Because the pore plays the primary role in strength development of blended cement paste, the role of filler-hydrates adhesion properties has attracted very little attention. The purpose of this study is to investigate the effect of filler-hydrates adhesion properties on strength development of cement paste. In this study, the development of compressive strength of portland cement paste and cement paste blended with limestone powder and micronized sand was studied experimentally. Parallel with this experimental study, the contact area in these cement pastes was quantified numerically. The relationship between the measured compressive strength and simulated contact area was then analyzed. With this relationship, the effect of filler-hydrates adhesion properties on strength development of cement paste was quantified. The contact area between hydrating cement particle and micronized sand particle had no contribution to the compressive strength. In contrast, the contact area between the hydrating cement particle and limestone particle had a substantial contribution to the compressive strength.

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The interface between filler and hydration products can have a significant effect on the mechanical properties of the cement paste system. With different adhesion properties between filler and hydration products, the effect of microstructural features (size, shape, surface roughness), particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system is supposed to be different, as well. In order to understand the effect of the microstructural features, particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system with either strong or weak filler-matrix interface, microscale simulations with a lattice model are carried out. The results show that the strength of the filler-matrix interface plays a more important role than the microstructural features, particle distribution and area fraction of filler in the crack propagation and the strength of blended cement paste. The knowledge acquired here provides a clue, or direction, for improving the performance of existing fillers. To improve the performance of fillers in cement paste in terms of strength, priority should be given to improving the bond strength between filler particles and matrix, not to modifying the microstructural features (i.e., shape and surface roughness) of the filler.@en

Fillers, such as limestone or quartz powder, are used as a replacement of Portland cement. Their application can make concrete more environment friendly and possibly cheaper. Additions of limestone or quartz powder have been reported to exert a limited chemical effect on cement hydration. The main quasi-chemical effect of added limestone and quartz powder is that they accelerate cement hydration by facilitating nucleation and growth of reaction products at their surfaces. Fine fillers in cement paste can result in improvements in strength because of a lower porosity and a denser packing. At the same time, however, the use of fillers results in dilution of Portland cement particles and their strength-providing reaction products in the paste. This ‘dilution’ effect will lead to an increased porosity. Above a critical amount of fillers, the effect of dilution exceeds the effect of packing, resulting in a lower strength of the hardened paste or concrete. These effects (porosity, packing and dilution) on the strength of cement paste have been studied intensively, but they (porosity, packing and dilution) cannot explain difference in performance of different fillers. Also adhesion between filler particles and reaction products has an influence on the strength of blended cement paste. However, filler-hydrates adhesion properties and their effect on the strength of blended cement paste are not quite well understood yet. The basic questions why filler particles and reaction products adhere to each other in blended cement paste, and how the chemistry and surface characteristics of fillers affect this adhesion, are rarely addressed and need further research. The aim of this project is to study the filler-hydrates adhesion properties in blended cement paste system. Firstly, the influence of the filler-hydrates adhesion properties on the strength of blended cement paste has been analysed. The compressive strength of cement paste blended with limestone powder and micronized sand was studied experimentally. The contact area between different solid phases in these cement pastes was quantified numerically. The relationship between the measured compressive strength and simulated contact area was analysed (“contact area concept”). Based on this relationship, the influence of the filler-hydrates adhesion properties on the strength of blended cement paste was quantified. It was found that micronized sand-hydrates contact area had no contribution to the compressive strength. By contrast, the limestone-hydrates contact area in cement paste had a substantial contribution to the compressive strength. Secondly, the filler-hydrates adhesion properties were studied at the microscale. Crack paths and fracture surfaces of loaded cement pastes were investigated by scanning electron microscopy (SEM) observation. Parallel with the SEM observations, the influence of interface’s mechanical properties on crack propagation, tensile strength and fracture energy was studied numerically by using a lattice model. Based on these SEM observations and simulation results, the mechanical properties of the interface between filler particles and hydration products were evaluated. Meanwhile, this provided a validation of the ‘contact area concept’. Then, the filler-hydrates adhesion mechanisms in blended cement paste system were investigated. The influence of the chemical nature of fillers on the interaction between main ions, i.e., Ca2+, SO42-, in the pore solution of blended cement paste and filler surfaces was investigated via zeta potential measurements. Meanwhile, microscopic observations of the nucleation and growth of C-S-H on the surface of these filler particles were performed by SEM. It was concluded that Ca2+ ions chemically adsorbed at limestone surfaces led to the formation of a relatively strong bond (most likely, ‘ionic-covalent’ bond) between a limestone particle and C-S-H. By contrast, Ca2+ ions electrostatically adsorbed at micronized sand surfaces resulted in an attractive ion-ion correlation force and hence a relatively weak bond between a micronized sand particle and C-S-H. This information about the filler-hydrates adhesion mechanisms is very important for the search for new fillers and for improving the performance of existing fillers. For example, based on the knowledge acquired in this study, carbonation can improve the performance of hardened cement paste powder when it is used as a filler in cement paste. This is because carbonation can turn the silicate and CH phase of the surface of the recycled hardened cement paste powder into calcite phase. This can enhance the adhesion properties between hardened cement paste particle and new hydration products. Finally, the fracture behaviour of cement paste with strong and weak filler-matrix interfaces was simulated at microscale by using a lattice model. The simulations indicated that the bond strength between filler particles and C-S-H matrix plays a more important role in the crack propagation and the strength of blended cement paste compared to the role of the particle size distribution, size (5, 10, 15 and 20 µm), shape, surface roughness and volume fraction (5, 15, 25, 35 and 45%) of the filler. These findings provided support to the previous findings that the strong interfaces between limestone particles and hydration products are due to the superior bond between limestone and hydration products rather than the physical surface properties, such as shape and surface roughness. Moreover, this study indicated the direction for optimization of the performance of fillers in cement paste in view of strength. To improve the performance of fillers, priority should be given to improving the bond strength between filler particles and hydration products. Modifying microstructural features (i.e., shape, surface roughness) of filler is less effective.@en

