## 1 Introduction  
It is well known that the manufacturing of clinker--the product traditional Portland cements (PC) are based on--is responsible for about 12-15% of energy consumption in the industry [1]. The limestone transformation into CaO during cement manufacturing emits 900 kg of CO\({}_{2}\) into the atmosphere for each ton of PC obtained [2], being a high-energy demanding chemical reaction that requires temperatures higher than 1400 \({}^{\circ}\)C [3]. One of the most promising alternatives for replacing traditional PCs with less polluting binders are hybrid cements [4, 5]. These cements are manufactured with only 20-30% of clinker, with 70-80% being by-products from other industries (that act as Supplementary Cemertitious Materials). These formulations can represent a reduction of up to 80% of CO\({}_{2}\) emissions in comparison with PCs that are richer in clinker [6, 7]. Unlike alkali-activated materials (AAMs), which have been extensively studied during recent decades, these more innovative hybrid formulations offer several advantages that can facilitate their in-construction implementation. For hybrid cements, binder hydration can be activated by solid salts with low alkalinity, instead of the highly alkaline, dangerous liquids required for the AAM hydration. Moreover, hybrid cements have setting times compliant with the EN 196-1 standard [5]. In contrast, AAMs have shown, in this aspect, significant technical drawbacks that limit their use, such as, among other issues, too rapid setting times [8] and high-dimensional instability during curing [9], which can easily lead to cracked microstructures [10,11]. Different precursors have been explored for the manufacturing of hybrid cements, such as red mud [12], natural pozzolans [13,14], fly ash [1,15], blast furnace slag [16,17] or other wastes [18]. Mixes of slag and fly ash [19,20] or mixes of fly ash with other products [21,22] have also been explored as precursors. The present study has focused on slag-based hybrid materials, for which better results than those for fly ash-based hybrid mortar have been obtained in our laboratory after activation with sulfates [23]. Initially, the curing reactions in hybrid cements generate a mixture of C-S-H gel, due to clinker hydration, and N-A-S-H and C-A-S-H gels are also formed (depending on the precursor composition) as a result of the Supplementary Cementitious Materials hydration [24]. The heat generated by PC hydration, together with the increase in the pH that the added activating salts can generate, favors the hydration of the Supplementary Cementitious Materials even at room temperature. The addition of a highly soluble alkaline salt, acting as activator, favors the progressive transformation of poorly soluble calcium hydroxides into highly soluble alkaline hydroxides that create favorable pH conditions for the formation of geopolymers during the curing [6]. The co-existence of geopolymer gels with C-S-H gel in the microstructure of the hybrid binders has been suggested as positive for their performance [25]. The type of activator used can affect not only the kinetics of the hydration process, but also the nature and relative amounts of the formed reaction products [26]. NaOH [2,27] and NaOH + Na\({}_{2}\)SiO\({}_{3}\) mixtures [26] have been studied as activators for hybrid mortars, following the activation procedures of AAMs, despite the clear economic, environmental and setting time-related disadvantages associated with the use of NaOH with or without silicates. Up to now, most of the pioneer studies in these alternative cements have used Na\({}_{2}\)SO\({}_{4}\) as activator [7,13,20,28]. The use of sodium carbonate [3,15] or potassium carbonate [3,29] has also been considered for activating these cements, showing moderate curing shrinkages [30]. Five percent Na\({}_{2}\)CO\({}_{3}\) has been reported as an activator concentration promoting rapid hydration [31]. Wear performance is an aspect that can affect potential applications of the alternative binders if they are used in floors, tiles or pavements.