Course:EOSC311/2020/Green Concrete: Paving Our Way to a Sustainable Future

Introduction

The Vancouver skyline to illustrate urbanization and how much concrete is utilized in a single city.

Concrete is the second most used substance in the world; placing behind water, an excessive amount of concrete is constantly produced and utilized to develop our industrial world . The size of the concrete industry has increased 25-fold since 1950 and is consuming the Earth; our urban environment may have already outgrown our natural one .

In addition, concrete production and implementation is not sustainable by any means, and due to the scale of the industry, the ecological drawbacks have an enormous impact upon the environment. Evidentially, the concrete industry exudes more carbon dioxide than every country, aside from China and the United States .

Fortunately as a result, several sustainable concrete technologies have been researched and developed. Some of these technologies make minor modifications to conventional concrete to reduce its ecological harm, while others involve innovative ideas that have the potential to lead to a sustainable revolution.

The Relations Between Sustainable Concrete and Geology

Sustainable concrete technologies relate to our course, EOSC 311: The Earth and its Resources, due to significant involvement of geological materials and their impact on Earth. Both conventional and advanced concrete technologies utilize raw materials like limestone, shale, and clay, and the process of using these materials to manufacture concrete often involves fossil fuel energy resources .

My major is Geography with a focus on Environment and Sustainability. Green concrete directly correlates to my major as it examines sustainable alternatives to conventional concrete technologies. Not only is this topic extremely pertinent to my major, geography, but it also possesses the same overarching theme or idea of environment and sustainability.

In addition, I hope to pursue a Master’s degree and career in Architecture, which will involve sustainable design. Ecological concrete relates to my prospective studies as sustainable building materials are incredibly important for the future of design and our planet.

I chose this topic because I am absolutely fascinated by the revolutionary and sustainable technologies currently being developed in a wide-variety of industries. I am particularly interested in sustainable energy and urbanization, due to my desire to pursue a career in architecture and design. I hope that I am able to make a positive contribution in the future by creating sustainable architectural designs, and that I am able to witness an environmental revolution in my lifetime.

Conventional Concrete

What is Concrete?

In simple terms, concrete is a strong stone-like material made by a combination of paste and rock aggregates. The aggregates are covered by the paste— a mixture of Portland cement and water— and after it hardens through hydration, the resulting product is concrete .

Concrete is a strong and durable substance that is utilized for industrial processes and urbanization. Concrete is used to create the built environment around the world; the buildings we live, work, learn— exist— in, the roads we walk on, the bridges we cross, the industrious structures used to create energy, and many more. Consequently, an astounding amount of concrete is produced annually: enough concrete to cover the entire country of England .

Clinker pellets for cement manufacture recently removed from the kiln.

Portland Cement

Contrary to common perception, cement and concrete are not analogous. Cement is a component that acts as a binder in the production of concrete. The standardized "portland cement" is made through the combination of elements— such as calcium, aluminum, and silicon— through the crushing and burning of rock materials . Limestone, clay, and shale are combined with other components, ground down, and inserted in a kiln . Powered by fossil fuels, the kiln heats the mixture to extreme temperatures, producing harmful gaseous emissions. The resulting product is small pellets of clinker, which is ground into dry cement powder and used to make concrete .

A graphic illustrating the ingredients in cement.

Production

Concrete is composed of roughly up to 8% air, 7-15% cement, 14-21% water and 60-75% aggregates . The combination of water and portland cement creates a paste that is used to cover and fasten the aggregates together. The aggregate-paste mixture will then endure the chemical process of hydration to harden and form concrete .

It is important that the proportions of each ingredient are correct, as the composition of the concrete can drastically affect the properties of the final product. For instance, the strength of the concrete is determined by the water-cement ratio; optimally, the ratio of water to cement is as low as possible, while still being able to utilize the concrete mixture prior to hydration .

In addition, the components have to be carefully selected. The water utilized must be similar to that of drinking quality, as an overabundance of minerals can cause defects and impurities . Also, the aggregates employed must be clean and of appropriate magnitude; a range in the size of aggregates is necessary, but it is important to consider the scale of the structure when selecting materials .

After the all materials have been combined, the concrete mixture is placed into an allocated form where it will undergo hydration and harden in the appropriate shape . It is critical that this is done in a timely manner, otherwise the concrete mixture will become too stiff to be molded into the required arrangement. Next, the concrete is consolidated; vibrations and gravity are utilized to eliminate possible faults, such as air bubbles and honeycombs . Finally, once the mixture has sufficiently solidified, the concrete is cured. The purpose of curing is to prolong the hydration of the cement, as this increases the strength of the final concrete product . Curing is accomplished by covering the cement form with moisture-retaining fabrics, plastic, or water-fog, to preserve the moisture within the mixture . Furthermore, an additional step is required for concrete slabs prior to curing; "floating" involves using a metal or wood hand-float to even-out and smooth the top of the surface .

