9th International Conference on Fracture Mechanics of Concrete and Concrete Structures STUDY OF SHRINKAGE RESTRAINT EFFECTS AT EARLY-AGE IN ALKALI- ACTIVATED SLAG MORTARS

Abstract: Alkali-activated materials are being increasingly studied nowadays as hydraulic binders. In order to be enrolled in different civil engineering applications, several properties must be characterized. This study focuses on their cracking risk by shrinkage restraints. The current paper summarizes the experimental and numerical results of a project assessing the development of early age properties of an alkali-activated slag mortar. First, an experimental campaign was held in order to quantify hydration heat release and hydration kinetics, to determine the time evolution of the Young modulus and the tensile strength and to measure free shrinkage strains, all in autogenous conditions. Second, two modelling approaches for mechanical properties and shrinkage development were compared. The results showed that the classical approach to characterize the hydration kinetics based on semi-adiabatic calorimetry results isn't suitable for the studied binder because of slow hydration and low heat release. Numerical work was finally conducted in order to predict stress development of a massive structure of alkali-activated slag mortar subjected to internal (self) and external strains' restraints (at mesoscopic and macroscopic scales)

Thermo-chemo-mechanical behavior of alkali-activated slag materials - Focus on early-age

International audienceThis study addresses the development of a modelling approach predicting the behavior of waste packages made with an alkali activated slag mortar. Durability of such structures is studied regarding the restriction of hydraulic binder's strains by the rigid inclusions and the wasteforms' container. The current work focuses on early-age behavior. First, the evolution of material's mechanical properties and delayed strains (autogenous shrinkage and basic creep) are determined experimentally. Results show that equivalent time approach should be used for simulations to address correctly the time-dependent evolution of mechanical parameters, rather than an approach based on hydration degree (determined by calorimetry). For basic creep, the use of an aging dashpot seems to be sufficient. Then, finite element calculations at meso-scale are performed to determine homogenized properties of binderwaste assembly. Results are used for macro-scale simulations

UNGG Waste Retrieval Comparison of general and galvanic corrosion of Magnesium alloy coupled to graphite in ordinary Portland cement and alkali-activated slag binders

International audienceGraphite and magnesium alloys wastes were generated during the reprocessing phase of spent fuel assemblies of the former nuclear reactors in France. Conditioning of these low to intermediate level wastes in cementitious materials is being addressed here. The study is aiming to develop a numerical model able to predict the generation of stresses in such waste packages during their lifetime. Magnesium is one of the most reactive metals with a standard potential of -2.37V/SHE [1]. Once embedded in a hydraulic binder with alkaline pH and high internal humidity, oxidation reactions occur. The subsequent formation of corrosion products around the alloy may result in tensile stresses development in the surrounding binder that could lead to cracking risks. Thus, general and galvanic corrosion (when coupled with graphite) of the metal in the packages should be properly addressed.Hence, weight losses, electrochemical techniques and microscopic observations together with chemical analysis methods (Raman and SEM/EDX) are used to characterize the metal's corrosion in three binders. These latter consist of two different ordinary Portland cement and an alkali-activated blast furnace slag mortars [2]. The results prove that the use of alkali-activated slag (AAS) is beneficial for the metal's galvanic corrosion while the general corrosion behavior is comparable in all studied mortars.Additionally, the electrical conductance of the hydraulic binders was determined using electrochemical impedance spectroscopy technique (EIS). These tests were performed on graphite and magnesium electrodes embedded in the binders, from 7 to 180 days of hydration. The analysis of the results, using an equivalent electrical circuit, showed that the electrical conductance of AAS binder was lower than ordinary Portland cement binders, at a given hydration age.In order to explain this lower conductance of the AAS binder, an investigation of its porosity (total porosity and porosity size distribution) and a characterization of the pore solution are undertaken. Therefore, on one hand, mercury intrusion porosimetry and BET nitrogen adsorption tests are performed on the different binders in order to compare the pore structure. On the other hand, pH measurements and liquid ion chromatography techniques are used to monitor the evolution of the AAS binder's pore solution. The latter results are compared to those for OPC binders found in the literature

Early-age cracking tendency of Alkali-activated slag binders compared to Ordinary Portland Cement

