I am a CMU producer located in the northeast, and have spent a lot of time of this very topic. We have EPDs for almost 400 of our different CMU mix designs.
In general, CMU uses less cement compared with cast in place concrete. This is due to it being made from dry cast concrete, and some of it’s strength is gained by the compaction and vibration of the block machine. As such, the strength of CMU is not dependent on cement content alone. In addition to this, there are several factors that affect the GWP of CMU, and the biggest is cement content.
I’d like to preface this by saying I am not speaking for the whole industry here, only for our CMU. There are regional similarities, but there are also regional differences. Our structural CMU is usually around 2700 psi, which exceeds the ASTM C-90 structural requirement of 2000 psi. We do make high strength units for special cases, which could be up to 3750. The global warming potential of our high strength units (3750) is almost 10% higher than our regular structural block of (2700) psi. Another factor for us, and this is a regional difference as well, is light weight aggregate. We use a manufactured light weight aggregate, made from expanded shale. The GWP of our lightweight CMU is substantailly higher compared to our normal weight CMU. BUT, I know of a CMU producer on the west coast that uses a natural light weight aggregate generated by volcanic activity, and their GWP is similar for both light weight and normal weight. Another fun fact, we found that a CMU using recycled concrete aggregate brought our GWP up.
Now, on to the topic of carbon sequestration. First, a quick/simplified overview of why this happens…when cement is produced, limestone is heated and chemically broken apart releasing CO2. When water and aggregate is added to the resulting cement powder to make concrete, CO2 is converted back to limestone within the matrix of the concrete. This is part of the reason concrete gets hard, and is also one of the reasons concrete shrinks. Sequestration has been historically called carbonation. Due to the unique structure of dry cast concrete, CMU has more potential to sequester CO2 during its manufacturing and use phase than poured in place concrete. The CMU industry is in the process of updating our industry PCR with UL Environment, and sequestration language will be added to our PCR. There are several sequestration technologies currently on the market that can increase the amount of CO2 that CMU sequesters.
Another thing to think about….depending on the climate zone, thermal mass benefits can also effect the embodied carbon of the assembly materials. Due to thermal mass, CMU walls use less insulation to meet energy code requirements. Take a look at this comparison for climate zone 5. The same amount of insulation in a metal stud wall vs a CMU wall has very different results for energy code compliance.