Permeation and Carbonation of Nano-HVFA Composites: Long-Term Studies

The development and modification of high volume fly ash (HVFA) concrete using nanomaterials to enhance the strength has advanced significantly of late. However, the long-term behavior of HVFA concrete with nanomaterials, including permeation and durability characteristics, have yet to be studied in detail. This study reports the long-term permeation and diffusion characteristics and the carbonation of nanoengineered HVFA concretes utilizing up to 80% cement replacement with fly ash, hydrated lime, and nanosilica. The 450-day compressive strength of the HVFA concrete was 73 MPa. The air and water permeability indices of HVFA-65 and HVFA-80 concretes (65% and 80% cement replaced) decreased from 0.058 to $0.039\mathrm{Ln}(\mathrm{mbar})/\min$ and $0.3–0.1(\times {10}^{-7}{\mathrm{m}}^3/{\min }^{0.5})$ between 28 and 450 days, and $0.078–0.043\mathrm{Ln}(\mathrm{mbar})/\min$ and $0.5–0.2(\times {10}^{-7}{\mathrm{m}}^3/{\min }^{0.5})$, respectively. These permeation values were significantly lower than the specified indices for low permeability concretes. The presence of pores, ranging from $30\mathrm{\mu m}$ to 1 mm, was observed to be the principal influence on the permeability properties. The pore size and the quantity of pores decreased significantly in the HVFA composites over time. The C─ A─ S─ H gel products formed [$\mathrm{Ca}/\mathrm{Si}=0.43\mathrm{and}\mathrm{Ca}/(\mathrm{Si}+\mathrm{Al})=0.25$] increased both pore discontinuity and the strength of the interfacial transmission zone (ITZ). This increased the packing density in the gel matrix correlated with enhanced durability. The chloride diffusion coefficients of HVFA-65 and HVFA-80 decreased from 3.88 to $0.60(\times {10}^{-12}{\mathrm{m}}^2/\mathrm{s})$ and from 4.06 to $1.61(\times {10}^{-12}{\mathrm{m}}^2/\mathrm{s})$, respectively, between 28 and 450 days. Furthermore, both HVFA-65 and HVFA-80 composites displayed low carbonation coefficients, 2.93 and $4.73{\mathrm{mm}/\mathrm{month}}^{0.5}$, respectively. Calcite was observed at 28 days and was attributed to the carbonation of the hydrated lime. Calcite intensity in the HVFA concretes was observed to decrease up to 90 days; this was attributed to dissolution in pore water to form the more soluble calcium bicarbonate. However, from 90 to 450 days, carbonation led to the accumulating of additional undissolved calcite (calcium carbonate) within the HVFA composites over the long term.