Influence of Cement Particle-Size Distribution on Early Age Autogenous Strains and Stresses in Cement-Based Materials (original) (raw)
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HAL (Le Centre pour la Communication Scientifique Directe), 2005
A micro-macro experimental study has been performed, from the end of mixing up to several months, on a set of plain cement pastes prepared with the same type 1 ordinary Portland cement (OPC) and various water-to-cement ratios (W /C), and cured at various constant temperatures. Chemical shrinkage, volumetric and one-dimensional autogenous deformations have been measured and analyzed in relation to the hydration process (degree of hydration of the cement a, Ca(OH)2 content, ...) and to the microstructural characteristics of the material. The effects of the curing temperature at early age (<24 hours) in the range 10-50°C, and ofW/C in the range 0.25-0.60, have been investigated. The temperature-induced changes recorded on both the magnitude and the kinetics of volumetric autogenous shrinkage clearly show the irrelevance of using the usual maturity concept to describe such phenomena within the whole early-age period. In addition, a threshold is pointed out at about a = 7%, both defming the range where autogenous shrinkage is linearly related to a and corresponding to the precipitation of Ca(OH)2• Moreover, a W/C threshold is pointed out both at the macro-level (autogenous deformations, ...) and at the micro-level (characteristics of the hydration products, MIP porosity and pore size distribution, ...). The chemical and (micro)structural basic effects of calcium hydroxide are in particular distinguished.
Mathematical Models for Assessing Hydration and Microstructure of Cement Pastes
1997
The durability of concrete is intrinsically related to the properties of the cement paste present in a concrete element. Properties such as permeability and diffusion depend on the microstructural characteristics of hydrated cement. These characteristics are influenced by the mixing and curing conditions, particularly the w/c ratio, the curing temperature and the relative humidity, and the degree of hydration. The study of cement microstructural parameters such as total porosity, pore size distribution and nature and amount of hydrated products as well as the engineering properties such as permeability, absorption and diffusion have been the main concern of researchers who intended to improve the durability of concrete. The current literature indicates that new experimental techniques and methods have been developed for the characterisation of the cement microstructure. This research presents a study of the characterisation of hydrated cement. The hydrated cement products and porosity of the pastes are quantified with the object of developing quantitative models to predict the degree of hydration with relation to the age of the cement paste. These predictive models will form the basis for predicting properties related to the performance and durability of concrete. The hydrated cement products were quantified with backscattered imaging and the degree of hydration was obtained from this study. A new method to calculate the degree of hydration using backscattered imaging is proposed. The degree of hydration obtained with backscattered imaging is comparable to the degree of hydration obtained with traditional techniques such as thermogravimetric analysis and specific gravity measurements. A model to predict the degree of hydration was developed in this research work. This model uses modified conceptual equations based on the classical theory of nucleation and diffusion growth which have been used to describe the development of compressive strength. Finally, the cement microstructure was characterised using the theory of Fractals. The morphology of the different products of hydration observed in cement microstructure was associated with a single number. This number is called the fractal dimension. These fractal dimensions were found to be statistcally correlated with the degree of hydration and thus could be used as characteristic parameters.
Early Age Autogenous Deformations of Cement-Based Materials
Proceedings of the 3 rd International Symposium GeoProc'2008, 2013
Autogenous shrinkage is considered to be one of the main phenomena involved in the early-age cracking of cementitious systems with low water-to-cement ratio. It results from the volume difference between the hydration products (hydrates) and the corresponding reactants (cement and water) and the self-desiccation of the porous network caused by the water consumption in the course of hydration. The objective of this paper is to model the autogenous shrinkage of a cement paste from the first contact between cement and water up to 2 days of hydration by means of a multi-scale homogenization model. The inputs of the model are the chemical composition and the Blaine fineness of cement, the mechanical properties of the main cement phases and the water-to-cement ratio (W/C). The outputs are the evolution of the cement phase volume fractions and chemical hydration reactions, the Young's modulus, the capillary tension, the chemical and the autogenous shrinkage. Numerical and experimental results are analysed in order to validate the model and propose improvements.
