# Steel Corrosion Behavior in Light Weight Fly-Ash Based Alkali Activated Mortars
## 1 Introduction  
Alkali activated materials (AAMs) are one of the possible routes for producing more sustainable construction materials compared to ordinary Portland Cement (OPC), especially when aluminosilicate precursors and/or activators with low environmental impact (i.e., industrial byproducts) are used [1; 2; 3]. In general, tailoring the mix designs allows to produce AAMs with high mechanical performances [2; 4], good durability [5; 6; 7; 8] and outstanding resistance to high temperature [9; 10; 11]. In recent years, many research efforts were spent in investigating the corrosion mechanism of steel reinforcements in alkali activated concretes and mortars, due to the carbonation that induces pH lowering [12; 13; 14] or due to the presence of aggressive species, especially chloride ions [15; 16; 17]. Compared to OPC-based materials, parameters, such as precursor and activator chemistry and concentration, rather than pore distribution, microstructure and composition of the pore solution, can strongly influence the steel corrosion behavior [18]. When low-calcium fly ashes are applied, a strongly alkaline and weakly oxidative environment is provided by pore solution and the passivation mechanism is similar to the one observed in cementitious materials [18; 19; 20]. Scattered results have been obtained among different studies that introduced chlorides in the material during mixing or through exposure to chloride-containing solutions [15; 16; 21; 22; 23]. This confirms that factors related to the binder microstructure and to the precursor chemistry, regulate the corrosion behavior. To better investigate these behaviors, electrochemical corrosion tests performed in simulated pore solutions highlighted that the alkalinity strongly influences the chloride-induced corrosion of the embedded steel [17]. So, due to the fact that low-calcium AAM binders do not contain Ca(OH)\({}_{2}\) species performing as pH buffer, as in OPC,long-term durability of steel embedded in low-Ca AAMs is ensured only if the binder is able to retain the alkalinity of the pore solution in contact with steel [18; 19]. Furthermore, AAMs have been positively considered when foams and light weight aggregates are applied for high temperature applications. This is related to the fact that light weight AAMs can exhibit low density and thermal conductivity values [9; 24], as well as great thermal stability [25; 26; 27; 28]. In addition, compared to cement-based materials that are prone to spalling when heated, AAMs (especially low-calcium based-ones) are not subjected to significant damages thanks to different microstructures and to the chemistry of water contained in the matrix. During high temperature exposure, the rapid dehydration of weakly bound water allows the retention of residual mechanical strength and dimensional stability [11]. Recently, light weight AAMs have been specifically applied as fire protection for steel structures producing passive fire protection showing encouraging results [29; 30; 31; 32]. In previous studies [24; 26; 33], the development of light weight mortars based on the alkali activation of low-calcium fly ashes was achieved as passive fire protection. The use of expanded perlite as light aggregates and hydrogen peroxide as foaming agent allowed the preparation of mortars with performances comparable to commercial cement based-passive fire protective coatings [24]. In addition, protection against cellulosic fire was obtained when a 20-mm thick light weight mortar coating was applied on steel, demonstrating the promising performances of these materials as passive fire protection [26; 33]. In addition to the measured fire protection, these innovative coatings should also exhibit durability properties, especially in terms of prevention and/or mitigation of steel corrosion phenomena. For this reason, the present study aims at the assessment of the chloride-induced steel corrosion when light weight alkali activated mortars (LVM) were applied.