A new lightweight concrete made from coal ash, discarded polystyrene packaging and seawater has achieved the unusual combination of structural strength, thermal insulation and sound absorption without using freshwater or heat curing. In laboratory tests, the strongest formulation reached a 28-day compressive strength of 26.8 megapascals while weighing 1,498 kilograms per cubic metre—substantially less than ordinary concrete. The material was developed by researchers in Thailand as an alkali-activated alternative to conventional Portland-cement concrete, whose manufacture is responsible for a major share of global industrial carbon dioxide emissions. The findings suggest that waste streams and seawater could be combined to produce lighter building components for walls, floor systems and façade panels, although the material is not yet suitable for use with ordinary steel reinforcement.
The researchers built the composite around high-calcium fly ash collected from the Mae Moh coal-fired power plant in Lampang, Thailand. Fly ash is a fine industrial residue whose glassy aluminosilicate phases can be chemically dissolved and reorganized into a hardened inorganic binder. X-ray fluorescence analysis showed that the ash contained 45.23 percent silicon dioxide, 19.94 percent aluminium oxide and 15.50 percent calcium oxide, along with iron, magnesium and smaller quantities of other oxides. Microscopy revealed mostly spherical particles, which can act like tiny ball bearings and improve the workability of fresh mixtures. X-ray diffraction identified a broad amorphous halo—evidence of reactive glass—together with crystalline quartz, mullite, magnetite, hematite and free lime. The ash was classified as a Class F pozzolan under ASTM standards, but its relatively high calcium content gave it a more complex reaction pathway than low-calcium fly ash.
To activate the ash, the team combined sodium hydroxide with liquid sodium silicate, replacing freshwater entirely with untreated seawater collected from the Gulf of Thailand near Chonburi. The seawater contained dissolved sodium and chloride as its dominant ions, as well as sulfate, magnesium, potassium and calcium. These ions are not chemically passive spectators. In an alkali-activated binder, hydroxide ions attack the glassy aluminosilicate structure, releasing silicon and aluminium into solution. Those species then polymerize into a three-dimensional sodium aluminosilicate hydrate, or N-A-S-H, network. Calcium from the fly ash and Portland cement can simultaneously promote calcium silicate hydrate and calcium aluminosilicate hydrate, commonly written as C-S-H and C-A-S-H. The resulting hybrid gel structure can harden at room temperature, while chloride and sulfate ions from seawater alter dissolution, precipitation and setting reactions.
The solid binder contained 90 percent fly ash and 10 percent ordinary Portland cement. That small cement addition was designed to provide extra reactive calcium and improve early strength and setting under ambient conditions without displacing the predominantly waste-derived binder. River sand supplied the fine mineral skeleton, while mechanically crushed expanded polystyrene, or EPS, replaced part of the denser matrix. The recycled particles measured between 2.36 and 4.75 millimetres and had a bulk density of only 26.84 kilograms per cubic metre. EPS is mostly closed-cell polymer foam, so it contributes little mass while trapping air—an arrangement that can slow heat flow through a wall. The researchers tested five EPS contents, from 0.80 to 1.60 percent of total binder mass, and compared activators containing either 5-molar or 10-molar sodium hydroxide.
The central surprise was that the lower-alkalinity mixture performed better. In conventional freshwater geopolymer systems, increasing sodium hydroxide concentration often improves precursor dissolution and compressive strength. The researchers therefore treated 10 molar sodium hydroxide as a conventional benchmark. But seawater changed the balance. At high alkalinity, the combination of hydroxide, chloride, sulfate and magnesium accelerated reactions so aggressively that the paste began to coagulate before it could be properly mixed and compacted around the hydrophobic EPS particles. This rapid setting trapped large air voids and disrupted the formation of a continuous binder network. By contrast, the 5-molar seawater activator slowed the reaction enough for the aluminosilicate network and calcium-rich gels to develop more evenly, producing a denser and stronger microstructure.
