For a long time I have had a certain uneasiness when it came to concrete. On the plus side, it is an incredibly versatile material, easily cast into a near-infinite variety of forms, and it is also a structural material with excellent compressive strength. It's also fairly cheap for what you get. A 2x4 stud at the local building supply might be close to $4.00 these days, whereas the equivalent mass of concrete would be about $1.64 by my calculations (given an ave. price of $100/cu.yard of concrete). with a forming/production cost factored in, the concrete 2x4 might be half the price of the wood one. I might note that the contractor down the road from me with the ICF house claimed a price for the 2x4-equivalent concrete volume to be $0.17, but I'm not so sure of his basis for that calculation. Concrete is also quite a bit cheaper than steel and has a much lower environmental footprint (<<-- link to a .pdf download) than steel. All that said, it is pretty common knowledge in the green building community that concrete isn't exactly among the greenest of materials out there. Depending upon which source you look at, with the concrete industry generally tending to understate the issue from what I've observed so far, and others overstating it, concrete production accounts for roughly 7% of all greenhouse gas production. By comparison, transportation accounts for about 15% of greenhouse gases. Sounds on the surface of it that concrete is a material to avoid using, or at least minimizing, on the environmental basis alone. However, the picture is a little more complicated than it first appears, and in today's piece I'd like to look into material matters a little further.
Why does concrete account for such a significant percentage of greenhouse gas production? After all, concrete is only composed of cement, along with aggregate (gravel, small stones) and sand. The stone and sand are surface-mined, so there are some obvious negative environmental effects there - especially if you are unfortunate enough to live in the vicinity of a gravel pit and get to hear the sounds of dump trucks moving back and forth all day long (ask me how I know). The production of the cement portion of concrete is really where the bulk of the greenhouse gas emissions come from, and this is particularly due to two factors, which I'll discuss shortly. Cement itself is most often of one type these days: Portland Cement. Also called OPC (Ordinary Portland Cement). This class of cement, more properly called hydraulic limes, developed in England in the early 1800's, and the name 'Portland' derives from the similarity of the material to a type of building stone quarried on the Isle of Portland, near Dorset, England.
Portland cement originates in a powder called 'Rawmix' in the industry, which is often from locally-sourced stone, and this powder essentially combines minerals commonly found in limestone and clay. These minerals, namely oxides of aluminum, calcium, iron, silicon and magnesium, are ground to a small size and mixed in a precise manner: in fact, the mix is controlled to an accuracy of 0.1% per component. Calcium and silicon combine to form strength-producing calcium silicates. The aluminum and iron oxides act as a flux or solvent to help the formation of the aforementioned silicates at lower temperatures (which are more economical).
The Rawmix is then heated in a cylinder within a cement kiln, and there it gradually reaches temperatures of 1400~1450˚C. It is this heating, and the consumption of fossil fuels that associates to it, that produces a significant quantity of CO2 associated to cement production. The limestone also undergoes chemical changes during the heating process as the calcium silicates are formed and this also releases significant amounts of CO2. The end result of the heating of the Rawmix in the cement kiln is the production of a material called clinker. The clinker can be stored for a number of years if need be, though when it is ready for use it is further modified by adding calcium sulfate (i.e., gypsum), typically 5~8% by volume, and then the mixture is finely ground to form a fine cement powder. This is the stuff sold by the bag at the local building supply outlet.
As I mentioned, it is the heating and chemical conversion processes that produce the bulk of the greenhouse gases, the gases from vehicles and machinery used in the cement plants and the distribution of cement accounting for only a very tiny percentage of gases. All in all, for every kilogram of cement produced, there is on average and depending upon whether the power to the cement plant comes from a nuclear plant or fossil-fuel powered electricity generating plant, about 0.8 kg of CO2 produced. That is close to a 1:1 ratio. The cement plants also produce hazardous airborne dust, noise, and the gases from the kilns can contain dioxins and silicon oxides, which are of course health hazards.
Keep in mind that cement is only about 15% of what goes into a typical concrete mix, but is a potent addition from an environmental perspective.
Portland Cements as such devolve into 5 basic types, depending upon the application:
Type 1: Common purpose cement used for general construction and precast, not meant to be in contact with soils or ground water.
Type 2: This is similar to type 1 in that it is general purpose, however it has additives to give it increased sulfate resistance, and thus can be used in contact with ground water or soil. Most of the Portland Cement sold in the US is of this type.
Type 3: Again, similar to type 1 but with the clinker mix ground more finely, which imparts greater early strength to the concrete, at some sacrifice of the materials long-term strength. This type of cement is used for a lot of precast work as it allows for fast mold turnover in the plant, along with application such as for repairs, temporary machine bases, and so forth .
Type 4: This is virtually the reverse to Type 3 in that it sets up slowly with low initial strength, but over the long term (one or two years curing) develops higher compressive strength than the other types of Portland Cement. this type of cement is used in places such as hydroelectric dams and other large concrete structures which have a low surface to volume ratio. curiously manufacturers do not generally stock this type of Portland Cement and it has generally been replaced by Portland-pozzolan mixes (a subject I will deal with next time) which are cheaper.
