One of the zHome materials benchmarks is for all concrete to include 30 percent flyash by volume.  This is an aggressive percentage, reflecting the nature of zHome’s pushing the envelope. 

Flyash is the byproduct from coal burning power plants.  Flyash is a recycled material that reduces the need for cement.  Another benefit to using flyash beyond simply reducing material use is that the production of cement produces a significant amount of CO2 – so using less is good.

High flyash concrete is different than typical cement.  It is stickier and harder to form and work.  It takes longer to cure, which particularly in structural situations can mess with your timeline.  On the upside, in addition to its environmental benefits, it has less voids than typical concrete (the flyash is like little ball bearings and helps the concrete flow) and is stronger once fully cured. 

Closeup of 30% flyash wall - almost void free.

Centralia power plant, source of our flyash

An interesting sidenote is that flyash locally has historically come from the Centralia power plant.  However, during the boom times, local demand for flyash resulted in it being brought in by rail from Alberta, which created its own carbon issues.  Now, with construction significantly reduced, we are back to Centralia flyash being easily available.