The work tackles a looming supply problem at its root. Alkali-activated materials, which harden when alkaline solutions dissolve aluminosilicate-rich precursors into binding gels, have long relied on ground granulated blast furnace slag, a steelmaking byproduct prized for its rapid reactivity and its ability to form dense calcium-aluminosilicate-hydrate gels. But global slag production sits at only about 330 million tonnes per year, and its share of total cement production has fallen from roughly 17 percent in 1980 to nearly 8 percent in 2014. As steelmakers decarbonize and slag gets snapped up for cement blending, researchers are racing to find locally available substitutes—and the Algerian team turned to materials that would otherwise clog landfills.

The first substitute, ceramic sanitary ware waste, is chemically ideal: it is rich in silicon dioxide (62.7 percent) and aluminum oxide (28.5 percent), the raw ingredients of aluminosilicate reaction networks. The second, eggshell powder, is almost pure calcium carbonate, with an oxide-equivalent calcium oxide content of 55.4 percent. Both were crushed and ground for one hour, with the ceramic powder emerging as the finest precursor at a median particle diameter of about 10 micrometers and the eggshell powder the coarsest at roughly 30 micrometers. Scanning electron microscopy revealed sharp, flaky ceramic fragments and porous, loosely packed eggshell platelets—morphologies that would prove decisive in how each material behaved in the fresh and hardened mortar.

Rather than testing mixes haphazardly, the researchers employed a central composite design, a response-surface statistical framework that maps the effects of two variables—ceramic and eggshell replacement levels, each varied from 0 to 25 percent of the slag content—using just nine experimental runs. Crucially, the alkalinity of the activator, a blend of 10-molar sodium hydroxide and sodium silicate, was held nearly constant across all formulations, at an alkali equivalent of about 17.1 percent. That design choice ensured that any differences in performance could be attributed to the precursor chemistry itself rather than to varying activator dosages, giving the study its statistical teeth.

The results revealed a striking trade-off between early and long-term performance. Ceramic waste slowed early strength gain, because its stable aluminum-oxygen-silicon bonds dissolve sluggishly in alkaline solution, delaying the release of reactive species. Yet by 28 days the picture had reversed: the mix containing 12.5 percent ceramic waste reached a compressive strength of 84.82 megapascals, exceeding the pure-slag reference at 80.41 megapascals, as the slowly dissolving ceramic steadily fed silica and alumina into the growing gel network and densified the microstructure. Eggshell powder behaved differently—moderate amounts boosted early strength through a filler effect, with one mix hitting 47.94 megapascals at just 7 days, but at 25 percent replacement strength collapsed to as low as 59.81 megapascals, a casualty of the eggshell’s low reactivity and its porous, irregular particles that introduce weak zones into the matrix.

Durability against carbonation—the process by which atmospheric carbon dioxide neutralizes the alkaline phases that protect steel reinforcement—emerged as the study’s most nuanced finding. Carbonation chemistry in alkali-activated systems differs fundamentally from that in Portland cement: in calcium-rich slag gels, carbon dioxide extracts calcium ions, progressively decalcifying the binding phase and shifting it toward brittle, silica-rich residues, while in low-calcium systems the pore solution is neutralized with less structural damage. After 28 days in a chamber at 3 percent carbon dioxide, the ceramic-rich mixes fared worst, with several showing near-complete alkalinity loss and strength losses up to 30.79 percent. The eggshell-containing mixes, by contrast, resisted degradation remarkably well: the best one lost only 12.05 percent of its strength, because the abundant calcium supplied by the eggshell buffered the carbon dioxide attack, stabilizing the gel structure and even plugging pores with newly precipitated calcium carbonate.

Spectroscopic and thermal analyses laid bare the molecular drama unfolding inside the carbonated samples. Infrared spectroscopy showed the main silicon-oxygen stretching band migrating to higher wavenumbers after carbonation, and Gaussian deconvolution revealed that the fraction of highly polymerized silicon units jumped from 31 to 47 percent in the pure-slag mix—an apparent increase in connectivity that actually signals degradation, as calcium-rich units are stripped away and replaced by condensed but mechanically feeble silica residues. Energy-dispersive X-ray analysis confirmed that calcium-to-silicon ratios in the binding gels plummeted from values as high as 4.98 down to roughly 0.7 to 0.82 after exposure, while carbon-to-silicon ratios soared, marking wholesale carbonate formation. In the pure-slag sample, the team even identified elongated sodium-carbonate crystals containing about 23 percent sodium and 56 percent carbon by atomic content—direct evidence of alkali ions migrating out of the destabilized gel and reacting with the invading carbon dioxide.

Feeding all measured responses into a multi-objective desirability optimization, the models converged on an ideal formulation containing roughly 19 percent ceramic waste and 16 percent eggshell powder, with a composite desirability of 0.876. This predicted optimum promises a 28-day strength of about 81.6 megapascals, porosity of 16.4 percent, and a carbonation-induced strength loss capped near 17.7 percent—figures the authors caution remain model predictions pending experimental validation. A sensitivity analysis across fifteen weighting scenarios barely moved the outcome, suggesting the optimum is robust. The environmental accounting, estimated using Bond’s law for grinding energy under both nuclear-based and gas-based electricity grids, placed the optimized binder’s embodied carbon at roughly 136 to 230 kilograms of carbon dioxide per cubic meter, averaging about 183 kilograms—well below ordinary Portland cement, calcium aluminate cement, calcium sulfoaluminate cement, and even limestone calcined clay systems when normalized by strength.

The implications ripple well beyond the laboratory. If a third of the slag in an alkali-activated binder can be swapped for bathroom ceramics and breakfast leftovers without sacrificing strength or durability, the approach could simultaneously relieve landfill pressure, hedge against future slag scarcity, and slash the carbon intensity of construction in regions where industrial byproducts are scarce but household waste is abundant. The authors are careful to note the caveats: the carbonation indicator used does not directly measure true corrosion risk, the optimum has not yet been experimentally cast, and the carbon analysis excludes transportation and collection. But as a proof of concept, the study makes a compelling case that the future of low-carbon construction may be hiding in demolition dumpsters and kitchen compost bins—waiting only for the right chemistry to set it in stone.

Subject of Research: Multi-performance optimization of alkali-activated mortars using ceramic and eggshell waste precursors with carbonation durability and carbon footprint assessment

Article Title: Multi-performance optimization of ceramic and biogenic wastes-based alkali-activated mortars with environmental benefits and carbonation durability assessment

Article References: Berkouche, A., Belkadi, A. A., Chiker, T., Cousture, A., & Aggoun, S. (2026). Multi-performance optimization of ceramic and biogenic wastes-based alkali-activated mortars with environmental benefits and carbonation durability assessment. Case Studies in Construction Materials, 25, Article e06554. https://doi.org/10.1016/j.cscm.2026.e06554

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06554

Keywords: alkali-activated materials, ceramic waste, eggshell powder, ground granulated blast furnace slag, carbonation, compressive strength, central composite design, desirability optimization, carbon footprint, sustainable construction, C-(N)-A-S-H gel, microstructural analysis