Anhydrous Cement Grains Research Articles

Implementation Variants of the Lineal Path Function Applied to Hardened Cement Paste Microstructure

The microstructure of hardened cement pastes comprises of a heterogeneous agglomeration of distinct quasihomogeneous domains with variable physical, chemical, and morphological features, denoted here as material phases. Accurate material characterization rests on a precise description and quantification of underlying principal phases, focusing, in particular, on their volumetric proportions and spatial configuration within the microstructure, as these affect, to a large extent, the macroscopic properties of the composite material. A realistic cement paste microstructure used in this study was obtained from micro-computed X-ray tomography, following the application of suitable segmentation filters, highlighting and isolating the sought phase – anhydrous cement grains – for statistical analysis. The present paper then compares and assesses several implementations of a lineal path function, all applied to quantifying the phase connectedness and short-range order characteristics of the grains. The main emphasis rests on assessing the accuracy and evaluation speed of the implemented algorithms.

Hydration and microstructural development of calcined clay cement paste in the presence of calcium-silicate-hydrate (C–S–H) seed

The microstructure of Portland and calcined clay cement paste incorporating calcium-silicate-hydrate (C–S–H) seed was investigated to understand the role of nucleation seed in cement hydration, and to study the potential synergy between C–S–H seed and calcined clay. The development of compressive strength of samples with and without seed was monitored until 28 days of hydration. Microstructural analysis techniques including thermogravimetric analysis (TGA), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and quantitative X-ray diffraction (QXRD) were used to study the phase assemblage and composition of the cement paste specimens. Image analysis (IA) of backscattered electron (BSE) micrographs was used to determine the hydration degree of cement at the age of 1 day and 28 days. The results showed that the introduction of C–S–H seed clearly accelerated the hydration of cement and enhanced the hydration degree of cement even at the later age. The presence of seed also modified the composition of the inner products (IP) formed around the anhydrous cement grains. The combination of calcined clay and C–S–H seed enabled high replacement levels (up to 40 wt%) of cement without compromising its mechanical properties.

Century-long expansion of hydrating cement counteracting concrete shrinkage due to humidity drop from selfdesiccation or external drying

A physically based model for auotgenous shrinkage and swelling of portland cement paste is necessary for computation of long-time hydgrothermal effects in concrete structures. The goal is to propose such a model. As known since 1887, the volume of cement hydration products is slightly smaller than the original volume of cement and water (chemical shrinkage). Nevertheless, this does not imply that the hydration reaction results in contraction of the concrete and cement paste. According to the authors’ recently proposed paradigm, the opposite is true for the entire lifetime of porous cement paste as a whole. The hydration process causes permanent volume expansion of the porous cement paste as a whole, due to the growth of C–S–H shells around anhydrous cement grains which pushes the neighbors apart, while the volume reduction of hydration products contributes to porosity. Additional expansion can happen due to the growth of ettringite and portlandite crystals. On the material scale, the expansion always dominates over the contraction, i.e., the hydration per se is, in the bulk, always and permanently expansive, while the source of all of the observed shrinkage, both autogenous and drying, is the compressive elastic or viscoelastic strain in the solid skeleton caused by a decrease of chemical potential of pore water, along with the associated decrease in pore relative humidity. As a result, the selfdesiccation, shrinkage and swelling can all be predicted from one and the same unified model, in which, furthermore, the low-density and high-density C–S–H are distinguished. A new thermodynamic formulation of unsaturated poromechanics with capillarity and adsorption is presented. The recently formulated local continuum model for calculating the evolution of hydration degree and a new formulation of nonlinear desorption isotherm are important for realistic and efficient finite element analysis of shrinkage and swelling. Comparisons with the existing relevant experimental evidence validate the proposed model.

Microstructural aspects in steel fiber reinforced acrylic emulsion polymer modified concrete

Scanning electron microscope observations of polymer-free and polymer-modified cements have shown that the polymer particles are partitioned between the inside of hydrates and the surface of anhydrous cement grains. For optimum dosage of acrylic emulsion polymer with 2.5%, the C-S-H gel in this structure is finer and more acicular. Some polymer adheres or deposit on the surface of the C-S-H gel. The presence of acrylic emulsion polymer confines the ionic diffusion so that the Ca(OH)2 crystallized locally to form fine crystals. The void in the structures seems to be smaller but no polymer films appears to be bridging the walls of pores although many polymer bonds or C-S-H spread into the pore spaces. In addition to porosity reduction, acrylic emulsion polymer modified the hydration products in the steel fiber –matrix ITZ. The hydration product C-S-H appeared as a needle like shape. The needle-shaped C-S-H increases and gradually formed the gel, with needles growing into the pore space. The phenomenon is more obvious as curing age increased.

