Fly ash, a by-product of coal combustion in industrial energy production, is generated in substantial quantities at industries in Brunei Darussalam, with an estimated daily output of approximately 9 tonnes. This study presents a detailed mineralogical and physicochemical characterisation of fly ash samples collected from the facilities. X-ray diffraction analysis reveals quartz as the dominant mineral phase, with subordinate zeolites present. Thermogravimetric analysis confirms the thermal stability of the material, supporting its potential for reuse in construction applications. Physicochemical properties of the fly ash samples reveal moisture contents ranging from 2 to 60 wt%, pH values between 7.5 and 8.9, electrical conductivity (EC) from 450 to 3670 µS/cm, and redox potential (Eh) ranging from 163 to 250 mV, indicating moderately oxidising conditions. The material is classified as Class F under ASTM standards and Type F under CSA guidelines. BET surface area measurements indicate moderate porosity, which enhances its reactivity and supports its potential use in cementitious systems and as a sorbent material. These characteristics make the fly ash suitable for incorporation into bricks, concrete, and other geotechnical applications, contributing to sustainable construction practices. Moreover, due to its buffering capacity and mineral composition, the fly ash may also serve as a soil conditioner, potentially aiding in pH regulation and improving soil structure in poor agricultural soils. Trace element analysis indicates relative enrichment of certain elements, which may pose environmental concerns if not properly managed. While current data suggest manageable levels, further geochemical and leaching studies are required to fully assess long-term environmental risks and inform safe reuse strategies. This research highlights the importance of developing integrated waste valorisation frameworks and regulatory support to promote circular economy principles and sustainable industrial development in Brunei Darussalam.

Authigenic pyrite is prevalent in the sedimentary deposits of Brunei Darussalam, occurring in well-defined morphologies such as cubes, pyritohedra, octahedra, and framboidal textures. These sulphide minerals are commonly associated with clay-rich matrices and lignitic horizons, indicative of anoxic depositional environments enriched in organic matter that facilitate pyrite formation. Upon exposure, pyrite undergoes oxidation, often accelerated by microbial activity, producing native sulphur and gypsum. Pyrite oxidation releases Fe²⁺ which is oxidised to Fe³⁺ subsequently forming Fe-oxides such as goethite, hematite and limonite. These phases contribute to the reddish pigmentation of soils and act as cementing agents in arenitic sands, forming ferricrusts. The geochemical alteration of pyrite significantly lowers the pH of surrounding soils, resulting in acidic conditions that pose serious challenges to agriculture and aquatic ecosystems due to the discharge of acidified waters. This study elucidates the mineralogical evolution and environmental consequences of pyrite-bearing sediments in Brunei, underscoring the need for targeted mitigation strategies in acid sulphate soil landscapes.