**1. Introduction**

Low-cost materials that result from binders using activating solutions represent a sustainable option in the construction sector, not only for compatible and durable repair works but also as alternative solutions for new constructions. The "soft" nature of earthen materials is well-known, and up till now, various methods have been used to achieve stabilization, such as adding different binders, including Portland cement, fly ash, and lime [1,2]. Various issues emerge, however, such as the reduction of cement consumption in total and the reinforcement of earthen structures without the use of cement, that need to be examined to move towards sustainable building practices. The mechanism of alkaline activation of clays and soils as precursors, is under investigation in the last period, to enhance the physical-mechanical properties which are essential in determining the long-term durability of mortars [3]. As abundant materials, the development of alternative products could lead to many environmental, social, and economic advantages [4,5].

Chemical modification of earthen materials is likely to be achieved through inorganic polymerization. Specifically, alkali-activated products occur by dissolving through thermal treatment of the aluminosilicate network contained in a solid material which is called the precursor by using an alkali solution called the activator [6]. Nevertheless, the suitability of these aluminosilicate solid materials as precursors for the alkali activation process is a subject that should, in all cases, be examined before application. Usually, the preferable heating temperatures to achieve the polymerization of an aluminosilicate precursor such as clay are between 60 ◦C and 90 ◦C [7]. However, scientific studies prove that the alkali activation reaction of calcium–aluminosilicate systems can also take place in ambient conditions [6,8,9].

Moreover, different activators can be used according to the precursor in hand, thus, more suitable for earthen materials are considered the solutions of Ca(OH)2, NaOH, and KOH. These activators are chosen for dissolving the clay minerals efficiently while modifying the binding networks [6,10,11]. In general, the nature of the precursors and activators determine the treating conditions applied to avoid efflorescence and achieve higher stability [12–14].

As for alkali-activated materials, capillary absorption tests have proven that their pore networks are sufficiently helical, leading to a low capillarity [15,16]. Other researchers have found that their permeability to pure water or solutions of different ions is higher or similar than that of cement concrete [17,18].

Nevertheless, the long-term behavior and the durability of alkali-activated clay mortars in wet–dry and freeze–thaw cycles are of interest since the longevity of these materials is crucial for their utilization both in old and in modern structures and requires further studying. Additionally, the lack of applicable regulations constitutes their use more difficult. Most studies concerning such durability tests deal with alkali-activated mortars and concretes, with admixtures of industrial waste binders, such as fly ash and slag [3,19]. Moreover, the durability tests usually follow up the standards for Portland cement and concern resistance to acid attack, high temperatures, fire, and freeze–thaw [19]. The performance of such alkali-activated materials under freeze–thaw durability experiments has been proven beneficiary, presenting high resistance [19].

The frost resistance of cement depends mainly on physicomechanical rather than chemical factors, such as the porous structure [20,21]. There is a variety of reports of the performance of alkaline activated materials in the freeze–thaw process, which in some cases perform better than comparable cement concrete exposed under the same conditions [22,23].

However, there is a little reference for wet–dry cycling in alkali-activated materials, since such a test is proven not to cause a significant deterioration in cement products [3]. Furthermore, different testing methods have been reported for wetting and drying, each one being adapted to simulate different environmental conditions [3,24]. Although the specimens were exposed to different environments (temperature, relative humidity) at different ages, dimensional and/or mass measurements did not show a significant effect on materials performance [25–27]. However, some damage may be attributed to calcium and alkali leaching of the material with efflorescence effects [28].

This study focuses on examining the long-term effect of various alkali activators in earthen mortars. The mortars were submitted in low thermal treatment (40 ◦C) and ambient conditions. Since state-of-the-art research is dealing with alkali-activated materials, their long-term behavior after exposure at durability cycles is of interest. For this reason, the mechanical and physical properties of the mortars are presented.

## **2. Materials and Methods**
