Silicate (Sequester CO2 from the air in recycled concrete used as soil additives)

Silicate (also Silicate Carbon), an Irish climate tech company founded in 2022, has used recycled concrete as soil additives to permanently remove CO₂ from the atmosphere by enhancing the weathering process. Compared to other carbon removal soil additives, such as basalt and olivine, recycled concrete also reduces carbon emissions by requiring less energy for transport and processing. Silicate has demonstrated that the recycled concrete as soil additive is hardly harmful to soil chemistry, even beneficial to soil pH and a valuable source of base cation fertilization. Silicate is one of the top 60 groups in the  XPRIZE Carbon Removal competition, a $100 million global competition funded by Elon Musk and the Musk Foundation.

Challenges: carbon emissions and carbon mineralization

Carbon emissions

Since the early 1900s, carbon dioxide (CO₂) levels in the atmosphere have increased by 50% due to human activities. When fossil fuels (such as coal, oil, and natural gas) are burned for energy production, transportation, and industrial processes, CO₂ is released into the atmosphere. This excess CO₂ acts as a greenhouse gas, trapping heat and causing the air and ocean temperatures to rise. CO₂ emissions play a crucial role in driving climate change.

This warming effect has caused the global average temperature to rise by about 1.1 ºC since the pre-industrial period. This has led to rising in the frequency and intensity of extreme weather events, melting of polar ice caps and glaciers and rising sea levels, shifts in species ranges and increased risk of species extinction, agriculture and food security,  and ocean acidification.

To mitigate these impacts, the Paris Agreement aims to limit global warming to well below 2 ºC above pre-industrial levels. The Intergovernmental Panel on Climate Change (IPCC) estimates that a “carbon budget” of about 500 GtCO₂, which corresponds to about ten years at current emission rates, provides a 66% chance of limiting global warming to 1.5 ºC.

Soil carbon sequestration

Soil respiration returns CO₂ fixed by plants to the atmosphere.  It produces CO₂ by the biological activity of soil organisms. The CO₂ concentration in soils can be up to 10 times that of the atmosphere. Soil respiration emits about 10% of global CO₂ emissions.

Soil additives, such as basalt and olivine powders, can be used to enhance soil weathering processes, which sequesters CO₂ from the atmosphere.

These soil additives consume CO₂ by neutralizing carbonic acid (H₂CO₃) which is produced when rainfall dissolves atmospheric CO₂. This enhanced weathering converts carbonic acid to stable bicarbonate ions (HCO₃⁻). Excess bicarbonates in the soil eventually drain into the oceans. Bicarbonates can trap carbon in the oceans for around 80,000 years.

Basalt rock, for example, is highly reactive, weathering quickly to speed up CO₂ sequestration. The powdered basalt as soil additives would rapidly release minerals and alkalinity into moist soils, thereby restoring degraded soil fertility and enhancing ecosystem carbon storage. Enhanced weathering by applying basalt to agricultural land is a proposed strategy for removing CO₂  from the atmosphere.

To achieve a sufficiently high net CO₂ removal, however, it will require upscaling of basalt mining, deploying systems in remote areas with a low carbon footprint, and using energy from low-carbon sources. Additionally, unknown side-effects of basalt soil additive and limited data on field-scale deployment need to be addressed first.

Silicate Technology

Silicate has discovered that recycled concrete, which previously has no use and was mistakenly considered a potential pollutant, is in fact a valuable soil additive product that effectively sequesters CO₂, increases soil pH buffering capacity, and provides valuable base cation fertilization.

Recycled concrete is abundant. On every construction site, unused concrete is typically recycled to the concrete production site. 3-5% of produced concrete is recycled to its producer, according to the US National Ready Mix Concrete Association (NRMCA). In 2021, the global concrete production is around 4.2 billion tons.

Compared to basalt and olivine, the crushing of recycled concrete is relatively efficient and can be done with low-carbon electricity. Additionally, crushed recycled concrete from a recycle plant is transported to farms over a shorter distance, reducing energy consumption and carbon emissions.

How does Silicate identify recycled concrete as soil additives?

Silicate carried out experiments in a trial tillage field in southeast Ireland to demonstrate that crushed recycled concrete can be used as soil additive for carbon sequestration.

The crushed recycled concrete with a pH value of 10 was spread at a rate of 10 tons per hectare as a soil additive to the trial tillage field. At monthly intervals, shallow soil-water solutions were extracted  from the trial tillage field and the untreated control sites adjacent to the trial tillage field. The geochemical impact of crushed recycled concrete on soil water was determined by analyzing the soil-water solutions.

Here are the main findings of Silicate:

  • The treated sites have higher soil-water pH levels than the untreated sites (by 0.2 to 0.5 pH units). This indicates the weathering of portlandite (Ca(OH)₂) and calcium silicates in the crushed recycled concrete. Carbon present as carbolic acid in soil water is converted to stable dissolved bicarbonate (HCO₃⁻). Thereby, CO₂ dissolved in solid water is captured from the air;
  • The acidity of soils with a moderately low pH is decreased. Soil acidification is caused by the depletion of base cations, such as Ca²⁺, through natural weathering processes. It was discovered that crushed recycled concrete can replenish depleted base cations, thereby preventing soil acidification; and
  • The content of heavy metals such as Ni and Cr is well within the regulations for ground limestone, making its application to agricultural land completely safe.

Silicate concluded that crushed recycled cement can be used safely as soil additive to sequester CO₂ from the atmosphere, reduce soil acidity, and provide valuable base cation fertilization.

How to produce crushed recycled concrete?

Concrete contains two principal components: cement and aggregate. Cement contains high calcium concentrations, which are primarily hosted in minerals that are prone to rapid weathering, such as portlandite and amorphous calcium silicates. The aggregate (chunks of rock) that makes up the rest of concrete is determined by the local rock sources.

De-aggregated crushed recycled concrete is ideally used as soil additives. It is produced by crushing de-aggregated recycled concrete cakes, which are composed of recycled concrete from which the aggregates have been removed.

How are cakes of de-aggregated recycled concrete made?

When recycled concrete still has plasticity, it is transported over a short distance to a recycling plant. The recycled concrete is mixed with water and then stirred. As a result, the cementitious component is separated from the sand and aggregates. The aggregates are removed from the mixture using sieves. The remaining mixture (i.e., water, cement, and sand) is delivered to a press with a sieve to squeeze the water out. Under the pressure from the press, the remaining residue of the mixture, which comprises sand, cementitious components, and water that cannot be mechanically removed, forms a cake. Cakes of de-aggregated recycled concrete are advantageously dried to reduce weight, transport costs, and transport-related CO₂ emissions.

The de-aggregated recycled concrete cake has a low compressive strength. It is not as strong as other soil additives, such as basalt and olivine. Crushing the cakes is easy and requires much lower energy, resulting in less CO₂ emissions.

The resulting de-aggregated crushed recycled concrete is suitable for use as a soil additive for carbon sequestration, according to the following cement weathering reaction:

2H₂CO₃ + Ca(OH)₂ → Ca²⁺ + 2HCO₃⁻ + 2H₂O

Silicate Patent

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