In situ fracture tests in the scanning electron microscope

The experimental arrangement for conducting fracture tests in the scanning electron microscope (SEM) are described together with the particular problems in applying this technique to polymers and composite materials. Several examples are given of mechanisms of fracture in epoxy resins both unfilled and reinforced with particulate and fibrous fillers. The paper is illustrated by still photos taken from the video recording of the fracture experiments and by photos taken from the SEM raster.     

A device for direct observation of cracking of cement paste or mortar under compressive loading within a scanning electron microscope

A device is described which permits the compressive loading of small specimens of hardened cement paste or mortar within the specimen chamber of a scanning electron microscope. The crack patterns which develop may then be seen at fairly high magnifications while the specimens remain under load. The loading frame itself is about 80 mm long by about 50 mm in diameter, and is activated by a pressurized nitrogen cylinder. Some preliminary results of observations made with the device are described.     

Low-temperature scanning electron microscope analysis of the portland cement paste early hydration

The use of the frozen hydrated scanning electron microscopy (FHSEM) in the study of cement paste is described. This technique permits analysis of the fractured surface of cement paste in a fully hydrated state with water present as ice in a low temperature scanning electron microscope. At 110 K the paste has a substantial increase in mechanical strength, because water is converted from liquid to a solid state, and this permits the use of bulk specimens at very early hydration. Some preliminary results for 1 hour hydration are presented and future applications of this technique are discussed.     

Application of the scanning electron microscope to the study of the initial hydration of portland cement paste

The preparation of samples for electron microscope studies of Portland cement paste at very early ages requires special techniques for forming soft paste samples, halting hydration without altering the structure of the paste and coating fracture surfaces with a conductive material. Freeze-drying the paste and sputter coating the fracture surface with gold has produced satisfactory results.     

The shrinkage of hardening cement paste and mortar

This paper is an abstract from the report of the commission B10: “The influence of the shrinkage of cement on the shrinkage of concrete”, of the Netherlands Committee for Concrete Research.Measurements of pulse velocity, volume shrinkage and heat of hydration on hardening portland cement support the idea that the formation of ettringite is an important link in the mechanism of shrinkage in the plastic stage of cement paste and mortar.Mechanical tests on prisms of 4 × 4 × 16 cm³ gave some information about the difference in sensitivity to surface corrosion of different types of cement.     

X-ray absorption study of drying cement paste and mortar

X-ray absorption was used to observe water evaporation with hydration time in paste and mortar specimens, with the aim of studying the influence of water/cement (w/c) ratio, presence of aggregates, curing conditions on drying during early hydration. For the samples subjected to surface drying immediately after mixing, there exists a moisture gradient within the internal part of the specimen. However, obvious top-down drying only occurs within a small zone near the surface for early age cement pastes and mortars. The evaporation rate of water is very high in the first day after casting and is drastically reduced afterwards due to the formation of a microstructure that greatly improves specimens resistance to moisture loss. Mortars reveal a slightly lower evaporation rate since the aggregate increases the length of the transport route because of a larger tortuosity. However, the effect of sealed curing is much more important than the tortuosity effect of the aggregates.     

Identification of hydrated cement constituents using a scanning electron microscope energy dispersive X-ray spectrometer combination

Preliminary studies of the chemical composition of individual particles and local areas in hydrated cement pastes were carried out using a scanning electron microscope-energy dispersive x-ray spectrometer combination. Calibration curves prepared from a series of homogeneous cement and hydrated cement compounds were used, the analysis being carried out in terms of ratios of calcium oxide to silicon, sulfur, and aluminum oxides. Sulfur was distributed throughout the calcium silicate hydrate gel, except for regions thought to represent “inner” hydration product. Apparent mole ratios of calcium oxide to silica ranged generally between 2 and 3, considerably higher than had been expected. Various morphological features were interpreted in the light of their chemical compositions.     

X-ray diffraction and scanning electron microscope studies of processed rice hull silica

The structure and composition of processed rice hull ash were studied to better understand its adsorption of the soy oil components, lutein and free fatty acids. The silica-rich ash was composed largely of low order cristobalite and tridymite in the form of opal CT, with variable particle size. On heating of the ash there was conversion to a more ordered form of opal C, fusing of particles, and loss of potassium. Acid washing, following heating, also removed potassium and slightly reduced particle size further. Crystal form, particle size, particle fusion and potassium content may be important factors influencing ash adsorption of soy oil components.     

