Use of Fly Ash, Slag, or Silica Fume to Inhibit Alkali-silica Reactivity

About the report:

This study had two objectives: (1) to evaluate the effectiveness of particular mineral admixtures when combined with portland cements of varying alkali content in preventing expansion due to alkali-silica reactivity (ASR), and (2) to determine if set minimum amounts of various types of mineral admixtures could be established for use with cements of varying alkali content to provide protection from ASR. Mortar bars made with Pyrex glass aggregate were stored moist at 38 ° C for 366 days and measured periodically for length change. Control mortars were mixed with portland cements having an alkali content ranging from 0.18% to 0.92% Na20 equivalent. Test mortars were mixed replacing various percentages of portland cement with Class F fly ash (15%, 25%, and 35%), slag (20%, 35%, and 50%), or silica fume (3% and 7%). Test mortars were also mixed using IP cement produced using Class F fly ash (18%, 20%, and 22%). Expansions of control mortars with alkali contents of 0.4% or less were negligible through the 336-day test period. With cements having an alkali content above 0.4%, equilibrium between cement alkali content and mortar bar expansion was reached at 56 days, when a strong linear relationship between increasing expansion and increasing alkali content developed. A regression analysis was performed on data from the control batches, and the resulting equation was used to normalize data from the test batches. The expansion of mortars containing fly ash, slag, and silica fume was lower than the expansion of the control batches for portland cement having an alkali content of 0.6% and above for all replacement levels of the tested materials. The amount of expansion varied with the type of material, percentage replacement, and cement alkali content. Using an expansion of 0.10% as the maximum indicative of acceptable performance in inhibiting ASR, minimum amounts of Class F fly ash, slag, and silica fume were established for use with portland cement depending on the alkali content of the cement.

Authors