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 CO₂ 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