Alkali-activation (or geopolymer) technology has gained a great deal of interest for its potential applications in water and wastewater treatment during the last decade. Alkali-activated materials can be prepared via a relatively simple and low-energy process, most commonly by treating aluminosilicate precursors with concentrated alkali hydroxide and/or silicate solutions at (near) ambient conditions. The resulting materials are, in general, amorphous, have good physical and chemical stability, ion-exchange properties, and a porous structure. Several of the precursors are industrial by-products or other readily available low-cost materials, which further enhances the environmental and economic feasibility. The application areas of alkali-activated materials in water and wastewater treatment are adsorbents/ion-exchangers, photocatalysts, high-pressure membranes, filter media, anti-microbial materials, pH buffers, carrier media in bioreactors, and solidification/stabilization of water treatment residues. The purpose of this review is to present a comprehensive evaluation of the rapidly growing prospects of alkali-activation technology in water and wastewater treatment.

Traditionally fly ash is thought to be glassy, spherical particle originating from pulverized coal combustion (PCC) at temperature up to 1700 °C. However, nowadays fluidized bed combustion (FBC) technology is spreading quickly around the world as it is an efficient and environmentally friendly method. FBC is also able to utilize mixtures of low-grade solid fuels (e.g., coal, lignite, biomass, and waste) that have fluctuating quality, composition, and moisture contents. However, this leads to a high variation in the produced fly ash quality, unlike PCC fly ash, and hence challenges when attempting to utilize this fly ash. In this study, the utilization of fluidized bed combustion fly ash (FBCFA) was reviewed using the Scopus database. The most promising utilization target for FBCFA from biomass combustion is as a fertilizer and soil amendment. In construction, the FBCFA from various fuels is utilized as cement replacement material, in non-cement binders, as lightweight aggregates and cast-concrete products. Other types of construction applications include mine backfilling material, soil stabilizer, and road construction material. There are also other promising applications for FBCFA utilization, such as catalysts support material and utilization in waste stabilization.