From waste to construction material: in Córdoba they developed a paving block without natural sand and cement
The construction industry today is under strong pressure to reduce carbon dioxide emissions and the consumption of non-renewable raw materials, while still having to deliver enough material for the growing needs of housing and infrastructure. Cement is particularly problematic: production is energy-intensive and chemically “locked in” to processes that generate CO2, so the sector is cited in numerous decarbonization strategies as one of the hardest areas for rapidly reducing emissions. In that context, researchers are increasingly trying to combine two needs—managing industrial by-products and reducing the share of traditional, high-carbon binders—through solutions that belong to the circular economy.
It is precisely along that line that a new type of pervious paving block has emerged, developed by a team from the School of Engineering Sciences in Belmez (Escuela Politécnica Superior de Belmez, EPSB) at the University of Córdoba. Their goal was ambitious: in the mix design, replace both the aggregate (sand/gravel) and cement, so that the final product contains no natural aggregates and no conventional cement, using exclusively recycled materials and industrial surpluses.
What they wanted to change and why
In traditional mortars and concretes, natural aggregates make up the largest volume. This means that every ton of concrete “demands” a large amount of sand and gravel—resources that are locally limited, increasingly expensive, and linked to environmental pressures on waterways, coasts, and quarries. In parallel, cement as a binder represents the largest part of concrete’s carbon footprint. According to industry and public reports, global cement production is associated with a large share of total CO2 emissions, making cement a key target for emissions-reduction and carbon-capture technologies.
The combination of those two problems—high demand for aggregates and high emissions associated with cement—explains why research is increasingly turning to substitutes: instead of “fresh” raw materials, waste and by-products from other industries are used, which are often difficult and costly to dispose of.
Aggregate from seashells: waste from the canning industry as a replacement for sand
The most recognizable element of the Belmez solution is the replacement of natural aggregate with material obtained from the shells of the marine mollusk Acanthocardia tuberculata. This is an edible bivalve species that is commercially produced and consumed, among other forms, in canned products. The canning industry, as the study’s author Ágata González-Caro notes, generates large quantities of this type of waste, which often ends up in landfills because it lacks stable industrial value.
The researchers mechanically crushed the shells and prepared them to obtain a calcareous (limestone) aggregate that can take over the role of natural sand in mortars and concrete. A scientific paper published in the journal Materials and Structures shows that “seashell sand” can be used as a complete replacement for natural sand in pervious vibro-compacted paving blocks, without the need to add any natural aggregate.
Such an approach has a double effect. On the one hand, it reduces demand for natural sand and pressure on extraction. On the other, industrial waste gains a market “address”—instead of a disposal cost, it becomes a raw material. In areas with developed seafood processing, this can be especially interesting, because the logistics of collecting shells already exist, and the disposal problem is ongoing.
Without cement: alkaline activation of ash and mining residues
The second, more technologically demanding step was eliminating conventional cement. Instead, the team uses a mixture of industrial by-products—fly ash and residues from coal mining tailings—from the broader Guadiato area. In a standard scenario, such materials often become an environmental burden: tailings occupy space, can cause problems with leachate water and dust, and ash requires controlled disposal.
The key process by which “waste is turned into a binder” is called alkaline activation. Put simply, it is a chemical reaction in which aluminosilicate materials, in contact with a highly alkaline solution, reorganize and create new binding phases similar to those that give strength to conventional cement. The paper describes combining alkaline activation with vibro-compaction, which matters because pervious paving blocks must have controlled porosity: enough voids for water to pass through, but also enough strength for loads.
CO2 as part of the process: accelerated carbonation curing
An interesting element of the study is also the curing strategy in the presence of CO2 (accelerated carbonation curing). Instead of the blocks simply “drying” or being cured in a standard way, the authors apply accelerated carbonation to improve mechanical properties while simultaneously promoting the binding of part of the carbon dioxide into carbonate phases. In the article, this step is presented as one of three “pillars” of sustainability: complete replacement of sand with shell aggregate, a binder made from ash and mining residues, and CO2 curing to increase strength and potentially capture CO2.
