The pore systems of cement-based materials are studied by N
2 sorption and mercury intrusion porosimetry. A new interpretation of the different data sets leads to coincident pore size distributions.
...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.
The chloride resistance of concrete mixtures produced with different binders and water-to-binder ratios is determined by three different methods (natural chloride diffusion, accelerated chloride ...migration and conductivity measurement). The influence of mix design and type of binder are evaluated and related to porosity. The effect of chloride binding on chloride resistance is assessed by thermodynamic modeling and compared with chloride content measured with acid and water extraction.
Chloride resistance depends on the type of binder and on water-to-binder ratio. Chloride content measurements and thermodynamic modeling both show that chloride binding is strongly related to the hydration degree of the cement and of the mineral admixtures. However, the decisive parameter for chloride resistance in all the tests is the permeability while the influence of chloride binding is less important.
► Free shrinkage, creep and restraint stress of CVC and SCC mixtures are compared. ► SCC reaches lower values for E-modulus, compressive and flexural strengths. ► Shrinkage, creep and strain under ...restrained conditions are higher compared to CVC. ► Cement type considerably affects creep. ► Composite models can predict the differences in E-modulus and shrinkage between CVC and SCC.
In this study, the stress–strain-behaviour of self-compacting concrete (SCC) and conventionally vibrated concrete (CVC) is compared. The influence of the paste volume and the cement type on E-modulus, flexural and compressive strengths, drying shrinkage, creep and stress development under restrained conditions is investigated on three SCC mixtures and three mixtures of CVC with identical water-to-cement-ratio. SCC reaches lower values for E-modulus, compressive and flexural strengths but higher values for shrinkage, creep and strain under restrained conditions. Besides the paste volume, the cement type plays a fundamental role regarding creep. Composite models were applied for comparisons and were able to predict the differences in E-modulus and shrinkage between CVC and SCC.
With the increasing use of self-compacting concrete (SCC) its durability has come into focus. Concerning the microstructure of concrete, the porosity in the interfacial transition zone (ITZ) is ...regarded as a key feature for permeability and durability. Generally, a combination of cement and mineral admixtures is used for the production of SCC. In the present study, ITZ porosity of four SCC mixtures produced with ordinary Portland cement, Portland limestone cement, slag cement and ordinary Portland cement combined with fly ash is analyzed. Additionally, the chloride migration coefficient is determined. ITZ porosity and width of the SCC mixtures are similar. The substantial differences in the chloride migration coefficients show that the binder type has a stronger influence on permeability than the pore volume in the ITZ.
Knowledge of the coefficient of thermal expansion (CTE) is of paramount importance for the determination of the cracking risk of concrete structures at early ages. This paper presents a novel ...technique which is suitable to measure the CTE of hardening materials with high accuracy starting from casting time.
The technique consists of casting a small amount of cement paste or mortar into flexible membranes. The specimens are immersed in an oil bath, whose temperature is rapidly changed and then kept constant in repeating cycles. By suspending the sample from a high-precision balance and reading the change of mass after each temperature step, the CTE is calculated with high accuracy from the measured temperature and strain.
Results on cement pastes and mortars (water/cement 0.3) showed a good repeatability. In particular, a sudden decrease in the CTE at setting time, followed by a gradual increase as the cement paste self-desiccates, was measured.
•The moisture state of RA before concrete production has no impact on carbonation.•Carbonated or porous RA can lead to an increase of carbonation coefficient K.•Carbonation coefficient K of RC ...decreases with decreasing clinker content of the cement.•Accelerated carbonation of RC provides a good assessment of natural carbonation.
The environmental impact of concrete production can be decreased by the use of cement with reduced clinker content or the reuse of concrete waste provided by the demolition of buildings in recycled aggregate concrete (RC). In this study, the carbonation resistance of RC is investigated in order to clarify the influence of recycled aggregate percentage, moisture state of the aggregates prior to concrete production and type of cement used. RC with two different replacement levels of natural aggregates is produced using three different blended cements and compared to reference concrete mixtures produced with natural aggregates.
The carbonation resistance of RC can be decreased by particles of recycled aggregate that are either very porous or already carbonated before concrete production. There were no systematic differences in compressive strength and carbonation resistance between dry and pre-saturated aggregates. The carbonation resistance at a given compressive strength decreases with decreasing clinker content of the cement.
