Hydration and durability of fast hardening binders incorporating supplementary cementitious materials

Sarra El Housseini
EPFL, 2022
Thèse EPFL

Résumé

The use of supplementary cementitious materials (SCMs) is widely considered the most promis-ing approach to reduce CO2 emissions relative to cement production. SCMs are not commonly present in binders used for special applications. On the other hand, due to their low reactivity at early age, cements incorporating SCMs such as Limestone calcined clay cement (LC3) exhibit low early age strength when compared to PC. This could restrict their field of applications and therefore solutions have to be found.This thesis focused on two main axes: The first one is to lower the CO2 footprint of ternary blends composed of Portland cement (PC), calcium aluminate cement (CAC) and calcium sulfate (Cs) using calcined clay as clinker substitute. The hydration of simple systems composed of PC with an increas-ing amount of CAC and Cs with and without calcined clay from 3 hours up to 90 days of hydration helped to understand and identify the contribution of each component. The results showed that in-creasing the CAC and Cs content in a PC system with and without calcined clay led to an increase of the ettringite content and a decrease of the portlandite content due to the portlandite consumption to form ettringite. The C3S hydration was not delayed in presence of CAC and Cs. However, the pres-ence of CAC, Cs and calcined clay led to slightly lower degree of hydration especially at later ages. The hydration and strength of a more realistic system "a fixing mortar formulation" with and without calcined clay was investigated. Overall, results showed that calcined clay can replace Portland cement by up to 20% with acceptable strength results. The second aspect on which this thesis focused was the influence of incorporating CAC and Cs as limestone substitute in LC3. The effect of the C/A molar ratio of the used CAC and its content were the main parameters investigated. The incorporation of 10% (CAC, Cs) improved the strength com-pared to LC3 at all ages. The massive ettringite formation during the first hours of hydration explained the early age strength improvement. Results showed that the C3S hydration was hindered when the C/A ratio of the CAC is equal to 1.7. The calculation of saturation indices indicated that a super satu-ration with respect to portlandite is likely needed to trigger the C3S hydration. The high late age strength in LC3-CAC blends was explained by a high volume occupied by hydrates in comparison with LC3 blends. In terms of durability, the resistance to chloride ion penetration was investigated through the mini-migration method. LC3-CAC blends showed a better chloride resistance than LC3, which is mainly explained by the porosity refinement compared to PC, and the decrease of the total porosity in comparison with LC3. On the other hand, the natural carbonation study showed that LC3-CAC blends have a higher carbonation rate when compared to PC. Contrary to PC, accelerated car-bonation of LC3 and LC3-CAC blends resulted in a decrease of the strength. However, the strength of carbonated LC3-CAC specimens is still at least similar to the non-carbonated PC specimen.

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https://infoscience.epfl.ch/record/295392?ln=fr

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Sarra El Housseini
EPFL, 2022
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/295392?ln=fr

À propos de ce résultat

Concepts associés (13)

Concepts associés

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Publications associées (8)

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Resistance of cementitious materials to sulfate attack: quantifying performance with a reliable unidirectional approach

Qiao Wang

Depending on the environmental conditions, concrete materials can come into contact with sulfate ions which are widely present in rivers, underground water, sewers, seawater and soil. Sulfates can react with the cement paste in concrete and cause damage which impacts the service life of concrete structures. Cement chemistry, transport of sulfate ions, and physical properties of the host material all interact in the complex process of sulfate degradation. Given different conditions of concrete exposure, sulfate attack is commonly divided into "chemical" and "physical" sulfate attack. However, the processes are not fully understood and there is still a need for in-depth investigations of the whole deterioration process. The current standard testing method is adapted for Portland cement systems affected by the "chemical sulfate attack" in submerged conditions. However, only expansion is monitored, which is not suitable for characterizing the increasingly common usage of supplementary cementitious materials in the cement. Moreover, several key factors (e.g., RH & temperature) are usually overlooked so lab tests do not well represent the field conditions. Particularly, the understanding of the damaging mechanism of "physical sulfate attack" is very limited even if different testing approaches have been established. The division into "chemical" and "physical" attack is in itself misleading. In this context, this research project was launched to establish a unidirectional penetration approach for the in-depth comprehensive investigation of joint sulfate attack mechanisms. First, the feasibility of this newly established approach was studied for Portland cement mortars and pastes. A high concentration of sulfate solution and a high water-to-cement ratio was chosen to accelerate the degradation and to obtain results in a reasonable period. Results showed that this new concept of combining chemical and physical sulfate attack mechanisms into a single experiment was viable. Moreover, results showed that using cement paste samples can accelerate the degradation, compared to mortar samples. The sulfate degradation process was more rapid with the capillary rise and water evaporation. A comparison of the macroscopic manifestation of damage (visual appearance and expansion) with the underlying microscopic changes was made considering different key parameters, i.e., water-to-cement ratio, exposure solution concentration, cement type, temperature and humidity conditions (constant or cyclic). Regarding the different cement types, Ordinary Portland cement pastes and sulfate-resisting cement pastes showed higher expansion and cracking, whereas, spalling damage was more pronounced in blended-cement pastes/mortars (slag Portland cement and limestone calcined clay cement). Damage from the chemical attack was more extensive and rapid when high concentrations and cyclic exposure conditions were applied. However, lower water-to-cement ratios leading to lower porosity and/or higher mechanical strength could usually reduce the extent of the damage. Thermodynamic modelling was also conducted with GEMS to better explain the pore solution environment and the link between supersaturation and expansion. The interacting factors of porosity and strength were decoupled. The effect of the curing temperature on the expansion mechanism was also investigated in sulfate-resisting cement mortars under submerged sulfate exposure.

