Synthesis and characterization of calcium silicate hydrate from biomass fly ash

The purpose of this research is to assess the possibility of producing calcium silicate hydrates from biomass ash using techniques that include milling, washing, and subjecting the waste to hydrothermal treatment. The effect of the water/solid ratio and milling parameters on the chemical composition of the fly ash was investigated. Subsequently, hydrothermal synthesis was employed at a saturated steam temperature of 200 °C, with an isothermal holding time of 2–24 h on the unwashed milled ash, another with milled washed ash at a water-to-solid ratio of 5, and a third with modified milled washed ash by adding additional CaO. During the investigation of changes in sample composition, the following methods were used: XRD, XRF for the identification of mineral and chemical composition of solid materials, DSC-TG for determination of thermal properties of materials, and ICP-MS for determination of chemical composition of liquid media. It was found that the duration of the intensive milling process did not affect the mineral composition of the ash. The Ca²⁺, K⁺, Na⁺, Mg²⁺ cations and the SO ₄ ²⁻ , PO ₄ ³⁻ anions exhibited predominant leaching from the milled ash. It was found that the unwashed biomass fly ash is a suitable material for the synthesis of calcium silicate hydrates, because after hydrothermal treatment the main product of synthesis was tobermorite. Within 2–72 h of the hydrothermal treatment, the intensities of the diffraction peaks of the mentioned compounds increased twice. The additional washing of biomass fly ash inhibits the formation of calcium silicate hydrate compound in the synthesis products. After 2–24 h of isothermal curing, only traces of tobermorite were identified. The addition of CaO to the washed biomass fly ash sample changed the reactivity of the primary compound and accelerated the formation of tobermorite in the synthesis products.