Research makes the world go round

Professor Frank McDermott
June 17, 2024
5 minutes

Results from our pilot study

The results of Silicate’s first ‘proof of concept’ enhanced weathering trial in an arable field were published recently as a peer-reviewed paper in Applied Geochemistry.

Silicate spread crushed returned concrete (CRC) as a soil amendment at a rate of 7.5 tonnes/hectare on an arable field in the southeast of Ireland in early June 2021 using a conventional tractor-drawn lime spreader. The amendment was mixed into the upper 15-20 cm of the soil by the no-plough minimum tilling seed-sowing operation in the days following spreading. In collaboration with researchers at University College Dublin and iCRAG, soil pH, soil leachable cations and soil-water bicarbonate and cation concentrations were measured over the course of the study until April 2022. Soil waters were sampled regularly (approximately monthly) using ceramic-cup suction lysimeters that were installed immediately post-amendment. No chemical or organic fertilisers were applied over the course of the study, and the site had a history of low-intensity management over several years.

Fig. 1: Picture shows the amended field in summer 2021. The field was used to grow a mixed cereal/seed crop (barley, oats, linseed mix) for a European Union scheme to create habitat and forage for wild birds.

Why is this study important?

Our approach is different, and arguably important, because unlike previous experiments that generally deploy basalt or silicate minerals as soil amendment materials, we used CRC, a carbonate-bearing material, the composition of which is discussed in detail in our paper. CRC is produced by crushing solidified returned concrete, a surplus alkaline product of the ready-mix concrete industry. Globally, it is estimated that returned concrete comprises 1-4% of all concrete produced, depending on the jurisdiction (Ren et al., 2022; Xuan et al., 2016). Global annual concrete production is estimated at 25 billion tonnes, which implies returned concrete supplies of 250 to 1000 million tonnes/year that could be re-purposed for carbon dioxide removal (CDR). Unlike construction and demolition waste, CRC is a virginal product and is free from potentially harmful building-related contaminants (e.g., asbestos, insulation fibres) that can occur in construction and demolition waste.

Our methodology to monitor CDR differs from many other companies because instead of focusing on the solid (soil) phase to measure weathering rates, we measured the product of CDR, namely an increase in soil-water bicarbonate concentrations in treated parts of the field, relative to control areas (delta bicarbonate).

Fig. 2: Picture shows the amended field in summer 2021, shortly after the crop had emerged following soil amendment. The location of one of the soil-water samplers can be seen near the middle of the picture (three bamboo stick markers).  
Fig. 3: Close up picture of soil surface taken on 16 July 2021, approximately six weeks after application of the CRC material. Some residual coarser material (up to 4mm) remained un-weathered on the surface at this point. In subsequent work the material was milled to <1mm diameter (referred to as milled returned concrete).

So, what did we find?

Soil pH increased rapidly by 0.3 to >1 pH units with the greatest increase in soils that had the lowest initial pH (< 6.0), demonstrating the efficacy of the material as a soil pH amendment. More importantly, from a CDR perspective, we found enhanced bicarbonate in soil waters at amended sites compared with controls (by up to ~5,5 mmol/litre). Furthermore, enhanced bicarbonate provided the balancing anion for observed increases in soil-water calcium and magnesium, indicative of carbonic-acid driven weathering of the CRC at most sites as shown in Fig. 4 (green symbols represent control sites, and red symbols amended sites).

Fig. 4: Molar bicarbonate vs. molar concentrations of the major cations (Mg and Ca) in soil waters. Most of the data lie close to the 2:1 molar line as expected by bicarbonate-divalent cation charge balance, with the exception of some samples from sites BF3 and BF8 which are interpreted to reflect nitric acid weathering. As discussed in our paper, these two sites tend to have high soil-water nitrate levels.

An important insight was that at some sites (e.g., BF3 represented by the red stars in Fig. 4), nitrate rather than bicarbonate was the balancing anion for the cations (calcium and magnesium), indicating strong (nitric) acid weathering at these sites. This effect would not have been noticed if we had not collected and analysed soil waters.  

A simple model that calculates the downward flux of bicarbonate through the soil based on annual effective rainfall (rainfall minus evapotranspiration) coupled with a stoichiometric correction for nitric acid weathering, indicated gross CDR rates in the range zero to 0.52 tonnes CO2/ha (69 kg CO2 per tonne of CRC), limited by bicarbonate export rather than weathering rates.

A preliminary life cycle assessment (LCA), including crushing and transport-related emissions based on measurements and estimates from the literature are presented in our paper. Briefly, this simplified LCA indicates combined CO2 emissions from crushing, loading, road transport and on-farm spreading of approximately 7.32 kg CO2 per tonne of CRC, or approximately ~11% of the 69 kg CO2/tonne inferred for the ‘best performing’ BF6 field site.

And, what is next?

Work is ongoing at Silicate, in collaboration with researchers at University College Dublin, to optimize rates of CDR to achieve values that approach the theoretical maximum value of the material (~270 kg CO2/tonne CRC). This optimised removal objective could come from milling the material so that it has a larger surface area, or other strategies to improve bicarbonate yield per increment of weathering.

Importantly, carbonate-bearing minerals are much more soluble than silicate-bearing minerals, and they dissolve orders of magnitude faster than silicate minerals. Therefore, enhanced weathering solutions employing returned concrete or other carbonate-bearing materials like limestone, could offer advantages for CDR over silicate rocks, such as basalt, which is an area Silicate will continue to explore.

Professor Frank McDermott