Processing of Optimized Cements and Concretes Via Particle Packing
- PDF / 1,654,871 Bytes
- 5 Pages / 576 x 777.6 pts Page_size
- 62 Downloads / 208 Views
H
Introduction It has been well-recognized for many years that the particle-size distributions of the cement and the grading of the aggregates play an important role in determining the properties and characteristics of cement and concrete products. DSP (densified with small particles) type cements and concretes, to a certain extent, MDF (macro-defect-free) cements, and optimized concretes are recently recognized outstanding examples of the application of this principle. The preset characteristics of the cementitious slurry are also strongly influenced by these factors. Both the workability of the fresh material, and the microstructure development are controlled to a considerable extent by these geometric parameters. Two seminal works in the areas of continuous particle size distributions and particle packing are those of Andreason13 and Furnas,4"5 respectively. Furnas deals mainly with discrete systems and Andreason with continuous distributions. As early as 1907, the concept of idealized particle packing6 was being used to optimize cements and concretes. Figure la shows an idealized cross section of a simple cubic packing of monodispersed spheres. This system has a maximum packing density of 0.65%. In an ideally packed system of discrete size ranges, the size of the next smallest particles would be such that they just fit in the gaps between the largest size particles, and so on for subsequent particle sizes; this system is represented schematically in Figure lb. Not only the sizes but also the relative numbers of particles are important; Figures lc and Id show systems where some fraction of the smaller and larger particle sizes, respectively, are missing. Figure le shows a system where
MRS BULLETIN/MARCH 1993
the size of the second largest particles is too large to fit into the gaps between the largest particles, resulting in a lower packing efficiency. Thus, both the particle size and fractions are important when considering packing efficiency. Often, the cementitious portions of a cement or concrete product are specified in terms of surface area. Figure 2 suggests that specifications based on equal surface areas or equal volumes (or masses) alone may not be enough when considering reactions in these systems. The figure shows that equal volumes of materials (mass) may have vastly different surface areas and, conversely, equal surface area (i.e., m2) can result from different volumes of materials. Packing, Processing, and Properties of Cement Pastes In recent years, developments in cement and concrete have taken advantage of increased packing efficiency in the cementitious portions, if only indirectly. Figure 3 shows the relationship between the percentage of silica fume additions and modulus of rupture (MOR) for pastes with a water-to-solids ratio of 0.20 cured at 38°C.7 The figure shows that there is a strong relationship between the theoretical packing density in the system and the f lexural strength observed. In blended cement systems, the effects of increased packing efficiency may be difficult to separat
Data Loading...
