Shotcrete is often in direct contact with groundwater containing sulfate. The resistance to sulfate attack of modern sprayed concretes produced with non-alkali aluminate-based accelerators in combination with different cement types is related to their microstructure, transport properties, porosity and phase composition. A characterization of these properties allows a comprehensive assessment of the sulfate resistance of shotcrete with the identification of the relevant physical and chemical influencing factors. As with conventional concrete, the cement type and the water to binder ratio are also decisive for the sulfate resistance in shotcrete. On the one hand, capillary porosity and diffusion coefficient show cement-specific differences, and accordingly the sulfate solution uptake and the sulfur profile in the test specimen with cement type and w/b also are different. On the other hand, the type of cement defines the potential for ettringite formation and thus for damage to the concrete. Sulfate expansion is caused by the conversion of monocarbonate/hemicarbonate/monosulfate (AFm phases) to ettringite. The application of sulfate-resistant cements does not guarantee the sulfate resistance of sprayed concretes in each case. Their use, however, minimizes the damage potential. In the sulfate resistance tests, damage in shotcrete causing a drop in the dynamic modulus of elasticity occurs at a higher sulfate expansion compared to conventional concrete due to the higher volume of hardened cement paste and the smaller maximum aggregate size. The studied alkali-free accelerators, currently used in practice, are found to have no direct adverse effect on the sulfate resistance in the dosage investigated and behave more favorably than the aluminate based alkali-containing accelerators. However, the accelerators influence the sulfate resistance indirectly by causing higher porosity and permeability and thus by an increased sulfate solution uptake of the 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. (C) 2021 The Author(s). Published by Elsevier Ltd.

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. (C) 2020 The Author(s). Published by Elsevier Ltd.

The pore connectivity of tight shale reservoirs plays an essential role in the movement of shale gas and oil, however, the characteristics of connected pores in shale with a multi-scale and coupled pore-fracture system are poorly constrained. Working with typical American (Barnett and Eagle Ford) and Chinese (Longmaxi) shale samples in 2D/3D spaces at nano- to mm-scales, connective pores were intruded with a molten alloy (Wood’s metal; WM) under a temperature of ~85 °C and high pressure (60, 300, and 600 MPa) conditions. After solidification of the alloy at room temperature, polished sections were used to map WM components by field emission-scanning electron microscopy (SEM), micro- and nano-X-ray tomography and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These tests were supplemented with mercury intrusion porosimetry (MIP) for pore-fracture throat size distribution. The shale matrix is generally characterized by low pore connectivity; however, the extent of connectivity within μm-sized and dispersed organic matter (OM) particles is high, with the observed WM-filled pore space ranging from 10% to 70% (averaged at 43%) for the Barnett Shale sample. The grain-edge fractures are important channels to connect multiple OM-hosted pore systems dispersed in shale matrix. Our work illustrates that shales exhibit a dual-connectivity behavior, with the effective porosity decreasing sharply as the distance from the sample boundary increases; the good pore connectivity zone away from the edge of sample is 500 μm under a pressure of 600 MPa for the Barnett Shale sample.

The production of concrete requires large quantities of sand. At first sight, sand seems to be a cheap, widely available and abundant resource, however, in some regions of the earth sand is getting rare because of the high demand. An alternative might be desert sand, which is available from many sources around the world, but is not yet widely used in concrete due to its narrow particle size distribution. In this study, a new concept of combining desert sands with a binder based on calcium sulfoaluminate cements (CSA) and gypsum is tested to obtain a suitable workability and packing density and hence allows for a high sand replacement level in standard concrete. Unlike typical CSA applications, a high gypsum content is used in order to produce a large amount of ettringite, which is able to bind large quantities of mixing water. The concept is applied on a series of pastes and concretes with different standard sand replacement levels. A very fine quartz sand is used as a model desert sand. Mechanical properties (compressive and bending strength) are determined and compared to reference systems based on ordinary Portland cement (OPC). It is shown that the formation of ettringite is capable to fill the voids between the small and narrow-sized desert sand particles leading to a very dense microstructure. Drying shrinkage data show volumetric stability and reduced shrinkage. Owing to the low CSA dosage and the low carbon dioxide emissions related to CSA production, the proposed concrete mixtures can be regarded as eco-friendly. (C) 2020 Elsevier Ltd. All rights reserved.

