fly ash: using fly ash in concrete generally decreases permeability, improves sulphate resistance and other durability aspects of concrete, and allows lower water content in the mixture.
Concrete is basically a mixture of two components: aggregates and paste. The paste is usually composed of Portland cement and water, and it binds together the fine and coarse aggregates. Supplementary cementing materials may also be included in the paste. A typical mix is about 10 to 15 % cement, 60 to 75 % sand/aggregate, 10 to 20 % water and 5 to 8 % air. When freshly mixed, it is plastic and malleable, allowing it to be poured into place and finished. Then, through a chemical reaction called hydration, the mixture hardens and gains strength to form the concrete we see in buildings, sidewalks, bridges and other structures. Concrete is the most commonly used construction material in the world.
Image courtesy of the Portland Cement Association.
What makes EcoSmart concrete different from conventional concrete is that it uses an optimum percentage of supplementary cementing materials to replace cement in the mix. Depending on the application and the SCM used up to 80 % of cement can be replaced with supplementary cementing materials. These materials are industrial by-products, so EcoSmart concrete is generally cheaper and can lower construction costs. In laboratory tests and field applications, EcoSmart concrete often outperforms conventional concrete in strength development and durability. It also offers significant environmental benefits, since each tonne of cement replaced by a supplementary cementing material reduces CO2 emissions by approximately one tonne.
Portland cement is the key component of concrete and is produced by intergrinding clinker and gypsum into a fine grey powder. Clinker is a granular product produced by intergrinding raw materials such as limestone, shale, clay and sand in predetermined proportions, and heating the ground materials at very high temperatures (>1500 °C) in rotating kilns. Gypsum (a mineral) is added to regulate the setting time of the cement after the clinker is cooled, prior to conversion into cement. Producing one tonne of cement emits approximately one tonne of CO2 gas due to the calcination of the raw materials and the combustion of fuels.
Supplementary Cementing Materials
Supplementary cementing materials (SCMs) (i.e. pozzolans and cementitious hydraulic slags) are used to partially replace cement in concrete. They are often added to concrete to make the mixtures more economical, reduce permeability, increase strength, or influence other properties. Typical examples include natural pozzolans (like volcanic ash), fly ash, ground granulated blast furnace slag, rice husk ash, and silica fume. Pozzolans react chemically with calcium hydroxide (Ca(OH)2) from the hydration of Portland cement to form calcium silicate hydrates or CSH. CSH is the strong binder that hardens in concrete. SCM + Ca(OH)2 = CSH.
The glassy particles in pozzolans react slowly with the Calcium Hydroxide salts to form CSH, so the strength and impermeability characteristics generally take longer to develop than they do in conventional concrete.
One of the most commonly used pozzolans in concrete is fly ash, a by-product from coal-fired power plants. Using fly ash in concrete generally decreases permeability, improves sulphate resistance and other durability aspects of concrete, and allows lower water content in the mixture. Using fly ash improves the plasticity and workability of fresh concrete, and produces a warmer coloured concrete. The annual production of fly ash in the US and Canada is 60 million tonnes per year, and there will be an estimated 600 million tonnes produced worldwide by the end of this year. Currently, about 80 % of the fly ash produced ends up in landfills. In North America, fly ash is typically used to replace an average of 8 % of the cement in concrete, while in many European countries, the replacement rate is greater than 25%.
Ground Blast Furnace Slag
Consisting of silicates, aluminosilicates of calcium, and other compounds, blast-furnace slag is the by-product of molten iron production in a blast furnace. The slag is then rapidly chilled to assure a high percentage of glass, and then ground to produce a fine powder for use as a supplementary cementing material in concrete. The ground granulated blast furnace slag acts similarly to cement since it possesses hydraulic properties. Ground granulated blast furnace slag, also known as cementitious hydraulic slag, is often used in concrete requiring maximum durability, higher strength, fire-resistance, better insulation, and lighter weight.
Ground Slag Cement (GS-Cem)
The prime ingredient in GS-Cem is barren slag, which is produced from Teck Cominco’s Trail Operations, in 1British Columbia, Canada, as a by-product of the lead smelter. The slag undergoes a slag cleaning process, resulting in a “barren” slag which is rich in reactive silica and chemically active metal oxides. The barren slag is transported to a specialty plant in Calgary, Alberta, where it is interground with a concrete additive called Econo-Set—a proprietary product which enhances early strength development, bleed water control and concrete finishability. When used as an SCM in concrete, GS-Cem increases concrete durability due to its highly pozzolanic properties. In addition, the use of GS-Cem also increases sulphate resistance of concrete, controls alkali-silica reactivity, increases the ultimate strength of concrete, and improves the water retention of the plastic concrete mix. GS-Cem is typically added to concrete at a rate of 15-20% by the weight of cement, replacing an equal amount of Portland cement in the concrete mix with no other mix adjustment required.
Metakaolin is produced by calcination of kaolin (clay mineral) at 650-800°C. Kaolin is the by-product of oil sands operations. Metakaolin is a highly reactive pozzolan with a high specific surface, which makes it very suitable as a cementing material in concrete. Metakaolin particles are nearly 10 times smaller than cement particles, which results in a denser, more impervious concrete. Using metakaolin in concrete increases the durability of concrete (i.e. resistance to chemical attacks, sulphate, ASR expansion, and freeze-thaw cycles). Metakaolin also enhances several mechanical properties (i.e. early-age compressive strength, and flexural strength) of concrete.
Pozzolan is a siliceous or alumino-siliceous material that, in finely divided form and in the presence of moisture, chemically react with the calcium hydroxide that is released by the hydration of Portland cement to form compounds possessing cementitious properties. Natural pozzolans include diatomaceous earth, metakaolin, rice husk ash, volcanic ash, or calcined shale, all of which are natural materials that may also be calcined and/or processed.
Silica fume is a residue from the manufacturing of silicon and ferro-silicon metals. Silica fume particles are 100 times smaller than cement grains, so they can be used to fill in the spaces between cement grains, minimizing permeability in concrete. Silica fume particles have a high surface area, making this pozzolanic material an ideal supplement to produce high-strength concrete. Using silica fume also results in improved freeze-thaw resistance in concrete.