An electron microprobe study of a mature cement paste

A portland cement paste 23 years old, and essentially fully hydrated, was studied by electron probe microanalysis. X-ray images indicated that the shapes of the original, largely polymineralic cement grains, and those of the regions occupied by the individual phases within them, are substantially preserved in the hydrated material. This was shown especially clearly by the Mg and Fe images, probably because these elements do not readily migrate in the alkaline medium. Estimation of individual phase compositions is rendered uncertain because of possible admixture of phases on or below a micrometer scale, but the atomic ratios relative to Ca are, approximately: Mg 0.03, Al 0.08, Si 0.60, S 0.03 and Fe 0.015 (Ca:Si 1.67) for the C-S-H, and Al 0.40, Si 0.33 and Fe 0.27 for the hydrogarnet.     

Interaction between two anti-graffiti treatments and cement mortar (paste)

Much of the twentieth century's built heritage is made of concrete, which is particularly susceptible to graffiti “attacks”. Since the traditional methods used to remove spray paint are not particularly effective on this type of material, the adoption of preventive measures by applying a new type of protective treatments, known as anti-graffiti coatings, may be a good way to tackle the problem. The present study aimed to evaluate the physical properties of a cement mortar coated with two such products and to study the possible molecular interactions between the coatings and the cement paste. Neither product induced chromatic changes on the surface of the material, while both made it water repellent. ²⁹Si MAS NMR analysis detected a lower Q¹/Q² ratio, indicative of a longer chain length, in the C-S-H gel in the cement paste mixed with the anti-graffiti products.     

Estimation of the degree of hydration of blended cement pastes by a scanning electron microscope point-counting procedure

A scanning electron microscope (SEM) point-counting technique was employed to study the hydration of plain portland and blended cement pastes containing fly ash or slag. For plain portland cement pastes, the results for the degree of cement hydration obtained by the SEM point-counting technique were consistent with the results from the traditional loss-on-ignition (LOI) of nonevaporable water-content measurements; agreement was within ±10%. The standard deviation in the determination of the degree of cement hydration via point counting ranged from ±1.5% to ±1.8% (one operator, one sample). For the blended cement pastes, it is the first time that the degree of hydration of cement in blended systems has been studied directly. The standard deviation for the degree of hydration of cement in the blended cement pastes ranged from ±1.4% to ±2.2%. Additionally, the degrees of reaction of the mineral admixtures (MAs) were also measured. The standard deviation for the degree of fly ash reaction was ±4.6% to ±5.0% and ±3.6% to ±4.3% for slag. All of the analyses suggest that the SEM point-counting technique can be a reliable and effective analysis tool for use in studies of the hydration of blended cement pastes.     

On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine

Recent work on the strength-enhancing mechanism of triisopropanolamine (TIPA) suggested that TIPA enhances the mechanical properties of mortar and concrete by acting on the interfacial transition zone (ITZ) between paste and sand or aggregate rather than improving the properties of the hydrated binder. This paper presents compressive strength data for 10 Portland cements tested as cement paste as well as two different kinds of mortar after 28 days hydration, so that these two mechanisms could be compared directly. The average strength improvement with TIPA was 10% in the hydrated portland cement paste and 9% in the mortar, clearly showing that the strength enhancement is not dependent on an ITZ mechanism.     

Effect of ice formation on the elastic moduli of cement paste and mortar

The effect of ice formation on the elastic moduli of cement paste and mortar is analyzed using a modified direct method which is based on both micromechanical and classical elasticity theory. Experimental studies were conducted on saturated and frozen cement paste and mortar specimens. The experimental data showed a good agreement with the predictions of the modified direct method.     

