In this study, a number of different fibers- namely kenaf, jute, abaca, coir and sisal- were investigated as natural alternatives to polypropylene (PP) fibers for reducing plastic shrinkage cracking. The risk of plastic shrinkage cracking of mortars with water-to-cement ratio 0.5 containing either 0.6 or 0.9 kg/m3 of natural fibers was assessed according to the ASTM C1579-21 standard and compared with plain mortars and mortars with PP fibers. The water absorption of the natural fibers was low enough that (at the employed dosages) the effect on the workability and on other fresh properties was small. The natural fibers also had no measurable influence on cement hydration in the examined mortars, as revealed by isothermal calorimetry. The best performance in reducing the width of plastic shrinkage cracks was shown by kenaf and jute fibers at the dosage of 0.6 kg/m3, which outperformed even a higher dosage of PP fibers (0.9 kg/m3). Kenaf fibers in pellets, which are advantageous for dosing and mixing, performed similarly as loose fibers. The distribution of both loose and pelletized kenaf fibers in the mortars was studied by X-ray tomography, showing no substantial difference between the two ways of delivering the fibers.

A detailed microstructural investigation of a concrete expanding due to alkali-carbonate reaction (ACR) shows that the cement paste adjacent to reactive aggregate particles is carbonated, which leads to a sulfur redistribution resembling internal sulfate attack. Simultaneously to dedolomitization, partial dissolution of illite occurs in aggregate particles leading to the formation of brucite, hydrotalcite, magnesium-silicate-hydrate (M-S-H) and calcium-aluminum-silicate-hydrate (C-A-S-H), in addition to calcite and thus to a substantial increase in solid volume. Thermodynamic modelling indicates that the simultaneous presence of illite and dolomite can accelerate the reactions within the aggregates. No alkali-silica reaction (ASR) products are observed. Dolomite, illite and all reaction products display a negative zeta-potential at high pH generating repulsive forces during dedolomitization. Together with the substantial increase in molar volume, the concrete expansion can be mainly attributed to the solidification pressure of hydrotalcite and M-S-H formation.

During alkali-carbonate reaction (ACR), dolomite present in the aggregate particles reacts with the hydroxide ions present in the concrete pore solution. The mechanism of expansion of ACR has generated controversial discussion in the scientific community. A detailed microstructural investigation was performed to characterize the changes in a concrete containing ACR-susceptible Kingston carbonate rock as aggregates. Four types of reaction products are present in the aggregates resulting from the dissolution of dolomite, illite and quartz. However, no alkali-silica reaction (ASR) products were observed. The cement paste adjacent to reactive aggregates is altered with the formation of a patchy layer of calcite embedded in de-calcified C-S-H with a high alkali content. A decrease of pH together with this alteration was indicated by the absence of S-containing phases within the reaction rim.

This research quantified the temporal mesostructural evolution of bitumen emulsion-cement composites using the time-lapse high-energy X-ray tomography of a fine-aggregate matrix. The image post-processing and analysis showed that the mastic's significant temporal decrease in volume complemented the expansion of the pore space. Nevertheless, the volume fractions determined by the image analysis essentially differed from the physical composition of specimens and were several times less sensitive to curing. Because of the extremely heterogenous microstructure and the abundance of calcium, the mastic phase had the highest attenuation of X-rays, but the attenuations of bitumen- and cement-dominated systems experienced contrary temporal trends. The sand and pore space had typically smooth and oppositely evolving axial distributions with the highest-density plateau in the middle, while the mastic was distributed uniformly. The radial distributions evolved less irregularly and notably interfered with the post-processing of beam hardening. The temporal increase in the pore space's local thicknesses was extremely unevenly distributed across diameters. Except the sand particle size distributions, all results were almost excellently repeatable. Finally, the interaction with X-rays was identified as crucial for the interpretation and validity of the results. Moreover, although water could not be segmented using the conventional X-ray tomography, its discrete signature was present throughout the behaviour of other phases.

