ABSTRACT
The chemical and physical properties of supplementary cementing materials (SCMs), the chemical reactions that occur with these materials, and the impact that they have on concrete vary widely among the different types of SCM. Table 2.1 describes the nature of ve commonly used SCMs: lowcalcium y ash, high-calcium y ash, slag, silica fume, and metakaolin (thermally-activated kaolin clay). The chemical composition of particular samples of these same SCMs is shown in Table 2.2. Figure 2.1 shows electron micrographs of y ash, slag, silica fume, and metakaolin. The chemical and mineralogical composition, the particle size, and the morphology of the particles vary signicantly among SCMs. Both silica fume and y ash are predominantly comprised of spherical particles, but the average particle size of silica fume is about 100 times ner than that of y ash. Slag and metakaolin both require grinding to make the material suitable for use in concrete, and this produces angular particles. The neness of these materials depends on the extent of grinding. Table 2.2 shows typical chemical compositions for different SCMs, and the wide variation between the materials is shown on a CaO-SiO2-Al2O3 ternary plot in Figure 2.2. Silica fume is almost entirely composed of silica (typically >90% SiO2) and is a purely pozzolanic material with no hydraulic properties. Slag, on the other hand, has a composition relatively close to that of portland cement (but is glassy rather than crystalline) and is hydraulic in nature with little or no pozzolanic behavior. Table 2.3 shows how, generally, the hydraulicity of an SCM increases with its calcium content. Table 2.1 and Figure 2.2 show how the composition of y ash can vary considerably, ranging from materials with very low calcium contents (<1% CaO) that only react pozzolanically to materials with much higher calcium contents (>30% CaO) that display both pozzolanic and hydraulic behavior.
