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.