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Blast furnace slag?
Finely milled blast furnace slag (GGBS hay GGBFS, short for English Ground-granulated blast-furnace slag). It is a by-product of the blast furnace smelting industry. Obtained by rapidly cooling cast iron slag with water or steam to produce granulated slag, a glassy structure (without time to crystallize) – called blast furnace granulated slag. Then blast furnace granules are dried and finely ground to produce finely ground blast furnace slag. Finely ground blast furnace slag is highly cohesive and rich in CSH (calcium silicate hydrate). As an additive to increase the strength, durability and smoothness of concrete.
The chemical composition of cast iron slag varies considerably depending on the composition of the input materials in the production of cast iron. Silicate and aluminate impurities from iron ore and coke . Combined in a blast furnace with fluxes that reduce the viscosity of the slag. In the case of iron and steel production. The flux mainly consists of a mixture of limestone and forsterite (Mg2SiO4) or in some cases dolomite. In the blast furnace, the slag floats on top of the liquid cast iron and is decanted for separation. Slow cooling of the molten slag produces inactive crystalline material. Consists of a combination of Ca-Al-Mg silicates.
To obtain good water reactivity or hardening properties of the slag. The molten slag should then be cooled rapidly or tempered below 800 °C to prevent crystallization of merwinite (Ca3Mg(SiO4)2) and melilit ((Ca,Na)2(Al,Mg,Fe2+)[(Al, Si)SiO7]). To cool and fragment the slag, a granulation process can be applied. Accordingly, the molten slag is subjected to high pressure air or water currents. In addition, in the pelletizing process, the liquid slag is partially cooled by water and then directed into the air by a rotating drum/drum. For proper reactivity, the resulting debris is finely ground to achieve a fineness like that of Portland cement.
The main components of blast furnace slag are CaO (30-50%), SiO2 (28-38%), Al2O3 (8-24%), and MgO (1-18%). In general, increasing the CaO content will lead to an increase in the alkalinity of the slag and an increase in the compressive strength. MgO concentrations between 10-12% and Al2O3 to about 14% also lead to similar results. Outside of this range there is no additional effect. Some ingredient ratio or so-called freezing index. Was used to correlate the slag composition with hardening activity. The latter is mainly expressed in terms of adhesive compressive strength.
The content of the amorphous phase (glass phase) of the slag suitable for mixing with Portland cement is 90-100% and depends on the cooling method and the cooling starting temperature. The glass structure of tempered cast iron slag is largely dependent on the ratio of lattice-forming elements such as Si and Al to lattice modifiers such as Ca, Mg and to a lesser extent Al. The increased number of network modifiers leads to a higher degree of network reactivity and depolymerization.
The common crystalline components of blast furnace slag are merwinite and melilite. Other minor components that can form during advanced crystallization are belite (Ca2SiO4), monticellit (Ca(Mg,Fe)SiO4), rankinit (Ca3Si2O7), wollastonite (CaSiO3) and forsterite. A small amount of commonly reduced sulfur is oldhamit ((Ca,Mg)S).
Finely ground blast furnace slag is used as a mineral admixture for concrete. It has the effect of taking advantage of slag, which is a waste product of the iron and steel industry. Replace the use of cement, increase durability, heat resistance of concrete, waterproof, anti-corrosion. Using finely ground blast furnace slag does not generate heat of hydration, so large concrete blocks can be cast.
Finely ground blast furnace slag is classified according to fineness, according to 28-day active strength.
Characterizing the fineness, the concept of “specific surface” (aspect ratio) is used. Usually determined by the Air Permeation Method (Blaine). Finely ground blast furnace slag is classified into S60, S75, S95, S105 . With minimum fineness of 2750, 3500, 5000, 7500 cm2/g, respectively.