Supplementary Cementing Materials can significantly reduce the embodied energy of precast concrete products by substituting waste materials for relatively high energy hydraulic cement. SCMs are mostly by-products of other industrial processes. Their judicious use in concrete production is desirable both for environmental and energy conservation as well as for the technical benefits they can provide. SCMs are added to concrete as part of the total cementitious system, either as an addition or partial replacement of Portland cement.
When properly used, the SCMs can enhance the following properties of concrete:
- Generally improve the workability and finishing of fresh concrete
- Reduce bleeding and segregation of fresh concrete
- Lower the heat of hydration beneficial in mass pours
- Improve the pumpability of fresh concrete
- Generally improve the long term strength gain
- Reduce permeability and absorption (especially silica fume)
- Reduce alkali-aggregate reactivity
The effect of replacing cement with supplementary cementitious materials on the embodied energy of concrete is appreciable. For example, a 1% replacement of cement with fly ash results in an approximately 0.7% reduction in energy consumption per unit of concrete.
Silica fume is a waste product recovered from the reduction of high-purity quartz with coal in electric furnaces in the production of silicon and ferrosilicon alloys. Silica fume improves the quality, strength and durability of concrete by making the concrete much less permeable and more resistant to corrosion of the steel reinforcement.
Fly ash is a pozzolan waste product collected from coal-fired power plants. Fly ash contains some heavy metal (normally more than silica fume), so the heavy metal content of the concrete will increase. Fly ash refines the pore structure of the concrete, making it more resistant to chloride penetration. Not all fly ash is suitable for use in concrete. Fly ash shall conform to the requirements of CSA A3000 and shall be specified in accordance with Tables 7 and 8 in CSA A23.1.
Normally 10% to 20% of the cement can be replaced with fly ash to reduce the environmental burden of the concrete. Substitution with fly ash at levels exceeding 25% is considered to be a high volume SCM application. Appropriate testing to ensure desired performance should be carried out. The use of fly ash can increase setting times. This may be an economic factor in precast concrete manufacturing if casting cannot be maintained on a daily cycle.
Although fly ash offers environmental advantages, it also improves the performance and quality of concrete. Fly ash affects the plastic properties of concrete by improving workability, reducing water demand, reducing segregation and bleeding, and lowering heat of hydration. Fly ash increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction. Fly ash reaches its maximum strength more slowly than concrete made with only Portland cement. The techniques for working with this type of concrete are standard for the industry and will not impact the budget of a job.
Blast Furnace Slag
Production of blast furnace slag consumes about ¨÷ of the energy required to produce cement. Substitution of slag at levels exceeding 35% for the Portland cement in precast concrete is considered a high volume SCM application, and its suitability for intended use must be prequalified. The addition of slag cement usually results in reduced need for water, faster setting time, improved pumpability and finishability, higher 28-day strength, lower permeability, resistance to sulfate attack and alkali-silica reactivity (ASR), and lighter color. Blast-furnace slag shall conform to the requirements of CSA A3000 and shall be specified in accordance with Tables 7 and 8 in CSA A23.1.
One of the important lessons from LCA is that decisions should not be based on either embodied or operational energy use in isolation. The key is to optimize total energy use over the full life cycle, recognizing that a higher embodied energy may pay dividends in the form of lower maintenance and operating energy. Athena Institute studies show that, over the lifetime of a building, operating energy is the more significant of the two. The embodied energy of the materials may represent only 3% to 13% of total energy use over a 75 to 80 year building life.