METHODS TO DECARBONIZE CONCRETE

ABSTRACT: Our planet Earth is in peril due to severe climatic changes. Due to the increasingly huge amounts of CO2 emissions by different industries, the temperature of Earth is increasing. Unless urgent measures are taken on a war footing, these problems will result in catastrophic consequences. In the recently concluded Glasgow Climate Change Conference, several Nations took a decision to limit the global temperature rise to 1.5 degrees. India has also committed to net-zero emissions by 2070. This requires several strategies and research efforts. Several such strategies are discussed and a recent research effort undertaken at the Washington State University, USA is also discussed.

INTRODUCTION

Our planet Earth is in peril due to severe climatic changes. The increase in population coupled with urbanization has resulted in unprecedented problems for our cities. Unless urgent measures are taken on a war footing, these problems will result in catastrophic consequences. The ever-increasing demand for energy, due to population and urban growth, has resulted in an energy crisis all over the world. The current use of fossil fuels, which may be depleted in another 40-50 years, has resulted in the release of huge amounts of greenhouse gases (GHG), especially carbon dioxide (CO2), which is harmful to the environment. In 2015, the global energy-related CO2 emissions were at the level of 49 gigatonnes per year (Gt/yr), with over 80% coming from fossil fuel combustion (In April 2023, CO2 concentration in the atmosphere reached 423?ppm, a 32% increase from the 1958 levels; 280-300 ppm may be considered as the ideal level of CO2?for human life)[1]. Although CO2 alone is singled out in many publications, there are other gases like methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (HFCs), that have a much greater effect on global warming than CO2, but their concentration in the atmosphere is less –collectively they are called as greenhouse gases (For example, methane is 22 times potent in global warming effect than CO2). It has to be noted that CO2 and other gases, which exist naturally in the atmosphere retain the Sun’s heat and create an atmosphere that sustains life on Earth. The primary source of human-generated CO2 emissions is fossil fuel power plants, which contribute to 35 % of all CO2 emissions.

Due to the increasing and huge amounts of CO2 emissions, the temperature of Earth has risen by 0.08 °C per decade since 1880. The rate of warming over the past 40 years has more than doubled to 0.18 °C per decade. Due to these climatic changes several other catastrophic changes are also predicted, which may include reduced snow cover and sea ice which may result in a rise in sea level (several cities along the seashore are at risk- as per current trajectory, sea level rise could exceed 2 m by 2100, which could displace some 630 million people worldwide), regional and seasonal temperature extremes and intensified heavy rainfall (causing drought and flooding), changed habitat for plants and animals (several plants and animals have already become extinct and some are even expanding). Thus, things that are valuable and humans depend upon such as water, energy, transportation, wildlife, agriculture, ecosystems, and human health, are all affected.?People in many parts of the world are already experiencing such effects and are suffering.

CONSTRUCTION INDUSTRY AND THE GLOBAL CO2 EMISSIONS

The construction industry accounted for 38% of total global energy-related CO2 emissions [2]. Next to water, concrete is one of the most widely used materials in the world, due to its many advantages like high strength, low cost, mouldability, etc. ?But, concrete is one of the largest single sources of carbon dioxide footprint. The production of cement, the main ingredient in concrete, is responsible for about 7% of global CO2 emissions, and steel is another 8% (steel is more recyclable than concrete).

?The process of making cement requires very high temperatures, and that usually requires burning fuels which, of course, emit CO2. That can be partly offset by switching to renewable energy sources, but chemical reactions in the mixture also release huge amounts of CO2, and this is harder to avoid.

Concrete Industry Initiatives to Achieve Net-Zero CO2 Emissions By 2050

According to the Global Cement and Concrete Association (GCCA), around 14 billion m3 (the equivalent of 33.6 billion tonnes) of concrete was cast from 4.2 billion tonnes of cement produced in 2020 (www. gccassociation.org). As per the GCCA, the global CO2 emissions from the cement and concrete sector today are in excess of 2.5Gt. In order to manufacture one tonne of cement, the raw materials are heated in a kiln up to 1,400 oC, resulting in the emission of 667 to 990 kg of CO2 for every 1000 kg of Portland cement produced. This depends on the fuel type, raw ingredients used, and the energy efficiency of the cement plant.