Type 5: This type has high sulfate resistance and is more common in the western US where soil sulphates are more of a problem and cause through reaction with the concrete a disruptive expansion and spalling.
In Europe, there are also 5 types of Portland Cement, however these types are not the same as the 5 set out in North American standards (ASTM).
For ICF construction, the other component besides the concrete mix is the foam form itself. For fun, try saying 'foam form' ten times fast. This rigid foam is most often made from expanded polystyrene. Polystyrene, it may surprise the woodworkers out there, originally was derived from a tree. In 1839, Eduard Simon in Berlin, distilled an oily substance from Storax, the resin of the Turkish Sweetgum Tree. Storax resin was sometimes used for incense, as many tree resins are, and also as a perfume additive and as a medicinal. Eduard Simon termed the oily substance, or monomer, that he distilled styrol. This residue apparently thickened up in a few days, and Simon dubbed this jelly-like substance styrol oxide, or "Styroloxyd". It was later determined by other chemists that the formation of Styroloxyd from styrol was a polymerization process, in which monomers, like styrol, undergo a chemical process in which they form three-dimensional chained molecules.
The process of converting styrol to Styroloxyd can be accelerated by heating, and in time (well, 80 years went by in fact before this was determined) the product of this process of polymerizing styrol came to be known as 'Polystyrene'. In 1931, the I.G. Farben company of Berlin (the same company later closely associated to the Nazis and guilty of numerous wartime atrocities, including using more than 80,000 inmate laborers at Auschwitz to manufacture synthetic oil and rubber from coal) began manufacturing polystyrene as a possible substitute for die-cast zinc.
Polystyrene is a ubiquitous consumer product these days, finding applications in such things as CD cases and disposable razors. Oh, I guess I might add that polystyrene is also a significant component in Napalm® and Nuclear bombs along with other polymer-bonded explosives. That's one of the dark sides of the material to be sure.
Styrene itself has been assessed by the EPA (as of the year 2000) in the following manner,
"Styrene is primarily used in the production of polystyrene plastics and resins. Acute (short-term) exposure to styrene in humans results in mucous membrane and eye irritation, and gastrointestinal effects. Chronic (long-term) exposure to styrene in humans results in effects on the central nervous system (CNS), such as headache, fatigue, weakness, and depression, CSN dysfunction, hearing loss, and peripheral neuropathy. Human studies are inconclusive on the reproductive and developmental effects of styrene; several studies did not report an increase in developmental effects in women who worked in the plastics industry, while an increased frequency of spontaneous abortions and decreased frequency of births were reported in another study. Several epidemiologic studies suggest there may be an association between styrene exposure and an increased risk of leukemia and lymphoma. However, the evidence is inconclusive due to confounding factors. EPA has not given a formal carcinogen classification to styrene."
Polystyrene is also highly flammable and building codes prohibit its use in any exposed locations, unless it is treated with flame-retardant chemicals such as good ole' Hexabromocyclododecane, or HBCD (doesn't that just roll off the tongue?). HBCD is persistently toxic and under consideration for restriction on the Stockholm Convention of Persistant Organic Pollutants. I pity the people that have to work with that stuff. Any exposed polystyrene in a building must be covered with concrete, metal, or sheet drywall.
Expanded polystyrene foam (EPS), of which most of today's ICF forms are composed, was developed in 1959 by the Koppers Company in Pittsburgh, Pennsylvania. Polystyrene is expanded by the use of chemical agents, most often hydrocarbons such as pentane, a solvent. These hydrocarbons have a relatively mild environmental impact overall. For the builder, one important detail is that the EPS foam as such has a thermal performance of R4 per inch.
A smaller percentage of ICF forms are made using Extruded Polystyrene, or XPS. A generic name for EPS foams is 'Styrofoam®', however this was originally a term and Dow trademark for XPS foam only. This material was re-invented in 1941 by a research engineer named Ray McIntire, drawing upon patents first claimed by a Swedish inventor Carl Jorg Munters. Dow was seeking to create a flexible electric insulating material, and developed ways to produce XPS in quantity. The insulating properties and buoyancy of XPS led to its adoption by the US Coast Guard in 1942 for 6-person life rafts, and of course the diversity of applications mushroomed from there. The Dow product, as an insulative material, is identified by its blue color. I note that ICF Builder Magazine, in a past article summary, seems to confuse EPS and XPS in a short 'Evolution of the ICF Block' article, as it states that Dow came up with the EPS when in fact they did not.
In part three of this series, I'll take a closer look at the environmental impacts of concrete and EPS foam, combined as they are in ICF construction technology and see if there are any ways that the negative aspects of these materials, some already mentioned, might be ameliorated, at least to some extent. As I mentioned at the outset, concrete is a tremendously versatile material and it would be jolly nice if it could be used with a cleaner conscience.