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

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. -- Highlights: •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