Cement hydration from hours to centuries controlled by diffusion through barrier shells of C-S-H

Although a few good models for cement hydration exist, they have some limitations. Some do not take into account the complete range of variation of pore relative humidity and temperature, and apply over durations limited from up a few months to up to about a year. The ones that are applicable for long durations are either computationally too intensive for use in finite element programs or predict the hydration to terminate after few months. However, recent tests of autogenous shrinkage and swelling in water imply that the hydration may continue, at decaying rate, for decades, provided that a not too low relative pore humidity (above 0.7) persists for a long time, as expected for the cores of thick concrete structural members. Therefore, and because design lifetimes of over hundred years are required for large concrete structures, a new hydration model for a hundred year lifespan and beyond is developed. The new model considers that, after the first day of hydration, the remnants of anhydrous cement grains, gradually consumed by hydration, are enveloped by contiguous, gradually thickening, spherical barrier shells of calcium-silicate hydrate (C-S-H). The hydration progress is controlled by transport of water from capillary pores through the barrier shells toward the interface with anhydrous cement. The transport is driven by a difference of humidity, defined by equivalence with the difference in chemical potential of water. Although, during the period of 4–24h, the C-S-H forms discontinuous nano-globules around the cement grain, an equivalent barrier shell control was formulated for this period, too, for ease and effectiveness of calculation. The entire model is calibrated and validated by published test data on the evolution of hydration degree for various cement types, particle size distributions, water-cement ratios and temperatures. Computationally, this model is sufficiently effective for calculating the evolution of hydration degree (or aging) at every integration point of every finite element in a large structure.

Micromechanical modelling of deformation and fracture of hydrating cement paste using X-ray computed tomography characterisation

Cement paste is the basic but most complex component in cement composites, which are the dominant construction material in the world. Understanding and predicting elastic properties and fracture of hydrating cement paste are challenging tasks due to its complex microstructure, but important for durability assessments and life extension decisions. A recently proposed microstructure-informed site-bond model with elastic-brittle spring bundles is developed further to predict the evolution of elastic properties and fracture behaviour of cement paste. It is based on microstructural characteristics of hydrating cement paste obtained from X-ray computed microtomography (micro-CT) with a spatial resolution of 0.5μm/voxel. Volume fraction and size distribution of anhydrous cement grains are used to determine the model length scale and pore-less elasticity. Porosity and pore size distribution are used for tuning elastic and failure properties of individual bonds. The fracture process is simulated by consecutive removal of bonds subjected to surface energy based failure criterion. The stress–strain response and elastic properties of hardened cement pastes with curing ages of 1, 7 and 28 days are obtained. The simulated Young's modulus and deformation response prior to peak stress agree very well with the experimental data. The proposed model provides an effective tool to evaluate time evolution of elastic properties and to simulate the initiation, propagation, coalescence and localisation of micro-cracks.

Microstructure-informed modelling of damage evolution in cement paste

Cement paste is a binder for cementitious materials and plays a critical role in their engineering-scale properties. Understanding fracture processes in such materials requires knowledge of damage evolution in cement paste. A site-bond model with elastic-brittle spring bundles is developed here for analysis of the mechanical behaviour of cement paste. It incorporates key microstructure information obtained from high resolution micro-CT. Volume fraction and size distribution of anhydrous cement grains are used for calculating model length scale and elasticity. Porosity and pore size distribution are used to allocate local failure energies. Macroscopic damage emerges from the generation of micro-crack population represented by bond removals. Effects of spatial distribution, porosity and sizes of pores on tensile strength and damage are investigated quantitatively. Results show a good agreement with experiment data, demonstrating that the proposed technology can predict mechanical and fracture behaviour of cementitious materials based exclusively on microstructure information.

A Lattice-spring Model for Damage Evolution in Cement Paste

To understand better the fracture processes in cement-based materials, it is essential to predict the evolution of damage in cement paste. A recently proposed site-bond model is developed further to take into account the key microstructure data, such as pore size distribution, porosity, and size distribution and volume fraction of anhydrous cement grains obtained from high resolution X- ray tomography. The grains are associated with lattice sites linked by deformable bonds. The bonds are bundles of elastic-brittle springs, resisting normal and shear relative displacements between grains with potential for failure. The model length scale and thence spring constants are controlled by grain statistics. The spring failure properties are controlled by pore statistics. Macroscopic damage develops by a succession of local failures, represented by spring removal. The model is used to simulate the stress-strain response and damage in cement paste under uniaxial tensile loading. The influence of porosity on tensile strength and damage evolution is estimated in a quantitative manner. The predictions of the model are in a very good agreement with the available experimental data.

3D experimental investigation of the microstructure of cement pastes using synchrotron X-ray microtomography (μCT)

Cement pastes aged from 1 to 60 days were studied using synchrotron microtomography on the MS-X04SA beam line at the Swiss Light Source. This allowed three dimensional images to be obtained with a resolution approaching that of backscattered electron images in the SEM. From these images, several features can be extracted and studied, both quantitatively and morphologically. In this study, attention was focused on the reacting anhydrous cement grains and porosity. Three dimensional imaging of capillary porosity allowed the connectivity and tortuosity of the pore network to be studied. It is shown that the degree of connectivity of the pore network is very sensitive to both the spatial resolution of the images and the evolution of contrast resolution during ageing of the cement.

