CarbonBuilt ($10M to produce carbon-negative concrete bricks using steel slag, quick lime, or lime kiln dust and captured CO2)

CarbonBuilt, an American clean tech company founded in 2020, develops a low-carbon concrete by replacing carbon-intensive portland cement with low-carbon, low-cost, highly scalable materials, such as slag, quick lime, lime kiln dust, and flue gas desulfurization gypsum. Additionally, the cement-free concrete blocks are curved with CO₂ gas via concurrent hydration and carbonation reactions at ambient pressure in a chamber, thereby sequestering CO₂ permanently in concrete blocks. CarbonBuilt’s technology has won the Carbon XPRIZE. CarbonBuilt has partnered with Blair Block in Alabama to produce low-carbon concrete.

Challenges: concrete CO emissions and low carbon concrete

Concrete CO₂ emissions

Traditional concrete is a mixture of calcium silicate-dominant ordinary portland cement (OPC), mineral aggregates, water, and chemical additives. The reaction of OPC with water (hydration) forms calcium silicate hydrate (C-S-H) compounds. The precipitation of C-S-H between proximate particles induces cohesion/hardening. The result of this is porosity reduction and refinement that strengthens the concrete.

Cement production, particularly Portland cement, is responsible for a significant portion of carbon dioxide (CO₂) emissions in the construction industry, accounting for around 8% of global CO₂ emissions. This is because the production of cement involves a chemical reaction called calcination, in which limestone (CaCO₃) is heated to over 1,400 ºC in a kiln to produce lime (CaO). This process releases carbon dioxide according to the following reaction:

CaCO₃ + heat → CaO + CO₂

Additionally, the cement production also requires large amounts of energy to heat the kiln and grind the raw materials into the fine powder that is used to make cement. The energy required for cement production comes mainly from burning fossil fuels such as coal, oil, and natural gas, which also releases CO₂ into the atmosphere.

The CO₂ produced for the manufacture of structural concrete (using ~14% cement) is estimated at 180 kg/ton.

It is estimated that 75% of the infrastructure that will exist in 2050 has not yet been built, which will lead to a massive increase in demand for cement, which necessitates new approaches to producing low-carbon cement.

Low carbon concrete

Low carbon concrete, also known as green concrete or sustainable concrete, aims to reduce the carbon footprint associated with the production of traditional concrete. It is created using alternative materials, innovative production methods, and optimized mix designs.

Low carbon concrete reduces carbon emissions by replacing a portion of cement with supplementary cementitious materials (SCMs) such as fly ash, slag, or natural pozzolans. Additionally, low carbon concrete can be produced using alternative binders, such as geopolymers or calcium sulfoaluminate cement.

Low carbon concrete has diverse applications in the construction industry, including the production of precast concrete elements and building blocks for residential, commercial and industrial buildings, as well as pavements, parking lots, and sidewalks, which makes it a promising solution for sustainable development.

While low carbon concrete offers many benefits, there are also some challenges that need to be addressed to accelerate its adoption in the construction industry. These challenges include higher costs, lack of standardization, limited availability of alternative materials, concerns regarding durability, resistance to change, and energy consumption.

CarbonBuilt Technology

CarbonBuilt has removed carbon-intensive portland cement from the concrete mixture and replaced it with low carbon, low cost, highly scalable materials, such as slag, quick lime, lime kiln dust, and flue gas desulfurization gypsum. The cement-free concrete blocks are cured via concurrent hydration and carbonation reactions in a chamber filled with concentrated CO₂ vapor at ambient pressure. This carbonation process permanently sequesters CO₂ and hardens the concrete blocks. The CO₂ vapor source is supplied by a small, energy-efficient Direct Air Capture system. CarbonBuilt’s concrete blocks have a compressive strength between 15 and 20 MPa, comparable to conventional concrete products.

CarbonBuilt concrete composition

CarbonBuilt cementitious mixture replaces portland cement  with aluminosilicate industrial solid wastes (5-20%wt of the total solid mass of the concrete mixture), lime (0.5-10%wt), kiln dust (1-20%), and gypsum (0.5-5%). (Detailed in US202163264152P)

The addition of lime-based activators, such as quicklime (CaO), hydrated lime (Ca(OH)₂), or kiln dust, can promote pozzolanic reactions, resulting in increased strength.

The addition of sulfate activators such as gypsum results in ettringite formation, which favors microstructure densification and strength.

  • Aluminosilicate industrial solid wastes

The aluminosilicate industrial solid wastes are sourced from coal combustion residues (such as class C fly ash and class F fly ashes) and slag (e.g., basic oxygen furnace slag, electric arc furnace slag, ladle slag, or blast furnace slag).

  • Lime

The lime can be virgin quicklime (CaO), virgin hydrated lime (also known as portlandite), or hydrated lime.

  • Kiln dust

Lime kiln dust is derived from the lime manufacturing process. Cement kiln dust is sourced from the cement production process.

  • Gypsum

Natural gypsum is mined and ground into a fine powder. Synthetic gypsum is collected from environmental control systems that are installed in the stacks of coal-fired power plants. These systems capture particles and gasses including sulfur dioxide. The sulfur dioxide is reacted with limestone and water to become synthetic or flue-gas desulfurization (FGD) gypsum.

