Effects of ionic substitutions, defects and variation in polymorph

2 Effects of ionic substitutions, defects and variation in polymorph 

The complex relations between content of foreign ions, polymorphism, defects and reactivity are well illustrated by the results of Boikova and coworkers (B42,B4) on ZnO-substituted C3S. With increasing ZnO content, triclinic, monoclinic and rhombohedral polymorphs were successively stabi- 

Studies on the comparative reactivities of ( -, o - and a-C,S, reviewed by Skalny and Young (S34), have given contradictory results. As these authors conclude, reactivity probably depends on specimen-specific factors other than the nature of the polymorph, and in the lower temperature polymorphs may be affected by the exsolution of impurities. This latter hypothesis receives strong support from subsequent observations that attack begins at exsolution lamellae and grain boundaries, and that these may contain phases, such as CjA, that are much more reactive than the CjS (Cl). Without these lamellae, P-C2S might be much less reactive than it is. 

There is wide agreement that substitution of alkali metal ions retards the early reaction of the aluminate phase, which is thus less for the orthorhombic than for the cubic polymorphs (S35,B43,RI3). The effect has been attributed to structural differences, but the early reaction of pure C, A is also retarded by adding NaOH to the solution, and the OH ion concentration in the solution may be the determining factor (S35). The reaction of C,A is also retarded by iron substitution and by close admixture with ferrite phase formation of a surface layer of reaction products may be a determining factor, at least in later stages of reaction, and the retarding effect of such a layer may be greater if it contains Fe (B44). 

The reactivities of ferrite phases in the pure Ca2(Al Fe, series have 

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