The growing use of blends of low- and high-calcium solid precursors in combination with different alkaline activators requires simple, efficient, and accurate experimental means to characterize their behavior, particularly during the liquid-to-solid transition (setting) at early material ages. This research investigates slag-fly ash systems mixed at different solution-to-binder (s/b) ratios with sodium silicate/sodium hydroxide-based activator solutions of varying concentrations. Therefore, continuous non-destructive tests—namely ultrasonic pulse velocity (UPV) measurements and isothermal calorimetry tests—are combined with classical slump flow, Vicat, and uniaxial compressive strength tests. The experimental results highlight that high alkali and silica contents and a low s/b ratio benefit the early-age hydration, lead to a faster setting, and improve the early-age strength. The loss of workability, determined from the time when the slump flow becomes negligible, correlates well with ultrasonic P-wave velocity evolutions. This is, however, not the case for Vicat or calorimetry tests.

Alkali-activated slag (AAS) is emerging as a possible and more sustainable alternative to Ordinary Portland Cement (OPC) in the construction industry, thanks to its good mechanical and chemical properties. Conversely, the effects of its high drying shrinkage are still a concern for its long-term durability. This study aims to investigate the drying shrinkage behaviour of six AAS/sodium hydroxide mortar compositions and the main phenomena affecting their drying shrinkage behaviour. Specifically, the molarity, solution-to-binder ratio (s/b), autogenous shrinkage, creep compliance, microcracking, and carbonation are considered as possible causes of the differences between AAS and OPC. The results show that it is not possible to correlate the shrinkage magnitude with the molarity of the activating solution, while an increase in the s/b increases the drying shrinkage. Concerning the other factors, autogenous deformation remains significant even after a period of 112 days, while the creep compliance is definitely affected by the drying process but does not seem to affect the shrinkage magnitude. Furthermore, the presence of microcracks caused by the drying process definitely influences the drying shrinkage. Finally, carbonation depends on the molarity of the activating solution, even though its effects on the material are still unclear.

The study of alkali-activated slag (AAS) is motivated by the need for more sustainable alternatives to Portland cement (PC) within the construction industry. Specifically, AAS offers good mechanical and chemical properties. However, the influence of the activator on its pore structure and hydraulic conductivity remains unclear. Both pore structure and hydraulic conductivity are key parameters in understanding the drying process and could potentially explain the high drying shrinkage observed so far. The present study aims to investigate the pore size distribution and hydraulic conductivity of six distinct AAS/sodium hydroxide mortar compositions, with a particular emphasis on the effect of varying the activator’s molarity and the solution-to-binder ratio (s/b). This research uses the mass variation in different relative humidity (RH) conditions from experimental tests to model the pore surface area, the pore size distribution, and the hydraulic conductivity. From the results, it emerges that increasing the molarity from 0.5 to 8 M reduces the open porosity and refines the pore structure, while increasing the s/b from 0.5 to 0.8 increases the open porosity while refining the pore structure. In addition, high molarity compositions are not suitable for testing in high RH and natural carbonation conditions due to the occurrence of deliquescence. Moreover, the main drying mechanism in AAS is water vapour transport even at high relative humidity, contrary to what was observed in the literature for PC. Finally, the hydraulic conductivity of alkali-activated slag presents a minimum of around 85% RH against the 60–70% RH for PC, causing AAS to dry faster when the relative humidity decreases from 85 to 50%.

This research investigates the impact of solution concentration and solution-to-binder ratio (S/B) on the volume changes in alkali-activated slags with sodium hydroxide at 20 °C. Autogenous and thermal strains are monitored with a customized testing device in which thermal variations are controlled. Consequently, both the autogenous strain and coefficient of thermal expansion (CTE) are determined. Heat flow and internal relative humidity (IRH) are also monitored in parallel, making this research a multifaceted study. The magnitudes of autogenous strain and CTE are higher than those of ordinary Portland cement paste. Decreasing the solution concentration or S/B generally decreases the autogenous strain (swelling and shrinkage) and the CTE. The shrinkage amounted to 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 OPC remains between 20 and 25 µm/m/°C, can be explained by the high CTE of the solution in comparison with water. The IRH of paste cannot explain the autogenous strain’s development alone. Increasing S/B eliminates the self-desiccation-related decrease.

This research investigates whether metakaolin can be used as a partial substitution for slag to mitigate significant volume changes in alkali-activated slags. Its effect on compressive strength and workability (as well as on isothermal calorimetry, autogenous strain, and coefficient of thermal expansion (CTE)) were 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 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 8 M 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.