Admixture compatibility in metakaolin–portland-limestone cement blends

Despite potential benefits including enhanced mechanical properties, reduced permeability, and decreased environmental impact, higher rates of metakaolin substitution for cement (> 10% MK by mass of cement) have been limited practically because of lower workability in such mixtures. When metakaolin is combined with portland-limestone cement (PLC), potential synergies further enhance hardened concrete performance and decrease environmental impact, but the greater surface area of PLCs suggests even greater challenges for workability in such blends. This study evaluates four water-reducing admixture chemistries-polycarboxylate ether (PCE), calcium lignosulfonate (LS), naphthalene formaldehyde condensate (PNS) and polymelamine sulfonate (PMS)—to assess their effectiveness in metakaolin-PLC combinations at up to 30% MK. At that upper bound, only PCE and PMS impart adequate workability within their recommended dosage limits. While PMS delays tricalcium silicate hydration and LS shows significant incompatibilities at higher MK contents, PCE has the least effect on hydration and can be used at a consistent dosage rate up to 30% MK.