Heightened understanding of the molecular structure of a key ingredient in cement could enable manufacturers to dramatically reduce energy consumption via modest changes to the production process.
Cement production is a major source of greenhouse gas emissions, due the chemical processes it involves, the copious amounts of energy it consumes, as well as the sheer ubiquity of the material in human society’s built environments – particularly as a key ingredient in concrete.
As much as 5 of anthropogenic carbon dioxide emissions are the result of cement manufacturing – a figure which is on an upward trajectory given a rising production rate of 2.5 per cent per annum.
Researchers at Rice University believe that the impact of cement production on the environment can now be significantly reduced, via modest amendments to the manufacturing process that can increase its overall energy efficiency.
The primary ingredient in Portland cement – the form of the material most commonly used by the global construction industry, is clinker – tiny nodules of calcium silicate that are produced by heating up raw chemical components in a rotary kiln. These minute nodules are subsequently ground into a fine powder in order to form cement.
The creation of clinker is the chief source of CO2 emissions during the manufacture of cement, as a result of the intense heating process and calcination of limestone.
Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering and materials science and nano engineering at rice, and Arup structural engineer as well as Rice alumnus Lu Chen, believe that minor alterations to the clinker production process can save dramatic amounts of energy.
The researchers examined the crystal and atomic structures of clinker during the five phases of the cooling process once it exits the kiln, focusing in particular at internal stresses that can make clinker more brittle, and thus easier to grind, as well as defects referred to as screw dislocations that affect the way the ground powder reacts with water and the strength of the resulting cement.
“Defects form naturally, and you cannot do anything about them,” said Shahsavari. “But the more brittle the clinkers are, the better they are for grinding.”
Their analysis, which has been published in the American Chemical Society journal Applied Materials and Interfaces, concluded that the clinker is at its most brittle as well as most reactive while still within its maximum temperature range.
“We found that the initial phase out of the kiln is the most brittle and that defects carry through to the powder,” said Shahsavari. “These are places where water molecules want to react.”
Equipped with this simple knowledge manufacturers can increase efficiency by reducing the amount of grinding required – a process that comprises 10 – 12 percent of the energy consumed by the cement manufacturing process, or approximately 50 kilograms of CO2 emissions per ton of finished cement.