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Article

Hybrid Mortars Activated with Alternative Steel-Compatible Salts: Impact on Chloride Diffusion and Durability

by
Angily Cruz-Hernández
,
Francisco Velasco
,
Manuel Torres-Carrasco
and
Asunción Bautista
*
Materials Science and Engineering Department–IAAB, Universidad Carlos III de Madrid, Leganés, 28911 Madrid, Spain
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(14), 8055; https://doi.org/10.3390/app15148055
Submission received: 25 June 2025 / Revised: 7 July 2025 / Accepted: 16 July 2025 / Published: 19 July 2025
(This article belongs to the Special Issue Advanced Concrete Technology and Applications in Construction Engineering, 2nd Edition)
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Abstract

Eco-friendly mortars have been manufactured with hybrid binders made of blast furnace slag and a reduced amount of clinker. The objective is to explore new formulations suitable for reinforced structures. Previous studies are mainly focused on activation with sulfates, a salt that is corrosive to reinforcing steel. Sodium nitrate and sodium carbonate, easily implementable in construction, have been used as activators in two different concentrations that involve similar Na content. A Type II PC mortar is used as reference. The dimensional stability of the mortars during curing (at 99% RH) and subsequent drying at 40% RH, has been evaluated, as well as their porosity and mechanical properties. Böhme tests revealed that studied hybrid binders have lower wear resistance than PC mortar. Activation with Na2CO3 allows the obtention of mortars with reduced porosity and good compression resistance, but generates microcracking that favors chloride diffusion. Activation with nitrates favors precipitation of AFm phases identified through differential thermal analysis. Nitrates in moderate amounts (4% w/w) allow manufacturing hybrid mortars with good resistance to chloride penetration and reasonably good mechanical properties. Hence, this binder can be a promising option for reinforced structures. Higher amounts of nitrates (8%) for activation give rise to more porous mortars.

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 CO2 into the atmosphere for each ton of PC obtained [2], being a high-energy demanding chemical reaction that requires temperatures higher than 1400 °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 Cementitious Materials). These formulations can represent a reduction of up to 80% of CO2 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 + Na2SiO3 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 Na2SO4 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 Na2CO3 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. Slag-based hybrid mortars activated with 5% Na2SO4 have shown adequate wear properties [32]. However, the influence of the nature and amount of the activator is an aspect that remains unexplored, and it is of interest to obtain new information. On the other hand, wear can reduce the thickness of the concrete covering the outermost steel bars in reinforced structures (the reinforcements prone to corrosion).
Another interesting possibility for hybrid binders is their use in steel-reinforced structures. Thus, the ability of the mortars to limit chloride transport is a key factor that determines their durability in certain environments. The transport of chloride ions in concrete is a phenomenon that is strongly influenced by diffusion, capillary suction, convective flow, physical adsorption and chemical bonding through the pore system and the microcrack network [33]. The rate at which they penetrate depends on the binder formulation, which determines the following: (a) the pore volume through which the chlorides can penetrate; (b) the pore refinement and the amount of pore surface where the chlorides can become physically bounded [34]; (c) the chloride-binding ability of the products formed during hydration [35] or dissolved with aging [36]; and (d) the presence of defects in the binder that act as preferential diffusion paths [23].
The chosen activator clearly affects material porosity and hence the transport of chlorides. Moreover, it must be born in mind that sulfate can negatively affect the corrosion resistance of steel embedded in concrete [37]. Hence, to achieve an optimized durability of the structure, activators other than sulfate must be used.
In this work, as an alternative to Na2SO4, sodium carbonate was selected, due to its positive effect as activator that has been reported in the previous literature [2,15]. Moreover, another different sodium salt (sodium nitrate) has first been explored as a possible activator for hybrid binders. NaNO3 has already been analyzed as accelerator for preventing early frost damage in traditional concretes [38], and it is considered as a corrosion inhibitor for steel in concrete [39]. It is important to stress that nitrate is a non-carcinogenic salt, unlike nitrites or other traditional corrosion inhibitors [40,41], so it seems potentially very interesting for the activation in reinforced concretes, which could enhance the durability of the embedded steel. Hence, this work offers completely new data about the properties of hybrid mortars activated with nitrates and new information about microcracking issues, wear and chloride diffusion on hybrid mortars activated with carbonates.

