**1. Introduction**

Alkali-activated materials (AAMs) are binders that accelerate the reaction of mineral admixtures, such as ground granulated blast furnace slag (GGBFS) and fly ash (FA), using strong alkali activators with high pH (>12). As AAMs are highly reactive, the hydrates of mineral admixtures are generated earlier. This lowers the fluidity of the mortar and causes setting and hardening at the same time. This process occurs quite suddenly [1–4]. The acceleration of the mortar setting has a significant impact on the physical properties of mortar at early ages [5–9], especially the initial shrinkage stress of the AAM mortar. According to the studies of many researchers, the shrinkage of mortar and concrete is evaluated after the occurrence of the final setting [10–14]. This is because the final setting means that the AAM matrix has been hardened and the deformation that occurs after the hardening of the AAM matrix acts as stress. Therefore, if the final setting of mortar that uses AAMs as binders is accelerated, shrinkage stress occurs earlier. This may cause significant initial shrinkage of the AAM mortar and high shrinkage stress. In addition, as the hydration reaction of AAMs is fast even after the final set, the strength and modulus of elasticity of the AAM mortar also significantly increase at early ages. The increase in the modulus of elasticity of the mortar causes a larger initial shrinkage stress. In general,

additives, such as expansive additives (EAs) and shrinkage reducing agents, are used to control the shrinkage of mortar. These additives are effective in reducing shrinkage, but shrinkage stress can be evaluated differently if the setting and modulus of elasticity of mortar are considered [15]. In particular, when a calcium sulfoalumiante (CSA) expansive additive (CSA EA) is used, the formation of ettringite and calcium monosulfoaluminate at the initial stage can compensate for shrinkage, but it can also increase the modulus of elasticity and compressive strength [16–19]. If the setting is accelerated or the modulus of elasticity increases despite the reduction in shrinkage, the initial shrinkage stress of mortar can be evaluated to be larger [20]. Therefore, mortar and concrete that used AAMs require a comprehensive evaluation of the properties that can calculate the stress caused by deformation, such as setting, modulus of elasticity, and shrinkage at early ages.

In general, it is possible to measure the shrinkage of mortar before setting using an embedded strain gauge and a data logger. However, to measure the shrinkage stress of mortar, the modulus of elasticity at the time of the occurrence of shrinkage must also be considered.

The elastic modulus is determined from the load–displacement curve, as it is the slope of the curve. Nevertheless, the strength of concrete and mortar specimens is too low to be measured with mechanical tests, making it difficult to determine elastic modulus. In addition, as the strength and modulus of elasticity vary dramatically over time at early ages, the limited number of specimens to perform continuous monitoring may cause larger experimental errors [21–23]. The use of the ultrasonic pulse velocity (UPV), a type of non-destructive testing, can detect the physical properties of a specimen continuously and successively without causing damage. With respect to the analysis of the properties of concrete, UPV has been verified by many researchers [24–27]. Öztürk et al. detected the early hydration status of cement materials through ultrasonic reflectance measurements. They showed that changes in reflected waves responded well to various stages of hydration [28]. Voigt et al. conducted studies on changes in microstructure during cement hydration using ultrasonic reflection and transmission techniques [29,30].

In addition, the use of UPV makes it possible to quantitatively analyze the hydration process of concrete. In particular, many researchers have reported that the setting time can be predicted using changes in the UPV curve [31–33]. Reinhardt and Grosse developed an ultrasonic device for concrete quality control testing that can continuously observe the characteristics of concrete. The results obtained using this device reported that the initial setting time can be determined by UPV [34]. Krauß and Hariri reported that the degree of hydration of concrete at the initial stage can be analyzed using UPV and proposed a method to predict the setting time [35]. Belie et al. conducted research on the setting and hardening of shotcrete mortar, for which the hydration reaction of the binder is significantly fast. They found that shotcrete mortar was immediately hardened in the fluid state, and they could analyze the sudden setting and hardening process of the mortar using UPV [36].

The use of UPV makes it possible to analyze the overall hardening process of concrete and mortar, and UPV has been used to analyze the static and dynamic elasticity characteristics as well as the tensile and compressive strengths of concrete and mortar [35,37–41]. Rajagopalan et al. reported that they identified highly reliable concrete characteristics at early ages using the relationship between the UPV and compressive strength of concrete at early ages [42]. Anderson and Seals conducted research to predict the long-term compressive strength of concrete through non-destructive testing using UPV and proposed a non-destructive test method for predicting the long-term strength [43]. Abdel-Jawed and Afaneh investigated various factors in concrete that affect the ultrasonic pulse. They mentioned that the compressive strength characteristics over time according to the water/binder (W/B) ratio can be analyzed using UPV [44]. Trtnik et al. investigated material and mix properties that affect the UPV of concrete and verified the relationship between UPV and the static/dynamic modulus of elasticity based on the results [39].

There have been attempts of measuring UPV as a means of exploring the fresh state and setting properties of ordinary Portland cement (OPC)-based concrete and mortar, while it has been rarely applied to the binders that set very rapidly. Determination of elastic modulus after the setting has

occurred is particularly important for controlling the shrinkage-induced stress and crack formation. This study attempted to evaluate the initial physical properties of the AAM mortar using CSA EA, which affects the setting and modulus of elasticity of mortar despite its shrinkage compensation effect. It was possible to find the initial and final setting times of the AAM mortar in the UPV inflection section by measuring the UPV for 48 h and to identify changes in its modulus of elasticity from the final setting to 48 h through the relationship between the UPV and dynamic modulus of elasticity. Based on this, the relationship between the setting and modulus of elasticity of the AAM mortar was analyzed according to the content of CSA EA, which is used for shrinkage reduction.
