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Reassessing Cement: Choosing an Optimal Particle Size Distribution to Mitigate Shrinkage

Concrete – a mixture of cement, sand, stone and water that hardens to a stone-like mass – is the most common construction material in the world. When mixed, the water and cement undergo a chemical reaction – called hydration – that ‘glues’ the sand and stone together, yielding a semiliquid mixture that hardens in less than 24 hours but requires 7 – 28 days of ‘curing’ to reach its design strength.

As the concrete cures, the excess water in the mixture will evaporate, causing it to shrink. This shrinkage results in an increase in tensile stress, which may lead to cracking, internal warping and external deflection, before the concrete is subjected to any kind of loading.

“Water is necessary to hydrate the cement, but for a complete reaction, only about one part of water is required for every three parts of cement,” explains Prannoy Suraneni, an assistant professor in the College of Engineering’s Department of Civil, Architectural and Environmental Engineering (CAE). “In practice, however, a larger water-to-cement ratio is utilized. This additional water is needed to improve the workability of concrete for proper mixing and pouring. Therefore, about a third of the water added during mixing is chemically unnecessary and will evaporate, causing shrinkage as it leaves the concrete.”

Shrinkage – and the subsequent cracking – is a major issue for concrete infrastructure because the cracks provide a pathway for deleterious species to seep into the concrete, reducing the expected service life. Much has been done to prevent shrinkage, including the use of supplementary cementitious materials, internal curing and the use of shrinkage reducing admixtures. However, these methods may be expensive and impractical.

“Altering the particle size distribution of the cement to improve shrinkage behavior while not adversely affecting workability and hydration has not been explored in detail,” says Suraneni. “If optimal cement particle size distributions that reduce shrinkage – without compromising workability and hydration – could be found, then this would result in more durable infrastructure without significantly affecting the cost.”

In the coming months, Suraneni, along with Sivakumar Ramanathan, a first year PhD student in CAE, will evaluate whether there exist particle size distributions that result in optimal properties for the cement paste and mortar, specifically in terms of workability, hydration and shrinkage.

The project will focus on using the commercially available cement to make several different cements (in terms of particle size distributions): the reference cement, finer cements, coarser cements and bimodal cements. These cements will be tested, evaluated and compared to determine the effect of the particle size distribution on the workability, hydration, strength and shrinkage behavior of the pastes and mortars. Particle size distributions which provide optimal workability, hydration, strength and shrinkage behavior will be identified.

“By identifying the optimal particle size distributions, we may be able to significantly reduce cracking due to shrinkage, increasing service life and decreasing repair costs of concrete structures worldwide,” Suraneni says.

The study, funded by Portland Cement Association, a non-profit organization that promotes the use of concrete, is entitled, “Cement particle size distributions that optimize workability, hydration, strength, and shrinkage properties.”

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