CarbonBuilt concrete carbonation system

The slurry of the cementitious mixture is pressed into molds to form concrete blocks. The blocks are placed inside a carbonation chamber for producing curved solid concrete. CarbonBuilt has developed a system that combines a Direct Air Capture (DAC) device and carbonation chamber via a CO₂ processing unit for producing low-carbon concrete products, as depicted in the diagram below.

CarbonBuilt concrete carbonation system (ref. US20230120088A1)
CarbonBuilt concrete carbonation system (ref. US20230120088A1).
  • Direct Air Capture system

The small-scale and energy-efficient DAC system supplies a low-cost CO₂ gas stream (>5 vol.% for CO₂) for the carbonation of concrete.

The DAC system captures CO₂ from the air using an electrochemically-induced pH-swing process and an amine-based absorption solution.

A concentrated (20-50%) aqueous amine solution is in contact with the airflow. Under basic conditions (pH > 10), CO₂ reacts with the amine (e.g., RNH₂ where R═CH₂CH₂OH) to form carbamate anions (RNHCOO⁻, RNCOO²⁻), protonated amines (RNH₃⁺), and protons.

In order to release CO₂ and regenerate the amine adsorbent, the pH of the CO₂-saturated amine solution is lowered below 7 (acidic conditions) to promote the decomposition of the carbamate anions into amine and CO₂.

An electrodialysis cell or an electrolyzer can be utilized for the pH swinging process. The swinging process can occur at ambient temperature, and therefore offers several advantages: (1) simpler process equipment requirements; (2) utilization of the amine’s maximum working capacity; and (3) reduced solvent loss.

  • CO₂ gas processing unit

The stream of concentrated CO₂ vapor from the DAC system passes through the gas processing unit. The gas processing unit  adjusts the temperature, relative humidity, and flow rate of the concentrated CO₂ vapor.

Temperature: 15-100 ºC

Humidity: 0-95%

Flow rate: 50-5,000 SCFM

  • Carbonation chamber

The CO₂ vapor stream that has been conditioned is introduced into a chamber. The calcium ions of concrete blocks react with CO₂ inside the chamber to form CaCO₃, which hardens the concrets. The carbonation process permanently sequesters CO₂.

CarbonBuilt Patent

  • US202163264152P Methods and compositions for low-carbon concrete production using carbon dioxide and solid waste streams
  • WO2022256456A1 Methods for reactivating passivated mineral residues
  • US20230120088A1 Integration of direct air capture system into co2 mineralizaton process of concretes and aggregates

CarbonBuilt Technology Applications

  • Low-carbon concrete production

The primary application of CarbonBuilt’s technology is in the production of concrete. The company’s technology replaces traditional Portland cement with calcium-rich, low-carbon industrial waste materials, reducing the carbon footprint of the concrete products. The technology is designed to be flexible, customizable, and easy to retrofit into existing masonry plants, using off-the-shelf equipment and locally sourced materials and CO₂.

  • Precast concrete products

CarbonBuilt’s technology can also be applied to a wide range of precast low carbon concrete products, including pavers, segmented retaining walls, hollow core slabs, pipes, manholes, and other types of dry- and wet-cast products. The company has announced a collaboration with Master Builders Solutions to develop admixtures for use with CarbonBuilt’s low-cost and low-carbon technology platform.

  • Construction industry

CarbonBuilt’s technology can be used in the construction industry to produce ultra-low carbon concrete blocks. The company has partnered with Blair Block for the commercial introduction of this technology, which will reduce the carbon footprint of Blair Block’s products by over 80%. The retrofit of one production line will enable the annual manufacturing of approximately 2.3 million 8-inch block-equivalents, each block permanently sequestering around 0.55 lb (0.25 kg) of CO₂.

  • Carbon credit market

CarbonBuilt’s technology can also be used to generate high-quality carbon credits that avoid and permanently remove CO₂ while accelerating concrete’s decarbonization.

CarbonBuilt Products

Global concrete market

The global concrete market is vast, with a market size of approximately $700 billion in 2022. This market is expected to reach a value of nearly $1 trillion by 2028.  This growth is driven by the rising demand for sustainable and environmentally friendly construction materials.

CarbonBuilt concrete

CarbonBuilt’s technology can reduce overall CO₂ emissions by 70-100%. The company have the following products:

  • Precast products

Precast products can be sold to traditional supply chains, providing a sustainable alternative to traditional concrete products.

  • Concrete blocks (CMUs)

Carbon concrete blocks have a significantly reduced carbon footprint compared to conventional concrete blocks while maintaining product performance.

  • Carbon credits

CarbonBuilt sells carbon credits. When a concrete producer mineralizes CO₂ in concrete blocks and receives a verified carbon credit, the producer can generate additional revenue by selling the credit to another organization.

CarbonBuilt shares with concrete producers all revenue generated from the sale of carbon credits. To date, CarbonBuilt has distributed over $1.7 million to producers from carbon credit sales. The exact amount of carbon credit revenue that a producer can earn depends on various factors, such as the volume of CarbonBuilt concrete produced.

CarbonBuilt Funding

CarbonBuilt has raised a total of $10M in funding over a Series A round raised on Oct 6, 2021.

CarbonBuilt Investors

CarbonBuilt is funded by 6 investors:

Foundamental and Tony Pritzker are the most recent investors.

CarbonBuilt Founder

Rahul Shendure and Gaurav N. Sant are Co-Founders.

CarbonBuilt CEO

Rahul Shendure is CEO.

CarbonBuilt Board Member and Advisor

Neil Renninger is Board Member.

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