Environmental Impact

Due to the wide variety of industrial applications, a tremendous amount of concrete is constantly being produced. To provide some perspective, 8 billion tonnes of plastic were produced over the last 60 years, while 8 billion tonnes of concrete were produced over the last 2 years .

Although concrete is used so extensively in industrial processes around the world, half of the carbon dioxide (CO₂) emissions associated with concrete are actually produced in cement fabrication; more specifically, clinker . The chemical reaction required to make clinker— the calcination of limestone and silica-aluminous materials— produces a significant amount of carbon dioxide (CO₂) as the excess reagent :

5CaCO₃ + 2SiO₂ → ( 3CaO, SiO₂ ) ( 2CaO, SiO₂ ) + 5CO₂

Numerically, 1 tonne of portland cement yields between 0.6-1 tonne of carbon dioxide (CO₂) . Over 4 billion tonnes of cement are produced annually, emitting an enormous amount of carbon dioxide (CO₂) into the atmosphere . This is in addition to the harmful gaseous pollution expended by the stone quarries, transportation, and implementation of the concrete in construction activities . In fact, the concrete industry is responsible for roughly 8% of global carbon dioxide (CO₂) emissions .

An illustration of an example of the heat-island effect.

The concrete industry is also accountable for other environmental sabotage. By building and developing the industrial world, natural habitats and ecosystems are destroyed. Not only does industrialization obliterate the current environment of the developing site, but it also prevents any of the naturally occurring ecological functions from happening in the future. This is devastating for habitats, organisms and the critical processes of nature, like oxygen production, pollination, sea level control, and water and soil purification. Furthermore, the concrete industry is also accountable for approximately 10% of Earth’s industrial water usage and significantly contributes to the heat-island effect in urban cities .

The rapid development of the urban world is not slowing down. Countries such as Indonesia and India are beginning to enter the stage of mass-concrete expansion, which contributes to the projection that the global urban “floor” will double over the next 40 years . If developing countries around the globe meet the average level of infrastructure, the global construction carbon dioxide (CO₂) levels are going to drastically transcend the proposed goal of the Paris agreement: 470 gigatonnes of carbon dioxide (CO₂) emitted by 2050 as opposed to a 16% annual decrease in levels by 2030 . By continuing to urbanize at the current pace, the future of the natural world is quickly diminishing and life on Earth is in jeopardy.

Bioconcrete

Bioconcrete is a broad term used to describe microorganisms that have grown into a brick-like product . Bioconcrete is an innovative concrete alternative that uses biomimicry and nature to create a sustainable building material.

bioMASON biocement binding to concrete aggregates.

bioMASON Concrete

bioMASON is a company that uses bacteria to grow bioconcrete that is strong, durable and sustainable. bioMASON concrete production involves zero carbon dioxide (CO₂) emissions and uses recycled materials for concrete aggregates . The production of bioMASON concrete is very similar to conventional concrete, and mainly differs in the manufacturing of “cement”. bioMASON concrete is composed of aggregates, bacteria and a calcium-abundant solution . The bacteria and calcium solution act as the "cement" by reacting to grow calcium carbonate (CaCO₃); the rigid component of both shells and portland cement . Portland cement is made by the calcination of limestone and silica-aluminous materials, burning calcium carbonate (CaCO₃) and emitting excessive amounts of carbon dioxide (CO₂) . “Biocement”, however, is made by biologically growing calcium carbonate (CaCO₃), which is then mixed with aggregates, placed in a form, and hardened to construct bioMASON concrete bricks .

An archway formed out of cyanobacteria concrete in Dr. Srubar's lab.

A chart demonstrating the manufacturing process of LBMs.

Cyanobacteria Concrete

Researchers at the University of Colorado have utilized photosynthetic microorganisms to grow bricks that reduce carbon dioxide (CO₂) emissions, repair themselves, and generate additional materials . The concrete bricks, or living building materials (LBMs), are made from cyanobacteria, sand, gelatin, water, and calcium nutrients . Biomineralization will occur once the materials are combined together and placed in a temperature environment of about 35 ºC, initiating photosynthesis and the genesis of the LBMs . Similar to conventional concrete, LBMs get their solidity through the formation of calcium carbonate (CaCO₃); a byproduct of the carbon dioxide (CO₂) absorbed through photosynthesis and the calcium-rich nutrients . Subsequently, the gelatin and calcium carbonate (CaCO₃) adhere the components together as the material cools, hardening into a brick form .