Thousands of homes in County Donegal, Ireland, built from concrete blocks, are damaged by extensive cracks and crumbling that occurred a few years after construction. Recently, research has shown that pyrrhotite oxidation triggering internal sulfate attack (ISA) is the cause. In this study, samples from the strip foundations, the rising blocks, the outer and inner leaf of one undamaged reference home and three affected homes are investigated. As these four structural components differ by concrete quality, exposure condition or both, their effect on ISA is investigated. All three damaged homes contained pyrrhotite in the aggregates, while it was absent in the reference home. ISA in the foundations is in an initial state. It has progressed further in the rising blocks and is most advanced in the outer leaf. Whilst carbonation limits the effects of ISA in the inner leaf, further pyrrhotite oxidation will create expansion leading to ongoing deterioration.

Cement production is linked to a substantial CO2 emission contributing to about 5-8% of the man-made emissions. However, hardened cementitious materials can absorb CO2 in the process called carbonation, both during the service life of the structures and during their demolition and recycling phase. As experimental data of CO2 absorption during the recycling phase are scarce, the goal of the study is to experimentally determine the CO2 absorption of recycled concrete aggregates (RA) from the point of crushing until reuse in recycled aggregate concrete (RC). Samples from three plants producing both RA and RC were collected and stored both in the plants and in the laboratory for several months. The change in the amount of uncarbonated cement paste of the RA with time was determined by using a novel method: image analysis of samples embedded in epoxy, ground and sprayed with phenolphthalein. This allowed to calculate the CO2 absorption during the storage of RA. The amount of absorbed CO2 corresponds to about 5.4-12.6% of total CO2 emission originally stemming from the production of cement.

In County Donegal, northwest Ireland, thousands of homes built with concrete blocks show an increasing degree of severe structural defects attributed to high mica content in the aggregates. Consequently, the problem is popularly known as the "Mica Crisis". In this project the concrete blocks of four affected homes are investigated by microstructural and chemical analysis combined with thermodynamic modelling. Apart from mica, the aggregates contain iron sulphides mainly in the form of pyrrhotite. The sulfur content of the aggregates considerably exceeds the limit value defined by the European standard for concrete aggregates (EN 12620). The results of the microstructural analysis coupled with thermodynamic modelling demonstrate that the concrete suffers from internal sulfate attack triggered by pyrrhotite oxidation. The comparison of the results of this investigation with the data collected by chartered engineers on almost hundred damaged homes shows that the four investigated cases are representative of the situation in Donegal.

During the demolition and recycling process the specific surface area per volume of concrete is significantly increased. As a result, the potential for CO2 absorption by carbonation of the cement hydrates is enhanced. In this study, recycled concrete aggregates (RCA) with varying moisture contents are carbonated with 100% CO2 at atmospheric pressure. The changes in the properties of RCA are characterized and they are used for the production of recycling concrete. There is only a minor impact of carbonation on bulk density and water adsorption of the RCA. The accelerated carbonation leads to the formation of calcium carbonate and decalcified calcium-silicate-hydrate (C-S-H) present in patches on the surface of RCA particles. Using slurry instead of tap water decreases concrete flow and generally increases compressive strength. Using carbonated instead of uncarbonated RCA leads to a faster decrease of flow with time and to a higher compressive strength of the recycling concrete compared to uncarbonated RCA. The higher strength linked to the use of carbonated RCA offers the potential to decrease the clinker content of the recycling concrete.

Carbonated wollastonite clinker (CS) may be suitable as supplementary cementitious material (SCM) for mortar and concrete. The microstructure of unground CS clinker, carbonated CS slurry and a mortar blended with carbonated CS are investigated by scanning electron microscopy. Additionally, a reference mortar with pure Portland cement and one with a cement replacement level of 30 mass-% by carbonated CS are produced to assess its contribution to compressive strength development. The calcium silicates are decalcified during carbonation resulting in CaCO3 and amorphous SiO2 . The latter reacts when used as SCM in mortar influencing the Ca/Si ratio of calcium-silicate-hydrate and contributing to compressive strength development.

