Comparative Study of the Microstructures of Normal and Very High-Strength Concretes

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Abstract
The microstructure of a normal concrete with water/cement ratio (W/C) = 0.56 (f′c at 28 days = 31.8 MPa or 4610 psi) was compared with two series of very high-strength concretes (up to 118 MPa or 17 120 psi), one made with Type III cement and the other with the same Type III cement plus 6 to 10% silica fume. These concretes were water cured for 91 days. Their microstructural examination was undertaken using mercury porosimetry and scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDXA). Similarities and differences are described in detail. Basically, both are composed of calcium silicate hydrates (C-S-H) and calcium hydroxide (C-H), but their proportions and morphology are quite different. The composition and types of C-S-H also vary. Significant differences in porosity and aggregate-cement bonding are present. Chloride ion permeability tests indicate the very high-strength concretes to be as impervious as polymer-impregnated types.

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