New study finds fly ash reduces expansion in cement

According to a new research report by the Portland Cement Association, substituting fly ash for part of the sand or cement in concrete mixtures can help control expansion in cold weather.

The report, “Substitution of Fly Ash for Cement or Aggregate in Concrete: Strength Development and Suppression of ASR,” takes an in-depth look at how fly ash affects the strength and expansion of concrete in different climates.

The report evolved over questions about the effectiveness of Class F fly ash in concrete mixtures and concern about the deterioration of pavements due to alkali-silica reaction. While substituting fly ash in concrete does help halt cold-weather expansion, it also generally reduces the rate of strength gain. It is for this reason that many highway departments usually allow its use only during warm weather.

In its research, PCA tested concrete mixtures with the addition, substitution or partial substitution of Class F fly ash at rates of 20 and 30 percent by mass of cement. The findings showed that the fly ash retarded the strength gain at the early ages, especially when cured at low temperatures. The report also found that maintaining the cement content mitigated that effect. In some cases, additional or partial substitution of fly ash for the cement was adequate to control expansion, but that was not always the case.

In its final results, PCA’s report concluded that for late fall concreting, the use of Class F fly ash with increased cement contents offer a good possibility of controlling expansions due to alkali-silica reaction with adequate rates of strength gain. However, PCA suggests tests using local materials should be done to determine which mix would bring the right rate of strength gain.

PCA’s report is available for $17 at or by calling 1-800-868-6733.