In situ electron microscope study on the formation and morphological evolution of carbon aggregates

Two phenomena were observed when a copper specimen was irradiated by normal electron beam in a transmission electron microscope. One is the growth of the rod-like carbon aggregates normally stemmed from the substrate of specimen, with their unique fir-like shape and pattern caused by the deposition of hydrocarbon molecules. These molecules come from the boiling and evaporating of the diffusion pump oil in the high vacuum system. The other is the evolution of the stacked morphologies from the fir-like to spindle-like, mushroom-like and ball-like. The experimental findings and theoretically analyses lead us to believe that the carbon aggregates go through an evolution process which is governed, to some degree, by the irradiation of electron beam and by the surface tension and the instability of the amorphous carbon. Moreover, during the growing process of the carbon aggregates, the magnitude of electric charges and their distributions on the deposited carbon dominate the side branching and the tip splitting. For the evolution process of the morphologies, the most important driving force is the surface tension of the carbon aggregates under the irradiation of electron beam in a vacuum system.     

Thermal expansion of Portland cement paste,mortar and concrete at high temperatures

Results of measurements of thermal expansion of Portland cement paste, mortar, concrete and a dolomitic rock aggregate are presented. Test temperatures ranged from 27 to 871°C. Cement paste contracted when subjected to temperatures from 204 to 871°C. Thermal expansion of mortar and concrete was dominated by thermal expansion characteristics of the mineral aggregate. However, expansion was moderated by contraction of the cement paste matrix at elevated temperatures. Average coefficients of expansion, over specific temperature ranges, are given for the materials tested. Equipment developed to control automatically the rate of heating and record strain vs. temperature relationships is described.     

Double torsion tests for studying slow crack growth of portland cement mortar

For studying slow crack growth in portland cement mortar 32″ (812.8 mm) long double torsion specimens were tested. During testing, the loading and reloading compliances, permanent (or inelastic) deformations and crack growth were measured. It was observed that the strain energy release rates calculated from elastic, secant or reloading compliances do not accurately represent the fracture behavior of this material. A modified definition of the strain energy release rate is developed here to include both the elastic and the inelastic strain energy absorbed during crack extension. For this method, in addition to the reloading compliance, the knowledge of the rate of change of permanent deformations with crack growth is necessary. Details of the analytical and experimental procedure are described in this paper.     

Shielding effect of double microelectrode tips in a scanning electrochemical microscope

Electrochemical interaction of coaxial double microelectrodes, in which a ring microelectrode was surrounded by another ring microelectrode, was investigated. Mass-transfer reactions that occurred on both inner and outer microelectrodes interfered with each other and showed a “shielding” effect depending on potentials and geometries of microelectrodes. Application of the inner microelectrode of the double microelectrodes for a probing tip of a scanning electrochemical microscope (SECM) revealed that the shielding effect by the outer microelectrode affected the electrochemistry on the inner microelectrode in the vicinity of the substrate surface. The effect was intensified above the insulator but attenuated above the conductor as the microelectrodes approached in feedback mode of the SECM. Approach to a critical interelectrode distance also intensified the shielding effect in the substrate generation/tip collection mode. An SECM line-scan using a platinum/epoxy resin-model substrate was carried out to investigate the shielding effect on current sensitivity and lateral resolution of an SECM image.     

Experimental chemo-mechanics of early-age fracture properties of cement paste

The risk of early-age fracture of cementitious materials in ever more challenging environments provides a unique opportunity to employ an experimental chemo-mechanical platform to develop functional relations between hydration degree, fracture and strength properties, assessed by isothermal calorimetry, micro-scratching, splitting and microindentation on white cement paste at various curing ages from 7 h to 28 days. We show that the modulus, tensile strength, fracture toughness and energy all evolve with a natural logarithmic dependence on the hydration degree. These trends are linked to the densification of the material during the hydration process, explained by compaction mechanics and free volume theory. We show that while the fracture process zone size is essentially constant during the hydration process, the ductility of the material, quantified by M/H, decreases, and is consistent with the evolution of K_c/H. Both quantities provide a convenient way to experimentally assess the fracture sensitivity of early-age cement-based materials.     

Optical and scanning electron microscope study of brown coals

Mid-European brown coals, especially topographical features, structure as revealed in polished surfaces, and the elemental composition of inorganic constituents, have been examined. It is suggested that the SEM and EDAX methods should be useful in future studies of their petrographic classification and in cataloguing petrological components.