Carbon sequestration in the built environment: Initial development of biochar wallboard (BWB)

The carbon impact of residential and commercial building materials is substantial. Wallboard is used in enormous quantities and comes with a considerable carbon footprint. Adding biochar to wallboard (BWB) could convert walls from high embodied carbon facades to large-scale, above-ground carbon sinks while also potentially offering enhanced features over contemporary wallboard options. The purpose of the research was to formulate and test different types of biochars combined with different binders and identify composites that meet or exceed performance of commercially available wallboard.

A total of 23 different composites were formulated and tested. Ten of these composites used reduced amounts of gypsum and commercially available biochars. Eight of the composites included reduced gypsum, two different types of biochar and recycled glass. Five of the composites eliminated gypsum entirely with an organic binder. All BWB samples as well as three commercially available wallboard materials were tested following ASTM standards. The three most promising composites based on performance, manufacturability and carbon sequestration potential were scaled-up. Based on our extensive characterization protocol, we determined that biochar blended with gypsum at mass loadings up to 20% yielded very low embodied carbon with minimal effects on mechanical, moisture or thermal properties. Both wood and rice husk biochar produced BWB samples that met our specification, whereas sugarcane bagasse biochar did not. Adding recycled glass in either pellet or powder form did not offer significant improvements in physical properties of BWB. We identified seaweed-derived alginate as an alternative binder that can completely eliminate gypsum and achieve even lower embodied carbon.  We conducted a down-select process to identify our top three biochar wallboard candidates: BWB1 (90% gypsum, 10% wood biochar), BWB2 (80% gypsum, 20% rice husk biochar), and BWB3 (80% alginate, 20% rice husk biochar). These three formulations were used to produce prototypes compared to two common commercial products, Type X and EcoSmart. BWB1 & BWB2 provided comparable properties to the commercial benchmarks, but with significant reduction in embodied carbon.

Our research has shown that certain types of BWB could meet or exceed performance criteria for traditional wallboard while significantly reducing the carbon footprint. Composites that completely eliminate gypsum would be able to not only sequester carbon, but could be beneficially returned to soil at the end-of-life (or unused portions could go immediately into surrounding soils instead of landfilled). BWB3 showed thermal conductivity comparable to or lower than expanded polystyrene, which introduces the possibility of also using it as an insulation material. 

This work was supported by the EPA SBIR Program under contract number 68HERC22C0028.