Durability of reactive powder composites: influence of silica fume on the leaching properties of very low water/binder pastes

Reactive Powder Composites are new cement-based materials which could be used for the storage of nuclear wastes thanks to their excellent microstructural properties. This paper studies their durability when submitted in the laboratory to a water leaching attack. In order to understand the behaviour of the hydrates (CSH), the study was carried out on a pure cement paste and on a cement + silica fume paste. The beneficial effect of silica fume is demonstrated from considerations on the calcium leaching, from XRD analysis, from SEM observations and from the tritium diffusion and pore distribution analysis. It was found that the leaching greatly affects the microstructure, especially that of the anhydrous cement grains remaining in the paste.

Microstructural aspects in a polymer-modified cement

Scanning electron microscopic observations of polymer-free and polymer-modified cements have shown that the polymer particles are partitioned between the inside of hydrates and the surface of anhydrous cement grains. Differential thermal analysis, thermogravimetric analysis, and porosimetry experiments show the retardant effect of the polymer and provide information on changes in porosity.

The Leaching Of The Reactive Powder Concretes: Results On Transfer Properties

ABSTRACTThe properties of concrete can be altered in aggressive media such as pure water. A leaching test was performed for Reactive Powder Concretes (RPC) in order to evaluate their durability when used as storage of nuclear wastes. Emphasis is placed on the changes of their microstructure, on the tritium diffusion coefficient and on the porosity after leaching. Using mainly non destructive techniques such as SEM, BET and diffusion cell, we found that the leaching affects the RPC microstructure, particularly the anhydrous cement grains. The tritium diffusion coefficient, even lower than that of granite, is however ten times higher after a three month-leaching.

The microstructure of steam-cured precast concrete

This work reports on the microstructure of concrete cubes manufactured at 16°C and steam-cured at 42°C, 60°C and 80°C. Increase in curing temperature leads to the development of distinct hydrate rims around both anhydrous cement grains and Hadley grains. In concrete cured at or above 60°C, 20%-30% of the C-S-H rims had a dark inner zone and light outer shell which were not present at room temperature, and up to 80% of the Hadley grains had distinct hydrate product rims, many being partially or completely in filled with either ettringite or monosulphate-type hydrates. As curing temperature increases the proportion of ettringite-type hydrates decreases, while those associated with monosulphate become more predominant. Sulphate concentrations were about twice as large in the light outer C-S-H as in the dark inner C-S-H for specimens cured at 80°C and, within the hydrate rims of the Hadley grains, the sulphate content at 80°C was about twice that observed at 60°C.

Effect of Composition on Basic Creep of Concrete and Cement Paste

A two-level composite model for predicting the basic creep of aging concrete from its composition and the properties of its constituents is proposed. On the macroscale, concrete is treated as a composite of elastic aggregate embedded in the matrix of creeping hardened cement paste. The composite action is described by a combined series-parallel model in which a portion of the paste acts in parallel with the aggregate and the remaining portion in series with this parallel coupling. The portion of the paste coupled in parallel is determined as the amount of paste needed to fill the voids when the aggregate is at its maximum possible compactness, and the remaining portion of the paste then corresponds to the series coupling. On the microscale, the hardened cement paste is considered as a composite of elastic anhydrous cement grains embedded in a matrix of cement gel with voids filled by water and air. The aging is considered by an extension of the previously proposed solidification theory, in which the creeping constituent, the gel, is considered to have nonaging viscoelastic properties, and the aging caused by the chemical reaction of cement hydration is totally ascribed to the volume growth of the load-bearing (bonded) portion of hardened cement gel. The model is calibrated and verified by means of a comprehensive data set reported by Ward, Neville and Singh.

A quantitative structural study of fresh cement paste by image analysis: Part 2 : Measurements and their application

This paper is the second part of a study of cement paste structure by image analysis. Solid concentration and granulometric measurement have been carried out on the original binary image, then on the same image whose morphology had been modified. These results revealed anhydrous cement grains dispersed in the fresh paste. The effect of concentration and the state of the solid phase can influence this dispersion and we suggest that our method can be used to investigate both these phenomena.

A quantitative structural study of fresh cement paste by image analysis part 1: Image processing

Flocculation and deflocculation have a fundamental role to play in the use of mortar and concrete. In order to monitor these phenomena more fully, we need to have a better understanding of the structure of fresh cement paste. The work presented in this publication is part of the process of meeting this need. By applying a technique of cryosublimation together with epoxy resin impregnation to fresh cement paste samples, we can obtain SEM images of test-piece cross-sections where the initial arrangement of anhydrous cement grains is modified neither by hydration nor by experimental procedure. Paste structure can thus be investigated by quantitative image analysis. By using a new method of image processing during filtering and binarization, we can obtain a binary image which faithfully reproduces the original image and on which we can then carry out geometrical parameter measurements.

Portland Cement: Pseudomorphs of Original Cement Grains Observed in Hardened Pastes

Scanning electron micrographs show that a hydration product forms inside the anhydrous cement grains and forms a pseudomorph of the original cement grain. The originally water-filled space between the grains is partially filled with calcium hydroxide crystals that appear to be responsible for the strength of the cement paste.