Limestone calcined clay cement (LC3) is a sustainable binder that has been increasingly studied as an alternative to Ordinary Portland Cement (OPC). However, one of the technical barriers to large scale application of LC3 is its low workability. Although the creation and application of tailored superplasticizers (SPs) has become one of the most common solutions to this problem, the over-reliance on such chemicals will give rise to other problems, including high embodied energy in these SPs.

This study aims to offer a more sustainable solution by valorizing abundant waste wood, in the form of biochar, to replace 2wt% and 10wt% of OPC content in LC3; this is done to increase the overall sustainability of the LC3, while increasing compressive strength, shortening setting times and improving workability. To ensure that our results are relevant to actual construction conditions, all samples were subjected to air-curing.

It was found that all LC3 that contained biochar were significantly stronger than OPC control at 28 days. In particular, incorporating 2wt% biochar (dry or pre-soaked) could maintain compressive strength of the LC3 but yield significant better workability than OPC mortar. 

A model was proposed to explain this phenomenon - specifically about how biochar modifies the water distribution by reducing the amount of gel pore water and at the same time, increasing the amount of free or bleeding water available when the LC3 samples were mechanically agitated; this enhances the movement of particles over one another during mixing or vibration, thus lowering the viscosity and improving the workability. 

In summary, these results can potentially point the way to improving the sustainability of LC3 while reducing wood waste, using biochar as a pathway to waste valorization in the creation of high-performance concrete.