Concrete That Cleans Water: Steel and Sugar Waste Turned Into Phenol-Scrubbing Building Material

Every year, industries that refine oil, manufacture pharmaceuticals, process coal and produce pulp and paper discharge phenol into waterways around the world. The compound is persistent, toxic and capable of accumulating in living tissue, making it one of the more stubborn pollutants in aquatic ecosystems. Conventional treatment options each carry a catch: biological methods struggle at high phenol concentrations, chemical approaches consume large quantities of reagents and can spawn secondary pollutants, and advanced techniques such as membranes, electrochemical treatment and oxidation demand heavy energy inputs and complex operation. Against that backdrop, a research team in India has reported an unusual candidate for the job: a geopolymer concrete made almost entirely from industrial waste that not only meets structural strength targets but also pulls phenol out of water.

The study, published in Results in Chemistry, describes a dual-functional geopolymer concrete composite synthesised from ground granulated blast-furnace slag, a byproduct of steel processing sourced from Ballari, and sugarcane bagasse ash collected from a sugar factory in the Belagavi district. The researchers combined the two precursors in equal mass and activated them with an alkaline solution of 8 molar sodium hydroxide and sodium silicate mixed at a 1:1.5 mass ratio. Binder, fine aggregate and coarse aggregate were blended at a 1:1.72:2.59 mass ratio, cast into moulds, compacted and cured at room temperature. The result was a workable concrete, with a slump of 112 millimetres, whose compressive strength climbed from 8.53 megapascals at three days to 20.44 megapascals at seven days and 28.97 megapascals at 28 days, comfortably meeting the M25 structural grade target.

What makes the material interesting chemically is the reactive aluminosilicate matrix that forms during geopolymerization. When the alkaline activator dissolves the silica and alumina in the slag and ash, the dissolved species polycondense into a gel network rich in silicon, aluminium and calcium. Scanning electron microscopy revealed a heterogeneous surface studded with irregular particles, agglomerated regions and interconnected voids, the kind of porous architecture that offers both mechanical integrity and abundant access points for pollutant molecules. Energy-dispersive X-ray spectroscopy confirmed a composition dominated by calcium oxide at 48.34 percent, silicon dioxide at 28.09 percent and aluminium oxide at 12.06 percent, with smaller amounts of iron, sodium, magnesium, potassium and sulfur oxides traceable to the precursor wastes and activators.

Spectroscopic analysis filled in the molecular picture. Fourier transform infrared spectroscopy identified broad hydroxyl stretching bands between 3200 and 3600 reciprocal centimetres, H-O-H bending near 1630 to 1650, and the strong asymmetric Si-O-Al and Si-O-Si vibrations between 950 and 1100 that fingerprint the geopolymeric network. X-ray diffraction showed crystalline quartz and a calcium-sodium aluminosilicate phase called bytownite embedded within a broad amorphous hump between 20 and 35 degrees two-theta, the signature of alkali-activated binders. X-ray photoelectron spectroscopy found the surface dominated by oxygen at 37.95 atomic percent, followed by carbon, silicon, sodium, calcium, aluminium and magnesium, indicating a calcium-aluminosilicate surface rich in oxygen-containing groups. Thermogravimetric analysis showed the composite retained roughly 79.2 percent of its mass at 800 degrees Celsius, a mark of good thermal stability.

Porosimetry added the final structural detail: a specific surface area of 49.51 square metres per gram, a total pore volume of 0.076 cubic centimetres per gram and an average pore diameter of 8.03 nanometres, placing the material firmly in the mesoporous regime under IUPAC classification. The nitrogen sorption isotherms displayed an H3-type hysteresis loop between relative pressures of roughly 0.4 and 0.9, associated with slit-shaped pores and irregular, interconnected networks formed by particle aggregation and gel development. Rather than a neatly ordered framework, the pore system is a heterogeneous mix of mesopores, interparticle voids and surface irregularities, which the authors argue is well suited to ferrying phenol molecules toward adsorption sites buried inside the matrix.

Batch adsorption experiments, run in duplicate in 250-millilitre flasks agitated at 150 revolutions per minute, mapped how the composite performs under varying conditions. Adsorption surged during the first hour, reaching 12.21 milligrams per gram, then slowed as internal pores filled and sites became occupied, with equilibrium effectively reached at 180 minutes. Dosage told a classic trade-off story: at 1 gram per litre the capacity peaked near 18.92 milligrams per gram but removal was only about 19 percent, while at 10 grams per litre removal rose to 51.56 percent even as capacity fell to 8.59 milligrams per gram, a consequence of particle aggregation and sites left idle by the fixed phenol concentration. The team settled on 3 grams per litre as the practical optimum, balancing removal efficiency of about 37 percent with efficient use of the material.

