectively. For Petrurgic method, only

kc is needed. Sometimes to obtain a high level

crystallinity, TC and tC may be introduced. The different

kinds of heat treatment schedules used by four methods

are described in above mentioned three figures. To the

best knowledge of the author, the determination of these

heat treatment parameters is still a tough problem,

because present methods such as thermal analysis and

image processing methods [12–15] are of high labor

intensity and of low accuracy. Therefore, it is urgent

and necessary to develop some more advanced techniques to

determine heat treatment parameters.

2.3 Kinetics of Crystallization

For a given parent glass, the determination of heat

treatment schedule to a great extent depends upon the

knowledge of kinetics of crystallization. As a matter of

fact, the knowledge of kinetics of crystallization mainly

includes two aspects, one is heat treatment parameters

and the other involves the calculation about the

nucleation rate, the crystal growth rate as well as

activation energy for crystallization.

Generally speaking, the most common method of determining

heat treatment parameters is by making full use of the

results of thermal analysis (DTA/DSC). As described in

references [16–18], some researchers prefer to apply

DTA/DSC results directly, that is: TN is somewhat 10–20 K

higher than Tg, and TC corresponds to the exothermic

crystallization peak. Sometimes, the occurrence of more

than one single exothermic peak leads to several crystal

growth steps. Of course, in order to find the most

suitable heat treatment schedule, some other pre-

determined schedules might be introduced to make

comparisons. Depending on advanced analysis techniques

such as scanning electron microscope (SEM) equipped with

energy dispersive spectrometer (EDS) and X-ray

diffraction (XRD), researchers are capable of finding the

optimum schedule. The procedures for the determination of

heat treatment parameters are summarized as followed:

(1) DTA/DSC is applied to find the glass transition

temperature and crystallization peak temperature.

Sometimes, to determine the glass transition

temperature, dilatometry experiment is also utilized.

(2) the determination of TN and tN: according to the

obtained glass transition temperature, several

nucleation temperatures are selected. Various heat

treatment schedules for DTA/DCS runs are utilized. The

plot of the height exothermic crystallization peak

versus the nucleation temperature allows the

specification of the optimum nucleation temperature

from the maximum of the curve. The determination of the

optimum nucleation time using the same conditions as

those for the optimum nucleation temperature

determination except that the sample in crucible is

held at the specified nucleation times for different

nucleation times. Typically, the plot of the height

exothermic crystallization peak versus the nucleation

time reaches a plateau with the increase of the

nucleation time, and the onset of the plateau

corresponds to the optimum nucleation time. It is worth

noting that if “one-step” method is applied, the work

of the determination of TN and tN would be removed.

(3) the calculation about the activation energy for

crystallization E, and Avrami constant n: in order to

get these two values, Kissinger equation (1) and

equation (2) are applied. Various heating rate are

using after holding optimum nucleation time at optimum

nucleation temperature. The calculation of E is

obtained by analyzing the slope of the ln[ φTC2 ]− 1TC curve.

(4) the determination of TC and tC: instead of using

DTA/DSC results, the estimation of the optimum crystal

growth temperature and the optimum crystal growth

holding time renders the utilization of XRD. More

detailed information can be found in reference [3]

[15].

Apart from the determination of heat treatment

parameters, thermal analysis is also applied to rapidly

obtain the crystal nucleation and growth rates. As

described in references [16–18].

2.4 Nucleation Agents

Nucleation agents are key minor constituents to obtain

fine-grained glass-ceramics. For slag-glass ceramics, the

main difficulty is initiation of internal nucleation

within parent glass. The selection of proper nucleation

agents for SCAM system is always the problem with which

worldwide researchers try to deal. It is already well-

known that TiO2 is a promising nucleation agent for many

glass composition systems. However, surface

crystallization which is unfavorable to mechanical

strength of final glass-ceramic products might happen in

some temperature ranges according to the results of

Ovecoglu [12]. Other studies [19–21] also verify that TiO2

is not the most effective nucleation agents for SCAM

systems to induce bulk nucleation. Revani et. al [22]

have conducted extensive research work about the

selection of nucleation agents for SCAM system. Their

results show that complex nucleation agents composed of

Cr2O3, Fe2O3 and TiO2 are extremely suitable for producing

internal nucleated glass-ceramics. Moreover, Williamson

[23] and Barbieri [24] et al. also confirm that the

effectiveness of Cr2O3 for inducing bulk nucleation.

Except for the nucleation agents such as Cr2O3, Fe2O3 and

TiO2, other compounds such as phosphorus oxide, sulfide

and fluoride might also be useful.

The function mechanism of effective nucleation agents is

summarized as follow:

(1) the nucleation agent crystals directly precipitate

during the cooling of molten glass or the reheating

process of parent glass, then major crystalline phase

grows on the surface of these nucleation crystals.

(2) intermediate phases resulting from the solid

solution reaction between nucleation agents and other

components are likely to crystallize first followed by

subsequent major crystalline phases.

(3) phase separation resulting in the formation of

numerous tiny droplets within parent glass might occur

when the glass is reheated.

3. Conclusion

This paper presented a brief review about the

manufacture of slag-glass ceramics. The methods of making

slag-glass ceramics were essentially the same as those of

making other kinds of glass-ceramics. It should be noted

that different applications of slag-glass ceramics

required certain properties rendering different chemical

composition formulations and microstructures. Besides,

from the point of view of saving cost, “one-step” method

and Petrurigic method seemingly had a bright future. For

“one-step” method, it was interesting to investigate the

nucleation process during the cooling of molten slag,

because the nuclei produced were favorable for crystal

growth on reheating. In order to realize “one-step”

method, except for adjusting the bulk composition of

parent glass, the temperature at which parent glass

possessed both satisfied glass-shaping ability and good

nucleation ability was a key factor to be controlled. For

Petrurigic method, if corresponding equipment was built

near blast furnace, it would be very attractive from the

viewpoint of saving cost and protecting environment.

In a word, the excellent mechanical strength and

chemical durability ensure a bright future of slag-glass

ceramics products. Extensive work is still need including

theoretical and practical aspects.

Acknowledgement

The author is thankful for the kind help from Dr. Guo

Hongwei, Zhiwen Shi, and Xinyu Liu.

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