Investigations on the influence of cement type on thaumasite formation

A high sulfate resistance is required if cements are to be used in sulfate bearing waters and soils especially under conditions favouring thaumasite formation. A long period program of laboratory investigations was carried out on CEN cements to assess thaumasite and ettringite formation. The experimental concept involved mixing ground cement pastes with stoichiometrical components of gypsum, calcite and water. The specimens were stored at 6 °C whereby chemical worst case conditions for thaumasite formation were simulated. At time intervals XRD analysis was conducted. Apart from pure cements mixtures containing additives, pure C₃S pastes with and without Al₂O₃ addition were investigated. The results confirm that thaumasite formation can be accelerated by Al₂O₃ bearing components in cements. However, thaumasite formation is also possible without active participation of Al³⁺. The assessment of sulfate resistance of cements only from the chemical point of view apparently gives results which are contrary to the field experience.     

Extent of immiscibility in the ettringite–thaumasite system

Solid solutions between thaumasite and the related phase ettringite were prepared and characterised by X-ray diffraction and infrared spectroscopy. A miscibility gap in the solid solution is identified and defined. Crystallographically, the miscibility gap is identified as a gap in the unit cell dimensions of the solid phases formed between c20.95 (10.475) and c21.3 Å. A combination of quantitative X-ray powder diffraction and infrared spectroscopy enabled us to define the miscibility gap in terms of Al:Si ratio. Ettringite can accept the replacement of 1/2 its Al by Si, while thaumasite tolerates little or no Al in its structure.     

Microstructural changes in alkali activated fly ash/slag geopolymers with sulfate exposure

Sulfate attack is recognized as a significant threat to many concrete structures, and often takes place in soil or marine environments. However, the understanding of the behavior of alkali-activated and geopolymer materials in sulfate-rich environments is limited. Therefore, the aim of this study is to investigate the performance of alkali silicate-activated fly ash/slag geopolymer binders subjected to different forms of sulfate exposure, specifically, immersion in 5 wt% magnesium sulfate or 5 wt% sodium sulfate solutions, for 3 months. Extensive physical deterioration of the pastes is observed during immersion in MgSO₄ solution, but not in Na₂SO₄ solution. Calcium sulfate dihydrate (gypsum) forms in pastes immersed in MgSO₄, and its expansive effects are identified as being particularly damaging to the material, but it is not observed in Na₂SO₄ environments. A lower water/binder (w/b) ratio leads to a greatly enhanced resistance to degradation by sulfate attack. Infrared spectroscopy shows some significant changes in the silicate gel bonding environment of geopolymers immersed in MgSO₄, attributed mostly to decalcification processes, but less changes upon exposure to sodium sulfate. It appears that the process of ‘sulfate attack’ on geopolymer binders is strongly dependent on the cation accompanying the sulfate, and it is suggested that a distinction should be drawn between ‘magnesium sulfate attack’ (where both Mg²⁺ and SO₄ ^2? are capable of inducing damage in the structure), and general processes related to the presence of sulfate accompanied by other, non-damaging cations. The alkali-activated fly ash/slag binders tested here are susceptible to the first of these modes of attack, but not the second.     

Occurrence of thaumasite in deteriorated concrete

The first case histories of thaumasite in concrete products in the United States were identified by the writer in the 1960s. There included two underground sanitary pipes, a grout in an underground lead/zinc mine, and the bottom of a concrete pavement on grade. In these cases, thaumasite was first identified by optical properties in powder mounts in a petrographic microscope, then further confirmed by X-ray diffraction and characterized in an electron microscope. Morphological similarities and positive distinction between thaumasite and ettringite are considered.     

The resistance of metakaolin (MK)–Portland cement (PC) concrete to the thaumasite-type of sulfate attack (TSA)––Programme of research and preliminary results

The thaumasite form of sulfate attack (TSA), is a deleterious physico-chemical attack of the calcium silicate hydrate (C–S–H), binding phase of concrete. Water:binder ratios (w:b), are known to control ingress of potentially deleterious ions by pore structure refinement at low (0.40) values. Equally, a physical–chemical barrier exists at about w:b ratio of 0.45 regardless of the binder type. The inclusion of ultra-fine pozzalans (e.g. metakaolin) in the binder has been shown to impart such properties through densification of the matrix and removal of calcium hydroxide.

A small-scale experimental design programme to establish the potential resistance of metakaolin–Portland cement (MK–PC), blended concrete to the thaumasite-type of attack is ongoing. Results are presented for concrete incorporating dolomitic limestone aggregate and with 0% and 7% metakaolin replacement of a high-C₃A PC binder at w:b ratios to 0.40 and 0.46. Exposure to three environments containing a sulfatic clay, sulfate solution and water has produced evidence of deleterious reactions within these samples. Visual data are quantified by a wear rating and supported by compressive strength and expansion values for up to 280 days of exposure. These results confirm the importance of low water binder ratios in the resistance of ion ingress and give an early indication of the desirable durability-enhancing properties of MK replacement of PC. Finally, the results also show that the two sulfatic environments of exposure produce marked differences in the degradation modes, which may be important in the methodologies used to determine TSA in the laboratory with reference to field observations.     

