Effect of sodium silicate on Portland cement/calcium aluminate cement/gypsum rich-water system: strength and microstructure

Abstract

In this investigation, sodium silicate (SS) was mixed into rich-water (RW) materials consisting of Portland cement, calcium aluminate cement and gypsum for improved mechanical properties. The RW materials containing different amounts of SS were characterized by the compression test, mercury intrusion porosity, scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The results demonstrated that with the increase of SS additions, the early strength of the RW materials increases, and the long-term strength retrogression of the RW materials can be inhibited when the SS content is above 3%. Pore structures of the RW materials are improved significantly due to the filling effect of the calcium silicate hydration (C–S–H) gel from a reaction between silicate ions and Ca(OH)₂, thus increasing the early strength of the RW materials. For the RW materials containing SS and cured for 0 to 14 days, there are more hexagonal hydrates including CaO·Al₂O₃·10H₂O (CAH₁₀) and 2CaO·Al₂O₃·8H₂O (C₂AH₈), more C–S–H gel and less ettringite crystals, which is of benefit to the strength of the material. The strength retrogression can be attributed to phase conversions from hexagonal hydrates (CAH₁₀ and C₂AH₈) to cubic ones (3CaO·Al₂O₃·6H₂O) with lower intercrystal bonding forces. Furthermore, this phase conversion is inhibited effectively by the chemical reaction of silicate ions and CAH₁₀ (or C₂AH₈), improving the long-term strength of the RW materials.