International audienceGround granulated blast furnace slag (GGBFS) is a by-product of steel manufacturing, increasingly used as an alkali-activated hydraulic binder. The substitution of Ordinary Portland Cement (OPC) based binders by this type of materials represents essentially an ecological advantage due to the reduction in CO2 emissions. Additionally, these materials constitute relevant alternative of OPC binders in some specific industrial applications [1] [2]. Hydration advancement and microstructural properties' evolution of alkali-activated slag materials have been extensively studied in the literature. However, a large dependency both on the chemical and physical properties of the activated GGBFS, and the type and concentration of the alkaline activator used, can be observed. Additionally, few studies have addressed the evolution of the mechanical properties of these binders and the early-age creep and shrinkage characteristics. Thus cracking tendency of this type of materials, especially at early ages when the volumetric changes of the hydraulic binders are the most important, needs to be investigated in order to study the durability of any structural application. In this context, the current paper describes, first, an experimental campaign comparing the early-age behavior of an alkali-activated slag mortar to that of an OPC mortar, then thermo-chemo-mechanical simulations allowing comparing the cracking tendency of both materials.The developed experimental campaign covers the delayed strains (autogenous shrinkage and creep) and the mechanical properties (mechanical strengths and Young's modulus) evolutions of the mortars. Results show that the alkali-activated mortar undergoes autogenous shrinkage strains higher then OPC mortar and showing an increase even at long term. However, its basic creep strains are more important than OPC mortar ones tested in the same conditions. This implies a higher capacity of stress relaxation for the alkali-activated slag mortar. Regarding the evolution of mechanical properties, Young modulus, compressive and tensile strengths of the alkali-activated mortar are lower of those of OPC based mortar at all ages. A simplified cracking index comparison applied at this stage of study shows comparable cracking risks of both materials at 7 and 28 days.The performed numerical simulations are performed by the means of a thermo-chemo-mechanical model developed based on the experimental study. The equivalent time approach model is adopted in order to describe the early-age evolution of the binders. Basic creep strains are expressed using a visco-elastic model combining a Kelvin-Voigt chain and dashpot [3]. This model is revisited to add a time-equivalent evolution for the ageing parameters. Finally, finite element calculations are performed on a bar fixed at both ends in order to compare the cracking tendency of the studied mortars

Caracterisation du comportement et modelisation d'un colis de dechets magnesiens immobilises dans un liant hydraulique

National audienc

General and galvanic corrosion of magnesium coupled to graphite embedded in hydraulic binders corrosion rate, rust layer properties and binder mechanical strains

International audienceGraphite and magnesium alloys wastes were generated during the reprocessing phase of spent fuel assemblies of the former nuclear reactors in France. Conditioning of these low to intermediate level wastes in cementitious materials is being addressed here. The study is held in the framework of a PhD thesis aiming to develop a numerical model able to predict the generation of stresses in such waste packages during their lifetime.In fact, magnesium is one of the most reactive metals with a standard normal potential of -2.37mV/SHE [1]. Once embedded in a hydraulic binder with alkaline pH and high internal humidity, oxidation reactions occur. The subsequent formation of corrosion products around the alloy may result in tensile stresses development in the surrounding binder that could lead to cracking risks. Thus, general and galvanic corrosion (when coupled with graphite) of the metal in the packages should be properly addressed.Hence, weight losses, electrochemical techniques and microscopic observations together with chemical analysis methods (Raman and Energy Dispersive X-ray Spectroscopies) are used to characterize the metal's corrosion in three binders. These latter consist of two different ordinary Portland cement and an alkali-activated blast furnace slag mortars [2]. Also, the mechanical properties of these products are determined using nano-indentation technique. The results prove that the use of alkali-activated slag is beneficial for the metal's galvanic corrosion while the general corrosion behavior is comparable in all studied mortars.Additionally, corrosion induced binders' strains are measured using a coupled experimental setup specially designed. Linear variable differential transformer (LVDT) sensors are used to monitor global strains of cementitious samples with normalized dimensions containing coupled magnesium and graphite electrodes. The potentiostatic electrochemical technique is applied in order to determine the evolution of the simultaneous corrosion current intensity in the sample

Study of shrinkage restraint effects at early-age in alkali-activated slag mortars

International audienc