The research, conducted by Praseeda Dandu and Rithwik Reddy Gattu of VNR Vignana Jyothi College of Engineering and Technology in Hyderabad together with Ravi Sankar Cheela of MVGR College of Engineering in Vizianagaram, set out to answer a deceptively simple question: can vitrified tiles sludge powder, a by-product rich in oxides of silicon and aluminium, serve as a viable precursor material for geopolymer synthesis alongside fly ash? Geopolymers are inorganic binders formed when aluminosilicate materials react with alkaline solutions, bypassing the energy-intensive calcination process at the heart of ordinary Portland cement production.

To find out, the team formulated ten different mix proportions, systematically varying the variables that matter most in geopolymer chemistry. They tested sodium hydroxide concentrations of 3, 6, 9 and 12 molar, adjusted the ratio of sodium hydroxide to sodium silicate activator between 2 and 2.5, and blended the vitrified tiles sludge powder with fly ash at substitution levels ranging from 10 to 50 percent. Each mix was cast into specimens and evaluated for the three parameters that govern brick performance: compressive strength, water absorption and bulk density.

The chemistry behind these bricks is elegant in its simplicity. When the alkaline activator contacts the silica- and alumina-rich particles of fly ash and tile sludge, it dissolves those oxides and reprecipitates them as a three-dimensional network of hydration products. The study attributes the observed strength development primarily to the formation of C-A-S-H, a calcium-aluminosilicate-hydrate phase, and N-A-S-H, its sodium-based counterpart. These interlocking gel phases densify the matrix, filling the microscopic pores between particles and progressively locking the material into a hard, durable solid that mimics the binding action of cement clinker without any of its associated carbon emissions.

The standout formulation emerged from the middle of the experimental range rather than its extremes. A brick containing vitrified tiles sludge powder and fly ash in equal 50:50 proportions, activated with 6 molar sodium hydroxide combined with sodium silicate at a 1:2 ratio, delivered a compressive strength of 23 megapascals at 28 days, comfortably meeting the requirements for load-bearing masonry in most building codes. Water absorption in this optimised mix measured below 6 percent, an important threshold for durability, since porous bricks that drink up moisture are vulnerable to freeze-thaw damage, efflorescence and salt crystallisation over time.

That the highest sludge content proved optimal is one of the study’s most commercially significant findings. In waste-valorisation research, performance frequently degrades as waste substitution rises, forcing engineers to trade environmental benefit for mechanical quality. Here, the opposite occurred: a formulation that diverts the maximum tested quantity of ceramic sludge from the waste stream also produced the best brick. The fine, reactive silica and alumina in the vitrified tiles residue appears to complement the fly ash chemistry rather than dilute it, suggesting a genuine industrial symbiosis in which one factory’s problem becomes another’s raw material.

Strength alone, however, was only half of the story. The researchers also carried out a life cycle analysis comparing their geopolymer bricks with conventional cement bricks across both mid-point and end-point impact categories. Mid-point indicators measure specific environmental mechanisms such as global warming potential, acidification and eutrophication, while end-point categories aggregate those effects into damage to human health, ecosystems and resource availability. The results were striking: the geopolymer bricks showed essentially zero impact on climate change, whereas cement bricks accounted for contributions amounting to roughly 100 percent of the climate change impact in the comparison.

This near-total decarbonisation stems from the fundamental difference in how the two binders are made. Portland cement releases carbon dioxide in two ways: the combustion of fossil fuels to reach clinkering temperatures near 1,450 degrees Celsius, and the stoichiometric release of CO2 as limestone decomposes into lime during calcination. Geopolymerisation sidesteps both processes, requiring only moderately alkaline activation at ambient or mildly elevated temperatures. The sodium hydroxide and sodium silicate activators do carry their own manufacturing footprints, which is precisely why the optimisation of molarity and activator ratio matters, since a leaner 6 molar solution minimises the chemical input while still achieving full geopolymerisation.

Across the remaining mid-point categories, the optimised geopolymer mix showed minimal environmental disturbance compared with both conventional cement bricks and other synthesised alternatives tested in the broader literature. The authors conclude that the resulting green geopolymer brick can serve as a construction material with significant physical and mechanical properties while remaining environmentally competent, a combination that has long eluded waste-based masonry products. Because the bricks simultaneously address two environmental problems, ceramic sludge disposal and the carbon intensity of masonry, they fit squarely within the circular economy framework that regulators and certification schemes increasingly demand from the building sector.

The implications extend well beyond the laboratory. India, where the research was conducted, hosts one of the world’s largest ceramics industries alongside enormous coal-fired power generation that produces vast quantities of fly ash, meaning both precursor materials are available locally and in abundance. If manufacturers can adapt existing brick production lines to handle alkaline activation, the study’s 50:50 formulation offers a template for scaling up: no exotic additives, moderate activator concentrations and a binder built almost entirely from industrial by-products. As the construction sector faces mounting pressure to decarbonise, bricks forged from polished-tile waste and coal ash may prove that the greenest building materials are sometimes the ones made from what industry throws away.

Subject of Research: Synthesis and life cycle assessment of sustainable geopolymer bricks incorporating vitrified tiles sludge powder and fly ash

Article Title: Synthesis and environmental impact assessment of sustainable geopolymer bricks incorporating vitrified tiles sludge powder

Article References: Dandu, P., Cheela, R. S., & Gattu, R. R. (2026). Synthesis and environmental impact assessment of sustainable geopolymer bricks incorporating vitrified tiles sludge powder. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38247-0

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38247-0

Keywords: geopolymer bricks, vitrified tiles sludge, fly ash, life cycle assessment, sustainable construction, compressive strength, sodium hydroxide, sodium silicate, industrial waste valorisation, C-A-S-H, N-A-S-H, carbon footprint