A comparison of classification systems for aggressive ground with thaumasite sulfate attack measured at highway structures in Gloucestershire, UK

Part one of the BRE Special Digest 1 ‘Concrete in aggressive ground’ (2001) provides UK guidelines on assessing aggressive ground conditions with respect to the specification of concrete and protective measures required for concrete structures in contact with the ground. This supersedes the guidelines of BRE Digest 363 (1996) and the Thaumasite Expert Group Report (1999). All three publications rely in part on classifying the ground based on the chemical composition of the soil, backfill and groundwater at the location of the proposed structure. This paper provides an empirical assessment of the three classification schemes that was made possible following an extensive investigation into the thaumasite form of sulfate attack (TSA) in buried concrete highway structures in Gloucestershire. Ground classifications were made at 21 of the sites. The results of this study confirmed that whenever possible, the sulfate concentration in groundwater should be measured, and that as noted in SD1, the potential sulfate classification can be too conservative. The conclusions are limited geologically to the Lower Lias Clay Formation and to Recent Alluvium. The current classification system needs to be developed further by undertaking and reporting similar case studies from a variety of ground conditions.     

Microstructural changes in oxygen-irradiated zirconium-based alloy characterised by X-ray diffraction techniques

We have carried out irradiation with 116 MeV O⁵⁺ ions on Zr–1Sn–1Nb–0.1Fe (ZIRLO) alloy at different doses and the microstructural parameters of the irradiated samples have been characterised by X-ray Diffraction Line Profile Analysis (XRDLPA). The average volume-weighted and surface-weighted domain size, microstrain and dislocation density have been estimated as a function of dose. There was a drastic decrease in domain size from unirradiated sample to the sample at a dose of 1 × 10¹⁷ O⁵⁺/m², but these values saturated with increasing dose of irradiation. The values of microstrain were found to increase with dose. The dislocation density increased almost by an order of magnitude for the samples irradiated with 1 × 10¹⁸ O⁵⁺/m² and 5 × 10¹⁸ O⁵⁺/m² as compared to the unirradiated samples.     

Measurement of the reflectivity changes occurring in coated steels on elevated temperature exposure

The reflectivity changes which accompany coating/substrate interdiffusion in a variety of commercially available aluminium- and zinc-coated steels at elevated temperature have been measured using an optical film monitor. The resulting data have enabled the determination of the temperature at which significant coating/substrate inter-alloying is initiated in the materials and have provided useful information on the kinetics of alloy growth.     

Microstructural and electrical changes in nickel manganite powder induced by mechanical activation

Nickel manganite powder synthesized by calcination of a stoichiometric mixture of manganese and nickel oxide was additionally mechanically activated in a high energy planetary ball mill for 5-60 min in order to obtain a pure NiMn₂O₄ phase. The as-prepared powders were uniaxially pressed into disc shape pellets and then sintered for 60 min at 1200 °C. Changes in the particle morphology induced by mechanical activation were monitored using scanning electron microscopy, while changes in powder structural characteristics were followed using X-ray powder diffraction. The ac impedance spectroscopy was performed on sintered nickel manganite samples at 25 °C, 50 °C and 80 °C. It was shown that mechanical activation intensifies transport processes causing a decrease in the average crystallites size, while longer activation times can lead to the formation of aggregates, defects and increase of lattice microstrains. The observed changes in microstructures were correlated with measured electrical properties in order to define optimal processing conditions.     

Spontaneous Generation of Single Vortices and Anti-Vortices in Superconductors with Restricted Geometries

We study the nucleation of vortices in a thin (thickness ≪ penetration depth) mesoscopic superconducting disc in an applied magnetic field perpendicular to the disc (i.e., parallel to the axis of the disc). We write down an expression for the free energy of the system with an arbitrary number of vortices and anti-vortices at finite (non-zero) temperatures. For a given applied field, we minimize the free energy to find the optimal position of the vortices and anti-vortices (the configuration which minimizes the energy). We show that, whereas at zero temperature anti-vortices do not nucleate, anti-vortices do penetrate the disc at finite temperatures. We also calculate the magnetization of the disc as a function of the applied field and hence determine the different configurations possible in which a fixed number of fluxoids can penetrate the disc.     