The sulfate resistance of concrete was tested using drying-immersion cycles of varying duration in different sulfate solutions. The measured expansion in the different protocols showed a correlation ...to the sulfate profiles in the test specimens determined by EDX. Based on the magnitude of expansion and the test duration, a suitable protocol for testing job-site concrete was identified. A matrix of 20 concrete mixtures was tested with this protocol. The test permitted to distinguish the effect of cement type, w/c and paste volume on expansion. Measurements of the dynamic E-modulus made it possible to link expansion and mechanical damage and to define a limit value for expansion. As this test appears to be suitable to determine the potential of concrete for expansion induced by ettringite formation due to sulfate ingress, it was introduced into the Swiss norms.
When cement with mineral additions is employed, the carbonation resistance of mortar and concrete may be decreased. In this study, mortars containing mineral additions are exposed both to accelerated ...carbonation (1% and 4% CO2) and to natural carbonation. Additionally, concrete mixtures produced with different cements, water-to-cement ratios and paste volumes are exposed to natural carbonation. The comparison of the carbonation coefficients determined in the different exposure conditions indicates that mortar and concrete containing slag and microsilica underperform in the accelerated carbonation test compared to field conditions. The carbonation resistance in mortar and concrete is mainly governed by the CO2 buffer capacity per volume of cement paste. It can be expressed by the ratio between water added during production and the amount of reactive CaO present in the binder (w/CaOreactive) resulting in a novel parameter to assess carbonation resistance of mortar and concrete containing mineral additions.
•Very early formation of ettringite in the presence of an alkali-free accelerator.•Destabilization of ettringite in the presence of an alkaline accelerator.•At late ages significant more AFm phases ...compared to reference without accelerator.•Reaction of AFm phases to ettringite during storage in sodium sulfate solution.•Formation of high quantities of ettringite with alkaline accelerator lead to strong expansion.
Alkali-free, aluminium-based accelerators are often used for shotcrete linings in tunnels and mines, where external sulfate attack can occur. Therefore, it is essential to know, if the influence of the accelerators on the cement hydrate assemblage negatively impacts sulfate resistance. This study focuses on the effect of aluminium-based accelerators on cement hydration and the consequences for external sulfate attack. Aluminum sulfate-based accelerators cause rapid setting due to the very early formation of ettringite and accelerate alite hydration. At late ages, significant more AFm phases are formed compared to the reference without accelerator, which during storage in sodium sulfate solution react to ettringite. A significantly higher volume increase due to the formation of additional ettringite during sulfate exposure was calculated by thermodynamic modelling for the accelerated paste compared to the reference. Alkaline accelerators based on sodium aluminate form mainly amorphous calcium (sulfo-) aluminate hydrates at very early age, while ettringite seems to be destabilized. The ettringite quantities formed during storage in sodium sulfate solution are significantly higher than for the reference and the paste with the alkali-free accelerator. This finding compares well to experiments on concrete specimens, where in the mixture with the alkaline accelerator deleterious expansion was observed.
•Sulfate resistance testing of shotcrete.•New method for test sample preparation in the laboratory without spraying.•Comparison of laboratory-prepared with real on-site shotcrete.•Chemical phase ...assemblage of shotcrete before and after sulfate interaction.•Effect of alkali-free accelerators and different cement types on the sulfate resistance of shotcrete.
When shotcrete is in contact with sulfate containing ground water, there is a risk of a drastically reduced service life. In this case, the use of sulfate-resisting cement is recommended. Since physical characteristics of the cement matrix of shotcrete differ from conventional concrete, sulfate-resisting cements do not always guarantee sufficient shotcrete durability. Thus, testing of the sulfate resistance in accelerated tests is necessary. Providing test samples from on-site (real) shotcrete is time consuming and demanding. In this study, the sulfate resistance and physical properties such as compressive strength and transport properties of samples produced in the laboratory without spraying are compared to the properties of field samples. A new compaction method is proposed, which is apt to allow high accelerator dosages. For different alkali-free set-accelerators and cement types, the laboratory-produced samples not only reveal similar sulfate resistance test results as on-site shotcrete, but also exhibit comparable porosity, transport properties and chemical phase assemblage.
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