Sky Pillar: Characterization and prototyping of biochar-cement composites

Kevin Mockford, Laure Nolte, Preston Stronach, James Forren

Material, Body, Environment Laboratory (MBEL)

“Sky Pillar” is a suite of investigations of biochar-concrete, a cementitious composite using biochar – a carbon sequestering biomass particulate – as fine aggregate. The investigation valorizes waste-stream by-products from forestry, agriculture, and aquaculture as feedstock for building materials: validating their thermal, hygroscopic, and mechanical properties while also prototyping novel means and methods of construction. In a post-carbon world, innovative material systems integrating biomass waste with architecture holds the dual promise of climate-cooling carbon sequestration and transformation of conventional waste streams into circular ecologies. Validating performance and serviceability is a critical step in demonstrating the general viability of this innovative material system strategy. The investigation created multiple biochar-concrete specimens for physical testing and material characterization to understand the impact of biochar content on the composite. The specimens used a standard concrete mix, modifying fine-aggregate ratios of sand-to-biochar in percentages from zero to one hundred by volume. Some preliminary findings were: 1) a comparable strength of biochar-concrete to normal-weight concrete up to a percentage of twenty percent fine aggregate content by volume; 2) water absorption below four percent at eight hours at up to forty percent fine aggregate content; and 3) reduced thermal conductivity over normal weight concrete. The investigation also involved the design and construction of a full-scale permanent installation. The demonstrator – two parallel, seven-foot-tall free-standing pillars oriented towards solar noon – is a solar-alignment structure designed in collaboration with an astronomer and permaculture not-for-profit, and fabricated with a precast concrete manufacturer. Both a community resource and research instrument, a high biochar content face will be observed for possible freeze-thaw degradation in the cold, humid climate. As the outer face potentially weathers, the inner, structural face will maintain a machine-like edge for precise shadow-casting: creating a rich contrast of interior and exterior faces that is revealed over time.