Magnesium-to-phosphate (Mg/PO4) and water-to-cement (w/c) ratios are important factors controlling hydration and properties of magnesium potassium phosphate (MKP) cements. This study investigated the influence of Mg/PO4 molar ratios (2.7, 4 and 8) and w/c (0.25 and 5) on cement hydration, compressive strength and volume stability. Low w/c ratio slowed down cement hydration at lower Mg/PO4 beyond 1 day, and prevented the precipitation of intermediate hydrates at higher Mg/PO4. At lower Mg/PO4 expansion and strength loss were observed with time due to the continuing hydration within the already hardened cement. Higher Mg/PO4 resulted in faster hydration, higher strength, and in the precipitation of traces of brucite, which had no significant influence on the long-term volume stability and strength. Therefore, moderate Mg/PO4 molar ratios between 4 and 8 depending on w/c ratio used are recommended for the production of MKP cements with robust performance.

Calcined clays emerge as a promising source of supplementary cementitious material, which can provide a significant lowering of the Portland clinker content in blended cements. This study focusses on sulfate resistance of calcined clay (CC) - limestone (L) - Portland cements for mortars exposed to a 0.11 M Na2SO4 solution at 5 and 20 degrees C after a hydration period of 91 days. The pozzolanicity, compressive strength, pore structure, and sulfate resistance of mortars containing laboratory-made metakaolin or calcined montmorillonite and limestone have been investigated in cements with 35 wt% replacement of a white Portland or an ordinary Portland clinker. The results show that all mortars with CC/(CC + L) >= 0.5 exhibit excellent sulfate resistance. The consumption of portlandite by the pozzolanic reactions of the calcined clays and the dilution of the Portland clinker lead to a lower amount of calcium available for the secondary formation of gypsum and ettringite, which is identified as the main reason for the excellent sulfate resistance of the ternary blends. The results suggest that calcined clay limestone - Portland cements are included in standards as a new type of sulfate-resisting Portland pozzolana cement and Portland composite cement.

Seasonal storage of excess solar energy is a key factor for the enhancement of the energy performance of a decentralized energy supply of private homes. Furthermore, thermal heat storage allows to bridge demand or supply gaps and is thought to be an important element to increase the flexibility of future renewable energy supply systems. Cementitious materials, being widely available, cheap construction materials, have been proposed for thermal heat storage based on the utilization of their relatively high specific heat capacity. Seasonal heat storage in concrete or mortar has been achieved solely by the incorporation of phase change materials (PCM). In this contribution, special hardened cement pastes based on hydrated calcium sulfoaluminate cements (CSA), containing large amounts of ettringite as main hydrate phase, are proposed as seasonal heat storage material. Combining relatively high storage capacity and moderate loading temperatures (< 100 degrees C) with mechanical performance and low cost they possess significant advantages regarding the application in buildings. Theoretical ettringite content of fully hydrated blends based on CSA-OPC-gypsum was modeled and based on these results the thermal properties of some variants were studied. Furthermore, hygric characteristics, microstructure, mechanical and thermo-physical properties were derived. Different prototypes made with CSA-based hardened cement pastes as seasonal storage material ranging from small size to large scale are studied. Construction details and temperature and humidity distributions within a modular 6 m(3) ettringite heat storage unit during loading and deloading are presented.

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.