It is important to emphasize that this does not automatically mean “concrete that cleans the atmosphere,” but rather a controlled technological step that can improve performance and reduce part of the emissions burden—depending on the CO2 source, the energy of the process, and the overall life cycle of the product. That is precisely why detailed life-cycle assessments and comparisons with conventional solutions under real production conditions are crucial in such innovations.
What was achieved and what the limits are
According to the published results, the developed paving blocks meet the key requirements normally demanded for this type of product: mechanical resistance, durability, and safety of use. The authors emphasize that it is a “fully recycled” product in terms of composition—without natural aggregates and without conventional cement—which is a rare achievement because most “green” recipes still retain some conventional components.
At the same time, the researchers openly state that further optimization of certain production steps is needed. In particular, they mention issues of compaction and demolding, which in industrial production determine speed, cost, and the scrap rate. Another topic is “greener” activators: the alkaline solutions that drive the reactions are often chemically demanding and carry their own environmental footprint, so the literature and industry are looking for alternatives that would reduce dependence on conventional chemicals or enable the use of by-products in that part of the process as well.
Why this matters for cities and infrastructure
Pervious paving blocks are not merely an aesthetic element. In urban environments, they are part of a broader stormwater management policy: they enable infiltration, reduce surface runoff, and can help alleviate the load on sewer systems during heavy rains. In an era of increasingly frequent extreme precipitation, cities are looking for solutions that combine a transport function with the “sponge-like” capacity of the ground. If, at the same time, the carbon footprint of the material can be reduced, the benefit is multiplied.
At the level of the European Union, construction and demolition waste (C&DW) is recognized as the largest waste stream, with high recovery rates but also a range of problems in recycling quality—much of the recovery comes down to low-value applications such as backfilling. That is why innovations that turn waste into high-value products (upcycling) have strategic weight: they can raise the quality of recycling, relieve landfills, and stimulate local value chains.
Belmez as a research “point” on Andalusia’s mining map
The geographical context is not insignificant. Belmez is part of the Upper Guadiato Valley (Alto Guadiato), with a mining tradition, and the EPSB is a University of Córdoba campus located outside the city of Córdoba. The university notes that the school is located about 70 kilometers from Córdoba, in the Sierra Morena area, and that it is an important cultural and academic reference for the local community. In such an environment, the idea of using mining residues as a resource for new materials also gains a social dimension: science builds on local industrial heritage, but seeks to redirect it toward more sustainable practices.
Broader trend: waste materials as the standard, not the exception
Although the Belmez project is specific in its combination of shells, ash, and mining residues, it fits into a broader trend that has accelerated in recent years. International agencies and industry actors warn that cement and concrete will struggle to reach net-zero targets without a combination of measures: reducing the share of clinker, using alternative binders, increasing energy efficiency, using low-carbon fuels, and capturing, using, and storing CO2. This explains why laboratory and pilot projects with alkali-activated materials are increasingly numerous: they offer potential for lower emissions, but require standardization, long-term testing, and adaptation to industrial production lines.
In that sense, the Belmez paving block should be read as a step that demonstrates the feasibility of the concept. The key question for the next phase will be confirmation of performance under real conditions (freeze–thaw, de-icing salts, wear), stability of supply of waste raw materials, production economics, and a regulatory framework that allows and encourages such materials. Currently, according to the available information from the scientific paper, the research is moving toward process optimization and the search for activators with a lower environmental footprint, which will largely determine the possibility of wider application.
Sources:- Materials and Structures (Springer Nature) – scientific paper on CO2-cured alkali-activated pervious paving blocks with “seashell sand” ( link )- Materials and Structures (PDF) – full text of the paper and methodology, including a description of raw materials and process ( link )- University of Córdoba – EPSB, location and basic information about the Belmez campus ( link )- International Energy Agency (IEA) – analyses of the cement and concrete transition and the need for emissions-reduction technologies ( link )- U.S. Department of Energy – overview of the cement sector and the emissions context and CO2 capture technologies ( link )- European Environment Agency (EEA) – briefing on construction and demolition waste and recovery quality in the EU ( link )
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