29-12-2012, 03:03 PM
Ground granulated blast furnace slag.
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Types of blast furnace slag.
1. Air cooled blast furnace comes into picture when the solidification of molten slag takes place under atmospheric conditions, however, the cooling can be accelerated at the surface by subsequently applying water.
2. Expanded blast furnace slag is formed when the cooling of molten slag is controlled by water and other agents such compressed air or steam. The material so formed is often lightweight and cellular.
3. Granulated blast furnace slag is produced when the molten slag is rapidly cooled by a process called quenching by immersion in water.
GGBFS is used in three major ways as a cementitious application over the last hundred years, these are as under.
1. Blended cement. A hydraulic cement resulting essentially from a intimate and uniform blend of blast furnace slag and hydrated lime; or an intimate and uniform blend of blast furnace slag and Portland cement, Portland cement and pozzolan, or Portland blast furnace slag cement and pozzolan produced by inter-grinding Portland cement clinker with other materials or by blending Portland cement with the other materials, or a combination of inter-grinding and blending.
2. Portland- blast furnace slag cement. A hydraulic cement consisting of intimately interground mixture of Portland cement clinker and granulated blast furnace slag or an intimate blend of Portland cement and fine granulated blast furnace slag in which the proportion of mixing is specified.
3. Slag cement. A hydraulic cement consisting mostly of intimate and uniform blend of granulated blast furnace slag and hydrated lime in which the slag constituent is more than specified minimum percentage.
Production of blast furnace slag.
In the production of iron, the blast furnace is continuously charged from the top with the iron oxide (ores, pellets, sinters, etc), fluxing stone (limestone and dolomite) and fuel that is coke. Two products are obtained from the furnace: the molten iron that collects in the bottom of the furnace (hearth) and liquid iron blast furnace slag floating on the pool of the iron. Both are periodically tapped from the furnace at a temperature nearly about 1500.
Processing.
Quenching is the most common process for granulating slags to be used as cementing materials which involves high pressure water jet impinging on the stream of molten slag at a water-slag ratio of about 10 to 1 by mass. The blast furnace slag is quenched more instantaneously to a temperature below the boiling point of water, producing particles of highly glassy material. Another process sometimes known as air granulation which involves the use of pelletizer (Cotsworth 1981). In this process the molten slag passes over the vibrating feed plate, where it is expanded and cooled by water sprays. It then passes on to a rotating, finned drum which throws it into the air where it rapidly solidifies into the spherical pellets. The resulting product may have high glass content and can be used as a cementitious material, or in a larger particle size, as a light weight aggregate.
After the granulated blast furnace slag is formed, it must be dewatered, dried and ground before it is used as a cementitious material. Magnets are often used before and after grinding to remove the residual metallic iron. Typically the slag is ground to an air permeability (Blaine) fineness exceeding that of the Portland cement to increase the activity at early ages. As with the Portland cement and pozzolans the reactivity increases with fineness.
Chemical and physical properties.
The composition of the blast furnace slag are determined by that of the ores, fluxing stone and the impurities in the coke charged into the blast furnace. Typically calcium, silicon, alumunium, magnesium and oxygen constitute 95% of more of blast furnace slag. To maximize hydraulic or cementitious properties, the molten slag must be chilled rapidly as it leaves the blast furnace. Rapid quenching or chilling minimizes the crystallization and converts the molten slag into fine aggregate size particles composed predominantly of glass. This product is referred to as granulated iron blast furnace slag. The cementitious action of a granulated blast furnace slag are largely dependent on glass content, although other factors also have some influence. Slowly cooled slags are predominantly crystalline and therefore do not posses significant cementitious properties.
Hydraulic activity of slag.
It is a common agreement among researchers that the principal hydration product that is formed when GGBFS is mixed with Portland cement is more or less same as the principal product formed with the hydration of Portland cement, i.e., calcium silicate hydrate. GGBFS hydrates are generally found more gel like than the hydrates of Portland cement and hence denseness to the cement pastes.
