“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, July 21, 2013


My firm is what would, in general, be viewed as one of the larger closely held, locally based construction inspection, materials testing, and geotechnical engineering firms in California. Barriers to entry for, at least, several material segments of our business are very minimal, though to provide the full spectrum of services that we offer they are significantly greater. Unfortunately for us, the "meat and potatoes" of our service line can be served (with arguable effectiveness but very low price) by those who select to provide only those services with the aforementioned low barriers.

Thus, we're constantly looking for areas where the investment we've made in people and equipment of very high capability can provide value that will be recognized by a client base willing to pay for these. In this effort, we've engaged in significant research and development, led by Dr. Boris Stein. Much of this R&D work has been related to various sustainability issues regarding concrete and its use in the built environment.

 Concrete is a construction material composed of cement (typically portland cement and thus referred to as "portland cement concrete"), coarse and fine aggregates, water, and, possibly, various admixtures to customize the concrete's properties in various ways.

The manufacture of portland cement involves (at a very simplistic level) charging a kiln with limestone (calcium carbonate, Ca CO3) and various other constituents (chiefly clay as a source of alumino-silicate), and heating the charge to around 1450°C. This results in the emission of CO2 as CaCO3 is changed to CaO in the so-called "clinker." This process is called "calcining." This clinker is then ground to an extremely fine powder in a grinding mill, and a small amount of gypsum (CaSO4) is added.

The heat may be supplied, in some instances, by the burning of manufactured materials (discarded tires, for example) but is usually accomplished by the burning of fossil fuels. Thus, the production of portland cement is a twofold emitter - the combustion product of the fuel used to heat the kiln and the "cooked off" CO2 from the calcium carbonate. Though estimates vary, for each tonne of cement produced, approximately 750 kg of CO2 is emitted. It's estimated that somewhere around 5% of the CO2 emissions worldwide are a result of the manufacture of cement (I've seen estimates as high as 7%).

Thus, finding substitutes for portland cement in concrete is an active pursuit in the academic and industrial arenas. Some of these pursuits involve replacement of calcium carbonate with a different raw material that either is a non-carbonate, requires much lower temperatures for processing, or both.
Electrotatic precipitator for collecting fly ash

Another approach is partial replacement of portland cement in a concrete mix with various industrial byproducts, "supplementary cementitious materials" or "SCMs." Chief among these are fly ash (a product of coal combustion) and ground granulated blast furnace slag ("GGBFS" or just "slag," a product of the refining of iron and of steel making). It's also possible to use "natural pozzolans" literally mined (typically from Southern California desert locations for use here).

As much as 50% or even 70% of the portland cement in concrete can be replaced with these SCMs, and the resulting concrete mixes can, with suitable attention to proportions and admixtures, result in concrete with much lower carbon footprint and characteristics as good as, or even better than, mixes utilizing only portland cement.

Books can be (and have been) written on the various methodologies of designing mixes with desired properties and I'm not going to go into that here. But let me (as is typical) use some back of the envelope calculations to see what kind of CO2 emission reductions are possible.

In 2011, 3.6 billion tonnes of cement were produced worldwide and this resulted in the emission of over 2 billion tonnes (metric ton, 1,000 kilograms - about 10% larger than a U.S. "short ton" of 2,000 pounds) of CO2. Certainly, some of that cement went into mixes already utilizing SCMs but the fraction worldwide, would be fairly small. Typical U.S. mixes at this time might utilize 15% to 25% fly ash, and many mixes don't use any. If we assume that 10% fly ash might be typical for a worldwide average and that we could ultimately get to mixes with 70% SCMs (entirely possible from a technical point of view). If so, we could reduce the 3.6 billion tonnes to 1.2 billion tonnes (to the great despair of the cement manufacturers who would fight this tooth and nail) and save the emission of something like 1.8 billion tonnes of CO2 annually.

Ah, but what about the availability of the SCMs?  We certainly don't want to burn more coal or refine more iron for the purpose of providing the concrete industry with SCMs. In 2010, 777.1 Mtonnes (megatonnes - a million tonnes or a billion kilograms) of CCPs (coal combustion products, most of which is fly ash) were produced and 415.5 Mtonnes were utilized. While much of the utilized fly ash went into concrete, there are many other uses competing for it. Let's say that, by more effective harvesting and economically incentivizing use in concrete, we could use 500 Mtonnes/year.  We're currently using about 190 Mtonnes in concrete, so we could possibly increase our use by 310 Mtonnes.

As for GGBFS, the latest year for which I could find data was 2005, in which the world produced 110 Mtonne of slag, of which 60 Mtonne went to slag cement. Suppose that we could use 100 Mtonne (assuming both an increase in production and an increase in utilization), a 40 Mtonne increase.

Totaling, and assuming that appropriate mix designs enabled all of the 310 Mtonnes of fly ash and 40 Mtonnes of slag to be used, we could replace something like 350 Mtonnes of portland cement. Thus, rather than the 2.4 billion tonnes feasible with technological implementation of mix designs, supply constrains us to the replacement of 0.31 billion tonnes and the elimination of something like 230 Mtonnes of CO2, on the order of 0.7% of worldwide emissions.

Sadly, SCMs won't save the world, but we are certainly heavily engaged in pursuing them. They have benefits in the final concrete product, they reduce the industrial waste stream and, to a limited extent, can reduce industrial CO2 emissions.

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