With regards to the time value of carbon I took a crack at modelling the emissions balance for Type I OPC concrete vs. 20% GGBFS SCM, using the 30 MPa mix data from the cradle to gate paper, XC1 exposure at 300 ppm atmospheric CO2, square geometry [SA 3.79 m2, V 0.3 m3]. With no interest / discount rate applied the sequestration over 100 years looks like this:
Applying an inflation on emitted carbon of 1.4% (in alignment with the Stern review), the emissions balance looks like this:
I applied the inflation rate as follows:
E_y = (E_y-1 - deltaC_s) * (1 + r)
where: E_y is the CO2 emissions balance for year y,
E_y-1 is the CO2 emissions balance for year y-1,
deltaC_s is the change in carbon sequestered for year y,
r is the inflation rate.
The idea being that the CO2 is acting on the climate from the day it’s emitted, therefore emissions today compound into the future. I don’t know if this approach is appropriate, and what the inflation rate should be, and am happy to discuss. However, the underlying point is that, even with a modest inflation rate, the contribution of carbonation to sequestering CO2 is swamped by the emissions intensity of OPC.
One last point I’d make is that if carbonation of cement is to be included in LCAs, how can excluding sequestered carbon from wood products be justified?