Comparative study of different test methods for reinforced concrete durability assessment in marine environment

There are many different test methods to assess reinforced concrete durability. As in marine environment reinforcement corrosion due to chloride attack is the most important degradation process, chloride penetration rate has been compared with different durability tests results (concrete strength, porosity, water absorption, water penetration depth under pressure, capillarity, water and oxygen permeability) carried out on concrete cores obtained from the caissons of seven Spanish wharves. Data have been studied separately, depending on concrete location (chloride penetration rate is faster in submerged concretes than in tidal zone concretes) and cement type (mineral admixtures reduce permeation rate due to pore size refinement). Results show that it is advisable to control concrete water tightness through water penetration under pressure test; additionally, in order to make sure a slow corrosion rate, it should be advisable to control oxygen permeability in tidal zone concretes.     

Strength development and chloride penetration in rubberized concretes with and without silica fume

The study presented herein has been carried out in order to investigate the strength development and chloride permeability characteristics of plain and rubberized concretes with and without silica fume. For this purpose, two types of tire rubber, namely crumb rubber and tire chips, were used as fine and coarse aggregate, respectively, in the production of rubberized concrete mixtures which were obtained by partially replacing the aggregate with rubber. Two water-cementitious material (w/cm) ratios (0.60 and 0.40), three moist curing periods (3, 7, and 28 days), four designated rubber contents (0, 5, 15, and 25 by total aggregate volume), two silica fume content (0 and 10% by weight of cement), and five different testing ages (3, 7, 28, 56, and 90 days) were considered as experimental parameters. The results indicated that for a given w/cm ratio and moist curing period, the use of rubber significantly aggravated the chloride ion penetration through concrete but the degree of the rate of the increment of the chloride permeability depended on the amount of the rubber used. When the curing period was extended from 3 to 28 days, the reduction in the magnitude of chloride penetration depth was notably higher for the rubberized concretes, even at a rubber content of as high as 25%. It was also observed that silica fume may be considered as a remedy to enhance the chloride penetration resistance of the rubberized concretes.     

Microstructural degeneration of simulated heat-affected zone in 2.25Cr–1Mo steel during high-temperature exposure

The microstructural of simulated heat-affected zone (HAZ) in 2.25Cr–1Mo steels has been investigated. The experiments with different heat inputs were carried out in a high speed dilatometer; the thermal cycles used corresponded to the actual thermal cycles that occurred in the coarse-grained region of the real HAZ. It was found that with a heat input of 20 kJ cm⁻¹, the simulated HAZ microstructure gave larger amounts of lower bainite with significant amounts of martensite. The 20 kJ cm⁻¹ heat input specimens were tempered at 700 °C for different time intervals ranging from 1 to 50 h. A sequence for corresponding microstructural degradation has been proposed.     

Microstructural evolution of Fe/Ti multilayers submitted to in situ thermal annealing

The Fe/Ti multilayers of nominal bilayer thickness of 4.0, 8.0 and 18.0 nm with alternating Fe and Ti sublayers thickness ratio of 1:1 were deposited by direct current magnetron sputtering on Si(100) substrates. The bilayer thickness of as-deposited Fe/Ti multilayers was measured as a modulation wavelength of 4.8, 8.5 and 19.0 nm, respectively, by small angle X-ray diffraction and cross-sectional transmission electron microscopy (XTEM). The three Fe/Ti multilayers were composed of pure metallic -Fe and -Ti according to wide angle X-ray diffraction and selected area diffraction. The Fe/Ti multilayers were in situ submitted to thermal vacuum annealing at the temperatures ranging from 523 K to 723 K for the annealing time of 3 h. With annealing at the lower temperature of 523 K, the intermetallic FeTi appeared in the Fe/Ti multilayers with small modulation wavelength of 4.8 nm. At 623 K, the intermetallic FeTi was formed with modulation wavelength of 8.5 nm. At the higher temperature of 723 K, the intermetallic FeTi was detected with modulation wavelength of 19.0 nm. The modulation wavelength of the Fe/Ti multilayers remained during the thermal annealing. The mixture of intermetallic FeTi and -Fe phase was observed in the extended Fe sublayer of the Fe/Ti multilayers annealed at 723 K by XTEM, correspondingly the remaining Ti sublayer was obtained as a thinned sublayer. Coexistence of the intermetallic FeTi, -Fe and -Ti phases indicated that the dynamic factors have control of the intermixing between the Fe and Ti sublayers in the Fe/Ti multilayers during the thermal annealing.     

