Look around. Indoors or outdoors, chances are you’re surrounded by the silent hero of our civilization: concrete. As the second most widely used substance in the world after water, concrete is the most prevalent construction material today (Gagg, 2014). From the impressive ancient Roman structures that have endured for over 2000 years to the concrete jungles that decorate the urban skylines of many major cities, concrete has demonstrated tremendous versatility and durability. Despite its millennia-long legacy, ongoing advances in science and engineering are still unlocking ways to make this material better.
Most modern concrete consists of a simple combination of aggregates (usually sand, gravel, or crushed stone), water, and a binding agent, predominantly ordinary Portland cement (OPC). Various reactions in the OPC, upon mixing with water, result in calcium silicate hydrates (Al-Jabari, 2022). This compound reduces weakness-inducing pores and tightly binds the aggregates to give the concrete its incredible compressive strength of about 30 MPa—enough to withstand the pressure of around 600 elephants on a square meter area (Al-Jabari, 2022; Bajaber & Hakeem, 2021; Nawy, 2008).
By itself, concrete is admittedly inherently brittle and is ten times weaker in tension than in compression. However, reinforced concrete, paired with internal tensile steel bars that act as a skeletal frame, can sustain the complex load requirements of large-scale projects (Gagg, 2014). The concrete’s structural strength, affordability, fire resistance, and ability to be cast and cured into desired shapes make it essential in the construction industry (Gagg, 2014). However, this doesn’t mean that the beloved artificial rock is flawless.
Concrete has, quite literally, shaped the modern world, but this material is also a major factor in its demise. With the construction industry pumping out over 4 billion tons of carbon-intensive cement annually, its production accounts for a staggering 8% of global anthropogenic carbon emissions (“MINERAL…”, 2023; Miller et al., 2016). As climate change is an ever-growing threat, there is an urgent need for sustainable and resilient construction. We must change how we build with concrete.
One method could be to increase the strength and durability of concrete, consequently reducing the production volume needed for resilient structures. In recent years, researchers have formulated ultra-high-performance concrete (UHPC) by adjusting ingredient ratios and experimenting with supplementary materials. UHPC’s astonishing compressive strength of 200 MPa—roughly seven times stronger than that of conventional concrete—is projected to enable more efficient and streamlined construction designs, decreasing carbon emissions (Bajaber & Hakeem, 2021).
Other studies are exploring self-healing concrete (SHC) to battle high-maintenance concrete cracking, drawing inspiration from how the human body heals wounds and broken bones. Biomimetic SHC employs capsules of bacteria to facilitate the formation of calcium carbonate (limestone) to mend damages through a process called biomineralization (Seifan et al., 2016). These valuable modifications, like UHPC and SHC, have the potential to drastically decrease the overall carbon footprint of concrete, while elongating its service life.
Meanwhile, something must be done about the concrete structures that already exist. Deteriorated by evaporation-induced concrete shrinkage and heavy load stress, many buildings worldwide require costly repairs, replacements, or even demolitions. Rather than exacerbating environmental degradation, researchers are looking to recycle materials from demolished structures and waste from other industries. According to experimental investigations, industrial byproducts such as concrete and brick waste, fly ash, silica fume, and blast furnace slag all have great potential to be salvaged, recycled, and incorporated into green, OPC-free concrete, which can then be used for new projects or for retrofitting damaged structures (Boobalan et al., 2022).
But it doesn’t stop there. Scientists are developing innovative formulas for the age-old building material that can even have beneficial, carbon-negative effects on the environment. Biochar, a type of eco-friendly charcoal made from biomass, enables concrete to capture nearly 23% of its weight in CO2 once integrated into the concrete mix (Li & Shi, 2023). Amazingly, after proper treatment, the biochar-augmented concrete exhibits physical properties on par with normal concrete (Li & Shi, 2023). Replacing the annual 30 billion tons of traditional concrete production with this carbon-negative alternative could theoretically remove 6.9 billion tons of CO2 from the atmosphere (“Concrete needs…”, 2021).
Photocatalytic concrete is another attractive option, particularly for urban settings with high pollution levels. The titanium dioxide on the surface of this concrete can chemically break down nitrogen oxides (contributors to air pollution and climate change) when activated by ultraviolet radiation from sunlight, leading to air-purifying effects (Hüsken et al., 2009). If applied to pavements and building surfaces, it can neutralize harmful pollutants from automobile traffic, acid rain, and smog, cleaning up our cities and lowering respiratory health risks (Boonen & Beeldens, 2014).
Although these groundbreaking concrete technologies are still in the early stages of development, current research shows promising progress. Continued environment-conscious explorations offer hope for concrete not only to build our structures but also to build a more sustainable future.
References:
Al-Jabari, M. (2022, January 1). 1 - Introduction to concrete chemistry (M. Al-Jabari, Ed.). ScienceDirect; Woodhead Publishing. https://www.sciencedirect.com/science/article/pii/B9780128243541000015
Bajaber, M., & Hakeem, I. (2021). UHPC evolution, development, and utilization in construction: a review. Journal of Materials Research and Technology, 10, 1058–1074. https://doi.org/10.1016/j.jmrt.2020.12.051
Boobalan, S. C., Salman Shereef, M., Saravanaboopathi, P., & Siranjeevi, K. (2022). Studies on green concrete – A review. Materials Today: Proceedings, 65, 1404–1409. https://doi.org/10.1016/j.matpr.2022.04.392
Boonen, E., & Beeldens, A. (2014). Recent Photocatalytic Applications for Air Purification in Belgium. Coatings, 4(3), 553–573. https://doi.org/10.3390/coatings4030553
Concrete needs to lose its colossal carbon footprint. (2021). Nature, 597(7878), 593–594. https://doi.org/10.1038/d41586-021-02612-5
Gagg, C. R. (2014). Cement and concrete as an engineering material: An historic appraisal and case study analysis. Engineering Failure Analysis, 40(40), 114–140. https://doi.org/10.1016/j.engfailanal.2014.02.004
Hüsken, G., Hunger, M., & Brouwers, H. J. H. (2009). Experimental study of photocatalytic concrete products for air purification. Building and Environment, 44(12), 2463–2474. https://doi.org/10.1016/j.buildenv.2009.04.010
Li, Z., & Shi, X. (2023). Towards sustainable industrial application of carbon-negative concrete: Synergistic carbon-capture by concrete washout water and biochar. Materials Letters, 342, 134368. https://doi.org/10.1016/j.matlet.2023.134368
Miller, S. A., Horvath, A., & Monteiro, P. J. M. (2016). Readily implementable techniques can cut annual CO 2 emissions from the production of concrete by over 20%. Environmental Research Letters, 11(7), 074029. https://doi.org/10.1088/1748-9326/11/7/074029
MINERAL COMMODITY SUMMARIES 2023. (2023). https://pubs.usgs.gov/periodicals/mcs2023/mcs2023.pdf
Nawy, E. G. (2008). Concrete construction engineering handbook. Crc Press, Taylor & Francis Group, Cop.
Seifan, M., Samani, A. K., & Berenjian, A. (2016). Bioconcrete: next generation of self-healing concrete. Applied Microbiology and Biotechnology, 100(6), 2591–2602. https://doi.org/10.1007/s00253-016-7316-z