Semi-dry flue gas desulfurization (FGD) material is a byproduct generated from the desulfurization system of coal-fired power station. The beneficial reuse application for dry FGD material is relatively undeveloped. FGD was used as raw material for the synthesis of value-added materials by alkaline hydrothermal treatment. Different experimental conditions, such as, reaction time, temperature, alkali hydroxide concentration and solid/liquid ratio were studied. Raw material and synthesis products were characterized by XRD, XRF, particle size analyser, TG-DTG-DTA, and FGD was classified according to Brazilian Environmental Regulations. The results show that dry FGD has a higher CaO and SO3 content and the major minerals present in material are hannebachite, anhydrite, calcite, portlandite, gehlenite and sodium carbonate. The size of dry FGD ash particles is around 0.399-355.656 ??m with median diameter of 7.63 ??m. Thermal behaviour of FGD was characterised by the existence of six and four stages under air and inert atmosphere, respectively. FGD can be classified as Class II A material (non-dangerous/non-inert). The presence of zeolite hydroxysodalite confirms successful conversion of FGD into zeolitic material after activation in NaOH solutions. Along with the zeolitic product were obtained katoite, hydrocalumite and Al-substituted tobermorite. All synthesised materials present ionic exchange capacity and could be used as a low-cost adsorbent. FGD utilisation minimises the environmental impact of disposal problems and further enables an application for treatment of wastewater.