Research in biochar concrete has advanced considerably in the last 11 years or so.  However, there is not yet any study examining the effects of biochar on the mechanical strength, net embodied carbon and effectiveness of accelerated carbonation of a relatively new and more sustainable type of concrete - limestone calcined clay concrete (LC3). 

This work represents a pioneering effort in examining the influence that rice husk- and wood-based biochar, produced at a temperature range of 300-500C, have on the aforementioned qualities of LC3-70, in which 30% of the Ordinary Portland Cement (OPC) was replaced with the combination of limestone, calcined clay and sustainable additives. Four types of additives were examined: rice husk ash (RHA), rice husk biochar, wood sawdust protected with titanium dioxide shell, and sawdust biochar. These samples were further categorized into two groups - those dosed with CO2 (i.e. accelerated carbonation at 20% over 24 hours) and those that were not dosed. 

It was observed that accelerated carbonation marginally increased the compressive strength (by 2-5%) of all samples containing the aforementioned sustainable additives at 28 days. However, all these samples had marginally lower strength than the control LC3-70, except when RHA was used (a 1.6% increased strength was observed). Furthermore, the compressive strength of the samples was not correlated to the quantities of calcium hydroxide produced from hydration of the binder. 

The most notable result in this study was that carbonation of these LC3 samples made them more resilient to heat - thermogravimetric analyses reviewed that carbonation substantially reduced the disintegration of the calcium carbonate (calcite) present in all the LC3-70 samples. This unusual phenomenon can be attributed to the production of additional amounts of alumina and silica gels in the matrix, which increased the internal thermal resistance of the microstructure of the LC3-70. 

These results show that substituting OPC in LC3-70 with the proposed sustainable additives does not affect the strength significantly, but it can reduce the net carbon emission of the concrete while rendering the mix more resistant against thermal disintegration. This special characteristic implies that our mixes can potentially be used as indoor lightweight non-structural panels that can be used to prevent fire from spreading within a particular confined indoor space.