Consolidation behavior of cement-based systems: Influence of inter-particle forces

The microstructure of concrete is of paramount importance for almost all performance aspects of the material. Therefore, the ability to model both initial and hydrated microstructure is a key to the subsequent understanding and prediction of many macroscopic properties of the material. Correct modelling of the initial microstructure requires knowledge of, among others, the forces acting on the constituent particles.

In the hydrated state, the pore structure of a cement-based material is often assessed experimentally. However, the present methods are in no way perfect and there is a need for a robust method where both advantageous and drawbacks are well described. This thesis comprises theoretical and experimental investigations of the effect of inter-particle forces on the consolidation behavior of fresh cement-based materials.

Furthermore, the basis for interpretation of results obtained through an experimental method for pore structure characterization is assessed. A model material with surface properties similar to those of cement, magnesium oxide (MgO), was initially used to quantify inter-particle force changes by addition of different superplasticizers.

The experiments were carried out by means of centrifugal consolidation and packing density profiles were obtained. Inter-particle forces were modified by the addition of different anionic comb-type superplasticizers in full surface coverage dosages. One of the superplasticizers was added in varying dosages to obtain sub-surface coverage.

It was found that an increased length of the side chains of the adsorbed superplasticizers reduces the inter-particle bond strength and thus the yield stress, and results in an increased packing density. A relation between side chain length and inter-particle bond strength was determined. Furthermore, a linear relation was found between the obtained packing density of the centrifuged suspensions and the sub-surface coverage at low dosage.

A quantitative link between the molecular structure and resulting adsorption thickness of the superplasticizers and the compression rheology behavior was established. Following the quantification of inter-particle forces with a model material, combinations of cement and silica fume were investigated.

The relevance of the forces acting on and between cement particles of 10 μm and silica fume particle of 100 nm, was evaluated and it was shown that the surface properties of silica fume particles in a cement-based suspension should be able to keep them from agglomerating with each other. It was also shown that cement-silica fume agglomeration occurs but may be resisted by the addition of a superplasticizer.

The size and surface properties of cement may cause cement-cement agglomeration, the extend of which may be reduced by superplasticizer addition. Based on the inter-particle force calculations and considerations on geometrical packing, conceptual packing and pore size models were proposed. The validity of the models was investigated experimentally by compressive consolidation experiments.

The employed consolidation equipment enabled the simultaneous determination of flow through the material and obtained packing density. By modelling the consolidation process and using the Hagen-Poiseuille equation, a characteristic mean pore diameter was established for each of the studied systems. This approach enables the determination of initial permeability of cement-based systems.

Experimental results verified the proposed conceptual packing model. The pore size model overall correlated with experiments, although some deviation was observed. Assessment of the pore structure in a hardened cement-based material may be carried out experimentally. In this thesis, the basis for interpretation of results from the experimental technique, Low Temperature Calorimetry (LTC), was assessed in order to determine the possibilities and limitations of using LTC for characterization of the porosity of cement-based materials.

Supercooling, general lack of equilibrium, bottle neck pores, and ions being expelled during freezing, were treated theoretically and evaluated experimentally using model materials. It was found that the measured temperature should be corrected for non-equilibrium between the sample and the reference block.

Furthermore, corrections should be made for the effect on the freezing point depression of the varying ion concentration in the non-frozen part of the pore solution. Making use of the knowledge from the obtained results, it was proposed that LTC may be used for characterization of pore connectivity as well as pore size distribution.