At the recently concluded Advances in Cement-based Materials meeting organized by ACerS’ Cements Division and ACBM, Karen Scrivener of the Ecole Polytechnique Fédérale de Lausanne (Switzerland) was selected by the organizers to deliver the Della Roy Lecture. Scrivener is a highly respected expert in the field of cements and she was an appropriate pick, having followed in Della Roy’s footsteps as the editor of the Cement and Concrete Research journal.
Scrivener is also the founder of Nanocem, a Europe-based initiative working on collaborative approaches (not just among institutions, but also between institutions and industry) to cements questions. One of the aims of Nanocem is to spur a constant effort to reduce CO2 emissions:
“Evolving CO2 emission caps in Europe mean that cement manufacturers will have to find solutions, or pay more to produce cement, which will reduce their competitivity. Nanocem is sponsoring fundamental research that will support technological solutions, not only to help secure the long-term health of the European cement industry, but also to address global CO2 reduction by ensuring that cement manufacturing is not just shifted to regions of the world that have less stringent emissions regulations.”
The title of Scrivener’s Della Roy Lecture was “Modeling Hydration Kinetics of Cementitious Systems,” which was quite a good discussion about the what has and what’s yet to be done in the world of modeling cement microstructures.
But, as a non-expert in cements, I found Scrivener’s opening remarks, which provided the context for her technical presentation, a balanced and compelling discussion about the importance of cements and concrete to, well … the world!
A few of her salient points:
• Concrete is most used material in the world. It is the only material that can satisfy the demand for low-cost decent housing and infrastructure. There is no way to satisfy the demand for low-cost housing and infrastructure without concrete.
• The demand for concrete is growing and will continue to soar, especially in the developing nations. The demand may double or triple by 2050.
• Although cement production is energy intensive, the energy and CO2 emissions of concrete (per ton) is among the lowest of all building materials, even lower than wood.
• The CO2 problem associated with cements (5-8% of CO2 production world wide) is primarily because of the volume of demand.
• There has been talk of achieving 5-10% reduction in emissions per cubic meter of concrete through the use of substitutes for Portland cement. It may be more like 1-2% given the amounts and choices of supplements (see below), but even a 1% saving would be equal to removing all the CO2 emissions associated with steel production. “So that shows how much impact we can have by research to increase the sustainability of cement,” she said.
• The available elements in the earth’s crust imposes a fundamental limit on the options for substitute cementitious materials. Eight elements—oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium—make up 98 percent of earths crust. So, forget about making cement out of any other elements.
• The way this has been pursued over the last 20-some years is first of all process optimization. Cement kiln and other production operations have gotten much better and state-of-the-art plants are achieving 80% of their theoretical efficiency.
• So, recently the goal has shifted more to reducing the “clinker factor,” i.e., instead of grinding clinker and gypsum, add more and more supplementary cementitious materials. SCMs my be byproducts or waste products from other industries, such as limestone, fly ash, blast furnace slag, silica flume, natural pozzolans, etc.
• This has been a good and successful strategy (over the last 20 years 25% of the previous amount of clinker in cements is now substituted by SCMs).
• But … and this is a big “but” … this is going to be a difficult strategy going forward because under the best of circumstances, the amount of SCM available will be dwarfed by amount of cement produced.
• For example, people talk about using fly ash as an SCM and it is probably the most widely available. But, there simply isn’t enough fly ash worldwide to replace cement in any big amount. The availability of SCMs in underdeveloped countries, where the demand is going to be coming from, is small because by definition they don’t have the scale of industries to provide large volumes of slag and fly ash byproducts.
• So where are SCMs going to come from? More limestone, probably, but more calcined clays and natural pozzolans. Cuba may be a good source of calcined clays.
• There is no one single answer. All sustainability possibilities must be pursued in parallel. Eventually we are going to see a very diverse range of cements, which are adapted to locally available materials. But diversity means performance questions will be more and more complicated.
• For researchers to support sustainability, they must provide end users of concrete information that will make them confident about the use of the SCMs. This means having relevant performance tests. In particular, researchers need to know how to start with the variables—the composition of the cements, the SCMs, mixing techniques, curing times, relative humidities, etc.— and from this predict performance.
• Short term performance can be relatively easy to predict in the laboratory, but long term performance—where we expect structures to last 40-50 years without hardly any maintenance—is much harder to measure in the lab.
• Therefore, researchers have to pursue mechanisms to predict microstructure via advanced modeling techniques to pull all of this complex information together and have it make sense.