Early-age drying (immediately after casting) of mortars and the corresponding plastic shrinkage were studied using bimodal neutron/X-ray computed tomography. This novel, correlative 3D imaging mode enabled studying simultaneously and without any source of spurious perturbation the water migration and loss processes together with the corresponding deformations due to plastic shrinkage. Bimodal imaging opens up new possibilities for studying dynamic processes of coupled water transport and deformations in porous solids. The measurements were carried out on model systems (cylindrical mortar specimens with height of 19 mm). The study focused on the effect of a paraffin-based curing compound. Our results confirm that when the curing compound was applied directly onto the drying surface in a sufficient amount, both the evaporation rate and the rate of vertical displacement (settlement) were substantially reduced. The results shed a new light on the mechanisms of plastic shrinkage and the action of curing compounds.

The formation of alkali-silica-reaction (ASR) products in concrete aggregates generates stress leading to the formation of cracks proceeding from the aggregates into the cement paste. However, there is little knowledge on the initial ASR products formed in aggregates before the cracking occurs, as their small volume considerably complicates analysis. In this study, a new approach for identification and visualisation of ASR product formation leading to concrete damage is presented. Caesium is added as a tracer during concrete production. Because it is mainly incorporated in ASR products, their backscattering contrast in the scanning electron microscope is considerably increased. This makes it possible for the first time to follow the temporal and spatial progression of ASR with resolution in the nanometre scale, thereby delivering a coherent reaction sequence.

New quantitative relationships are established between effective properties (gas diffusivity, permeability and electrical conductivity) for a dry GDL (25 BA) from SGL Carbon with the corresponding microstructure characteristics from 3D analysis. These microstructure characteristics include phase volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. The latter two parameters include information from two different size distribution curves for bulges (continuous PSD) and for bottlenecks (MIP-PSD). X-ray tomographic microscopy is performed for GDL at different compression levels and the micro-macro-relationships are then established for the in-plane and through-plane directions. The predicted properties based on these relationships are compared with numerical transport simulations, which give very similar results and which can be summarized as follows:Gas diffusivity is higher in the in-plane than in the through-plane direction. Its variation with compression is mainly related to changes of porosity and geodesic tortuosity. Permeability is dominated by variations in hydraulic radius. Through-plane permeability is slightly higher than in-plane. Anisotropy of electrical conductivity is controlled by tortuosity, which is higher for the through-plane direction. A table with new quantitative relationships is provided, which are considered to be more accurate and precise than older descriptions (e.g. Carman-Kozeny, Bruggeman), because they are based on detailed topological information from 3D analysis. Furthermore, when using these relationships as input for macro-homogenous modeling, this enables to simulate microstructure effects of real GDL (SGL 25 BA) more accurately. In future, the same methodology can be used to study micro-macro relationships in wet GDL and to predict relative liquid permeability and relative gas diffusivity. (C) 2017 Elsevier Ltd. All rights reserved.

The diffusion coefficient D-O2, the porosity and the pore structure of mortars produced with a Portland cement and a range of blended cements containing limestone powder, microsilica, portlandite or slag were measured in the non-carbonated and the carbonated state. Additionally, the setup for measuring O-2 diffusion was adapted to measure also the CO2 diffusion of the carbonated mortars. The diffusion coefficient D-O2 and the total porosity were increased in the mortars containing microsilica and slag, while they were decreased in the other mortars due to carbonation. Invariably, the pore structure became coarser in all samples. The relationship between diffusion coefficients D-O2 and D-CO2 in the carbonated mortars was always linear, with D-O2 systematically higher by factor of 1.37. As this factor broadly agrees with what was found in the scant literature about CO2 diffusion, it could be used for estimating D-CO2 of carbonated mortar and concrete based on measurements of O-2 diffusion.