The best mechanical result came from the 5M-E0.80 mixture, containing 0.80 percent EPS and the 5-molar seawater-based activator. After 28 days of sealed ambient curing at approximately 25 degrees Celsius, it reached 26.8 megapascals. That is above the commonly cited threshold of 17 megapascals for structural lightweight concrete and exceeds the 13.1-megapascal minimum referenced for load-bearing lightweight masonry units. Increasing EPS content progressively reduced strength: in the 5-molar series, the 28-day value fell from 26.8 megapascals at 0.80 percent EPS to 9.6 megapascals at 1.60 percent. The polymer particles behave as soft inclusions rather than load-bearing aggregate. Under compression, stress concentrates around their boundaries, where the chemically incompatible interface can initiate microcracks. The 10-molar series was weaker at every EPS dosage; at 0.80 percent EPS, it reached 19.5 megapascals, about 27 percent below the matching 5-molar specimen.
The same pores that weakened the most highly expanded mixtures also gave the material functional advantages. In the 5-molar samples, increasing EPS from 0.80 to 1.60 percent raised apparent porosity from 34.8 to 53.6 percent and water absorption from 1.92 to 5.74 percent. EPS itself does not absorb water, but its water-repellent surface forms a weak interfacial transition zone with the surrounding hydrophilic paste. That boundary can create interconnected capillary pathways. Thermal conductivity declined as EPS content increased, reaching 0.387 watts per metre-kelvin in the most porous 10-molar mixture at 1.60 percent EPS. However, that sample suffered a major loss of strength. The more useful compromise was again 5M-E0.80, which combined 26.8 megapascals of compressive strength with a thermal conductivity of 0.718 watts per metre-kelvin.
The composite also displayed measurable acoustic performance. Using an impedance tube, the researchers examined sound absorption between 400 and 2,500 hertz. Most mixtures recorded absorption coefficients between 0.05 and 0.20, higher than the value below roughly 0.05 typical of dense concrete. The strongest response came from the 5M-E1.20 formulation, which reached a peak absorption coefficient of 0.358 near 500 hertz. The 5M-E0.80 sample reached 0.220 at the same frequency, while the best 10-molar mixture peaked at only 0.189. Sound absorption in the material arises as air motion enters pores and narrow channels, generating viscous friction that converts acoustic energy into heat. The researchers argue that controlled reaction kinetics in the 5-molar system preserve a tortuous network of micro-capillaries, whereas rapid setting in the 10-molar mixture creates irregular macropores that are less effective at dissipating sound.
The study presents a striking sustainability package: coal ash is diverted from waste storage, EPS packaging is reused, freshwater is removed from the mix and no elevated-temperature curing is required. Yet the material’s advantages come with important boundaries. Raw seawater introduces a high chloride load, making conventional steel reinforcement vulnerable to corrosion even if some chloride becomes chemically bound within the alkaline matrix. Near-term applications would therefore be limited to unreinforced blocks, precast wall elements, insulating façade panels and other plain-concrete components, or to structures using corrosion-resistant reinforcement such as fibre-reinforced polymer bars. The experiments also covered only one- and 28-day strength, leaving long-term questions about creep, drying shrinkage, efflorescence, fire behaviour, freeze–thaw resistance and degradation of the EPS interface unresolved. The results are consequently best viewed not as a ready-made replacement for all concrete, but as evidence that carefully tuned seawater chemistry can turn problematic wastes into a structural-grade, multifunctional building material.
Subject of Research: Ambient-cured lightweight alkali-activated concrete made with high-calcium fly ash, Portland cement, recycled EPS foam and raw seawater
Article Title: Lightweight FA–OPC Alkali-Activated Concrete Incorporating Recycled EPS Foam and Raw Seawater
Article References: Amornpinyo, P., Chindaprasirt, P., Posi, P., Tankasem, P., Rukzon, S., Srirueng, P., Sirisripetch, Y., and Bubpi, A. Original research article URL not provided.
Image Credits: AI Generated
DOI: Not provided
Keywords: seawater concrete, alkali-activated materials, fly ash geopolymer, recycled EPS foam, lightweight concrete, thermal insulation, sound absorption, sustainable construction
Tags: alkali-activated cement alternativeseco-friendly wall and façade panelsenvironmentally friendly building materialsfly ash concrete developmentindustrial waste reuse in constructionlightweight structural building componentslow-carbon footprint building materialsrecycled foam lightweight concreteseawater in concrete productionseawater-based construction materialssustainable construction innovationsthermal insulation sound absorption concrete
