A general ‘rule of thumb’ is that you need 4 to 5 tonnes of rock dust per tonne of carbon dioxide removed.  But this number is derived from theory and a central goal of our research is to understand what the number looks like in practice.  We are aiming to understand the carbon dioxide removal potential of crushed rocks applied to croplands under field conditions.  In this agricultural context, factors such as application of fertilizers, soil properties, climate and crop type, could change the efficiency of carbon capture.
At all of our research sites, we are assessing the CO2 emissions associated with mining, grinding, transporting and spreading the rock dust to enable us to calculate net CO2 removal after accounting for these costs.  All of our field sites use basalt rock dust as a by-product of working quarries that is too fine to have traditional commercial use.  This means there are no additional CO2 emissions from mining and grinding at our field sites.