Opportunities for the Portland cement industry to reduce CO2 emissions associated with the finished cement product include replacement of a portion of clinker in Portland cement with materials that do not require the same degree of fuel or electrical energy input for processing, or do not themselves emit CO2 (as does limestone when calcined as a component of cement raw mix). Certain clinker substitutes may improve cement performance while others may be neutral or detrimental.
Portland cement users and producers have long recognized the ability of natural and synthetic cementitious materials to contribute to cement and concrete performance . These materials have been used directly in concrete, incorporated as a component of cement, or both. Performance specifications have been developed for ground granulated BFS, fly ash, and silica fume in concrete [33-35]. BFS substitution requires grinding of the material to approximately cement fineness, a process that requires electrical energy comparable to, or greater than, that required for clinker grinding . Despite potentially higher grinding energy requirements, CO2 reductions may still accrue.
Processing requirements for adding non-clinker cementitious components in cement vary depending on the nature of the replacement material. Addition of finely ground materials (fly ash or separately ground BFS) may be made at the point of shipping or distribution. Alternatively, fly ash, BFS, or silica fume may be inter-ground with clinker and other cement components in cement mills.
In 2004, the ASTM revised its standard specification for Portland cement to allow the incorporation of up to a 5% mass fraction of limestone in ordinary Portland cements . CO2 reductions can accrue from using limestone as a filler replacing an equivalent amount of clinker.
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