**Gum-Sung Ryu 1,\*, Sung Choi 2, Kyung-Taek Koh 1, Gi-Hong Ahn 1, Hyeong-Yeol Kim <sup>1</sup> and Young-Jun You <sup>1</sup>**


Received: 31 August 2020; Accepted: 28 September 2020; Published: 5 October 2020

**Abstract:** This study investigated the hardening process of alkali-activated material (AAM) mortar using calcium sulfoalumiante (CSA) expansive additive (CSA EA), which accelerates the initial reactivity of AAMs, and subsequent changes in ultrasonic pulse velocity (UPV). After the AAM mortar was mixed with three different contents of CSA EA, the setting and modulus of elasticity of the mortar at one day of age, which represent curing steps, were measured. In addition, UPV was used to analyze each curing step. The initial and final setting times of the AAM mortar could be predicted by analyzing the UPV results measured for 14 h. In addition, the dynamic modulus of elasticity calculated using the UPV results for 24 h showed a tendency similar to that of the static modulus of elasticity. The test results showed that the use of CSA EA accelerated the setting of the AAM mortar and increased the modulus of elasticity, and these results could be inferred using UPV. The proposed measurement method can be effective in evaluating the properties of a material that accelerates the initial reactivity.

**Keywords:** alkali-activated material; CSA expansive additive; ultrasonic pulse velocity; setting time; modulus of elasticity
