Do Portland cement clinkers contain glass or

2 Do Portland cement clinkers contain glass or C12A2  

The applicability of these conclusions to production clinkers requires examination. The compositions of the aluminate and ferrite phases differ markedly from C3A and C4AF, respectively (Chapter 1) and, because of the presence of minor components, especially MgO, TiOj and alkalis, the compositions of the liquid phase differ from those in the pure system. We shall consider in turn first, whether glass is formed, and if so, under what conditions second, whether either C,2A2 or free lime crystallizes from the liquid phase under appropriate conditions and third, whether there is evidence of either equilibrium or independent cooling. 

The early literature contains many references to the presence in production clinkers of glass, often in substantial proportions. This view was based partly on observations by light microscopy however, this method cannot distinguish glass from crystalline solids of the cubic system unless crystals with distinct faces have been formed, nor from crystalline materials of any kind if the crystals are below a certain size. It was also found that if clinkers believed to contain glass were annealed, their heats of solution in an acid medium increased, and this method was used to obtain approximate estimates of the glass content (LI 3). This evidence, too, is inconclusive, because the same effect would arise from the presence of small or structurally imperfect crystals. 

Neither XRD nor electron or light microscopy indicates that C12A7 is present in any modern clinkers, and when free lime is found, microscopic examination does not suggest that it has formed by precipitation from the liquid. 

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