Pretty much all concrete producers have been supplementing cement powder in concrete formulations with other supplementary cementitious materials (SCM) for many years. This is done to reduce the cost of producing the concrete, and to provide a reduction in the emissions created while making concrete. Fortunately, the practice of adding SCM's also results in improvements to the final concrete’s strength, chemical resistance, and can often reduce the permeability of concrete. The most common three SCM’s are blast furnace slag, fly ash, and silica fume.
Blast Furnace Slag is a by-product of the iron industry. The material in rough terms, is the impurities and flux that floats to the top of the molten iron where it is then skimmed off. It often contains high concentrations of limestone, forsterite and in some cases dolomite. When incorporated, it is touted as increasing the durability and strength of concrete. It can also be used to extend the set times and reduces the risk of cold joints. One aspect I am particularly looking for in the concrete for my suspended garage slab, is its ability to resists the ingress of salts and therefore reducing the risk of reinforcement corrosion. A typical formulation replaces 40-50% of the cement powder with ground-granulated blast-furnace slag. One of the negatives of using slag for concrete is the large volume of water needed to quench and rapidly cool the molten slag to prevent the crystallization of the slag, and then the energy needed to dry and grind the finished granulated product prior to inclusion into concrete.
Fly ash is created by the coal power industry and is captured by precipitators or other filters within the Coal Thermal Plants before it is able to enter the atmosphere. It is substantially made up of silicone dioxide and calcium oxide. It also includes a concoction of toxic constituents like heavy metals in quantities from trace amounts to several percent. About 43% of fly ash is recycled, with the majority used as a constituent of concrete, with the rest is often land filled or stockpiled in ponds where if not carefully controlled, can leach into ground water supplies. The use of fly ash in concrete is closely regulated and is usually restricted to Class F ash. Class C ash can have volatile effects on concrete with entrained air, causing reduced resistance to freeze/thaw damage. Fly ash is often added in ratios of 30% by mass over Portland in concrete mixes. Fly ash, like slag, is also reported to increase concretes strength and chemical resistance and also improves the workability of concrete and can reduce water demand lowering shrinkage crack potential. Finally, it is reported that the use of fly ash to replace 1 ton of Portland cement, offsets one ton of Carbon Dioxide. Of course this does not take into account the 20-30 tons of CO2 created by the burning of the coal needed to produce one ton of Fly ash, but as the coal is being burned anyway to produce power, and this is a waste product that is not further transformed for use in concrete, we can ignore this fact.
Silica fume is an ultra-fine powder collected as a by-product of the silicon and ferrosilicon alloy production in electric arc furnaces. Silica fume, when added to concrete is reported to improve the concrete’s compressive strength, bond strength, and abrasion resistance. And like the above two SCM’s, it too reduces the risk of reinforcement corrosion. Silica fume is reported to reduce bleed water significantly due to the large surface area its particles represent in the concrete matrix. This property also blocks the concrete pores and prevents mix water from coming to the surface. Silica fume, like Fly ash also has the benefit of not requiring any further processing to be utilized in concrete. One down side to the incorporation if silica fume into a concrete matrix is its tendency to lower workability by making the concrete ‘stickier’ and therefore requiring increased volumes of water.
Were off to a good start, but how else can a cement producer reduce the embodied energy of the finished product – Concrete.
Lafarge’s cement plant in Richmond BC, currently the eighth largest carbon producer in the Province per Pacific Carbon Trust, is trying and succeeding in changing this statistic. They have and are implementing two programs that will significantly reduce their carbon output going forward.
The first project involves switching part of the boiler fuel needs from coal to construction waste that would have otherwise ended up in the landfill and released methane. This will result in a reduction of 83,000 tonnes of carbon output over a 6 year period (28%) or the equivalent of 16,275 cars being taken off the road for one year.
The second project involves evolving to a new generation of cement powder called Portland Limestone cement (PLC). Lafarge is able to reduce its fuel consumption and cut its GHG emissions by roughly 8% (or the equivalent of taking 4,667 cars of the road for one year) by displacing conventional clinker with finely ground limestone in a ‘raw’ state, up to a ratio of 15% when formulating its cement powder.
**Updated**
In Canada, the PLC product is made on the east coast by Holcim and St. Mary's and in the Lower Mainland is made by Lafarge - branded: Contempra and by Lehigh - branded EcoCem. While this formulation has been used in Europe of over 25 years, it was only introduced to Canada in 2009. These producers are to be congratulated for making this commitment to the future and reducing their global impact on our planet.
Additional Reading:
1) Concrete CO2 Fact Sheet produced by the NRMCA
2) Concrete and SCM use for sustainable future by Lafarge
3) Concrete in Practice - Why/What/How by NRMCA
4) PCA Manual - Design and Control of Concrete Mixtures, Chapter 3 hosted by University of Memphis
5) Understanding Supplementary Cementitious Materials and Their Benifits by Julie Buffenbarger
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