The performance of polymer electrolyte fuel cells (PEFC) strongly depends on a controlled water management within the porous layers. For this purpose we investigate liquid water transport in a commercial gas diffusion layer (SGL 25BA) on the pore scale. X-ray tomography experiments combined with pressure-induced water injection provide 3D images of the liquid water distribution inside the GDL at incremental pressure steps between 0 and 100 mbar. The breakthrough behavior of the liquid phase is highly anisotropic. In through-plane (tp) direction first bubble points appear at the outlet plane already at 5 mbar and the 'breakthrough' then evolves continuously over an extended pressure range up to > 30 mbar. For in-plane (ip) direction the breakthrough is discontinuous and takes place at 27 mbar. Simulations of the intrusion process reveal that the different breakthrough behaviors are mainly triggered by different ip- and tp-transport distances. Short tp-transport distances through the thin gas diffusion layer (ca. 100 mu m) are responsible for the characteristic continuous tp-breakthrough behavior, which is thus attributed to a so-called short-range effect.Dedicated methods for 3D-image analysis adapted to fibrous GDL microstructures were presented in part I. With these methods we quantify all microstructure characteristics that are relevant for liquid permeability. These characteristics of pore and liquid phases include size distributions of bulges and bottlenecks, connectivity, effective volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. Quantitative relationships are established between these microstructure characteristics and the liquid permeability, which provide a better understanding of the underlying microstructure limitations for injection and liquid transport.For the in-plane direction the liquid permeability is limited to roughly a similar extent by tortuosity, constrictivity and effective volume fraction. In contrast, for through-plane direction relatively low volume fractions of the liquid phase put stronger limitations to the liquid permeability than tortuosity, constrictivity and hydraulic radius.The curves for relative permeability vs. saturation (and vs. capillary pressure, respectively) achieved from 3D-analysis reveal complex but characteristic (reproducible) shapes with concave, linear and convex segments. The shape of these segments can be attributed to distinct microstructure effects. In contrast, the conventional macroscopic descriptions from literature cannot capture these complex shapes and the underlying microstructure effects. Future investigations with different GDL materials are necessary in order to understand whether these complex shapes for the relative permeability represent a general feature of gas diffusion layers or if they are specific to the investigated SGL material. (C) 2017 Elsevier Ltd. All rights reserved.

FIB-nanotomography is a new method for high resolution 3D-microscopy that was recently developed by the 3D-Mat group at Empa . With a resolution below 20nm, FIB-nt opens new possibilities for microstructure anal ysis of the complex cementitious materials, which can not be achieved with any other microscopy method at present. In this paper the potential of FIB-nt for quantitative microstructure analysis is demonstrated for porosity in hardened cem ent pastes and, in combination with cryo-preparation methods, for agglomerations and ear ly hydration products in fresh cement pastes. In combination with modern computational an alysis, important topolog ical and statistical information can be obtained from the high resolu tion 3D-data. As discussed in this paper, the new approaches of microstructure analysis will considerably improve our understanding of the micro-macro-link such as the relationship between porosity and the corresponding transport properties in hardened cementitious materials or the mechanisms of particle agglomeration and the influence on rheological properties in fresh cement pastes.

The geological storage of nuclear waste includes multibarrier engineered systems where a large amount of cement-based material is used. Predicting the long term behaviour of cement is approached by reactive transport modelling, where some of the boundary conditions can be defined through studying natural cement analogues (e.g. at the Maqarin natural analogue site). At Maqarin, pyrometamorphism of clay biomicrites and siliceous chalks, caused by the in-situ combustion of organic matter, produced various clinker minerals. The interaction of infiltrating groundwater with these clinker phases resulted in a portlandite-buffered hyperalkaline leachate plume, which migrated into the adjacent biomicrite host rock, resulting in the precipitation of hydrated cement minerals.In this study, rock samples with different degrees of interaction with the hyperalkaline plume were investigated by various methods (mostly SEM-EDS). The observations have identified a paragenetic sequence of hydrous cement minerals, and reveal how the fractures and porosity in the biomicrite have become sequentially filled. In the alkaline disturbed zone, C-A-S-H (an unstoichiometric gel of Ca, Al, Si and OH) is observed to fill the pores of the biomicrite wallrock, as a consequence of reaction with a high pH Ca-rich fluid circulating in fractures. Porosity profiles indicate that in some cases the pores of the rock adjacent to the fractures became tightly sealed, whereas in the veins some porosity is preserved. Later pulses of sulphate-rich groundwater precipitated ettringite and occasionally thaumasite in the veins, whereas downstream in the lower pH distal regions of the hyperalkaline plume, zeolite was precipitated.Comparing our observations with the reactive transport modelling results reveals two major discrepancies: firstly, the models predict that ettringite is precipitated before C-A-S-H, whereas the C-A-S-H is observed as the earlier phase in Maqarin; and, secondly, the models predict that ettringite acts as the principal pore-filling phase in contrast to the C-A-S-H observed in the natural system. These discrepancies are related to the fact that our data were not available at the time the modelling studies were performed. However, all models succeeded in reproducing the porosity reduction observed at the fracture-rock interface in the natural analogue system. (C) 2016 Elsevier Ltd. All rights reserved.

