Clinker Grains Research Articles

From micron-sized needle-shaped hydrates to meter-sized shotcrete tunnel shells: micromechanical upscaling of stiffness and strength of hydrating shotcrete

Knowledge on the stresses in shotcrete tunnel shells is of great importance, as to assess their safety against severe cracking or failure. Estimation of these stresses from 3D optical displacement measurements requires shotcrete material models, which may preferentially consider variations in the water–cement and aggregate–cement ratios. Therefore, we employ two representative volume elements within a continuum micromechanics framework: the first one relates to cement paste (with a spherical material phase representing cement clinker grains, needle-shaped hydrate phases with isotropically distributed spatial orientations, a spherical water phase, and a spherical air phase; all being in mutual contact), and the second one relates to shotcrete (with phases representing cement paste and aggregates, whereby aggregate inclusions are embedded into a matrix made up by cement paste). Elasticity homogenization follows self-consistent schemes (at the cement paste level) and Mori–Tanaka estimates (at the shotcrete level), and stress peaks in the hydrates related to quasi-brittle material failure are estimated by second-order phase averages derived from the RVE-related elastic energy. The latter permits upscaling from the hydrate strength to the shotcrete strength. Experimental data from resonant frequency tests, ultrasonics tests, adiabatic tests, uniaxial compression tests, and nanoindentation tests suggest that shotcrete elasticity and strength can be reasonably predicted from mixture- and hydration-independent elastic properties of aggregates, clinker, hydrates, water, and air, and from strength properties of hydrates. At the structural level, the micromechanics model, when combined with 3D displacement measurements, predicts that a decrease of the water–cement ratio increases the safety of the shotcrete tunnel shell.

The microstructure of dispersed and non-dispersed fresh cement pastes — New insight by cryo-microscopy

This study aims to give new insights to the microstructural development of fresh cement pastes without and with polycarboxylate-type (PCE) of superplasticizers. Qualitative comparisons of the particulate structure of such cementitious systems were carried out by using cryo-FIB and cryo-SEM techniques. The natural structure of fresh cement pastes was preserved by high pressure freezing. It is illustrated that non-dispersed cement systems tend to form hydration rims on the surface of the clinker grains immediately after mixing. This leads to an interlocking of the particles. In contrast, in the dispersed systems the early hydrates precipitate in the pore solution and thus, agglomeration is prevented. These microstructural observations are important aspects of the fluidization effect of superplasticizers.

Microstructure of 5-year-old mortars containing limestone filler damaged by thaumasite

This paper presents the findings of a long-term study on the microstructure of Portland cement mortar specimens containing 5%, 15% and 35% limestone filler, as cement replacement, after exposure to a solution of magnesium sulfate at a concentration of 1.44% SO 4, for 5 years at 5 °C. The findings are compared to results reported earlier, obtained from the same systems but after 1-year exposure. It was found that the deterioration due to thaumasite advanced with the increased exposure period and limestone content. Thaumasite solid solutions (Tss) formed as the dominant phases within the deteriorated cement matrix and at the paste-aggregate interface resulting in cracks and delamination. Thaumasite was also found as an inner product in various clinker grains. Interestingly, the control specimens, with no limestone filler, were found to exhibit cracks due to the formation of Tss, with atmospheric carbon dioxide being the most likely source for the carbonates.

Investigation of the microstructure and carbonation of CS¯A-based concretes removed from service

The microstructure, mineralogy and depth of carbonation of two concrete samples, one removed from a normal strength crane column and the other from a high-strength pile, are reported. The normal strength CS¯A cement concrete had a high w/c ratio; microstructural images show that clinker tends to hydrate almost completely. But for high-strength CS¯A cement concretes, made with low w/c ratios, large amounts of partially hydrated clinker grains remain as a microaggregate. CS¯A cements and concretes are subject to carbonation in service conditions. The usual method of determining depth of carbonation, the phenolphthalein test, does not work with aged CS¯A matrices. A new method, using infrared microscopy, has been used to determine carbonation depth of aging CS¯A cement concrete. It has been shown that carbonation of a normal strength CS¯A cement concrete exposed to open air for 16 years averages ∼0.5 mm/year, and is thus comparable with reported rates of carbonation of OPC concretes. The high-strength CS¯A concrete carbonated at a maximum rate of ∼60 μm/year.

