Autogenous deformations of cement pastes (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.
Cement and Concrete Research, 2004
A semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH) 2 content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH) 2 amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 jC, respectively. Hydration degree and Ca(OH) 2 content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method.
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.
Time-dependent load-induced deformation of Ca(OH)2
Advances in Cement Research, 2002
Short-term time-dependent strains were monitored on calcium hydroxide compacts having a porosity of 19•7%. The creep experiments were conducted in a controlled environment chamber maintained dry (0% RH) at 22 ± 2°C with nitrogen purging. Prior to loading, the compact specimens were vacuum dried at 105°e for 3 h (equivalent to D-drying). The applied stress for the creep test corref>ponds to 5% of the compaction pressure (386 MPa). The Ca(OHh compact specimens were in the form of T-shaped columns with a minimum thickness value (for the web and flanges) less than 1•2 mm. The AC impedance spectra of loaded and unloaded Ca(OH)2 compact specimens were also monitored in real time. This was achieved by the coupling of the impedance analyser to the creep experiment. An assessment of the relevance of the high frequency arc depression angle obtained from the impedance analysis to an understanding of the load-induced deformation behaviour of the Ca(OH)2 compacts was made.
Autogenous shrinkage of concrete: a balance between autogenous swelling and self-desiccation
Cement and concrete research, 2005
According to physical analyses, the driving force of autogenous shrinkage of concrete is the change in the capillary pressure induced by self-desiccation in its cement matrix. Self-desiccation is caused by the balance between the absolute volume reduction (chemical shrinkage) and the building up of the capillary network. The aim of this study was to quantify the influence of the cement characteristics on the chain of mechanisms leading from hydration to autogenous deformations. Four parameters were selected: (i) for clinker, the amount of C 3 A and free lime and the SO 3 /K 2 O ratio; (ii) for cement, the fineness. To master the experimental area, 16 cements were prepared at the laboratory from pure raw materials. An important number of characterizing techniques were used in the experimental study. Their choice was based on the important parameters drawn from the physical analysis: setting time, suspension-solid transition, hydration kinetics through isothermal calorimetry and nonevaporable water, chemical shrinkage, evolution of relative humidity, capillary porosity and autogenous shrinkage. Using different techniques allowed to determine the precise mechanism of action of each parameter. Results showed that these mechanisms are generally different, even if their macroscopic consequences may be identical. This point will probably be useful for modeling and determining the industrial keys reducing the autogenous shrinkage. The physical mechanisms involved in autogenous deformations were further understood. In particular, this study shows that initial autogenous shrinkage should be considered as a balance between the self-desiccation and an initial swelling phase. The influence of the four parameters considered on this last phenomenon were also characterized.
Growth of Calcium Hydroxide Islands in Tricalcium Silicate-Based Cements at Early Age
Journal of the American Ceramic Society, 2012
Microstructural characteristics of hydrated triclinic tricalcium silicate [C 3 S(t)], monoclinic tricalcium silicate (alite), and type I portland cement at ages between time zero and 28 days were observed using electron microscopy. Image analysis was used to follow the development of calcium hydroxide (Ca(OH) 2 ) and calcium silicate hydrate at micro-and meso-length scales. In all cases, easily recognizable islands of Ca(OH) 2 were noted to form, though the morphology and rate of formation differed for the three cement types. The formation of these islands was found to influence hydration by hindering the reaction of unhydrated cement particles that are embedded within their largely Ca(OH) 2 -bearing matrix. Such may be important features to include and validate in developing microstructural and other forms of computational models. The extent to which such microstructural features influence the local rates of reaction seems to depend, at least in part, upon the crystallography of the C 3 S.
Physico-chemical deformations of solidifying cementitious systems: multiscale modelling
Materials and Structures, 2010
At early stages of hydration and in autogenous conditions (no mass transfer with the outside), solidifying cementitious systems exhibit dimensional variations following two main processes: Le Chatelier contraction (also called chemical shrinkage) and self-desiccation shrinkage causing autogenous shrinkage. Chemical shrinkage results from the difference between the specific volumes of reactants (anhydrous cement and water) and hydration products. Early-age autogenous shrinkage
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.
Construction and Building Materials, 2018
h i g h l i g h t s Crack in concrete structures significantly increases ion diffusivity and permeability inside the material and allows chloride ions. When the aqueous environment, self-healing can occur where a part of the crack is filled by rehydration of the cement particles and precipitation of CaCO 3. Change in the crystal forms of CaCO 3 generated in the hardened cement paste through pH adjustment were monitored for the generation of denser CaCO 3 crystals. The pH conditions enabling the generation of the denser vaterite CaCO 3 are presented herein.
HARDENING OF CALCIUM HYDROXIDE AND CALCIUM SILICATE BINDERS DUE TO HYDRATION AND CARBONATION
Hardening of calcium hydroxide and calcium silicate binders composed of cement, rice husk ash (RHA) and lime in different compositions were studied with mechanical strength, mercury intrusion porosimetry, thermal analysis and SEM. When cement is partially replaced with RHA and lime, hardening occurs as a result of combined hydration, pozzolanic reaction and carbonation reaction. While hydration of cement contributes to the early strength development of the mortars, carbonation is much more pronounced at later stage with the decrease in the cement content in the mortar and the increase in the porosity of the mortars. RHA-cement mortars indicated a long-term strength development, which is lower than that of the reference cement mortar. This was attributed to the high water demand of the blended mortars due to the porous RHA grains, which resulted in an increase in their porosity. Strength reduction was recorded at the very early stage for RHA-cement-lime mortars containing 10%-wt ceme...