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Presumably the preponderance of completely empty Hadley grains over those with residual cement particles reflects the nearly complete hydration attained in these mortars by 14 days. V. ConclusionsThese results on mortars confirmed that microstructural features analogous to those previously established as occunhg nexs to glass slide surfaces occur near silica sand surfaces in mortars. Individual details are more difficult to define in the mortarspecimens, because of geometrical and other complexities of the fracture surfaces produced by transverse fracture and of the sand particle surfaces themselves.It seems reasonable to postulate that similar features are present in the zones surrounding both coarse aggregate and sand particles in ordinary portland cement concrete. ReferencesI B. D. Barnes. S. Diamond, and W. L. Dokh. "Contact Zone Between hrtland Cement Paste and Glass 'Aggregate' Surfaces,'' Cein. Contr. Rrs.. 8 [2] 233-43 ( I978 ). The chemical diffusion coefficient was measured for undoped, singIe-crystalline NiO at 900" to 1200°C and within an oxygen partial pressure of 10" to 0.21 atm. Electrical conductivity was used to monitor the reequilibration kinetics after the oxygen pressure was suddenly changed over the initially equilibrated NiO crystal. The chemical diffusion coefficient was calculated vs the reequilibration degree indicating the most stable range of investigations. The chemical diffusion coefficient value is virtually the same for t ! e oxidation and reduction experiments, giving, respectively: Dchem=(1.64X lo-') eXp[-(22,480+800)/RT] Dchem=(9.68X exp[ -(21y430?2600)/RT]It has been stated that &hem is independent of the oxygen pressure and thus of oxide composition. The electrical conductivity depends on the oxygen partial pressure in the power (lln) = (1/5.45), indicating that doubly ionized cation vacancies are the predominant defects. Deviation from the linear dependence of log v vs logp,, was observed at

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The reduction process of hematite to magnetite results in distinct changes in morphology of magnetite. These changes depend on structural properties of parent hematite and reduction conditions. The reduction experiments were performed in 3% CO and 97% CO2 gas mixture at 450 and 850°C on selected crystals of natural hematite. The phase composition and morphological characteristics of the reduced layer were determined on the basis of microscopic analysis. Singular blasts or blastoidal colonies of magnetite were formed in 450°C on the boundaries of the hematite grains. They began to grow and joined the layer. Magnetite formed at 450°C is distinctly microporous. Cracks and desintegration of hematite grains appear together with reduction of hematite. At 850°C nucleation of the magnetite is quite different then at 450°C. The formation of singular magnetite lamellae or a palisade of crystallographically oriented magnetite lamellae were observed. Their growth results in the formation of the magnetite layer. Tunnel‐shaped pores in magnetite layer show the same direction as lamellar front of reduction.

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