Microstructural and strength improvements through the use of Na2CO3 in a cementless Ca(OH)2-activated Class F fly ash system

• The use of Na2CO3 significantly improved strength and microstructure.
• The use of Na2CO3 induced more dissolution of raw fly ash at early ages.
• The use of Na2CO3 promoted more C–S–H [or C–S–H(I)] formation.
• The use of Na2CO3 reduced total porosity and refined pore-size distribution.
• The use of Na2CO3 produced neither geopolymer formations nor pore-filling actions from CaCO3.

This study explores the beneficial effects of Na2CO3 as an additive for microstructural and strength improvements in a Ca(OH)2-activated fly ash system. NaOH-activated fly ash samples were also tested to compare the effect of Na2CO3. Compressive strength testing, XRD, SEM/BSE/EDS, 29Si/27Al MAS-NMR, MIP and TGA were performed. The testing results indicate that the use of Na2CO3 for Ca(OH)2-activation led to a noticeable improvement in strength and microstructure, primarily due to (1) more dissolution of raw fly ash at an early age, (2) more formation of C–S–H [or C–S–H(I)], (3) porosity reduction, and (4) pore-size refinement. We also found that (1) an early high alkalinity from the NaOH formation was not a major cause of strength, (2) geopolymer was not formed despite the early NaOH formation, and (3) no visible pore-filling action of CaCO3 was observed. However, Na2CO3 did not produce any improvement in strength for NaOH-activated fly ash.