From sewage sludge to dairy by-products, technical experimentation opens new avenues for the production of more sustainable building materials

by Marco Arezio

At the heart of the ecological transition, the construction industry finds itself having to radically rethink its materials, supply chains, and the environmental impact of the entire production cycle. With cement alone contributing to approximately 8% of global CO₂ emissions and a demand for natural resources—sand, gravel, water—that exceeds any other sector, the need for a breakthrough is now urgent. In this scenario, a concrete and unexpected possibility is emerging: using agricultural, industrial, and even dairy waste in the production of concrete and mortar, transforming potentially polluting waste into technically sound and environmentally sound construction materials.

This is not a theoretical provocation, but a concrete line of research, with numerous ongoing experiments and pilot production already active in some contexts. Wastewater such as sewage sludge, digestate from biogas plants, combustion fly ash, dehydrated whey, and flotation sludge from the dairy industry are finding a place in materials engineering laboratories and, in some cases, on actual construction sites. The goal is not only to reduce the construction sector's environmental footprint, but also to offer a cost-effective alternative to traditional materials, in a spirit of industrial symbiosis.

Types of wastewater that can be used and their characteristics

The wastewater involved in these experiments is characterized by a surprising chemical and physical diversity, which allows its use in multiple stages of the production process. Urban sewage sludge, for example, rich in silica, alumina, calcium oxides, and iron, after heat treatment can become a valid replacement for part of the cement, acting as an artificial pozzolan. Fly ash from waste-to-energy plants or biomass combustion plants , once micronized, offers high specific surface areas and reactivity, improving the compactness of the bonded material.

Alongside these already well-known wastes from the construction industry, more innovative solutions are being explored, such as byproducts from the dairy industry. Used whey, particularly rich in mineral salts and protein compounds, can be dehydrated and used as a plasticizer additive or as an alkaline component in binding processes. Even more promising is flotation sludge, a byproduct of fat separation in the treatment of dairy wastewater: after drying and inertization, it proves useful as hydrophobic additives or partial fillers in the formulation of plaster mortars.

Agricultural digestates, from biogas plants, are also demonstrating interesting capabilities as organo-mineral fillers, capable of improving the breathability of mortars and providing thermal insulation characteristics to products.

Experimental status and application results

Ongoing experiments, conducted by universities, technology centers, and industrial consortia, have moved beyond the exploratory phase, often leading to the production of demonstration products and small industrial batches. In Italy, for example, the Polytechnic University of Turin has created self-compacting concrete with 15% fly ash from sludge and wastewater from the dairy industry as the mixing water, without experiencing significant losses in mechanical performance. The workability of the mix has even been improved, thanks to the presence of organic compounds capable of reducing internal friction in the mix.

In Puglia, the University of Bari conducted tests on mortars made from natural hydraulic lime with added whey powder. The results showed high adhesion to substrates and a reduced tendency to shrink, paving the way for potential use in architectural restoration and green building.

In the Iberian context , the combination of dried agricultural digestate and hydraulic lime has allowed the creation of plaster panels with high hygroscopic properties, suitable for improving the internal comfort of buildings in hot-dry climates.

More recently, some prototypes have also been tested in prefabricated elements—benchtops, road kerbs, and masonry blocks—made with a percentage of alternative binder derived from wastewater greater than 20%. Although their compressive strengths are generally lower than those of standard concrete (around 20-25 MPa at 28 days), they are perfectly suitable for non-structural uses.

Environmental, economic and territorial benefits

The use of wastewater in construction not only complies with the principles of the circular economy, but also offers quantifiable environmental benefits. Even partial replacement of Portland cement reduces greenhouse gas emissions by up to 30% per ton of material produced. Furthermore, the costs and environmental implications of disposal are avoided, which can be particularly costly for sludge and whey, both due to landfill restrictions and the risk of environmental contamination.

Another advantage is the ability to create short, regionally integrated supply chains. Farms or dairies can collaborate with construction companies, composting plants, and waste management consortia to fuel local production cycles, generating added value and reducing transportation costs.

Equally important is social acceptability. The growing focus on sustainable materials among designers, customers, and public institutions can become a powerful driver for the market introduction of these products, provided safety, traceability, and performance are guaranteed.

Cost-effectiveness of the process and the final product

From an economic standpoint, the recovery of construction wastewater can be advantageous in many ways. The organic and mineral wastewater used has virtually zero raw material costs, and in many cases, producers would be willing to pay for its collection to avoid disposal costs. The required treatments—drying, calcination, micronization—involve significant energy costs, but still lower than those of the cement clinkerization process.

Overall, the use of treated wastewater can reduce the unit cost of cementitious binders by 10-20%, especially when the entire supply chain (treatment + mixing + installation) is located within a small geographic radius. Studies conducted in Italy and Spain show that the production of prefabricated products (kerbs, blocks, street furniture) with a 15-25% recycled content is competitive with traditional products, even without considering any public incentives or tax benefits related to sustainability.

The real turning point could come when technical and environmental standards are recognized that allow the industrial-scale adoption and full commercialization of these products.

Reference legislation and environmental requirements

Current legislation is complex and constantly evolving. At the European level, Directive 2008/98/EC establishes that waste can be reintroduced into the production cycle only if it undergoes treatment that guarantees its safety and usefulness. The "End of Waste" concept is central to this process: wastewater ceases to be waste only when it demonstrates, through technical and environmental analyses, its ability to fulfill a specific function.

European technical standards (UNI EN 206 for concrete and UNI EN 197-1 for cement) place stringent constraints on composition, especially for products intended for structural use. There is still no explicit regulatory recognition of wastewater as additives or secondary aggregates, so each use must be assessed on a case-by-case basis, with a specific authorization procedure.

In Italy, the Ministerial Decree of February 5, 1998, although limited, permits the use of certain non-hazardous wastes for the production of construction materials, provided that release and chemical stability limits are met. Regional environmental protection agencies (ARPA) and ISPRA (National Institute for Environmental Protection and Research) establish analytical criteria and limits for heavy metals, eluates, and hazardous substances, which often represent the greatest obstacle to the use of organic wastewater.

Technical limitations and future challenges

Despite its potential, the use of wastewater in construction materials presents some technical challenges. The highly variable composition requires very thorough quality control systems, which are often still lacking. Some organic components, if not fully stabilized, can degrade over time, resulting in odorous emissions or reduced mechanical durability. Furthermore, the presence of inhibitory substances can interfere with the cement's hydration reaction, compromising setting and final strength.

Large-scale industrial integration requires the introduction of advanced treatment technologies (such as accelerated carbonation or vitrification) and the development of environmental certification systems (e.g., EPDs) that ensure transparency and traceability.

Conclusion

The future of sustainable construction also depends—and perhaps above all—on the ability to transform what we currently discard into useful resources. The use of agricultural, industrial, and dairy wastewater for the production of concrete and mortar represents one of the most fascinating frontiers of industrial symbiosis, where waste chemistry meets materials engineering.

However, coordinated action between scientific research, industry, and policymakers is needed, capable of supporting innovation with regulatory tools, economic incentives, and technical culture. Only then can these materials emerge from the labs and become an integral part of a new generation of buildings: more equitable, more local, more sustainable.

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