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Canadian use of ground granulated blast-furnace slag as a supplementary cementing material for enhanced performance of concrete R. Doug Hooton
Abstract: The performance of concrete, in terms of its placeability, physical properties, and its durability, can be enhanced by the use of slag-blended cements or separately added ground granulated blast-furnace slag. It also has advantages for architectural purposes due to the whiteness it imparts to concrete. Properly proportioned and cured slag concretes will control deleterious alkali–silica reactions, impart sulphate resistance, greatly reduce chloride ingress, and reduce heat of hydration. Setting times and early age strengths can be controlled through appropriate proportioning, while later age properties are typically enhanced. CSA and ASTM standards cover both slag-blended cements (CSA A362; ASTM C595; ASTM C1157) and slag as a supplementary cementing material (CSA A23.5; ASTM C989). Since Lafarge introduced the first large-scale slag grinding plant near Hamilton in 1976, slag has become the predominant supplementary cementing material in Ontario. Recently, its availability in the U.S. has expanded dramatically. Key words: blast-furnace slag, concrete performance, supplementary cementing material. Résumé : La performance du béton, en termes de sa malléabilité, de ses propriétés physiques et de sa durabilité, peut être amélioré par l’utilisation de mélanges de ciments avec scories ou par l’addition de granules de scories de hauts fourneaux. Cela a aussi des avantages pour des buts architecturaux à cause de la blancheur qu’ils donnent au béton. Avec une proportion des ingrédients et une cure adéquates, les bétons avec scories contrôleront les actions délétères des réactions alcali-silice, donneront une résistance au sulfates, réduiront grandement les introductions de chlore, et réduiront la chaleur de l’hydratation. Les temps de mise en place et les résistances précoces peuvent être contrôlés par une adéquate proportion des ingrédients, alors que les propriétés à un âge plus avancé sont typiquement améliorées. Les normes de la CSA et de l’ASTM couvrent les mélanges de ciments avec scories (CSA A362; ASTM C595; ASTM C1157) et les scories en tant qu’adjuvant (CSA A23.5; ASTM C989). Depuis l’introduction par Lafarge de la première installation à grande échelle de broyage de scories, en 1976, près d’Hamilton, les scories sont devenues l’adjuvant prédominant en Ontario. Récemment, sa disponibilité aux États Unis s’est grandement accrue. Mots clés : scories de hauts fourneaux, performance du béton, adjuvant. [Traduit par la Rédaction]
Hooton
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1. Introduction Iron blast-furnace slag, when granulated (i.e., quenched) and ground to cement fineness, has been used as a primary or secondary binder to produce concretes for over 100 years in Europe. Use of slag as an ingredient in quality concrete has expanded rapidly since the 1950s and has spread to Australia, the Pacific Rim, North America, and parts of Africa. In Canada, the 1976 opening of the Standard Slag Cement plant in Fruitland (now Lafarge), Ontario, marked the first major use of separately ground slag as a supplementary cementing material. The replacement level of portland cement depends on both climate and application. The choice of producing slag in blended cements or as a separate ingredient to the concrete mixer is based on economics and local Received July 6, 1999. Revised manuscript accepted January 21, 2000. R.D. Hooton. Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, ON M5S 1A4, Canada (e-mail:
[email protected]). Written discussion of this article is welcomed and will be received by the Editor until December 31, 2000. Can. J. Civ. Eng. 27: 754–760 (2000)
practice — both can be used to make quality concrete. In North America the trend has been to add slag separately at the concrete plants.
