Research & Development / 8/28/2024

Concrete: less carbon dioxide with recycled aggregates and GFRP

The use of supplementary cementitious materials (SCMs), geopolymers and innovative strengthening materials allows to improve structural performances and promote sustainability

The construction industry is adopting sustainable practices by using recycled aggregates, geopolymers, and innovative reinforcement materials like GFRP to reduce carbon dioxide emissions. A recent research from the University of Brescia demonstrates that hybrid reinforcement and geopolymer-based concrete significantly improve structural performance and sustainability of concrete.

Climate norms have imposed the European Union to reduce Greenhouse Gas emissions by 55% by 2023 and for the whole of the EU to become carbon neutral by 2050. These objectives are legally binding: individual states are, in fact, working to achieve these objectives within the specified timeframes.

This transition is also an opportunity to reappraise the European economic and industrial systems at every level. To achieve these objectives the construction industry, and the concrete sector in particular, needs to be radically transformed.

In this scenario Cembureau (the European Cement Association), together with the American Portland Cement Association and the Global Cement and Concrete Association, has mapped a pathway in order for the cement industry to achieve climate neutrality by 2050. The approach covers every phase of production, from the cement plant to the entire life cycle of structures, according to circular economy principles.

The production of concrete can provide a valid contribution to the development of a circular economy through the use of recycled aggregates (derived from construction or demolition activities) or artificial aggregates (from industrial waste) as substitutes for natural aggregates. Also, concrete itself may be recycled and then used again for new constructions.

Sustainable concrete solutions: reducing carbon emissions with recycled aggregates, geopolymers, and gfrp reinforcement

Sustainable alternatives

The benefits deriving from the use of Supplementary Cementitious Materials (SCMs) in the production of cement are well known: fly ash, slag, limestone, silica fume and natural Pozzolan all offer a concrete way of reducing the carbon footprint of the cement production process. Around the world the use of SCMs has reached a significant level, with peak values of up to 75%.

In Italy there is plenty of scope to reduce the carbon footprint of concrete, because of both the relatively low use of type III, IV and V concrete (with lower Portland cement content) and the relatively low use of SCMs in the production of concrete. The use of SCMs in the production of cement is regulated in Italy by UNI EN 197-1:2011 standard which prescribes the composition, specifications and compliance criteria for normal types of cement. The production of concrete, on the other hand, is regulated by UNI EN 206:2021 and UNI 11104:2016.

Geopolymers

In addition to these supplementary cementitious materials there are also geopolymers which, thanks to their low impact on the environment compared with traditional concrete made with Portland cement, are a promising alternative for sustainable construction practices.

Geopolymers, or alkaline-activated materials, are materials resulting from the reaction of a precursor, typically Ground Granulated Blast Furnace Slag (GGBFS) or fly ash (FA), and an alkaline activator, typically a mix of caustic soda and sodium silicate, with water.

These precursors are amorphous aluminosilicates characterised by latent hydraulicity, which only become reactive in the presence of highly alkaline substances. The reactions that lead to geopolymers becoming hardened are slower than those that regulate the hardening of Portland cement. However, whereas Portland cement matures after 28 days of curing, geopolymers continue to react for longer and their mechanical properties can continue to increase even after 60 days.

They also have a different composition. Going into detail, the microstructure of geopolymers is much denser than that of cementitious paste and this gives them excellent resistance to aggressive agents (sulphates, chlorides, acids), which makes them particularly suitable for the production of precast concrete elements used in areas with a high anthropic impact (sewage systems, drains for industrial waste and storm drains).

A new approach to reinforced concrete

Replacing traditional steel reinforcement (which causes considerable CO₂ emissions) with innovative materials, such as fiber-reinforced concrete (FRC) and GFRP Glass Fiber Reinforced Polymers (GFRP), is also a valid way of reducing the environmental impact of the concrete production process.

Hybrid structures, which employ a combination of steel reinforcement and FRP, are an example of this new approach to structural design. These are solutions that combine the strength and ductility of steel with the characteristic resistance to corrosion and low weight of FRP. These hybrids not only improve structural performance, but they also promote sustainability because such structures have a lower impact on the environment. What is more, the resistance to corrosion of FRP materials reduces the need for maintenance and extends the service life of structures, thereby increasing sustainability even further.

Research work at the University of Brescia

The object of research work carried out in collaboration with the University of Brescia and led by Prof. Giovanni Plizzari and Prof. Adriano Reggia was to carry out industrial scale tests on reinforced concrete beams aimed at testing and developing solutions with a lower carbon footprint. The tests analysed, on a full scale, the relationship between load-bearing capacity and displacement under loading.

The idea was, firstly, to promote sustainable solutions already compliant with technical standards and, secondly, to study the benefits from using new materials not yet covered by technical standards.

Going into detail, tests were carried out on reinforced concrete beams made from traditional concrete incorporating hybrid reinforcement (made of steel, GFRP glass fibers, FRC polymer fibers) and from concrete containing geopolymers. The aim of the tests was to assess structural performance, such as stiffness and load-bearing capacity.

The research also introduced criteria to assess the materials and their structural performance based on durability and environmental impact.

Research, in collaboration with the University of Brescia and Crezza S.r.l., has highlighted the advantages of hybrid reinforcement solutions

Materials

Three types of concrete by the concrete manufacturer Crezza S.r.l were used for the tests:

traditional reference concrete (REF) formulated with CEM II cement for concrete structures cast on site;
sustainable concrete compliant with Italian standards (ECO3): concrete formulated with CEM IV (Pozzolan) cement with artificial and recycled aggregates
an innovative type of concrete containing geopolymers (GEO), an important technological innovation in that it makes use of an alkaline liquid slag activator (MAPECUBE GEO) rather than sodium hydroxide (or caustic soda), which is non-hazardous and requires no heat-curing in order to develop strength. Also, unlike traditional activators, MAPECUBE GEO is compatible with all polycarboxylate ether (PCE) superplasticizers. With MAPECUBE GEO, therefore, it is possible to make geopolymer-based concrete in the same production plants and with the same methods used for Portland-based concrete.

With regards to the superplasticisers used for the tests, DYNAMON NRG 1012 admixture by Mapei, specific for precast concrete, was used to make the reference concrete (REF) and ECO3 concrete. DYNAMON CUBE 804 superplasticiser was added to the concrete containing geopolymers, which helps maintain a good level of workability, including with mixes containing slag. The strengthening materials used were traditional steel reinforcement rods and macro-synthetic fibers (MAPEFIBRE ST42 PLUS).

The mechanical properties of the materials were tested by carrying out compressive strength tests, structural performance tests and durability characterisation.

The results of these tests showed that hybrid strengthening solutions improve load-bearing capacity and displacement under service loads, while limiting reduction in ductility compared compared with traditional reinforced concrete structures, as one can see in the graphs in this articles.

Sustainability assessment

The results showed there was a significant reduction in Greenhouse Gas (GWP, Global Warming Potential) using geopolymer concrete with hybrid reinforcement. This solution, which corresponds to a GWP value of around 150 kgCO₂/m^3, leads to a significant reduction of around two thirds (66%) compared with that of the reference concrete. By using ECO3 concrete with hybrid reinforcement, a reduction of 44% of GWP was achieved. Because of this reduction in GWP, geopolymer concrete is the concrete with the best sustainability potential, as shown in the graph below.

Significant reduction (66%) of greenhouse gas emissions was achieved thanks to the use of geopolymers