Josef Kaufmann

Interaction of cement model systems with superplasticizers investigated by atomic force microscopy, zeta potential, and adsorption measurements

Polyelectrolyte-based dispersants are commonly used in a wide range of industrial applications to provide specific workability to colloidal suspensions. Their working mechanism is based on adsorption onto the surfaces of the suspended particles. The adsorbed polymer layer can exercise an electrostatic and/or a steric effect which is responsible for achieving dispersion. This study is focused on the dispersion forces induced by polycarboxylate ether-based superplasticizers (PCEs) commonly used in concrete. They are investigated by atomic force microscopy (AFM) applying standard silicon nitride tips exposed to solutions with different ionic compositions in a wet cell. Adsorption isotherms and zeta potential analysis were performed to characterize polymer displacement in the AFM system on nonreactive model substrates (quartz, mica, calcite, and magnesium oxide) in order to avoid the complexity of cement hydration products. The results show that PCE is strongly adsorbed by positively charged materials. This fact reveals that, being silicon nitride naturally positively charged, in most cases the superplasticizer adsorbs preferably on the silicon nitride tip than on the AFM substrate. However, the force-distance curves displayed repulsive interactions between tip and substrates even when polymer was poorly adsorbed on both. These observations allow us to conclude that the dispersion due to PCE strongly depends on the particle charge. It differs between colloids adsorbing and not adsorbing PCE, and leads to different forces acting between the particles.

Analysis of cement-bonded materials by multi-cycle mercury intrusion and nitrogen sorption.

The pore systems of cement-based materials are studied by N(2) sorption and mercury intrusion porosimetry (MIP). Pore size distributions and internal surfaces are derived. Especially in materials with a broad pore size distribution, these (and other) methods generally do not lead to coincident results. It is shown here, how the interpretation of the experimental data of the two methods may be modified in order to obtain coincident pore size distributions from both methods. The studied pore systems are described as array of chambers which are connected by smaller throats. N(2) adsorption is used to calculate the size of the pores, whereby no distinction between throat or chamber type is possible with this method. Assuming mercury entrapping in ink-bottle type pores (pores that are connected to an external surface through smaller pores only) being the dominant process for mercury snap-off during extrusion and applying multi-cycle MIP, the calculation of the size of the entrances of these ink-bottles is possible. It is shown that similar results also may be derived from mercury extrusion data by applying a contact angle correction for the retracting mercury meniscus. A good agreement of the pore size distribution of the connected, non-ink-bottle type pores derived from either N(2) sorption or mercury intrusion is obtained. Samples of cement paste and mortar are analysed. A significant difference between cement paste and mortar regarding the neck entrances of ink-bottle type pores is found and attributed to the coarse pore space around the aggregates, the interfacial transition zone.

Effect of the addition of ultrafine cement and short fiber reinforcement on shrinkage, rheological and mechanical properties of Portland cement pastes

Abstract The packing density of a powder can be improved by adding a fine powder to a coarse one. This conventional technique, frequently used in ceramic production, also can be applied to optimise the properties of cementitious binders, especially for the production of high performance concrete. In this paper the effect of mixing ultrafine cement and normal grain sized Portland cement is studied. The rheological properties of the fresh paste are influenced positively. An important dispersing effect is observed, decreasing yield value and plastic viscosity. This permits mixing of very low w / c -ratio cement pastes with low porosity and high strengths, applying conventional mixing procedures. Due to the low amount of water, that is available in the narrow pores, the hydration of the cement is not complete. At the same time, permeability is strongly reduced, leading to a lack of water with ongoing hydration. Self-desiccation especially at early age is the consequence. Shrinkage and as a consequence crack formation may be observed. In a special experimental setup early shrinkage (from nearly time zero after mixing) was monitored continuously. Fresh pastes of different mix proportions were put in a cone and the length change was measured by a laser system. Additionally, the shrinkage of hardened pastes was measured until 90 days by conventional technique. The influence of different surrounding climates was studied. Such dense materials generally are very brittle. The use of fibers increases the ductility significantly and leads to a further improvement of the shrinkage and strength properties. The excellent rheological properties of the cement matrix containing ultrafine cement also allows a conventional mixing of composites with a high fiber content. The effect of different amount and type (PP, carbon) of fibers on the shrinkage at very early age and the influence of different curing conditions at early age on the mechanical properties was studied.

Impact of particle size on interaction forces between ettringite and dispersing comb-polymers in various electrolyte solutions.

The inter-particle forces play a fundamental role for the flow properties of a particle suspension in response to shear stresses. In concrete applications, cement admixtures based on comb-polymers like polycarboxylate-ether-based superplasticizer (PCE) are used to control the rheological behavior of the fresh mixtures, as it is negatively impacted by certain early hydration products, like the mineral ettringite. In this work, dispersion forces due to PCE were measured directly at the surface of ettringite crystals in different electrolyte solutions by the means of atomic force microscopy (AFM) applying spherical and sharp silicon dioxide tips. Results show an effective repulsion between ettringite surface and AFM tips for solutions above the IEP of ettringite (pH∼12) and significant attraction in solution at lower pH. The addition of polyelectrolytes in solution provides dispersion forces exclusively between the sharp tips (radius ≈ 10 nm) and the ettringite surface, whereas the polymer layer at the ettringite surface results to be unable to disperse large colloidal probes (radius ≈ 10 μm). A simple modeling of the inter-particle forces explains that, for large particles, the steric hindrance of the studied PCE molecules is not high enough to compensate for the Van der Waals and the attractive electrostatic contributions. Therefore, in cement suspensions the impact of ettringite on rheology is probably not only related to the particle charge, but also related to the involved particle sizes.

