“In saecula saeculorum”, forever and ever
For almost two millennia the ‘Mercati di Traiano,’ or Trajan’s market, has existed next to the slopes of the ‘Quirinale’, surviving wars, storms and earthquakes. The vast complex, which dates back to the second century A.D., made with the ‘opus latericium’ technique (concrete covered with a brick facing), is only one amongst the many monuments of the Roman period that has remained structurally intact up to the present day.
Recently, a research team from the University of California, Berkeley, examined the fine-scale structure of Roman concrete. It described for the first time how the extraordinarily stable compound, calcium-aluminum-silicate-hydrate, binds the material used to build some of the most enduring structures in western civilisation .
The experts are aware of an ancient recipe for mortar, described for the first time by the Roman engineer Marcus Vitruvius Pollio around 27-29 B.C. in his treatise ‘De Architectura.’ The main ingredients of the compound were ‘pozzolana’ (a mixture of volcanic ash and silt extracted at the time in the ‘Campi Flegrei’ of Pozzuoli and Lazio) and lime, into which fragments of tuff, bricks and shards were inserted to form the so-called ‘cementitious,’ one of the first examples of concrete in history.
Thanks to the development of this technology, the ancient Romans were able to realise, for instance, the dome of the ‘Pantheon,’ the foundation and the inner walls of the ‘Colosseum,’ and water resistant structures such as wharves, breakwaters and other harbour structures.
The dome of the ‘Pantheon’ in Rome represents the most extraordinary and best-preserved work handed down to us by the ancient Romans. With a total diameter of 43.44 meters and a central oculus of 8.92 meters in diameter, it’s still the widest hemispherical dome ever built using unreinforced concrete.
But what surprises most is inherent in the technology adopted for the construction of the dome, with a tapered section towards the top and the use of the ‘opus caementicium’ composed of pozzolanic cement with lightened aggregates.
The Roman concrete doesn’t show only remarkable durability but its manufacturing leaves a smaller carbon footprint than does its modern counterpart, Portland cement, which accounts for 7 % of industrially emitted carbon dioxide .
According to a recent study promoted by the European Climate Foundation, ordinary Portland cement retains its popularity as the construction material of choice in the field of civil engineering .
But the pozzolan and its potential substitutes such as fly ash (an industrial waste product from the burning of coal) and volcanic ash, could replace only 40 % of the world’s demand for Portland cement .
Sustainability concerns for modern cement
The global production of cement has exceeded 3.6 billion tonnes due to massive urbanisation, especially in fast-developing countries such as China and India. The concrete market is currently the third-largest industrial energy consumer and second-largest industrial carbon dioxide emitter in the world .
According to the Carbon Disclosure Project, cement making accounts for 6 % of global carbon emissions and manufacturers must make sharp reductions if the Paris climate goals are to be met.
Concrete has traditionally been designed to meet a prescribed specification, including what is considered to be inevitable material degradation over time. Mitigating this heretofore unavoidable degradation necessitates expensive maintenance regimes and, in many cases, complete renewal .
Our cities face growing pressure from the global challenges of pollution, sustainable urbanisation, and resilience to catastrophic natural events, amongst others. To address these concerns, there is the need to design concrete structures to be both far more sustainable and resilient, so that the carbon emissions, costs, and societal impacts over their lifetime are significantly reduced.
The promising field of nano-engineered concrete
Concrete is widely appreciated in the construction industry. Indeed it is the most consumed material in the world after water but its major disadvantage is its brittle nature, which is attributed to its poor resistance to crack formation, low tensile strength, and strain capacities.
Many attempts have been directed at enhancing the performance of cement-based materials by manipulating the properties of cement composites with admixtures, supplementary cementitious materials and fibers.
Fiber-reinforced concrete provides increased toughness and tensile strength. In addition, fibers replace large cracks with a dense system of microcracks. Yet the fibers fail to stop crack initiation at the nanoscale.
There is a wide consensus in the research community that concrete has to be engineered at the nanoscale, where its chemical and mechanical properties can be truly enhanced. These materials have to exhibit enhanced durability and mechanical performance and have to incorporate functionalities that satisfy multiple uses in order to be suitable for future emerging structural applications .
Engineered nanomaterials exist in three principal shapes, namely 0D nanoparticles, 1D nanofibers and 2D nanosheets. There have been many recent studies on newly produced nanomaterials such as nano silica, nano titanium oxide, nano iron oxide, carbon nanotubes (CNTs), graphene oxide (GO) and graphene sheets .
The amazing properties of a new graphene-reinforced concrete
Researchers at the University of Exeter have described in a recent paper a new technique that uses nanoengineering technology to incorporate graphene into traditional concrete production.
The graphene-reinforced concrete is produced by the addition of water‐stabilised graphene dispersions, with high yield, low cost, and compatible with the large‐scale manufacturing required for the use of this material in practical applications.
They demonstrated an extraordinary increase of up to 146 % in compressive strength, up to 79.5 % in flexural strength and a decrease in the maximum displacement due to compressive loading by 78 %. At the same time, they found an enhanced electrical and thermal performance with an 88 % increase in heat capacity.
Moreover, they proved a remarkable decrease in water permeability by nearly 400 % compared to standard concrete. This is an extremely sought‐after property for long durability of concrete structures and makes this novel composite material ideally suited for constructions in areas subject to flooding.