Some of the most promising methods tried by the concrete industry and researchers all over the world, to achieve net-zero emissions of CO2 by 2050, include [3]:

• ?Reducing the amount of clinker in concrete.?Clinker is the main ingredient in cement, and its production is responsible for about 90% of the CO2 emissions from concrete. There are a number of ways to reduce the amount of clinker in concrete, including using alternative materials such as fly ash, slag, and calcined clay. The cement industry has already replaced some of its raw natural resources with waste and by-products from other industrial processes. These materials should contain elements such as calcium, silica, alumina, and iron [4]. They can be used in the kiln, replacing natural raw materials such as limestone, shale, and clay. Some of these waste materials will have both useful mineral content and recoverable calorific value. In this connection, the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland collaborated with researchers at Universidad de las Villa (UCLV) in Santa Clara, Cuba, Technology and Action for Rural Advancement (TARA), New Delhi, India, IIT Delhi, IIT Madras, and IIT Bombay, have developed a blended cement having reduced clinker, calcined clay, ground limestone, and gypsum.?This cement termed LC3 (Limestone calcined clay cement) has proven successful through industrial trials carried out in Cuba and India [5].
• Improving the efficiency of cement production. Another way to reduce the emissions from concrete production is to improve the efficiency of cement kilns. This can be done by using more efficient technologies, such as heat recovery systems, and by using renewable energy sources Using such technologies, reduction in the emissions of CO2 is possible and ranges from a low of 0.13 kgC/t clinker with the installation of efficient kiln drives to a high of 85 kgC/t clinker for maximum use of blended cement. Assuming a 90% clinker-to-cement ratio, a plant that produces 1 Mt cement per year could realize carbon emissions reductions of 144 tC to 94,444 tC, respectively, for these improvements (Galitsky and?Price, 2007) [6].
• Using renewable energy in concrete production.?The production of concrete requires a significant amount of energy, and most of this energy is currently generated from fossil fuels. Using renewable energy sources such as solar and wind power can help to reduce the CO2 emissions from concrete production.
• Using alternative materials. One way of reducing carbon footprint is to reduce the amount of cement in concrete by using alternative materials, such as fly ash, slag, silica fume, etc. These materials can provide similar strength and at the same time provide better durability to concrete, but they produce significantly fewer emissions. Use of industrial waste materials such as fly ash (a fine, gray powder obtained by burning coal in thermal power plants), ground granulated blast furnace slag (GGBS) (a byproduct of iron manufacture), silica fume (a byproduct of processing quartz into silicon or ferrosilicon metals in an electric arc furnace), high-reactivity metakaolin (HRM), and glass powder as partial replacement of ordinary Portland cement in concrete, has already been widely adopted. These industrial waste products, which would otherwise end up in landfills, are called supplementary cementitious materials (SCMs). These materials can provide similar strength to concrete at 28 days (or even more strength at 56 days) and at the same time provide better durability to concrete, but reduce the CO2 emissions significantly. Additionally, the use of SCM with modern-day cement also reduces the increase in the peak temperature of concrete (often limited to 70°C) and its early occurrence. Further, as many researchers have pointed out, the resulting microstructure of concrete with SCM is far superior to that of pure OPC concrete. GGBS is a hard, granular material that can also be used as a partial replacement for cement in concrete.
• ?Using recycled materials in concrete.?Concrete can be made with a variety of recycled materials, such as old concrete, fly ash, and slag. Using recycled materials can help to reduce the number of new materials that need to be extracted and processed, which can help to reduce the environmental impact of concrete production. Recycled concrete aggregates are obtained by crushing old concrete and by careful screening. It can be used as a replacement for natural aggregate in concrete.?However, higher amounts of replacement of natural aggregates by recycled aggregates can reduce strength and durability [7].
• Capturing and storing carbon dioxide. ?Carbon capture, utilization, and storage (CCUS) technology can be used to capture the CO2 emissions from concrete production and store them underground. This is a promising technology, but it is still in its early stages of development. CCUS may become significant after about 2030 when commercial viability and the necessary infrastructure are expected to be established. CCUS is the only large-scale mitigation option available to make significant reductions in CO2 emissions from industrial sectors such as cement, iron and steel, chemicals, and refining [8].

These are just a few of the methods that are being developed to decarbonize concrete. With continued investment and innovation, it is possible to make concrete a more sustainable material. It is important to note that these are just a few of the methods that are being developed to decarbonize concrete. With continued investment and innovation, it is possible to make concrete a more sustainable material.

CARBON NEGATIVE CONCRETE

To produce carbon-negative concrete, researchers have been tweaking the formula of concrete, by substituting limestone for volcanic rock, adding ingredients like titanium dioxide, construction waste, baking soda, or adding clay commonly discarded during mining. Some researchers have even tried using microalgae to absorb CO2 and grow biomass, which can later be used to fuel the kiln [9].

