Under open-system hydrothermal conditions, the observed growth habit for mullite composites is columnar. Based on theoretic calculations, performed with the Cambridge serial total energy package, of mullite crystal surface energies for facets (001), (210), (110), and (120), we analyze and discuss the relationship between growth rate and growth habit. Following the above facet order, these surface energies are 0.337, 0.022, 0.217, and 0.039 keV nm-2. According to the Curie-Woolf principle, growth rates would follow the sequence (001)>(110)>(120)>(210). Thus mullite crystals would favor theoretically a c-axis columnar growth habit. This result supports in this instance growth rate determinations based on theoretic calculations of surface energies.

It takes on the technical and economic double benefits that fly ash taking the place of cement. However, water/binder of modern concrete is generally low; the research on the concrete performance which is based on large water/binder is no longer suitable for analysis of modern concrete. As to this problem, using different proportions of W/B and FA dosage as influencing factors, specimen is compounded for the compressive strength test. Then, mechanical properties of HPC are studied systematically. Based on this, macro-performance is analyzed from a micro-mechanism point of view through taking the electron micrograph. As the study shows, strength of HPC mixed with fly ash has low characteristics at early period while high ones at later period. At later hydration, fly ash effect plays a significant role, the growth rate of concrete strength increases as the increase of FA dosage. Compressive strength of concrete decreases as the increase of W/B and FA dosage. When the W/B is high, compressive strength is not sensitive to the change of W/B and FA dosage; but when the W/B decreases from 0.30 to 0.25, the concrete strength transition occurs. The influence of W/B on concrete compressive strength is more significant than that of the FA dosage; in the preparation of HPC, fly ash can be added more and cement is added less relatively by decreasing the W/B, displaying the role of micro-aggregate filling and modification, improving the strength and other performance of concrete. The study on micro-mechanism proves well the macro-phenomena above.

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