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Volume changes of alkali-activated materials in sealed conditions

par Lacante, Maïté
Président du jury Berke, Peter
Promoteur Staquet, Stéphanie ;Aggelis, Dimitrios G.
Publication Non publié, 2025-10-14

Thèse de doctorat

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Résumé :

Concrete is the most widely used construction material in the world. Unfortunately, the production of its essential component, Portland cement, is responsible for 5 % to 8 % of the global anthropogenic CO2 emissions. To reduce this percentage, clinker-free alternative binders with a low associated CO2 footprint, such as alkali-activated materials, are emerging as a promising and necessary alternative to Portland binders. While researchers have already tested and studied some of their mechanical properties, alkali-activated materials suffer from significant early-age volume changes, such as autogenous and thermal strain, with very limited studies on the coefficient of thermal expansion (CTE). These high early-age volume changes can lead to internal tensile stresses, which in turn lead to micro-cracking, and structural instability if not properly handled. By exploring the early-age volume stability of alkali-activated materials, the aim is to contribute to the understanding of their potential as sustainable and durable alternatives. In this PhD thesis, the objective is to study the impact of various factors on the volume changes of alkali-activated materials, with a focus on their autogenous and thermal strains. Factors such as the amount of solution, activator type, concentration, scale, and temperature conditions as well as internal relative humidity, have an important influence on how the early-age volume develops. The mitigation effect of limestone filler, metakaolin, and curing temperature will be investigated as well. To address the issue of cracking, the cracking behavior of these materials will be studied with the acoustic emission technique. The following points represent the different research steps in this PhD thesis.First, general interest is given to the evolution of autogenous strain, thermal strain, heat release and internal relative humidity of blast-furnace slag activated with sodium hydroxide on paste scale. Then, the impact of the internal parameters (the precursors and activators) is studied by considering solution-to-binder ratio, the concentration of the alkaline solution, the effect of limestone filler and metakaolin. The magnitudes of autogenous strain and the coefficient of thermal expansion, related to the thermal strains, of alkali-activated materials are higher than those of Portland cement paste. Decreasing the solution concentration or increasing the solution-to-binder ratio generally decreases the autogenous shrinkage and increases the CTE. The shrinkage amounted from 87 to 1981 μm/m, while the swelling reached between 27 and 295 μm/m and was only present in half of the compositions. The amplitude of the CTE, which increases up to 55 μm/m/°C for some compositions while the CTE of PC remains between 20 and 25 μm/m/°C, can be explained by the higher CTE of the solution in comparison with water. The internal relative humidity of the paste cannot explain the autogenous strain’s development alone. Increasing the solution-to-binder ratio decreases the self-desiccation-related decrease.Then, the partial substitution of blast-furnace slag by limestone filler was investigated. The substitution rates of 15 % and 30 % emerged as the most optimal with a maximal reduction of the compressive strength of 23 %. The isothermal calorimetry revealed that the limestone filler wasprobably not entirely inert and showed the dilution effect (increase of the solution-to-slag ratio when the substitution rate increases). The autogenous shrinkage decreased when substituting 15 % of the slag while higher autogenous shrinkage was obtained when 30 % was substituted. In addition, its rate of development was reduced. Finally, the coefficient of thermal expansion was generally slightly reduced and delayed when slag was substituted. Meanwhile, the effect of the substitution of blast furnace by metakaolin was found to depend on both the type and concentration of the alkaline activator. When using 8 M and 10 M sodium hydroxide (NaOH), increasing the substitution rate increased the compressive strength. With sodium silicate (Na2SiO3), compressive strength decreased as the substitution increased. Isothermal calorimetry revealed the metakaolin’s dilution effect at 10 % substitution. With 8 M NaOH, a third reaction peak appeared, whose magnitude increased with the substitution rate, while the second peak decreased. The swelling was increased at 10 % substitution, followed by constant shrinkage in case of NaOH activation. Shrinkage was mitigated with Na2SiO3-activation. Higher substitutions with 8M NaOH resulted in a significant increase in the shrinkage rate and CTE, occurring when the third reaction peak appeared. A 10 % substitution delayed the CTE increase but resulted in higher later-age values (dilution effect). The 20 % substitution led to a similar final CTE value at 300 h, while 30 % substitution resulted in a decrease in CTE after the initial increase.The investigation into the impact of external parameters considered the influence of curing temperature (explored at 10 °C, 20 °C, and 30 °C) on the volume changes. The results indicate that increasing the curing temperature to 30 °C reduces autogenous shrinkage, likely due to changes in the elastic modulus and viscoelastic properties, while promoting swelling, especially for higher concentrations. The coefficient of thermal expansion is higher when the curing temperature is increased, but its development is delayed. The internal relative humidity is influenced more by the activating solution’s concentration than by the curing temperature, although the temperature does affect the initial internal relative humidity. The study also revealed that higher curing temperatures accelerate chemical reactions and reduce setting times. These findings contribute to the understanding of how the curing temperature influences the durability of AAS pastes, offering insights into optimized construction practices under varying environmental conditions.Blended systems like paste made from blast-furnace slag and fly ash activated by sodium hydroxide and sodium silicate were also investigated. Both the measured autogenous shrinkage and CTE are rather large; they amount to 4000–5000 μm/m and roughly 40 μm/m/ °C, respectively,at material ages of 2 weeks. An increase in solution-to-binder ratio leads to a decrease in autogenous shrinkage and an increase in CTE. An increase in the silica modulus (SiO2/Na2O) causes a decrease in both the autogenous shrinkage and the CTE. Most strikingly, autogenous shrinkage evolves linearly with the cumulative heat released by the binders.The evolution of the autogenous strain, coefficient of thermal expansion and heat release on mortar and concrete scales were studied. Both types of strain were reduced. The reduction was evaluated by determining a time-dependent restraining factor using the Pickett equation. Therestraint imposed by the sand on the paste seemed more significant than that of the coarse aggregate on the mortar.The long-term autogenous strain results revealed a separation into groups based on the solution concentration. The later-age thermal expansion coefficient revealed significantly decreased results compared to the early age, indicating the importance of early-age coefficient of thermal expansion determination for the cracking risk assessment.Different testing methods were also compared. The determination of the setting times (by the Vicat criterion, isothermal calorimetry or knee-point method applied on the autogenous strain) yielded somewhat similar results for some compositions between the two first-mentioned methods. In general, no consensus could be drawn between the methods. Autogenous and thermal strains were monitored with a customized testing device in which thermal variations are controlled, said devices were the corrugated tubes method (paste and mortar scales) and the BTJade device (mortar and concrete scales). Consequently, both the autogenous strain and the coefficient of thermal expansion were determined. Again, depending on the compositions (lower concentration), good correlations can be obtained. In addition, the autogenous strain of two different specimen sizes was also assessed manually with the DEMEC (initially for the long-term) but early-age comparison showed good correlation for lower solution-to-binder ratios.At early age, investigations revealed higher shrinkage and acoustic emission with high-intensity periods, as the solution concentration was increased for a 0.5 solution-to-binder. Using the measured strain and the E-modulus calculated based on the UPV measurements, the tensile stress and hence the cracking potential were evaluated for the different compositions, showing that high cracking potentials were escorted by high cumulative AE. The high strains might result in localized stress buildup higher than the material’s tensile strength, potentially resulting in micro-cracking.On the concrete scale, a correlation based on the modified equations from the standards was established between the compressive strength and the tensile strength, obtained from the splitting tensile test.Overall, this research aims to contribute to the development of clinker-free alternative binders that have lower CO2 emissions and are suitable for use in the construction sector.