Hail impact damage on External Thermal Insulation Systems (ETICS) is increasingly recognized by insurance companies owing to increased storm occurrence frequency and storm intensity. To develop hail resistant ETICS for houses and better understand existing admission tests, high-speed-camera recordings of ice ball impacts at an angle of 45 degrees and steel ball impacts at angles of 90 degrees and 45 degrees were used to characterize the impact process and to derivate the damaging mechanisms of impacts on facades. Recorded surface deformation is characterized by high indentation depth of the impactor and high flexural bending causing high surface parallel strain. Analyses of the impact process allowed the identification of the mechanisms and timing of fracture formation in different regions. Additionally, differences in the impact process of the European steel ball impact test (90 degrees, ETAG 004) and the Swiss ice ball impact test (45 degrees, VKF P. No 8) are discussed in detail. Caused by the difference in impact angle, the 45 degrees ice ball impacts lead to lower indentation depth and consequently to lower tensile strain and damage. However, surface parallel movement of the impactor caused the formation of an elongated damage pattern in the 45 degrees impacts. To avoid the observed brittle failure behavior, the development of flexible materials with the ability to elastically accommodate impact strains is favorable to reduce hail stone impact damage. (C) 2017 Elsevier Ltd. All rights reserved.

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.

Calcium sulfoaluminate cements are promising low carbon dioxide alternatives to Portland cement. Their main hydration products are ettringite and aluminium hydroxide. Ettringite has recently been identified as a potential heat storage material. The reversible dehydration of ettringite to metaettringite at elevated temperatures and under dry conditions involves a relatively high enthalpy, which can be used for (seasonal) heat storage. In this context, the effect of elevated temperatures (up to 110°C) and humidity conditions (steam curing and dry curing) on the stability of ettringite in calcium sulfoaluminate was studied experimentally using thermogravimetric analysis, X-ray diffraction and thermodynamic modelling. The experimental results show that ettringite decomposes under steam curing conditions at temperatures far below 100°C to monosulfate. This may lead to delayed ettringite formation when the temperature is lowered again under humid or wet conditions. Under dry conditions at low water vapour saturation, the expected dehydration of ettringite to metaettringite is found. A thermodynamic model for the stability of hydrated calcium sulfoaluminate cements based on cement composition and calculations of thermodynamic equilibria has been established. The modelled phase development under different hygro-thermal conditions agrees well with the experimental findings. Stable heating and drying conditions for calcium sulfoaluminate based heat storage materials could be identified.

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91days and exposure to 1% (v/v) CO2 at 20°C/57% RH for 280days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO2 binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO2 binding capacity. The limestone blend has higher CO2 binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO2 binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.

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.

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.

Observations of a nanometer-scale nodular morphology on differently prepared surfaces of thermosets have frequently been interpreted as a sign for an inhomogeneous molecular network, which would result in an inhomogeneous modulus distribution within those thermosets. In order to test this hypothesis, the Peak-Force Tapping atomic force microscopy (AFM) mode was used on fracture surfaces and ultramicrotome cuts of epoxy and other polymers using differently sharp AFM probes.

To track down potential sites of material failure in the tile–mortar–substrate systems, locations and intensities of stress concentrations owing to drying-induced shrinkage are investigated. For this purpose, mechanical properties were measured on real systems and used as input parameters for numerical modeling of the effect of shrinkage of substrate and/or mortar using the finite element code Abaqus. On the base of different geometrical set-ups we demonstrate that stress concentrations in the mortar can become critical when (i) substantial mortar shrinkage occurs, (ii) substrate shrinkage can accumulate over considerable spatial distances, particularly (iii) in situations where the mortar layer is not separated from the substrate by a flexible waterproofing membrane. Hence material failure in the system tile–mortar–substrate can be prevented (or reduced) by (i) an application of the tiles after the major stages of substrate shrinkage, (ii) the use of elasto-plastic deformable tile adhesives which can react elastically on local stress concentrations, (iii) the implementation of flexible membranes, and (iv) a reduction of the field size by the installation of flexible joints.