Influence of mix composition on early-age autogenous deformations of cement pastes
International RILEM Conference on Volume Changes of Hardening Concrete: Testing and Mitigation, 2006
The influence of the mix composition on the early-age deformations of the cement paste matrix of high-strength and self-compacting concretes has been investigated. Linear deformation measurements were started prior to setting. Setting (distinction between plastic and stress inducing shrinkage) was determined by analysis of deformation velocity. The measured deformations were linked to the hydration of the binders and to the structural properties of the hardening pastes. The effect of mineral and organic additives on the cement hydration was systematically assessed with isothermal calorimetry and thermogravimetric analysis. Based on hydration data the capillary tension (as the presumed main driving force of autogenous shrinkage) was estimated using pore structure data determined by mercury intrusion porosimetry. From the results conclusions were drawn on the deformation behaviour of the cement pastes.
The effects of particle size distribution and surface area upon cement strength development
Powder Technology, 2009
Particle size distribution, uniformity of the distribution and specific surface area (SSA) have a great influence on service properties of cement, particularly on strength. In this paper the effects of these physical parameters on strength development were studied using PC 42.5 R. In order to understand the significance of different particle size ranges in a distribution, samples having size distributions such as −10 µm, −20 µm, − 30 µm, −45 µm, −32 + 3 µm and −20 + 5 µm were prepared from PC 42.5 R by using a laboratory scale 3rd generation separator. Additionally −32 + 3 µm and −20 + 5 µm fractions were added to the original PC 42.5 R in varying amounts to study SSA and uniformity effects. Same strength values were obtained for samples with a narrower size distribution but smaller SSA. Fineness is very important for strength development, particularly in the early stages of hydration.
Materials and Structures, 2016
When cementitious materials are dried, internal stresses are generated that lead to desiccation shrinkage, a portion of which is irreversible. Previous research has indicated that, while a cementitious composite is subjected to a state of stress, dissolution of cement grains and precipitation of hydrates can yield irreversible creep strains, and it is hypothesized that the same process can lead to irreversible shrinkage during drying. To evaluate this hypothesis, a computationally implemented model integrating a microstructural evolution model with a finite element calculation routine was utilized. This computationally implemented model is capable of predicting the magnitude of shrinkage deformation of cement paste during drying conditions as a result of cement grain dissolution and hydrate precipitation. From the simulation results, the mechanism of cement grain dissolution and hydrate precipitation can lead to significant shrinkage behavior of cement paste, and it is also a potential mechanism resulting in the irreversible component of desiccation shrinkage at early ages (e.g., while the hydration rate is significant). The predicted irreversible shrinkage decreases with the age at which drying is initiated as a result of the decreasing hydration reaction rate.
Prediction of elastic properties of cement pastes at early ages
Computational Materials Science, 2010
Cementitious materials are known to be sensitive to cracking at early ages. During the first days which follow the contact between water and cement, the system is continuously evolving, as its mechanical characteristics follow a rapid rate of change and the material is prone to cracking. One of the parameters that highly influence the behavior of the material at early ages is the Young's modulus. Analytical calculations, based on existing homogenization models and finite element calculations, applied on a discrete generated microstructure, are first considered in order to predict the elastic properties of the material. As long as the cohesive role played by the hydrates is not taken into account, results at early age remain inaccurate, especially for low watercement ratios. The need of modeling an intrinsic characteristic of cementitious materialssettingarises. An approach, based on percolation and on the so-called "burning" algorithm, which takes into account explicitly the bonding role of hydrates and reveals a degree of hydration threshold below which the rigidity of the material is negligible, is therefore proposed.
Early-Age Properties of Cement-Based Materials: II. Influence of Water-to-Cement Ratio
ASCE Journal of Materials in Civil Engineering, 2009
The influence of water-to-cement mass ratio (w/c) on early-age properties of cement-based materials is investigated using a variety of experimental techniques. Properties that are critical to the early-age performance of these materials are tested, including heat release, semi-adiabatic temperature, setting time, autogenous deformation, and strength development. Measurements of these properties using a single cement are presented for four different w/c, ranging from 0.325 to 0.425. Some of the measured properties are observed to vary widely within this range of w/c ratios. The heat release and setting time behaviors of cement pastes are contrasted. While early-age heat release is relatively independent of w/c, the measured setting times vary by several hours between the four w/c investigated in this study, indicating the fundamental differences between a physical process such as setting and heat release which is purely a quantification of chemical reaction. While decreasing w/c certainly increases compressive strength at equivalent ages, it also significantly increases autogenous shrinkage and may increase semi-adiabatic temperature rise, both of which can increase the propensity for early-age cracking in cement-based materials.