“Metakaolin‐Slag‐Clinker Blends.” The Role of Na⁺ or K⁺ as Alkaline Activators of Theses Ternary Blends

Abstract

This article reports on research into the use of solid alkalis (Na 2 CO 3 and K 2 CO 3 ) 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% cl ınker + 40% blast furnace slag + 40% metakaolin. More specifically, the study aimed to determine the effect of activator dosage (5 and 8 wt%) 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% Na 2 CO 3 . According to the XRD, NMR, and SEM/EDX analyses conducted on the reaction products, the alkalinity and solubilized chemical species generated by adding 5% Na 2 CO 3 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 (3CaOÁAl 2 O 3 ÁCaCO 3 Á 11H 2 O-type) carboaluminates that evolved into the calcite or vaterite forms of calcium carbonate. When K 2 CO 3 was used (instead of Na 2 CO 3 ), a (3CaOÁAl 2 O 3 Á0.5Ca(OH) 2 Á0.5CaCO 3 Á 11H 2 O-type) hemicarboaluminate also formed. The study also revealed that Na + favors coagulation/precipitation more effectively than K + , generating gels with a wider range of Q n species.

Elucidating the Crack Resistance of Alkali‐Activated Slag Mortars Using Coupled Fracture Tests and Image Correlation

“…Recent NMR spectroscopy studies on alkali‐activated slags by the authors indicated an increase in mean silicate chain length (a measure of the degree of polymerization of the gel) with increase in M s in the ranges mentioned in this study . These effects have been reported to be responsible for better mechanical properties . Beyond a certain M s (typically 2.5), excess silicates interfere with gel polymerization, and the strengths have been shown to reduce.…”

“…Based on previous studies, the optimum n value was found to be 0.05. A lower n value results in a lower alkalinity and consequently reduced dissolution and reaction rates whereas a higher value contributes to leaching of excess alkalis and carbonation . The sodium silicate solution used here has a solids content of 36% and a molar M s of 3.3, while the potassium silicate solution has a solids content of 44% and a molar M s of 3.29.…”

Geopolymerization and Its Potential Application in Mine Tailings Consolidation: A Review

“…The alkali activators can be divided into two categories, namely alkalis such as NaOH, KOH, Na 2 CO 3 and K 2 CO 3 and silicate additives such as Na 2 SiO 3 and K 2 SiO 3 Fernández-Jiménez et al 2013;Xiong et al 2004). Table 3 summarizes some experiences for controlling alkali activators from various research groups.…”

“…However, conflicting results were reported when comparing the effects of Na þ and K þ . Fernández-Jiménez et al (2013, 2006a reported that Na þ has a greater capacity than K þ to accelerate the setting and hardening reactions in fly ash geopolymerization when all the other conditions are kept the same, and thus the sodium geopolymers had higher mechanical strength than potassium geopolymers. Based on the results of strength measurements and SEM and MAS NMR analyses, they attributed the observed differences to a balance of two opposing mechanisms: (1) the higher alkalinity of the potassium salts, which induces higher aluminosilicate solubility than sodium salts and (2) the smaller ionic radii of Na þ , which gives the sodium ion a higher charge density than K þ .…”

“…The Na þ ions can thus link the formed silicate and aluminate monomers more effectively for the crystallization of ''pre-zeolitic'' gel, while K þ favors the formation of larger and looser silicate oligomers. However, while Fernández-Jiménez et al (2013, 2006a considered the smaller ionic radii of Na þ ions as the main contributing factor for the higher mechanical strength of sodium geopolymer, van Deventer et al (Duxson et al, 2005b; van Jaarsveld and van Deventer 1999a) emphasized the higher alkalinity of potassium salt as the main contributing factor, and they stated that although potassium geopolymers (fly ash) have higher porosity and a lower degree of crystallinity, they show higher compressive strengths than sodium geopolymers. They explained this effect further by employing 2 H, 27 Al and 23 Na MAS NMR techniques to observe the structural properties and mechanism of (AlðOHÞ 4 À ) formation in metakaolin-based geopolymers.…”