The interface between filler and hydration products can have a significant effect on the mechanical properties of the cement paste system. Surface analysis techniques and mechanical model can be used to study the mechanical properties of the interface. These studies can provide insight into the adhesion mechanisms between filler and hydration products. In this paper, cement pastes blended with fillers (micronized sand and limestone powder) are discussed. Crack paths and fracture surfaces of loaded cement pastes were investigated by scanning electron microscopy (SEM) observation. Parallel with the SEM observations, the influence of interface properties on crack propagation, tensile strength and fracture energy was studied numerically by using a lattice model. With these SEM observations and simulation results the mechanical properties of the interface between filler and hydration products were evaluated. Limestone powder exhibited superior bond characteristics with hydration products compared with micronized sand. Furthermore, it suggests that the limestone-hydration products interface is even stronger than hydration products. This is likely due to the strong electrostatic interactions or the iono-covalent forces between limestone particles and C-S-H particles.

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A complete understanding of the mechanisms upon which a filler acts in a cement-based material, e.g. as a C–S–H nucleation and/or growth-inducing factor, is of high importance. Although various studies report on accelerated cement hydration in the presence of fillers, the reason behind these observations is not completely understood yet. This work contributes to this subject, by providing an experimental evidence on the (electro) chemical aspects of the filler surface modification in the model solution, simulating the pore solution of cement paste. The nature of the various interactions with regard to the affinity of a filler surface towards C–S–H nucleation and growth was discussed in detail in this work with regard to zeta potential measurements of micronized sand and limestone particles in the model solutions. These results are further supported by microscopic observations of morphology and distribution of hydration products on the filler surfaces, together with considerations on thermodynamic principles in view of hydration products formation and distribution. The C–S–H nucleation and growth appeared to be due to the interactions between a filler surface and calcium ions in the pore solution. These interactions were determined by the chemical nature of the filler surface. The interaction mechanisms were found to be governed by relatively weak electrostatic forces in the case of micronized sand. This was reflected by a non-significant adsorption of calcium ions on the filler surface, resulting in non-uniformly distributed and less stable C–S–H nuclei. In contrast, the nucleation and growth of C–S–H on limestone particles were predominantly determined by donor–acceptor mechanisms, following moderate acid–base interactions. Consequently, a strong chemical bonding of calcium ions to a limestone surface resulted in a large amount of uniformly distributed C–S–H nuclei.

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The adhesion mechanisms between C[sbnd]S[sbnd]H and fillers are of great significance when fillers are employed as a cement replacement. The affinity of a filler's surface towards ions in the pore solution of cement paste is reported in this work from the view point of the governing mechanisms. The discussion on various interactions is justified by results from zeta-potential measurements, and further supported by microscopic observations of hydration products on the filler's surfaces. The bond strength between the filler and hydrates is also evaluated. The C[sbnd]S[sbnd]H/filler adhesion appears due to the interactions between a filler's surface and calcium ions. In the case of calcite, the interactions between a filler's surface and calcium ions are predominantly determined by acid-base interactions which lead to the formation of a strong bond (most likely ionic-covalent bond). In the case of silica, the adhesion is found to be governed by an attractive ion-ion correlation force.

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