Concrete highway and airport pavements are designed to be long lasting; however, some concrete pavements have shown premature deterioration at the cracks and joints. It has been hypothesized that one cause of this deterioration is associated with fluid ingress, especially in cases where those fluids contain deicing salts. This paper examines fluid ingress in mortar using a cross-sectional geometry that is similar to a typical concrete pavement joint. Time-dependent and spatial aspects of fluid ingress are examined using neutron radiography (NR), which was performed using the thermal neutron radiography station at the neutron spallation source at the Paul Scherrer Institut (PSI). Specifically, this paper examines the role of the initial relative humidity (or degree of saturation) and air content on the fluid ingress. The work indicates that the initial fluid ingress reaches a specific degree of saturation relatively rapidly, where the large capillary and gel pores appear to be filled in (commonly referred to as the nick point in sorption tests) and the entrapped and entrained air pores fill in more slowly over time.

Polished section analysis was employed to study the pore structure of model tile adhesive mortars containing different types of cellulose ethers (CE, respectively Methyl Cellulose, Hydroxyethyl- and Hydroxypropyl-Methyl Cellulose) at different dosages (0.3% and 0.8% by dry mortar mass). Flat layers of hardened mortar applied on an absorbing substrate were studied. To enhance the visibility of the air voids, the polished cross-sections were colored in black and talcum powder was pressed into the voids. Next, flatbed scanner was used to take the images. Due to a high level of agglomeration of pores, digital detachment was necessary in order to retrieve their original shapes and sizes. A stereological reconstruction of 3-D pore size distributions was performed based on the 2-D data from section analysis. Pore shapes were analyzed by determining circularity of the 2-D pore representations, a parameter that allowed estimating the extent of pores agglomeration that occurred in the analyzed mortars. It was found that higher dosage of CE resulted in a slight volume increase and a clear coarsening of the air voids. As shown by the analysis of circularity, this is likely due to higher extent of agglomeration at higher CE dosage.

The Swiss concept for geological disposal of high-level radioactive waste foresees bentonite as buffer between canister and hostrock, as backfill material and for tunnel sealing. Shotcrete is proposed as tunnel supporting material. The emplacement of high-pH cementitious material next to clay generates a chemical gradient that drives diffusive transport. Both laboratory studies and reactive transport modeling predict significant mineral alteration near the interfaces [1, 2].

3D microstructure-performance relationships in Ni-YSZ anodes for electrolyte-supported cells are investigated in terms of the correlation between the triple phase boundary (TPB) length and polarization resistance (Rpol). Three different Ni-YSZ anodes of varying microstructure are subjected to eight reduction-oxidation (redox) cycles at 950 °C. In general the TPB lengths correlate with anode performance. However, the quantitative results also show that there is no simplistic relationship between TPB and Rpol. The degradation mechanism strongly depends on the initial microstructure. Finer microstructures exhibit lower degradation rates of TPB and Rpol. In fine microstructures, TPB loss is found to be due to Ni coarsening, while in coarse microstructures reduction of active TPB results mainly from loss of YSZ percolation. The latter is attributed to weak bottlenecks associated with lower sintering activity of the coarse YSZ. The coarse anode suffers from complete loss of YSZ connectivity and associated drop of TPBactive by 93%. Surprisingly, this severe microstructure degradation did not lead to electrochemical failure. Mechanistic scenarios are discussed for different anode microstructures. These scenarios are based on a model for coupled charge transfer and transport, which allows using TPB and effective properties as input. The mechanistic scenarios describe the microstructure influence on current distributions, which explains the observed complex relationship between TPB lengths and anode performances. The observed loss of YSZ percolation in the coarse anode is not detrimental because the electrochemical activity is concentrated in a narrow active layer. The anode performance can be predicted reliably if the volume-averaged properties (TPBactive, effective ionic conductivity) are corrected for the so-called short-range effect, which is particularly important in cases with a narrow active layer.