Since the recognition of cement production as a major contributor of CO2 emissions, alternate materials have ramped up anti-cement development and marketing. The wood industry, for example, is pushing hard to promote expanded use of its products as renewable materials in tall structures. However, even timber, unquestioned as the sustainable preference, has its issues and an inherent free pass in reporting its substantial impacts. While renewability is a great single attribute, reality is in the whole.
In such a view, concrete holds up well, and concrete masonry is even more attractive as it utilizes far less cement than poured-in-place concrete. (See Less Cement - the Choice is Concrete Masonry below.) For concrete masonry, the bulk of cement is contained in masonry grout that fills the cells of cmu, and by volume is the greater component in the wall. The good news is much work has been done to vet alternatives to Portland cement in masonry grout mix design. (See also Angelus Block Reduces Its Carbon Footprint, examining the lower impacts of replacing Portland cement with a blended cement.)
Below are downloads related to research conducted in supplemental cementitious material (SCM) replacement of Portland cement in masonry grout, and an article on the minimal use of cement in concrete masonry structures compared to solid concrete walls.
Replacement of Portland Cement with Supplemental Cementitious Materials in Masonry Grout, Fonseca, Fernando S., Ph.D., S.E., and Siggard, Kurt, Director, Concrete Masonry Association of California and Nevada. Presented to 15th International Brick and Block Masonry Conference, Florianópolis, Brazil, 2012
High Supplemental Cementitious Material (SCM) Grout Phase 2 and 3 Research, Masonry Chronicles Winter 2010-11, Maribeth S. Bradfield, P.E., LEED AP.
High Volume Fly Ash Masonry Grout, Fernando S. Fonseca, Ph.D., S.E., and Kurt Siggard, Structure Magazine, May 2012.
Less Cement - The Choice is Concrete Masonry, Kurt Siggard, Concrete Masonry Association of California and Nevada, March 2009.