The initial hydration of GGBFS is much slower than that of the Portland cement mixed with water; therefore Portland cement or lime or alkali is added to increase the reaction rate. Hydration of GGBFS in the presence of Portland cement largely depends on the breakdown of glassy slag structure by hydroxyl ions released during the hydration of Portland cement.
In the hydration of GGBFS the slag reacts with alkali and calcium hydroxide (CaOH2) to produce additional C-S-H. Researchers like Regourd (1980), Vanden Bosch (1980) and Roy and Idorn (1982) have suggested that, in general, the hydration of GGBFS with Portland cement is a two stage reaction.
Effects on properties fresh concrete.
Wood (1981) reported that workability or place ability of concrete containing GGBFS yielded improved characteristics when compared to the concrete not containing GGBFS. He further stated that this result was due to the surface characteristics of GGBFS. He also theorized that due to smooth and dense surfaces of GGBFS particles, little if any, water was absorbed by GGBFS during initial mixing, unlike Portland cement. Wu and Roy (1982) found that pastes containing GGBFS exhibited different rheological properties compared to the pastes of Portland cements alone. Their results indicate a better particle dispersion and higher fluidity of the pastes and mortars both with and without water reducing ad-mixtures.
Strength and its rate of gain.
Compressive and flexural strength gain characteristics of the concrete containing GGBFS can vary over a wide range. When compared to the Portland cement, the use of grade 120 slag typically results in reduced early strengths (1 to 3 days) and increased strengths at later ages (7 days and beyond) (Hogan and Meusel 1981). Similarly use of grade 100 results in lower strength at early ages (1 to 21 days) and equal or greater strengths at later ages. Grade 80 gives reduced strength at all ages. The extent to which the GGBFS affects the strength is dependent on the slag activity index of that particular slag and the ratio in which it is used in the mixture. Other factors that can affect the performance of GGBFS in concrete are water-cementititous materials ratio, physical and chemical characteristics of the Portland cement and curing conditions. It can be seen that the percentage of strength gain achieved with a grade 120 slag is greater in concrete mixtures in which have high water-cementitious materials ratio than in the mixtures with low water-cementitious materials ratio (Fulton 1974; Meusel and Rose 1983). The same trend was also noted by Malhotra (1980).
Effect of curing.
There has been considerable discussion on the effect of curing on the concrete containing Portland blast furnace slag cement and concrete containing GGBFS as a separate constituent. Mather (1957) studied the comparison between type 2 cement and Portland blast furnace slag cement; he found that both cements suffered loss to the same degree when curing was stopped after 3 days. Conversely Fulton (1974) reports that the concrete containing GGBFS are more susceptible to poor curing conditions than the concrete without GGBFS where the GGBFS is used in the percentages higher than 30%. He attributes this succeptibility to the reduced formation of hydrates leading to the increased loss of moisture which would otherwise be available to continue. It is obvious that, as with all the cementitious materials, the rate and degree of hydration is affected by the loss of moisture at an early age, with a decrease in strength gain.
Effect on temperature rise on mass concrete.
In almost all the cases, the incorporation of GGBFS reduced the early rate of heat generation; this reduction is directly proportional to the proportion of GGBFS used. In-situ measurements were reported by Bamforth (1980) comparing concrete without GGBFS to the concrete with 30% fly ash and 75% GGBFS. The heat of hydration is dependent on Portland cement used and the activity of GGBFS. Reduced heat of hydration can be expected when GGBFS are used to replace equal amounts of cement. studies by Kokubu, Tankahashi, and Anzai (1989) indicate that the blends of 35 to 55% GGBFS produced greater total heat than Portland cement mixtures, even though the rate of heat rise was less. Where blends of highly active GGBFS are used, proportion of at least 70% GGBFS may be needed to meet out the low heat of hydration requirements.