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.

This study introduces the concept of Bio-LC³ in which biomass waste is upcycled into sustainable ingredients in limestone calcined clay cement (LC³) by partially replacing cement. Specifically, rice husk ash, rice husk biochar, sawdust biochar and titanium dioxide (TiO₂)-coated sawdust were chosen as the partial replacement for Ordinary Portland Cement (OPC). 

The novelty of this study lies in, firstly, a high replacement rate of 5-15 wt% was applied to replace OPC with the abovementioned biomass waste. Secondly, Accelerated Carbon Curing was applied to these different types of LC³ so that we could evaluate the effects of the different waste on carbon mineralization, strength, water absorption and thermal stability of LC³. 

It was found that it is possible to replace 15 wt% of cement with rice husk ash or 5 wt% of cement with TiO₂-coated sawdust and achieve similar compressive strength to that of carbonated LC³ control, which was in turn significantly stronger than LC³ control without carbonation. Carbonating LC³ with TiO₂-coated sawdust enhanced the reaction between mineralized carbonates (calcite) and metakaolin. In contrast, carbonation of sawdust biochar reduced calcite-metakaolin and metakaolin-Portlandite (CH) reactions, thus lowering its 28-day strength. Presence of rice husk biochar enhanced capture of carbon, as well as the overall bulk thermal stability.

All in all, these results showed that it is possible to further increase the sustainability of LC³ by valorizing different types of bio-waste and develop special functions that enhance the overall usefulness of these sustainable materials.