?Researchers at Washington State University (WSU) have recently developed a new method to make concrete that will absorb more carbon than it emits during its manufacture. These researchers investigated a new method of making concrete involving biochar, a charcoal made from organic waste. While biochar has been added to cement by earlier investigators [10,11], Li and Shi (2023) treated it first using concrete washout wastewater [12]. This boosted the strength and allowed a higher proportion of the additive to be mixed in. Most importantly, the biochar was able to absorb up to 23% of its own weight in carbon dioxide from the air around it.

In their experiments, Li and Shi (2023) created a cement that contained 30% treated biochar, and found that the resulting concrete was carbon-negative - it actually absorbed more carbon dioxide than that was emitted during the production of concrete [12]. After 14 days of weathering, the biochar treated by concrete washout water captured 22.85 wt% air-borne CO2, and this CO2-weathered biochar at 30% by weight of Portland limestone cement made the paste carbon-negative. The microscopic evidence confirmed that the captured CO2 precipitated calcium carbonate onto/into the biochar. They calculated that 1 kg of the 30%-biochar concrete removed about 13 g of CO2 more than its production releases. That might not sound impressive, but considering regular concrete is usually responsible for the release of about 0.9 kg of CO2 per 1 kg of material, it is significant.

The total gains could be even better if downstream differences were accounted for in their analysis. For example, the beneficial use of biochar in concrete diverts carbon-rich biomass from alternative pathways that could potentially release more CO2. Plus, the new concrete would be expected to continue absorbing CO2 during its working life of several decades [12].

An additional and more important aspect of the biochar-concrete is that it also retained its strength. After 28 days, the compressive strength of the biochar concrete was 27.6 MPa, which was similar to that of conventional concrete.

REFERENCES

1.??????Subramanian, N., “Principles of Sustainable Building Design”, chapter in Green Buildings with Concrete-Sustainable Design and Construction, Gajanan M. Sabnis (Ed.), 2nd Edition, CRC Press, Boca Raton, FL, 2016, pp. 35-88.

2.??????UNEP (2019) Global Status Report for Buildings and Construction-Towards a zero-emissions, efficient and resilient Buildings, and construction sector, Global Alliance for Buildings and Construction, International Energy Agency and the United Nations Environment Programme, 41 pp.

3.??????https://gccassociation.org/concretefuture/getting-to-net-zero/

4. Subramanian, N. (2022), “Achieving Net-Zero CO2 Emissions in the Concrete Industry”, Civil Engineering & Construction Review (CE & CR), Vol. 35, No.4, Apr., pp. 32-41.

5.??????Scrivener, K., Martirena, F., Bishnoi, S., and Maity, S. (2018) "Calcined clay limestone cement (LC3)", Cement and Concrete Research, Vol.114, pp. 49-56. https://doi.org/10.1016/j.cemconres.2017.08.017

6.??????Galitsky, C., and?Price, L. (2007) "Opportunities for Improving Energy Efficiency, Reducing Pollution and Increasing Economic Output in Chinese Cement Kilns ", ACEEE Summer Study on Energy Efficiency in Industry, pp. 3-65 to 3-76.

7.??????Makul, N. (2023) Recycled Aggregate Concrete-Technology and Properties,?CRC Press, 420 pp.

8.??????IEA (2013) Technology Roadmap-Carbon capture and Storage, International Energy Agency, Paris, France, 63 pp.

9.??????CEMBUREAU-the European Cement Association (https://lowcarboneconomy.cembureau.eu/carbon-neutrality/our-2050-roadmap-the-5c-approach-clinker/)

10.???Aman, A.M.N., Selvarajoo, A., Lau, T.L., and Chen, W.-H. ?(2022) “Biochar as Cement Replacement to Enhance Concrete Composite Properties: A Review”, Energies, Vol. 15, 7662, https://doi.org/10.3390/en15207662.

11.???Tan, K.H., Wang, T.Y., Zhou, Z.H., and Qin, Y.H. (2021) “Biochar as a partial cement replacement material for developing sustainable concrete: An overview”, Journal of Materials in Civil Engineering, Vol. 33, No.12, 03121001.

12.???Li, Z., And Shi, X. (2023), "Towards sustainable industrial application of carbon-negative concrete: Synergistic carbon-capture by concrete washout water and biochar", Materials Letters, Vol.342, 1 July, 134368