"Metakaolin-Slag-Clinker Blends." The Role of Na plus or K plus as Alkaline Activators of Theses Ternary Blends

This article reports on research into the use of solid alkalis (Na2CO3 and K2CO3) as activators to obtain hybrid cement (cement whose hydration generates a mix of C-A-S-H and (N,C)-A-S-H gels) from a blend of 20% clinker+40% blast furnace slag+40% metakaolin. More specifically, the study aimed to determine the effect of activator dosage (5 and 8wt%) and type of alkaline cation (Na+ or K+) on the 2- and 28-d mechanical strength of the end materials. The findings showed that the highest mechanical strength values were obtained with 5% Na2CO3. According to the XRD, NMR, and SEM/EDX analyses conducted on the reaction products, the alkalinity and solubilized chemical species generated by adding 5% Na2CO3 to the system yielded a mix of (N,C)-A-S-H and C-A-S-H cementitious gels as the main reaction products. The secondary reaction products included metastable (3CaO center dot Al2O3 center dot CaCO3 center dot 11H2O-type) carboaluminates that evolved into the calcite or vaterite forms of calcium carbonate. When K2CO3 was used (instead of Na2CO3), a (3CaO center dot Al2O3 center dot 0.5Ca(OH)2 center dot 0.5CaCO3 center dot 11H2O-type) hemicarboaluminate also formed. The study also revealed that Na+ favors coagulation/precipitation more effectively than K+, generating gels with a wider range of Qn species.