EPFL2022

Chargement

Nanoscale modelling of ionic transport in the porous C-S-H network

Khalil Ferjaoui

To reduce the CO2 footprint of construction materials, concrete producers blend their cement with Supplementary Cementitious Materials (SCMs). SCMs such as fly ash or blast furnace slag are mostly the byproducts of other industries. And while SCMs are chosen to match the properties of the common Ordinary Portland cement (OPC), their addition to the cement recipe may alter the chemistry of the system. These changes may lead to different mechanical and transport properties of the cement-based structure which may, in turn, affect its long-term resistance to harmful external agents. Therefore, understanding the relationship between cementâs microstructure and the degradation mechanisms is key for optimizing the design of new cementitious materialsIn this context, chloride attack is the most common reason for steel rebars to corrode especially when exposed to external chloride (seawater, deicer saltsâŠ). At low w/c ratios (typically 13) pore solution, the C-S-H surface develops a negative surface charge density. Electrostatic interactions between the surface and the ions in the pore solution result in a redistribution of the ionic species in two layers of charge at the interface of the solid i.e. the Electrical double layer (EDL). In nanoscopic pores, the EDL is dominated by atomic phenomena which are thought to interfere with the mobility of ions and chloride in particular. In order to understand and quantify the surface effects and their influence on ionic transport, we firstly propose an atomistic model of the EDL formation based on the use of the Metropolis Monte Carlo algorithm. This model is used to compute the ionic distributions and electrochemical potentials of electrolytes at equilibrium in nanoscopic pores. These quantities constitute the main driving forces of ionic transport at the pore scale. The microstructure parameters including the surface charge density of C-S-H, the ionic strength (and pH) of the pore solution, and the pore size are equally investigated and their effect on chlorideâs behavior quantified. Among the other parameters, the model also provides quantitative information on the effect of calcium ions which are thought to play a major role in the binding of chloride on C-S-H. The next step consists in using the calculated atomic-scale properties of the EDL in order to resolve the transport problem at the pore scale and compute microscopic diffusivities of chloride. This is achieved by using the molecular computations from the Monte Carlo (MC) engine in order to implement a modified version of the classical Poisson-Boltzmann system. The method is compared to the classical Finite element analysis of the Poisson-Nernst-Planck (PNP) equations and the data are discussed in the light of established experimental results in the literature.

EPFL2022

Chargement

Placing concrete requires much more water than the cement needs for its hydration. This results in an important porosity in the hardened concrete, which accentuates the degradation of this material. By adding small amounts of polymeric admixtures, called superplasticizers, to the fresh concrete one can significantly reduce the amount of water required to obtain the suitable workability. The plasticizing effect of superplasticizers has been studied for many years, and remains today, an ongoing field of research. This research led to a better understanding of the forces that act during the deflocculation and dispersion of cement grains induced by superplasticizers. This research work was performed within the framework of the "Superplast" European project. Its main objective has been to determine which parameters (molecular structure, induced charge, adsorption mode, molar mass) influence the plasticizing effects of superplasticizers. The results of this study will allow synthesis new polymeric admixtures with better performances than those currently available. For this study, two types of superplasticizers were selected: lignosulfonates and polycarboxylates. For each one, different samples were synthesised and characterised. For each sample, adsorption, rheological and interaction forces measurements were performed by our partners while we have performed adsorption and electroacoustic measurements on model powder suspensions. Different cement model powders were investigated and a magnesium oxide was selected. The particle size distribution and reactivity were carefully characterized. The inert model system (MgO) allowed us to study adsorption mechanisms without the complexity linked to cement hydration reactions that modify surface and solution of the suspension. Particle surface charge and pH are the two main parameters that influence the polymer adsorption and the polymer conformation. Magnesium oxide which has a high isoelectric point (around at pH 12.4) allows a surface charge similar to cement suspensions at high pH. First, we have measured the adsorption isotherms of all superplasticizers on model suspensions in NaOH (0.01M). This study allowed us to evaluate the affinity of each polymer for MgO and its adsorption plateau. These measurements showed that the lignosulfonates have a higher affinity than the polycarboxylates. They also showed that lignosulfonate adsorption is mainly influenced by their molar mass and their carboxylic group content (for similar sulfonate group content) while polycarboxylate adsorption is mainly driven by the backbone length, the side chain length and the carboxylic group content. Adsorption plateaux allowed us to calculate the surface coverage ratio and to estimate the superplasticizer conformation on the surface. Adsorption isotherms were finally measured on a Portland cement. The polymer adsorption is influenced by the same parameters as on MgO. The model system MgO is representative for the polymer adsorption on cement. In a second step, different electrolytes were added to model suspensions. This practice allowed us to study separately the effect of the main ions present in cement suspensions (Na+, Ca2+, SO42-, OH-) and to mimic the ionic composition of the aqueous phase of the cement suspension. Lignosulfonate adsorption is neither influenced by the studied ions or the pH. Only an increase of ionic strength increases the adsorbed polymer mass. Polycarboxylate adsorption is influenced by calcium and sulfate ions and by the particle surface charge. Finally, superplasticizer adsorption was studied on different cements by our partners and on a fly ash and a silica fume by ourselves. Lignosulfonate adsorption isotherms on cements showed that affinity and adsorbed polymer mass increases with the C3A content and decreases with the alkalis content. The isotherms measured on industrial by-products showed that superplasticizers adsorb on fly ash, but not on silica fume.

EPFL2004

Chargement

Resistance of cementitious materials to sulfate attack: quantifying performance with a reliable unidirectional approach

Qiao Wang

EPFL2022

EPFL2004

Nanoscale modelling of ionic transport in the porous C-S-H network

Khalil Ferjaoui

EPFL2022