Finally, the researchers showed that the inclusion of graphene in currently used concrete would lead to a reduction of the amount of concrete material required by 50 % while still fulfilling the loading specifications for buildings. This would lead to a significant reduction of 446 kg per tonne of the carbon emissions from cement manufacturing .
The new era of smart concrete
As concrete in a dry state effectively does not conduct electricity, adding the function of high electrical conductivity has been a goal of material scientists for a long time.
Attempts typically relied on the addition of large amounts of magnetite, steel fibres and synthetic and natural forms of carbon including graphite, but did not achieve high levels of performance. In addition, the required high loadings of active materials tended to cause negative effects on cost, strength, corrosion and abrasion resistance, maintenance costs and weight .
The cement producer Italcementi, as part of the international HeidelbergCement Group and as a member of the Graphene Flagship Consortium, has exhibited a mock-up at the Mobile World Congress 2019 in Barcelona that shows the multiple potentials of a structural slab with a thin layer of a proprietary graphene-cementitious composite.
By incorporating graphene into concrete, Italcementi researchers have succeeded in
modifying the typical insulating behavior of cementitious compounds, allowing the passage of electric current.
The electrically conductive ‘graphene concrete’ could have application in underfloor heating, where it can provide a long-term, low-maintenance alternative to plumbed hot water installations.
Additionally, as a solid-state heater, the technology may enable more rapid, and environmentally friendly ways of clearing ice and snow from houses, key transport routes and airports without the need for corrosive salt and antifreeze chemicals, thereby safeguarding the safety of people, the durability of materials and the impact on the environment.
As part of the Internet of Things (IoT) trend, sensors are being mounted everywhere. In view of their proliferation, a new graphene-based smart concrete can create sensors across an entire surface; a whole building, road or piece of machinery can be a sensor, not just a designated area within a structure or an add-on device.
Using graphene to create smart sensors reporting real-time stress, temperature and moisture changes may be the next big breakthrough in green buildings.
Conductivity makes ‘self-sensing’ solutions possible, capable of monitoring stresses, deformations and cracking states of the concrete in real time without the need for special additional sensors or integrating them efficiently.
Among the wide variety of monitoring parameters (e.g. acceleration, temperature, pH and pressure), strain is an important indicator of structural response to external loads and for identifying damage. For instance, localised changes in structural strains can indicate the presence of cracks and impact damage.
These are solutions that will allow timely intervention at the beginning of initial phenomena of degradation of structures and infrastructures, with great advantages from the point of view of the durability of the structures .
Other current applications include the provision of anti-static flooring and EMI shielding (radio frequency interference) in buildings, and cost-effective grounding and lightning strike protection for a range of infrastructure from bridges to wind turbines.
Italcementi has also been at the head of an international project to further develop the depolluting of photocatalytic cements thanks to graphene. The aim of the project is to increase the effectiveness of the catalytic principle and extend its sensitivity in poor lighting conditions.
Electrically conductive concrete is also already bringing great innovations in mobility because it has a cost-effective role in enabling inductive (wireless) charging technologies for electric vehicles under dynamic (driving) as well as stationary (parking) conditions through the increased range of heating, sensing and other conductive concrete functions.
Challenges for real world applications
It’s possible to outline a significant commitment in the field of concrete re-engineering, in order to make concrete a more sustainable material.
Amongst the possible solutions to the problem of concrete sustainability, the addition of graphene nanosheets to the concrete mixture seems to be a very versatile and promising solution in terms of raw material consumption, durability and particularly for the development of applications for smart buildings and smart cities.
Graphene has shown to have a big potential also for protective paints, self-cleaning applications, more efficient LED lights, more resistant and durable steel, more efficient solar panels, etc.
Despite the predictions from the academic world and industry about a bright future for graphene in the construction sector, its adoption is not without challenges.
One amongst the most significant challenges for the real world application of graphene can be represented by the lack of confidence of this material in the supply chain. A barrier could be the difficulty to find a reliable graphene supplier. For instance, an issue that the existing suppliers are facing is to measure a statistically representative quantity of nanoscale flakes for each batch of material on the scale of tonnes .
Another challenge for graphene producers, which are significantly increasing their production volumes, is to increase the concentration of graphene in solvents and dispersions. The graphene concentration is typically 1g/L, consequently, its transport is not convenient neither economically nor environmentally .
It is essential to advance knowledge about the impact that graphene can have on the environment and health in order not to repeat the mistakes made in the past on the use of new substances such as the coolant PCB, the insecticide DDT, the biocide TBT, benzene, halogenated hydrocarbons, etc. For the purpose of developing comprehensive risk assessments, it’s necessary to consider, amongst others, the emissions from graphene for all its possible relevant applications, the toxicological aspects, persistence in environmental media, hydrophobicity and the evolution of the physical form of nanographene sheets .
Recently, extensive research has been conducted to correlate the physicochemical characteristics of graphene and related materials to biological effects, focussing on the potential interactions of graphene-based materials with key target organs as well as a wide range of other organisms including bacteria, algae, plants, invertebrates, and vertebrates in various ecosystems.
From these studies it has emerged that a classification based on lateral dimensions, the number of layers and the carbon-to-oxygen ratio has enabled a description of the parameters that can alter graphene’s toxicology. These considerations can guide future development and applications of this type of material .
It’s reasonable to believe that more research is still needed to be able to fully exploit the potential of the so-called wonder material without running risks for the environment and health, but, most likely, graphene or its variants will become part of our cities and our homes in the coming years, bringing significant benefits to everyday life.
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