Concentration and pH shaped the equilibrium behaviour in revealing ways. As the initial phenol concentration rose from 1 to 100 milligrams per litre, capacity climbed from 0.28 to 18.15 milligrams per gram while removal efficiency dropped from 82.96 to 54.44 percent, reflecting saturation of available sites. Adsorption increased as pH rose from 3 to 9, reaching 16.32 milligrams per gram, but fell back at pH 11. The explanation lies in acid-base chemistry: phenol, with a pKa near 9.9, stays mostly in its neutral molecular form below that threshold, while the composite’s point of zero charge sits around pH 10, so electrostatics play little role. Instead, the gains up to pH 9 reflect reduced competition from hydrogen ions and favourable hydrogen bonding between surface hydroxyl groups and phenol’s oxygen. At pH 11, deprotonation to phenolate ions, a negatively charged surface and hydroxide competition all conspire to suppress uptake. Because performance at neutral pH was nearly as good, the team chose pH 7 for practical operation.

Modelling pinned down the mechanism. Kinetic data fitted the pseudo-second-order model best, with a correlation coefficient of 0.9915 and a calculated equilibrium capacity of 13.81 milligrams per gram closely matching the experimental 12.21, pointing to chemisorption at the surface as the dominant rate behaviour. Intra-particle diffusion plots showed two linear stages, with a non-zero intercept indicating that diffusion alone does not control the rate, and liquid-film diffusion analysis confirmed that external mass transfer contributes during the early phase. On the equilibrium side, the Langmuir model won decisively with a correlation coefficient of 0.9985 and a maximum monolayer capacity of 11.08 milligrams per gram, while the Freundlich model’s strong fit of 0.9951 acknowledged some surface heterogeneity. The Dubinin-Radushkevich model’s poor performance suggested pore-filling is secondary to surface interactions. Taken together with post-adsorption changes in the hydroxyl band of the infrared spectrum and the partially filled pores seen in electron micrographs, the evidence points to hydrogen bonding with hydroxylated aluminosilicate sites, aided by pore diffusion and pore filling, as the principal uptake pathway, with sodium- and calcium-bearing sites playing possible supporting roles.

Reusability results were encouraging if not spectacular. Across six consecutive adsorption-desorption cycles using distilled water as the eluent, desorption efficiency declined from 74.58 percent in the first cycle to 57.33 percent in the sixth, a pattern consistent with a mix of reversible surface interactions and stronger, harder-to-reverse binding, plus some pore blockage. The authors suggest that alternative desorbing agents such as dilute acids, alkaline solutions or organic solvents, along with thermal or oxidation-based regeneration, could push performance further. Benchmarked against previously reported adsorbents, whose capacities range from 1.47 milligrams per gram for sawdust to 14.76 for African potato waste carbon, the composite’s 11.08 milligrams per gram at neutral pH is competitive, and it achieved that figure at a modest 3 grams per litre within 180 minutes.

The broader significance of the work lies in the fusion of two functions usually kept apart. Adsorbents are typically powders or granules with no structural role, while concrete is structural material with no environmental function; this composite is both, valorising two troublesome waste streams in the process. The authors are candid about what remains to be done: fixed-bed column trials to test continuous-flow performance, multi-pollutant experiments to probe competitive and synergistic adsorption, standardised heavy-metal leaching tests to confirm environmental safety, and optimised regeneration strategies for long-term reuse. But the concept, a load-bearing concrete that scrubs phenol from water while diverting steel slag and sugar ash from landfills, offers a glimpse of infrastructure that does more than stand up. It could, in principle, quietly clean as it carries.

Subject of Research: Dual-functional geopolymer concrete from agro-industrial wastes for phenol adsorption from water

Article Title: Evaluation of agro-industrial waste-derived concrete composite for phenol adsorption: Process optimization, kinetics, isotherm modelling and mechanistic insights

Article References: S.V., M., B.V., C., M.S., S., K., S., Y.M., P., S., S., Girish, C. R., Mohan, B., & A., S. M. (2026). Evaluation of agro-industrial waste-derived concrete composite for phenol adsorption: Process optimization, kinetics, isotherm modelling and mechanistic insights. Results in Chemistry, 31, Article 103954. https://doi.org/10.1016/j.rechem.2026.103954

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103954

Keywords: geopolymer concrete, phenol adsorption, ground granulated blast-furnace slag, bagasse ash, waste valorization, water treatment, adsorption kinetics, Langmuir isotherm, mesoporous materials, alkali activation, hydrogen bonding, sustainable construction