Particle packing of cement and silica fume in pastes using an analytical model

Abstract

ResumoWhen added to concrete in appropriate content, silica fume may provide an increase in the mechanical strength of the material due to its high pozzolanic reactivity. In addition to the chemical contribution, physical changes can also be observed in concretes with silica fume due to an improvement in the particle packing of the paste. This is a result of their small size spherical particles, which fill the voids between the larger cement grains. However, it is necessary to properly establish the cement replacement content by silica fume, because at high amounts, which exceed the volume of voids between the cement particles, silica fume can promote the loosening of these particles. Thus, instead of filling the voids and increasing the packing density, the addition of silica fume will increase the volume of voids, decreasing the solid concentration. Consequently, this will impair the properties of the concrete. The objective of this paper is to use a particle packing analytical model, the CPM (Compressible Packing Model), to verify the maximum packing density of cement and silica fume, which could be associated with the silica fume optimum content in pastes. The ideal content of silica fume in pastes, mortars and concretes is usually experimentally determined. However, a theoretical study to contrast experimental data may help understanding the behaviour of silica fume in mixes. Theoretical results show maximum amounts of silica fume in the order of 18 to 20% of the cement weight, which is high considering recommendations on literature of 15%. Nevertheless, the packing model does not consider the effect of silica fume high specific surface on the agglomeration of particles or water demand. Hence, the packing density predicted by this model cannot be used as the single parameter in determining the optimum amount of silica fume in pastes.Keywords: silica fume; packing of fine particles; wet packing; analytical model; CPM.Pela sua pozolanicidade e alta reatividade, a sílica ativa, quando adicionada ao concreto em porcentagens adequadas, pode proporcionar um aumento na resistência mecânica do material. Além da contribuição química, alterações físicas no concreto com sílica ativa também são observadas pela melhora no empacotamento de partículas na pasta, resultado de suas pequenas partículas esféricas que preenchem os vazios entre os grãos de cimento, de maior tamanho. Para tanto, é necessário prever o teor de substituição adequadamente, pois em teores muito elevados, que superem o volume de vazios entre as partículas de cimento, a sílica pode promover o afastamento destas partículas. Assim, ao invés de preencher os vazios e aumentar a densidade de empacotamento, a adição de sílica irá aumentar o volume de vazios, diminuindo a concentração de sólidos na pasta e, consequentemente, prejudicando as propriedades do concreto. Dessa forma, o objetivo deste trabalho é utilizar um modelo analítico de empacotamento de partículas, o CPM (do inglês Compressible Packing Model), para verificar a densidade de empacotame...

Comparison of the Silica Fume Content for High-Strength Concrete Production: Chemical Analysis of the Pozzolanic Reaction and Physical Behavior by Particle Packing

“…On average, the CPM 34 results were 4.6% higher than the experimental results. Similar values were found in the literature when the same model was used 23,41 . The average results obtained for the CIPM 18,35 were 7.8% higher than the experimental results.…”

“…Since the packing density is reached when all particles of the system are wet, the SF water demand will be higher due to its higher specific surface area. Additionally, in finer particles, the relation between surface area and volume is higher, which causes the forces of attraction between grains to be dominant, thus, favoring the formation of agglomerates where air can be entrapped and leading to a higher voids ratio 19,23 . For the maximum solids concentration of each individual material presented (Figure 4), the air ratio was calculated as previously described.…”

“…It may be observed from Figures 8 and 9 that increasing the SF content from 10% to 16% did not result in an increase in the experimental packing density. This is explained by the fineness of the SF, since the attraction forces between grains are dominant for finer particles and favor the formation of agglomerates 23,43 . Therefore, the packing density does not improve with the incorporation of SF in the values range tested.…”

“…Values above lead to considerable increases in the final cost of the concrete, while a lower compressive strength gain. Also, when added in levels above 15%, SF can increase the water consumption necessary to maintain the same level of consistency, since the volume of water released by the filler effect is not sufficient to involve the fine particles, whose specific surface area is high 23 . In terms of efficiency, the consumption of PC per MPa of compressive strength, 16% of SF in relation to the total of fine materials, appears to be ideal for HSC 24 .…”

A comparative work on the magnetic field-dependent properties of plate-like and spherical iron particle-based magnetorheological grease

“…Therefore, a high yield stress was required to break the bonds of the chain structures so that the fluid could flow. In contrast, the yield stress of the MRG-P incorporated with 30 wt% of plate-like CI particles was reduced at a higher applied magnetic field due to the increasing void ratio, where the particles did not completely fill in the grease medium, resulting in the loosening of the chains in the MR grease concentration [ 32 ]. The particles were not saturated to form pre-structures even though the gap between the particles was small to allow free orientation movement, defined as the damping effect [ 10 ].…”