CHAPTER 2. OVERVIEW OF MATERIALS AND EXPERIMENTAL TECHNIQUES
2.1. Blast Furnace Slag
Blast furnace slag (BFS) is a by-product of the steel industry. The BFS is produced through metallurgical smelting processes when coke, iron ore, iron scrap and some fluxes (e.g., limestone or dolomite) are blended and melted together in a blast furnace. The schematic process for BFS is represented in Figure 2- 1 [7]. Due to the nature of by-products, the properties of BFS are highly affected by the properties of the raw materials, the cooling and production process, and so on.
The BFS is granulated in a rapid cooling process, and thereafter is called granulated blast furnace slag (GBFS). The manufactured GBFS generally undergoes a grinding process for the engineering purpose of being used as a construction material. The ground form of GBFS has previously been called ground granulated blast furnace slag (GGBFS). The term “GGBFS” was replaced with the term “slag cement” by the American Concrete Institute (ACI) and American Society for Testing and Materials (ASTM) in 2003 at the request of GGBFS manufacturers [49]. Nevertheless, in this study, the term “GGBFS” will be used for the ground form of GBFS because the term “slag cement” is not used in Korea and Europe where standards use the term “GGBFS” for the ground form of vitreous granular BFS.
Figure 2- 1: Schematic production progress of blast furnace slag. [7]
2.1.1. Chemical composition of blast furnace slag
The chemical composition of GGBFS is highly dependent on the types of iron produced and iron ore used, which can be frequently presented in CaO-SiO2-Al2O3-MgO diagrams [7]. In general, the amount of CaO in GGBFS is smaller than Portland cement and higher than other types of supplementary cementitious materials (SCMs) such as fly ash, metakaolin, and natural pozzolans [50].
Chemical composition is one of the important parameters, determining the structure of amorphous phases and its hydraulic properties. Many researchers have tried to develop hydraulic indexes of GGBFS to predict GGBFS reactivity based on chemical composition [7]. Some hydraulic indexes are represented in Table 2- 1. Some hydraulic indexes presented in Table 2- 1 have been adopted and used by standard regulation in many countries to evaluate GGBFS reactivity.
Table 2- 1: Some examples of hydraulic indexes of GGBFS based on chemical composition. [7]
No. Hydraulic index Note
1 ଵൌ ͳͲͲ െ ଶ -
2 ଶൌͳͲͲ െ ଶ
ଶ -
3 ଷ ൌ ଶଷ
ଶ Korean Standard
4 ସൌ ଶଷെ ͳͲ
ଶ
- 5 ହൌ ͳǤͶ ͲǤଶଷ
ଶ -
6 ൌ ͲǤͷଶଷെ ʹǤͲଶ - 7 ൌ ͵ଶଷ
ଶ Ͷ -
8 ଼ൌ ͲǤͷ
ଶ -
9 ଽൌ ͲǤͷ ଶଷ
ଶ ሺሻଶ -
10 ଵൌ ଶଷ
ଶ -
11 ଵଵൌ ଶଷ
ଶ ଶ Chinese Standard 12 ଵଶൌ ͲǤ͵ଶଷ
ଶ ͲǤଶଷ -
13 ଵଷൌ
ଶ ͲǤͷଶଷ -
2.1.2. Cooling of blast furnace slag and its crystalline phases
The microstructures of BFS are highly affected by the cooling process of melted BFS. In general, slowly cooled BFS produces stable crystalline phases which have little or no cementing properties. Smolczyk [51] investigated the crystalline phases contained in slowly cooled BFS as shown in Table 2- 2. Among these crystalline phases, only dicalcium silicate (usually Ⱦ-C2S), which is one of the compounds in Portland cement, poesses cementing properties.
Table 2- 2: Crystallized phases in slowly cooled blast furnace slag. [51]
Crystalline phases Chemical formula Cement notation
Melilite (solid solution of
gehlenite and akermanite) ʹ ή ଶଷή ଶ ʹ ή ή ʹଶ C2AS + C2MS2
Merwinite ͵ ή ή ʹଶ C3MS2
Dicalcim silicate ʹ ή ଶ C2S
Rankinite ͵ ή ʹଶ C3S2
Wollastonite ή ଶ CS
Diopside ή ή ʹଶ CMS2
Monticellite ή ή ଶ CMS
Spinel ή ଶଷ MA
Magnesium silicate ʹ ή ଶ M2S
Sulphide ǡ ǡ
Others ǡ ଶଷ
The cementing properties of BFS can be obtained by quickly cooling melted BFS. Nowadays, the melted BFS is granulated by directly injecting high pressure water into the BFS. The rapid cooled BFS produces a large amount of amorphous phases with a very small amount of crystalline phases such as akermanite and gehlenite. The structures of vitreous BFS are not determined, yet, but have been considered as an akermanite glass [52] or melilite, which is a solid solution between gehlenite and akermanite, containing various cations [38].
2.1.3. Hydration features of ground granulated blast furnace slag
When the GGBFS is exposed to water without an activator, the GGBFS particles react with water, producing aluminosilicate-coated rims within a few minutes [35, 53-56]. The rims are little permeable to water, which means that further hydration of GGBFS is obstructed. Thus, the degree of hydration of GGBFS without an activator cannot be considerable. For further hydration of GGBFS, the rim should be physically or chemically attacked with proper methods. Portland cement, various
alkali chemicals, and alkaline earth chemicals can be used as chemical activators to destruct the rim of GGBFS.
The hydration of GGBFS is highly affected by the pH value in pore solution [35, 49, 53-55], which means that a higher pH value is associated with a higher degree of hydration. The relationship between pH value and degree of hydration is induced from the fact that the OH- ions can react with amorphous aluminosilicate i.e. coated rim, resulting in facilitating further hydration process. Thus, the proper activators, which can cause a high pH value in pore solution, should be selected for developing activated-GGBFS which can be an alternative cement to replace Portland cement.
The major hydration product of GGBFS is C-S-H [35, 49, 55] which is one of the main hydration products in Portland cements. The nature of C-S-H is also highly affected by the type of activator and pH value [35, 53-55]. A higher pH value produces a low Ca/Si ratio of C-S-H and a large amount of substitution of silicon tetrahedra by aluminum tetrahedra in the bridging site because a high pH value increases dissolution of aluminum and silicon ions as well as precipitation of calcium ions [35]. In addition, hydrotalcite can be produced from GGBFS hydration in case the of high pH or prolonged hydration time [35, 36, 53, 54].