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Cement, being a hydrated complex of lime and silicate, has the potential for being recycled by heating without emitting CO2.



It is known for some time that heating of cement material produces a new type of cement binder. However, the efficacy of this binder currently stands at about one-fifth of the strength of virgin cement [1].

Theoretically the addition of excess heat should, in chemical principle, be able to make the hydration process during curing of concrete a fully reversable process. In general, with enough energy input, you can chemically force-react matter into any composition, including that of virgin-grade cement.

Category of the action

Industrial Efficiency: Cement Industry

What actions do you propose?

The sun represents the most obvious source of practically unlimited freedom of heat energy. Using this unlimited energy one can heat pulverized waste cement to finer grain sizes and heat them to higher temperatures. 

A large solar concentrator is built whereby the focal point achieves a temperature just shy of melting lime (calcium oxide), a refractory ceramic. Existing mechanical methods can repeatedly grind and sieve sizes that are just a few microns in size. These particles, derived from waste concrete, are heated at high temperatures where they immediately dehydrate.

The recycling process applies for crushed concrete (including aggregate rocks) and the heating process requires 300 degrees Celsius. 

There are ways to do this economically. For example, the most economical way to re-use concrete is as aggregate itself, where in the splitting process only a large tumbler is needed. Gravity is very efficient at crushing concrete, whether it is from a tumbling process or random chunks being dropped from a high altitude. 

From as-yet unpublished research on lunar cements, one observes that a weight fraction of less than 5% including aggregate is more than sufficient to achieve a structurally intact solid. The same must hold true for terrestrial concrete. That is, the proportion of cement must be similar to or even below 5% in weight fraction if the particle size gradation is controlled and aggregates are included. 

Can size gradation be economically controlled? The answer is yes, because sieving is an inexpensive process applied to certain industrial processes. 

The 5% is key, and if the densities are similar between cement and aggregate (a reasonable assumption), then we can say that only 5% of volume of recycled concrete goes into a dehydration recycling process. The rest turns into raw aggregate. 

Generally it also that 5% which will be finely divided into a fine powder, which can be heated and water expelled from it quite fast. Suppose a very long cylindrical reflector is built, where the sunlight is focused on one line. These reflectors will be much larger than most personal solar cookers - because it is industrial-scale. Then, air carrying the particles can be blown across in a glass tube collinear with the focal line, and the gas speed is such that the small particles remain suspended in the air flow. Of course, there is the issue of particles accumulating on the glass tube, but I suppose this problem can be solved by an electrostatic "pulse" mechanism. 

The problem arises where one attempts to segregate the aggregate from the cement. For the substantial fraction of concretes using limestone as aggregate, the limestone itself turns into the lime-containing precursor for cement. For silicate aggregates, it is hypothesized that the tumbling process would tend to produce a gradation of particle sizes favoring the tricalcium silicate and dicalcium silicate phases of material, since the intrinsic (brittle linear-elastic) strength of those phases is lower than that of the bulk silicate rock.

However, even if the rock were to be non-segregated, a blind selection can be made where a certain cutoff size - say 1 cm - is applied to sieve out as relatively coarse aggregate, and any size less than that amount would be ball-milled into nanopowder sizes and subsequently roasted. In this case, the cement phase need only achieve 5% volume fraction to be structurally intact. Research on polymeric cements suggests that this is plausible.

Who will take these actions?

The cement industry stands to profit enormously from developing such technology. If the strength of recycled cement can approach that of virgin cement, it can slash most of the CO2 emissions related to heating limestone to form lime.

Where will these actions be taken?

Because cement and concrete are usually regionally sourced materials, the actions can be taken anywhere with cement production. There will be many such locations around the world.

How much will emissions be reduced or sequestered vs. business as usual levels?

Over the research and development life (about 10 years), as much as 25 - 50% of anticipated CO2 emissions can be prevented.

The analogy is that, by the end of the 10-year development push, the industry can completely retool itself to resemble today's asphalt industry (mostly recycling).

What are other key benefits?

The slowdown or cessation of the expansion of mines may positively affect local communities which live in proximity to limestone-to-cement facilities, which also produces a variety of emissions.

What are the proposal’s costs?

The development of massive solar facilities will cost a large amount of money. The anticipated rough-order-of-magnitude is 1 billion dollars (10^9) dollars to 10 billion (10^10) dollars.

However, the savings over the longer term - 10 to 20 years - will recover this cost.

Time line

Related proposals


1. Ma, X., Han, Z., and Li, X. Reactivity of Dehydrated Cement Paste from Waste Concrete Subject to Heat Treatment. Second International Conference on Sustainable Construction Materials and Technologies (June 2010). Retrieved November 21, 2012 from