The crystallisation of zeolite (X- and A-type) from fly ash at 25 °C in artificial sea water

The crystallisation mechanism of X- and A-type zeolites from fly ash, pre-treated by fusion with NaOH at 550 °C, was studied by analysing the role of cations during treatment with artificial seawater (salinity: 35‰) at room temperature (25 °C). The results indicated that solubilisation of sodium silicate and sodium aluminosilicate formed from the fusion of fly ash with NaOH determines the precipitation of geopolymers that capture Mg and Ca ions, significantly reducing their concentration in the starting solution. The formation and growth of well-developed crystals of zeolites occur from geopolymeric materials as result of crystalline domain formations that progressively grow both at the liquid–solid interface and within the geopolymeric material. Use of distilled water leads to the formation of geopolymers, but zeolites in this condition did not form despite more than 2 mol of Na remaining in solution after the precipitation of geopolymers, thus indicating that Mg (and possibly Ca) plays a fundamental role in determining the formation of zeolite crystals. A speculative hypothesis can be advanced: Mg induces the formation of zeolite domains that act as templates for the crystallisation and growth, at room temperature, of both X-type and A-type zeolites.The crystallisation of the zeolites in this study follows two distinct trends. In the shorter runs (<96 h), the formation of zeolite A is preferred to zeolite X, but for longer incubation times the X-type become progressively more abundant. The Mg necessary to form the mineral is provided by the geopolymers.

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► Fly ash was converted into zeolites at 25 °C by using artificial seawater.
► The formation of zeolites takes place from geopolymeric materials.
► Crystalline domains grow up at the interface liquid–solid and within the geopolymers.
► Mg (and Ca?) play a fundamental role in determining the formation of zeolite crystals.
► Use of distilled water leads to the formation of geopolymers but not of zeolites.