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An interesting feature of Fig. 4(A) is the rounding of the surface crystallite edges, which may be due to their reaction with the fluorine escaping from the glassceramic and atmospheric moisture (forming HF) during heating. Mass spectrometric profile datalo show that the surface-fluorine depletion is -125 to 150 p m deep, which is in good agreement with our XRD data.Structure of Glass-Ceramics," Yogyo Kyokai Shi, 76 [I I] 385-93 (1968).Theoretical considerations and test data indicate that, in the low-porosity range, the strength of set cement can be better predicted from its W,,* xG,*/P,ratio than from its porosity or gel concentration; in this range (Pf= 0.16 to 0.28), therefore, the ratio in question constitutes a more basic and general strength-determining factor. At advanced stages of hydration, however, the set cement is stronger than can be expected from its W,,,* xG,*IPf ratio, apparently due to aging effects.

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The purpose of this investigation was to determine the combined effects of fly ash as a partial replacement for portland cement and the use of high-pressure compaction in reducing the porosity and increasing the compressive strength of *prepared miniature compacts. Ac,,ordinq to current practice and theory, for specialized uses, portland cem:ent concrete materials with strengths approaching 20,000 psi (140 MPa) may be produced by reducing the amount of porosity present in the microstructure of the ". paste-natrix material. To accomplish this, both high-pressure coripaction and increased formation of c entitious products from a pozzolanic reaction have provided excellent results. However, judging from an extensive literature survey, the effect of conbining the two has not been examined.