Modelling the effects of waste components on cement hydration

Ordinary Portland Cement (OPC) is often used for the solidification/stabilization (S/S) of waste containing heavy metals and salts. These waste components will precipitate in the form of insoluble compounds on to unreacted cement clinker grains preventing further hydration. In this study the long term effects of the presence of contaminants in solidified waste is examined by numerically simulating cement hydration after precipitation of metal salts on the surface of cement grains. A cement hydration model was extended in order to describe pore water composition and the effects of cement grain coating. Calculations were made and the strength development predicted by the model was found to agree qualitatively with experimental results found in literature. The complete model is useful in predicting the strength and leaching resistance of solidified products and developing solidification recipes based on cement.

Hydrating cement pastes as a complex disordered system

In small-angle neutron scattering (SANS) experiments, realized on the MURN facility of the pulsed reactor IBR-2 of the Frank Laboratory of Neutron Physics at the Joint Institute for Nuclear Research, Dubna, the hydration processes in samples of ordinary Portland cement (OPC) and single clinker minerals are studied. The measured scattering curve contains information about the fractal behaviour of the interfaces and the size distribution of the scattering particles. Furthermore, a variation of the heavy and light water composition for the hydration water supports the selection of the observable microstructural objects. In dependence on the size distribution of the clinker grains a various time-dependent behaviour of the potential law of the scattering curve is shown. Considering the SANS results of hydrating OPC the exponents of the scattering curve in a given Q-range are varying in dependence on the hydration time and sample thickness. They lie in an interval from about −2 to −4. This is believed to be associated with fractal behaviour. A set of four hydrating C3S-samples is divided into 2 parts after an under water storage of 53 days. Then 2 samples were stored in an H2O/D2O-mixture for reducing the variety of the several hydration products by changing the neutron optical contrast. Considering the time-dependent change of the potential law of the scattering curves of hydrating C3S-samples some differences in contrast to hydrating OPC powder are visible. Within about 100 days after mixing the dry C3S powder with water the exponents of the SANS curves in the measured Q-range are higher than −3. If the hydration products of C3S are forming fractal structures then volume or mass fractals of some nanometers are shown.

A Re-Evaluation of Hardened Cement Paste Microstructure Based on Backscatter Sem Investigations

AbstractBackscattered electron imaging of polished cement paste specimens permits a re-evaluation of structural details of hydrated cement paste at the gim level. The primarily microstructural units comprise a highly porous groundmass and large distinct grains (“phenograins”) set in it. The groundmass is composed of several kinds of fine particles, with a significant content of easily detected gross pores. Phenograins are primarily large clinker grains hydrating in-situ, but may be distinct deposits of CH, or may be mineral admixture grains. Detailed EDS analyses indicated that hydrating cement in phenograins has a highly consistent composition, interpreted as C-S-H with a small but regular incorporation of sub-jim CH and calcium monosulfoaluminate. Groundmass particles are highly variable in composition, but appear to consist of C-S-H with variable and occasionally major contents of other hydration products on a sub-μm scale. Incorporation of fly ash does not appear to change the basic microstructure, but silica fume incorporated with superplasticizer drastically modifies the character of the groundmass. Some attempts at quantification of these features by application of image analysis are briefly described.

Delayed ettringite formation: a microstructural and microanalytical study

Cement pastes were cured at 80°C and examined by scanning electron microscopy, X-ray microanalysis and X-ray diffraction immediately or after storage in water for various periods at 20°C. Ettringite began to form a few days after the heat treatment, as very small crystals thinly dispersed throughout the paste in close admixture with C-S-H. It subsequently recrystallized in cavities typically 5-10μm in size formed by dissolution of clinker grains or in other ways. There were no indications that this latter process disrupted the surrounding material. The results favour the view that the expansion that can be associated with delayed ettringite formation is driven by processes occurring within the paste and not by the formation of ettringite at aggregate interfaces.