Durability of Portland Cement Blends Including Calcined Clay and Limestone: Interactions with Sulfate, Chloride and Carbonate Ions

The durability has been investigated for mortars made from a pure Portland cement (CEM I) and five Portland cement – SCM blends, using a cement replacement level of 35 wt% and the following SCM’s: (i) pure limestone, (ii) pure metakaolin, (iii) metakaolin and limestone (3:1 w/w), (iv) metakaolin and silica fume, and (v) metakaolin, silica fume and limestone. The blends with metakaolin and silica fume employ a fixed ratio for these components which mimics the alumina-silicate composition of a 2:1 clay (i.e., montmorillonite). All mortars were demoulded after hydration for one day and cured saturated in water at 20 °C for 90 days prior to exposure. Expansions induced by sulfate attack, chloride profiles, and carbonation depths were measured to investigate the durability performances of the mortars. Porosity and pore connectivity were analysed before exposure by mercury intrusion porosimetry. The results show that mortars incorporating metakaolin, independent of additional silica fume or limestone, all exhibit very high resistance towards sulfate attack and chloride ingress, but are vulnerable to carbonation. The binary Portland cement – limestone blend is most susceptible to all types of studied chemical attacks, as expected. The pure Portland cement exhibits poor resistance to sulfate attack and chloride ingress, but high resistance to carbonation. The observed performances for the different blends can be explained based on their microstructure and phase assemblages. For example, the presence of metakaolin increases the chloride-ion binding capacity and enhances chloride resistance by the low pore connectivity present in the hydrated blends with metakaolin.

Multiscale porosity estimates along the pro-and retrograde deformation path: an example from Alpine slates

Estimating the porosity of slates is of great interest for the industries dealing with sub-surface areas such as CO2 sequestration, nuclear waste disposal and shale gas but also for engineering purposes in terms of mechanical stability for underground or surface constructions. In this study, we aim at understanding estimates of the porosity of slates from the Infrahelvetic flysch units (IFUs) in the Glarus Alps (eastern Switzerland). Surface and sub-surface samples were collected along a temperature gradient from 200 to 320 C and therefore give the opportunity to link pore types along this temperature and deformation path. In addition, we indicate which porosity is the effect of surface processes and indicate the contribution of artificially induced porosity. The developed workflow consists of a combination of bulk rock measurements including helium pycnometry (He pycnometry) and mercury intrusion porosimetry (MIP) with image analysis. Image analysis was performed with high-resolution scanning electron microscopy (SEM) on broad ion beam (BIB) prepared cross sections (BIB-SEM). Different vein generations provide evidence of porosity formation at depth, as they present “paleo-porosity”. Towards peak metamorphic conditions (prograde path), porosity reduces to < 1 vol%, indicated by matrix porosity detected by BIB-SEM. During exhumation (retrograde path) porosity increases due to the formation of microfractures interpreted as the effect of unloading (open fractures). At the surface, porosity is further increased due to the formation of macro-fractures (fracture apertures up to 1 mm), which are interpreted as being either due to the effect of weathering processes such as freeze and thaw cycles or artificially induced by sample preparation. Additionally, porosity and pore morphology are strongly dependent on mineralogy, sample homogeneity and strain, which change dynamically in time and space.

Bi-component polyolefin fibers used for concrete and shotcrete applications

Abstract Cement and concrete are relatively strong when subjected to compressive forces, but are not able to take much stress or strain in tension. The obvious method to improve the tensile properties is to incorporate fibers. This paper describes the research program on the development of Bi-Component Polyolefin fibers, which covers the choice of a fiber type based on the interfacial bond, design of the optimum core and sheath combination, including surface plasma treatment. The most successful fiber was chosen from the varied tests performed and field tests were conducted in order to test their performance in concrete and shotcrete. The field tests and research proved to be very successful for industrial applications and the technology was bought by the company Brugg Contec AG in Switzerland. The bi-component fiber developed called Fibrofor is now marketed as Concrix by Brugg Contec AG. For this purpose a bi-component fiber with a core consisting of a low melt flow rate polymer and narrow molecular weight distribution and a sheath consisting of a polymer with a higher melt flow rate and a broader molecular weight distribution were manufactured.

Properties of Short Fiber Reinforced Cement Paste for Concrete Tubes Produced by a Centrifugation Method

This paper describes how concrete tubes are usually produced by a centrifugation method using steel bar reinforcements. It explains how the reinforcement of concrete with steel bars is expensive, susceptible to corrosion, and leads to rather thick and heavy structural elements. The application of short fiber reinforced cement (FRC) or mortar is a suitable alternative. This paper presents the development and evaluation of a suitable FRC for this particular application. First, the cement matrix was optimized for use in a conventional casting forming process. A mixture of ultra-fine cement and ordinary Portland cement improves the rheological properties of the fresh mixture and results in a very dense cement matrix with excellent mechanical properties. This optimized cement matrix was then reinforced with different kinds of carbon and polymeric fibers such as PVA and PP. Hereby, the carbon fibers primarily increase the flexural and tensile strength of the material, where the polymer fibers tend to improve the ductility of the cement matrix. Furthermore, the influence of water-reducing agents, of different constituents (microsilica, filler, sand), and the mixing process on the mechanical properties were studied. The mechanical properties were found to depend also on the curing conditions of the hydrated samples. The microstructure and the fiber-matrix interface were investigated by Environmental Scanning Electron Microscope (ESEM). In a further test series, the mixtures were optimized with regard to the flow properties needed for the centrifugation process. The mechanical properties and the microstructure were investigated. As a result, this work shows the possibility to apply the FRC for industrial production of centrifuged tubes.