2. Blast-furnace slags Blast furnaces and other furnaces used to produce iron from iron ore in the presence of added limestone or dolomite fluxes produce a molten slag, which floats above the molten iron and is tapped off separately. This 1500–1600°C molten slag has about 30–40% SiO2 and about 40% CaO, which is close in composition to portland cement. If cooled slowly in pits, the slag crystallizes into melilite or merwinite minerals and, although useful as a concrete aggregate or road base, it possesses little hydraulic value. However, if quenched rapidly, as by granulation in water (or by pelletizing), it forms a glass, which when dried and ground is latently hydraulic (i.e., it just needs an alkaline environment to hydrate, but does not require the lime that pozzolans such as silica fume and Class F fly ash do). The glass content does not have to be 100% to have a reactive slag nor is the chemical composition of major importance (Hooton and Emery 1983; Hooton 1987). The © 2000 NRC Canada
Hooton
chemistry of slag is controlled by the iron making process and is usually very uniform. Typical compositions are given in Table 1. Other than a 5.0% limit on sulphate content, the CSA A23.5-98 standard for supplementary cementing materials places no limits on the chemical composition of slag. The one item that can be controlled is the fineness of grinding, and similar to portland cements, finer, high surface area material hydrates more rapidly. Granulated blast-furnace slag has been used since the late 1800s, but its modern use as a separately added cement replacement was pioneered in South Africa about 40 years ago. This concept then spread to the U.K. (1960s) and Canada (1970s) and then to the U.S.A. (1980s). In the latter 1990s there has been a large increase in the amount of separately ground slag available in the eastern and central parts of the U.S.A., and this availability may soon impact on adjacent provinces. In Canada, separate additions of slag are covered under CAN/CSA A23.5 on supplementary cementing materials. Blended cements made by intergrinding granulated slag together with portland cement clinker tend to dominate the market in Australia, Germany, and other European countries. Blended cements reduce the need for a second silo at the concrete plant, but do not provide the flexibility to change replacement levels for multiple applications or for cold and hot weather concreting. Also, intergrinding allows the producer to optimize gypsum additions for set control and early strength. CSA has standards for blended cements (A362), but until recently no slag blended cements have been produced in Canada. However, with increasing pressure on the cement industry to reduce its CO2 emissions, the use of blended cements will likely increase. 2.1. Slag replacement levels When separately ground slag was first introduced in Ontario, in 1976, replacement levels were quite conservative: 20–25% in summer and 15–20% in winter. However, after over 20 years of ever widening acceptance, normal replacement levels for specified strength concrete range from 20% to 50%. For special requirements, such as mass concrete, 60–70% cement replacements have been used. Based on data developed in part by the author (Hooton and Emery 1990), typically 50% replacement or less is considered sufficient to obtain equivalent performance to a sulphate resisting portland cement, and 35% replacement for moderate resistance (CSA Type 20 equivalence) regardless of the C3A content of the portland cement. For control of alkali–silica reaction, 50% slag has been used in a series of dams in Ontario and is allowed under CSA A23.1 Appendix B.
3. Effects on fresh concrete 3.1. General Effects on fresh concrete properties will vary both with replacement level and depending on whether the slag is interground or used as an SCM. Slag as an SCM is often ground finer than the portland cement it replaces and often produces a mixture that is more cohesive and easier to place and finish. Because of its relative hardness, slag in interground (blended) cements can be underground relative
755 Table 1. Typical chemical compositions of Canadian separately ground slags (Hooton 1987). Dofasco slag Hamilton, Ont.
Algoma slag Sault Ste. Marie, Ont.
CaO SiO2 Al2O3 MgO Fe2O3 S* Na2O K2O MnO TiO2
40.04 37.08 8.76 11.52 1.93 1.99 0.36 0.44 0.72 0.28
32.34 38.35 8.76 18.64 0.61 0.95 0.22 0.71 1.41 0.36
Form
Pelletized
Granulated
*Sulphur is almost totally present as sulphide.
to the portland cement clinker, which will result in different rheological properties as well as hardened properties. 3.2. Workability Generally, for a typical mass replacement of slag for portland cement (which will affect yield because of the lower specific gravity of slag (2.92 vs. 3.15)), the slump will be unaffected. However, the slag concrete is generally much easier to compact by vibration and is therefore considered to be more workable. 3.3. Air content Because of its improved workability, entrapped air content is lowered. In air-entrained concrete, a slightly higher dosage of admixture (approx. 10–15%) is required to achieve the same total air content. 3.4. Finishing and setting As mentioned previously, because of higher fines content in the mixtures, slag concretes are easier to finish. However, at high replacement levels and low ambient temperatures (
