Numerical validation of a population balance model describing cement paste rheology

Rheology control is essential during the period in which cement and concrete pastes are encountered in the fresh state, due to the fact that it directly affects workability, initial placement and the structural performance of the hardened material. Optimizations of clinker formulations and reductions in cement-to-water ratios induced by economic and environmental considerations have a significant effect in rheology, which invokes the need for mechanistic models capable of describing the effect of multiple relevant phenomena on the observed paste flow. In this work, the population balance framework was implemented to develop a model able to relate the transient microstructural evolution of cement pastes under typical experimental conditions with its macroscopic rheological responses. Numerical details and performance are assessed and discussed. It was found that the model is capable of reproducing experimentally observed flow curves by using measured cluster size distribution information. It is also able to predict the complex rheological characteristics typically found in cement pastes. Furthermore, a spatially resolved scheme was proposed to investigate the nature of flow inside a parallel-plates rheometer geometry with the objective of assessing the ability of the model of qualitatively predicting experimentally observed behavior and to gain insight into the effect of possible secondary flows

Structural build-up of cementitious paste with nano-Fe3O4 under time-varying magnetic fields

The structural build-up of cementitious paste with nano-Fe3O4 under time-varying magnetic fields was experimentally investigated using small amplitude oscillatory shear (SAOS) technique. Several modes of magnetic fields, such as constant, sudden-changed and linearly-changed, were applied to the cementitious paste. Results showed that the structural build-up of the cementitious paste depended on the magnetizing time and magnetic field strength. Applying constant magnetic fields improved the liquid-like behavior during first minutes and afterwards the solid-like property was enhanced. Both the sudden-increased and sudden-decreased magnetic fields resulted in a sharp decrease in storage modulus. The linearly increasing magnetic field resulted in a slight increase in storage modulus but higher liquid-like behavior. When the magnetic field was linearly decreased from 0.5 T to approx. 0.25 T, the structural build-up was enhanced significantly, and with the continuously decreasing magnetic field from approx. 0.25 T to 0 T, a decrease in storage modulus was observed

Structure-property relationships for polycarboxylate ether superplasticizers by means of RAFT polymerization

Hypothesis: Polycarboxylate ether (PCE) comb-copolymers are widely used as water reducing agents in the concrete industry while maintaining a high fluidity via the polymer adsorption to the cement particles. PCE copolymers with a broad range of structures are well established by Free radical polymerization, however, understanding the structure-property relationship is still complex due to the high polydispersity of PCE copolymers prepared by conventional polymerization. The influence of different structural parameters using well-defined polymeric structures is yet to be explored. Experiments: In this study, two different types of comb-like random copolymers, namely polycarboxylate ether (PCE; poly(oligo(ethylene glycol) methyl ether methacrylate/methacrylic acid)) and polysulfonate ether (PSE; poly(oligo(ethylene glycol) methyl ether acrylate/sodium 4-styrenesulfonate)), were synthesized by RAFT polymerization to enable the synthesis of polymers with controlled features. The effect of charge types and side chain lengths on the adsorption, rheology, and dispersing ability of cement pastes have been studied. Findings: RAFT polymerization could be used to prepare PCE random copolymers with good control over the polymer molecular weight and narrow polydispersity (D < 1.3). Results revealed that the zeta-potential values depend on both the charge type and side chain lengths. Copolymers containing SO3- exhibited higher absolute negative zeta-potential values than COO- while PCE copolymers with shorter side chains developed higher absolute negative zeta-potential values. On the other hand, the adsorption study demonstrated that decreasing the side chain lengths lead to higher adsorption of PCE copolymers while Copolymers with COO- groups were found to be adsorbed more than SO3- counterparts. These results are further confirmed with the rheological studies and it is found that the shorter the side chain, the lower the yield stress and the higher the dispersion of cement pastes but to a limited effect. Additionally, the charge types have a major influence on the performance of superplasticizers. This study could make further progress in establishing superplasticizers with controlled architectures for better performance

Consequences of an adjusted slip layer thickness for the hardened properties of UHPC

The pumpability of a concrete mixture cannot be described by one “single parameter”. The mixture composition, the pump equipment, the pumping pressure,… these parameters all affect the pumpability of a mixture and the ability to form a lubrication layer. Especially in case of ultra-high performance concrete (UHPC), where an optimal packing density, a high amount of powders and a low water/binder-ratio are necessary, the formation of that layer is not evident. In this research, the particle mobility is improved by adding an excess amount of paste to a reference UHPC-mixture in order to create a slip layer. As the required slip layer thickness of a pumpable UHPC-mixture is still unknown, different values are taken into account, reaching from 1 to 10 mm. The varying thickness influences the concrete properties in hardened state. One can see that for a thicker lubrication layer, the durability properties decrease due to a higher porosity. The compressive strength is more or less the same comparing the different mixtures

CFD implementation of time-dependent behaviour : application for concrete pumping

Sorptivity describes the pore connectivity of cementitious materials. This property is widely used for the assessment of the resistance of concrete to the ingress of aggressive agents. However, anomalous behaviour of cementitious materials during water uptake is usually reported. This is possibly explained by the effect of the hygroscopic nature of cementitious materials on the dynamics of the process. Water affinity of C-S-H might turn it into an imbibant and cause swelling as the material is exposed to water. Development of swelling can cause a variable hydraulic diffusivity of the material with time, and this is consistent with the deviation from the progress of the water uptake with the square root of time in the short term usually reported in the literature for the particular case of cementitious materials. This paper provides experimental results in support of the occurrence of swelling during water uptake in mortar samples. Consequently, the term capillary imbibition instead of capillary absorption seems more appropriate for describing the water uptake by capillarity of cementitious materials, as imbibition is usually connected to swelling. The idea of cementitious materials as rigid materials during water uptake seems incomplete for a complete description of the process