Aggregates derived from industrial wastes
One major component of concrete sustainability is the use of recycled or waste materials. There are three principal classes of such materials:
• supplementary cementitiousmaterials, such as blast furnace slag, ﬂy ash, and silica fume, that can be used to replace a portion of the Portland cement, as discussed in detail in Chapter 5;
• used oils, spent solvents, rubber tires, and other wastes that can be used as fuels in the manufacture of Portland cement, as mentioned in Chapter 1;
• some construction and demolition wastes, including old concrete, that can be recycled to serve as coarse aggregate in concrete. Since the coarse aggregate generally makes up about 40% of the mass of a typical concrete, reducing the amount of natural aggregate required can have a signiﬁcant effect on preserving our natural resources.