Cyanobacteria LBMs have incredible potential for both environmental benefits and future industrial endeavours. Due to the photosynthetic properties of the microorganisms, these bioconcrete bricks reduce greenhouse gas emissions by absorbing carbon dioxide (CO₂) . Plus, the other materials involved do not need to meet the high standards of traditional concrete, meaning that recycled materials can be implemented . Additionally, the bacteria can continue to grow, self-repairing faults within the bricks and generating an exponential amount of new materials . The regenerating capabilities of these LBMs make building in harsh environments feasible; LBMs may have provided a viable method for industrialization beyond Earth.

Geopolymer Concrete

Geopolymer concrete is very similar to conventional concrete, however, geopolymer cement is utilized as the binding agent instead of portland cement . Geopolymer concrete is not difficult to implement into industrial operations as the similarities between conventional and geopolymer concrete allow for analogous usage of the two products.

Geopolymers have several properties that are advantageous for use as construction materials, including high heat resistance, weather resistance, strength, and durability . In addition, the adjustments made for geopolymer concrete allow consolidation to happen without high energy, high temperatures or a significant output of carbon dioxide (CO₂) .

A chart demonstrating the production process of geopolymer concrete.

Production

The production of geopolymers is referred to as geopolymerization and involves two main components: raw materials and an alkaline activator . Similar to the manufacturing requirements for clinker, geopolymerization requires raw materials rich in aluminum and silicon; recycled materials from industrial waste fulfill this requirement and can therefore be reused instead of gathering new supplies . Geopolymer concrete has the potential to reclaim industrial waste products and reuse them for future sustainable construction. In addition, the alkaline activator allows the geopolymers to be produced without intense heat and use of fossil fuels . These two modifications to the conventional procedure significantly reduce the greenhouse gases and waste materials produced by urbanization.

The specific substances used as the raw materials and alkaline activator will affect the physical and chemical properties of the geopolymer, influencing the manufacturing process. For example, where the raw materials would typically be inserted into a kiln at roughly 1400 ºC to make clinker, the geopolymer materials are able to combine at less than 100 ºC . This is significant in regards to the fossil fuels required to produce the intensive heat conditions and the carbon dioxide (CO₂) emitted as a product.

Once geopolymerization is completed, the process to manufacture concrete mimics normal convention; the geopolymer simply replaces portland cement to make the paste component of concrete . The sequential steps are slightly modified according to the different materials utilized. However, it is therefore difficult to standardize the full process of generating geopolymer concrete, as adjustments will always have to be made for the chemical properties of the varying ingredient options .

Modifications to Conventional Concrete

Carbon-Dioxide Injection

An illustration of the carbon capture process implemented by CO2 Concrete

CO₂Concrete LLC, a company developed by a professor from the University of California at Los Angeles, utilizes carbon dioxide (CO₂) emissions from industrial processes to create a more sustainable alternative to conventional concrete . In a similar process to the production of limestone, or "carbon dioxide (CO₂) mineralization", carbon dioxide (CO₂) is converted into solid calcium carbonate (CaCO₃) and combined with a cement mixture . The resulting product is a concrete alternative that has a lower concentration of cement— reducing the carbon footprint of production— and sequesters excess carbon dioxide (CO₂) emissions from industrial processes .

Additionally, the production process is able to be executed without the use of intense pressures, temperatures or energies, and the captured CO₂ does not need to “purified” prior to utilization; therefore, the production process itself is fairly sustainable .

Concrene

The incorporation of graphene into traditional concrete has created a new substance: concrene. Concrene is superior to conventional concrete in regards to both its physical properties and environmental impact . The addition of graphene improves the material’s strength and durability, decreasing the cost by extending the lifetime of the product and making typical concrete reinforcements unnecessary . Furthermore, as the volume of cement used in concrene is less than in traditional concrete, concrene provides environmental advantages; concrene utilization decreases the by-product of carbon-dioxide (CO₂), as well as reduces the amount of heat pollution emitted by the exothermic process of concrete solidification .

Conclusion

The future of urbanization has tremendous potential for innovative and sustainable technologies. The concrete industry is a significant contributor to global industrialization and the consequential environmental harms. The current concrete alternatives are not yet at a sufficient capacity to dethrone conventional concrete, but they provide insight on the importance and potential of sustainable concrete technologies.

I am thrilled to be pursuing a career in the urban industry in such an innovative time, and I am excited to see the brilliant solutions that arise in the future. Moving forward, I hope that a global shift in mindset occurs, so that instead of trying to lessen the environmental impact of industrial processes, we strive to achieve ecologically net-positive endeavors to actually benefit our Earth.