Influence of Mix Composition on the Microstructural Evolution of Leached Cement Pastes

Area of Science:

• Materials Science
• Civil Engineering
• Concrete Technology

Background:

• Calcium leaching in hydraulic concrete increases porosity and diffusion, compromising structural integrity.
• Fly ash and silica fume are common admixtures, but their impact on leaching resistance needs further study.

Purpose of the Study:

• To investigate the effects of fly ash and silica fume on the microstructural and transport properties of hydraulic concrete during calcium leaching.

Main Methods:

• Calcium leaching tests were performed on cement paste (CP), silica fume-cement paste (SF), and fly ash-cement paste (FA) using ammonium chloride solution.
• Analyses included phenolphthalein spraying, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM).
• Diffusion coefficients were calculated and analyzed at various depths.

Main Results:

• Silica fume and fly ash increased initial porosity and diffusion coefficients.
• After 28 days of leaching, porosity increased by 83.6% (CP), 11.0% (SF), and 39.0% (FA) in the surface layer.
• Diffusion coefficients increased by factors of 14.3 (CP), 6.1 (SF), and 13.6 (FA), showing SF and FA enhance leaching resistance.

Conclusions:

• Partially replacing cement with silica fume or fly ash improves resistance to calcium leaching.
• The pozzolanic reaction of admixtures with calcium hydroxide forms additional calcium silicate hydrate (C-S-H) gel, reducing capillary pores and enhancing durability.