Alteration of polished sections of free lime containing cement clinker by short-term atmospheric exposure

It was observed that clusters of free lime particles exposed on the surface of clinker grains embedded in epoxy, polished, and palladium coated in preparation for scanning electron microscopy underwent severe alteration following a few hours of exposure to laboratory air (60% RH). The visible signs of alteration included extensive swelling, cross-banding, and crack formation. The popcorn-like morphology of the altered grains resembled that previously described for “epezite”, an unusual form of calcium hydroxide reported to be formed by air slaking hydration of free lime in clinker. Epezite is said to have optical properties different from those of portlandite, and thus may constitute more than a morphological variant. Alteration does not take place for specimens kept in evacuated desiccators, but takes place very reproducibly on exposure to laboratory air. The easily recognizable alteration phenomenon might be deliberately induced to serve as a rapid check for the existence and extent of free lime in a given clinker.

Immobilization mechanisms in solidifiction/stabilization of cadmium and lead salts using portland cement fixing agents

The authors have investigated the behavior of Cd and Pb salts toward cement-based solidification using TCLP leaching tests, conduction calorimetry, and solid-state NMR as a function of time. Concentrations of Cd in leachates are very low, while Pb concentrations are considerably higher and would represent a serious threat to groundwater. The Cd/cement system involves Cd(OH){sub 2}, while provides nucleation sites for precipitation of calcium silicate hydrate (C-S-H) gel and calcium hydroxide, resulting in Cd being in the form of the insoluble hydroxide with a very impervious coating. On the other hand, the Pb/cement system involves hydroxide, sulfate, and nitrate mixed salts, which retard cement hydration reactions by forming an impervious coating around cement clinker grains. However, as pH in the cement pore waters undergoes fluctuations during the progress of hydration, the Pb salts undergo solubilization and reprecipitation on leachable surfaces of the cement matrix.

NMR studies of hydrating cement: A spin-spin relaxation study of the early hydration stage

The early stages of cement hydration were investigated by proton spin-spin relaxation time (T c) measurements because these experiments can be performed much faster (in several seconds) than the characteristic time for a change in hydration evolution. In addition, comparing the free induction decays of nuclear magnetization and its spin echoes allows the separation of solid-like and liquid-like proton groups. The values of T 2 and the associated magnetization fractions were monitored for cement pastes with different water/cement ratios in the first 20 hours of hydration. Results show that already at the time of the first measurement, i.e. six minutes after mixing dry white cement powder with water, a quasi steady state is reached with protons distributed in three groups: loosely bound water (in the gel coatings and interstital spaces between coated clinker grains with T 2 of several ms), tightly bound water (T 2 ∼ 100 μs) and water/hydroxyl protons in crystalline structure (T 2 ∼ 10 μs). The values of T 2 and their associated magnetizations are fairly constant during the first several hours with the solid-like components only slowly growing at the expense of the liquid one. The fraction of the loosely bound water as well as the absolute value of its T 2 increases with increasing w/c ratio. These data indicate that although nothing vigorous is happening in the dormant period of cement hydration there is a continuous growth of the solid matrix.

Microstructure and Fracture At the Cement Paste-Aggregate Interface

ABSTRACTThe mechanical behavior of concrete is thought to be affected by the microstructure of the paste-aggregate interface, which differs in several respects from the microstructure of bulk paste. General aspects of interfacial microstructure, which appear to be fairly well understood, are reviewed. Recent microstructural studies using backscattered electron imaging show that there is a zone along the interface (approximately 50 μm wide) within which there are generally few large unhydrated clinker grains, many large voids (greater than 5 μm), a generally porous microstructure, many hollow-shell hydration grains, and in some cases little calcium hydroxide. The tendency for cracks to develop or grow along the paste-aggregate interface is discussed in relation to specific features of the interfacial microstructure.

The Microstructure of High Strength Cement Pastes

AbstractThe microstructures of high strength pastes of OPC and high alumina cements prepared by the high shear mixing of a low water/cement ratio paste with water soluble polymers have been studied by transmission electron microscopy. In the case of high strength OPC, the usual hydration products are present, however, the CSH gel lacks the fibrillar morphology often observed in conventional cements. Pastes based on high alumina cement do not contain the normal crystalline calcium aluminate hydrates but a small quantity of gel containing the organic polymer forms a continuous network structure bonding clinker grains. Microanalysis of the polymer phase in high alumina cement revealed the presence of Ca and Al while Ca-rich interstitial gel material was found in OPC pastes.