Microstructural changes in the upper surface of austenitic steel induced by ion sputtering and gas nitriding

The results of investigations of the influence of ion sputtering parameters applied as a pre-treatment on the formation of a nitrided layer during gas nitriding are presented. Previous work had shown that a low reproducibility of layer composition occurred after gas nitriding, thought to be the result of uncontrolled variation of sputtering parameters during the plasma initiation. In the present studies a two-stage sputtering process in nitrogen at a pressure of 3-5 Pa was used: firstly for a few seconds to 1 min with the plasma parameters 1.8 kV and 3 mA/cm², and secondly followed by sputtering with the parameters 1.35 kV, 0.5 mA/cm² for 15 min. The samples were gas nitrided immediately following these processes in 100% ammonia. It was found that even a few seconds of the first plasma treatment enabled magnetic “expanded austenite” to be formed in the layer. Surface layers were analysed using grazing angle X-ray diffraction (GXRD), light microscopy (LM), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD), magnetic and atomic force microscopy (AFM and MFM) and electron probe micro analysis (EPMA) that revealed a complex microstructure. The results have emphasised the need for good control over plasma sputtering parameters in the initial process stage.     

Si cross-polarization magic-angle spinning NMR spectroscopy––an efficient tool for quantification of thaumasite in cement-based materials

²⁹Si{¹H} cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy is a powerful and reliable tool for the quantification of thaumasite in cement-based materials. The most efficient method for quantifying thaumasite from ²⁹Si{¹H} CP/MAS NMR spectra is described and it is shown that the method allows detection of thaumasite contents below approximately 10 wt.% with a relative precision of 15% and contents above 10 wt.% with a relative precision of 10%. The applicability of ²⁹Si{¹H} CP/MAS NMR for quantification of thaumasite is demonstrated for different Portland cement pastes and shows that thaumasite contents as low as 0.2–0.5 wt.% can be detected in cementitious systems with low concentrations of paramagnetic impurities. For a Portland cement containing various amounts of limestone dust and stored at 5 °C in a MgSO₄ solution, large quantities of thaumasite have been detected. Furthermore, the quantity of thaumasite is found to be less sensitive to the amount of added limestone dust. For samples of a Portland cement with a fixed content of limestone dust but different quantities of added gypsum, the increased contents of gypsum are observed to result in larger quantities of thaumasite after prolonged hydration.     

Fine ceramics replacing cement in mortars Partial replacement of cement with fine ceramics in rendering mortars

The main objective of the use of very fine red clay ceramic waste in rendering mortars is the reduction in the primary binder (cement) content made possible by the potential pozzolanic effect of this recycled material, with very clear environmental benefits in the reduction of overly-high energy consuming cement and economic benefits in the potential reduction of the cost of mortars. This paper presents an experimental program where ceramic waste crushed to very fine particles was used to partially replace cement in mortars manufacturing, acting as a secondary binder. A large number of tests of the most relevant characteristics of various mortars in which this principle was applied were performed and compared with the results of the same tests in a reference rendering mortar with no ceramic fines (and no reduction of the cement content). The results are most promising both from a performance-based and an environmental point of view.     

Microstructural changes of Pt/Ti bilayer during annealing in different atmospheres — an XRD study

The high temperature behaviour of Pt/Ti base electrode bilayers is crucial for the deposition of ferro-electric thin films on top of such metal films. Therefore the microstructural development during annealing has to be considered. These parameters are studied by X-ray diffraction (XRD). Comparing the microstructure before and after thermal treatment significant differences could be found. Pt₃Ti compound formation was observed in all coatings. The stress in the Pt phase changes from compressive to tensile during annealing. In contradiction the stress in the Pt₃Ti phase is compressive after thermal treatment. From measurements of the total reflected signal an amount of TiO₂ on the surface of the oxygen treated film could be found. A model based on diffusion of Ti along grain boundaries and considering thermodynamic aspects of stress development is predicted to describe the microstructural changes during annealing.     

Selecting exposure measures in crash rate prediction for two-lane highway segments

A critical part of any risk assessment is identifying how to represent exposure to the risk involved. Recent research shows that the relationship between crash count and traffic volume is non-linear; consequently, a simple crash rate computed as the ratio of crash count to volume is not proper for comparing the safety of sites with different traffic volumes. To solve this problem, we describe a new approach for relating traffic volume and crash incidence. Specifically, we disaggregate crashes into four types: (1) single-vehicle, (2) multi-vehicle same direction, (3) multi-vehicle opposite direction, and (4) multi-vehicle intersecting, and define candidate exposure measures for each that we hypothesize will be linear with respect to each crash type. This paper describes initial investigation using crash and physical characteristics data for highway segments in Michigan from the Highway Safety Information System (HSIS). We use zero-inflated-Poisson (ZIP) modeling to estimate models for predicting counts for each of the above crash types as a function of the daily volume, segment length, speed limit and roadway width. We found that the relationship between crashes and the daily volume (AADT) is non-linear and varies by crash type, and is significantly different from the relationship between crashes and segment length for all crash types. Our research will provide information to improve accuracy of crash predictions and, thus, facilitate more meaningful comparison of the safety record of seemingly similar highway locations.     

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC 718PLUS™ superalloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.