Interactions between inorganic surface treatment agents and matrix of Portland cement-based materials

• In cement paste, magnesium fluorosilicate may not be limited on the reaction with Ca(OH)2.
• Carbonation plays an important part in watergalss treatment.
• Sodium fluorosilicate can improve the effectiveness of watergalss treatment.

Magnesium fluorosilicate, waterglass, sodium fluorosilicate, and combination of waterglass and sodium fluorosilicate were used as surface treatment agents and their interactions with the Portland cement hydrates were studied. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR) were employed to analyze the phase changes in the hardened cement pastes after surface treatments. A scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) analysis was adopted to examine the changes in morphologies and chemical compositions. The TGA and FT-IR results showed that the inorganic surface treatments could reduce the content of Ca(OH)2 in the surface layer of hardened cement, whereas the amount of calcium silicate hydrate (C-S-H) and silica gel increased. Combined treatment with waterglass and sodium fluorosilicate could generate more gel products, and made the sample surface denser than other treatments used in this work, because sodium fluorosilicate could accelerate the hardening of waterglass, and both waterglass and sodium fluorosilicate could react with cement hydrates respectively.