The Feasibility of Biochar as a Cement Replacement to Reduce Concrete’s Carbon Footprint 

Lori E. Tunstall, Ph.D. 

Colorado School of Mines

In this presentation, we show that biochar-cement mortars have similar or improved 28-day strength compared to a control with no biochar (strengths ≥ 50 MPa). 

We assess 16 distinct biochars for these mortars, with cement weight replacements of 10%. The biochars are made from softwood, hardwood, and agricultural feedstocks processed at temperatures ranging from 400 to 1100 °C using various pyrolysis techniques. 

A subset of biochar-cement concretes were also assessed for permeability via water penetration depth measurements; the depth of water penetration decreases with increasing biochar content for each biochar assessed. Additionally, preliminary analysis indicates that biochar is compatible with fly ash as a cement replacement and can accelerate strength gains compared to mortars with only a fly ash-blended cement binder. 

To successfully integrate biochar into cementitious composites we recommend the following: 1) mill the biochar; and 2) use biochar with high available sorption capacity, low O/C, and high soluble silicon content. 

Finally, we demonstrate that we can produce a carbon negative biochar-cement mortar with strength parity to a traditional cement mortar and discuss the target biochar price to be competitive for use in the concrete industry.