2. Materials and Methods

2.1. Mortar Formulation and Manufacturing

Five different binders were considered in this study. Four hybrid mortars of ecological interest using slag and a Type I PC as precursors. A plain PC mortar (based on Type II/B-M PC, according to UNE-EN 197 standard) was also manufactured as commercial reference. Type I PC is considered a general, all-purpose cement, with >95% of clinker in its composition, thus being adequate for hybrid mortars to ensure reactivity. Type II/B-M cement was selected to manufacture the reference mortar, due to its high level of implantation in construction. Type II/B-M cement presents 65–79% of clinker in its composition, with the remaining portion consisting of other main constituents like limestone, fly ash, or blast furnace slag, and minor constituents like gypsum. Table 1 shows the chemical composition of these three materials, determined by X-ray fluorescence (XRF). The results for the particle size of the precursors used to manufacture the binders are summarized in Table 2.
For the formulation of the hybrid mortars, two different types of activators were used, both of them in two amounts that involve two similar Na contents: 0.05 and 0.10 moles of Na per 100 g of binder. Na2CO3 (3% and 6% by weight, from Panreac) and NaNO3 (4% and 8% by weight, from Labkem) were added to obtain those Na moles, with lower values yielding 0.05 moles of Na and the higher ones yielding 0.10 moles of Na, always per 100 g of binder. The two Na concentrations selected for both salts correspond to the range of those considered in previous studies using sulfates [42]. The Na/Ca ratio, an important factor for the further development of the hydration reactions, is also detailed in Table 3. The solid activator was introduced in powdered form and thoroughly homogenized with the slag and cement during the dry mixing stage. Subsequently, the required volume of water was incorporated into the dry blend to achieve the final mortar mixture.
The labels chosen for the five mortars considered in the present study, and used hereafter in the article, can be seen in Table 3, together with their dosages. For the mortar manufacturing, standardized sand (according to EN 196-1 standard) was added in the usual sand/binder (s/b) ratio of 3/1. The amount of water for the mortars was determined following the criterion of similar work abilities (which were measured according to the EN 196-3 standard).
Carbonate-activated hybrids (HC), nitrate-activated hybrids (HN) and commercial Portland cement (OPC) mortars were used for manufacturing the different types of samples for the experimental studies detailed in Section 2.2 formulations in Table 3. All the samples were cured for 28 days in a chamber at 99% relative humidity (RH) and 25 °C.

2.2. Mortar Characterization

For monitoring the dimensional changes that can occur during the mortar curing process, and during the drying that takes place during the subsequent exposure to the atmosphere, four samples of 285 × 25 × 25 mm size from each of the five mortars under study were manufactured. All of them were monitored during their curing at 99% RH and 25 °C, and then during their drying in conditions that simulate continental environments (40% RH and 25 °C). The monitoring of the dimensional changes during drying was also carried out for 28 additional days. Length changes were evaluated following the procedure described in the UNE-EN 80, 112 standard. A K02 length comparer from Shock Baker, with ±0.002 mm accuracy, was used for these studies.
For each mortar formulation, six prismatic mortar samples, 160 × 40 × 40 mm in size, were also manufactured to carry out the mechanical and microstructural characterization. Different characterization studies related to the chemical composition of the mortars such as pH and oxidation profile measurements and simultaneous differential thermal analysis (DTA) and thermogravimetry measurements (TG) were also carried out on samples obtained from this type of specimens.
For mortars under both conditions (cured and dried), open porosity was measured comparing helium pycnometry (apparent density), using an Accupyc 1330 helium pycnometer (Micromeritics, Norcross, GA, USA), and Archimedes technique (following UNE-EN 993-1 standard). A helium pycnometer allows the density of the material and the volume of closed pores (those the He is not able to penetrate) to be determined. Cubes (approximately 1 × 1 × 1 cm) obtained from the central region of each mortar were used for these measurements. Each measurement was performed in triplicate, and epoxy resin (Macdermid Enthone, Birmingham, UK) was used to seal the samples for the Archimedes method. Hence, open porosity (to He) was determined combining both methods.
The overall porosity and pore size distribution of the mortars after drying were assessed using Hg intrusion porosimetry utilizing a Micrometrics Autopore IV 9500 analyzer (Micromeritics, Norcross, GA, USA). This investigation was conducted with a multiplicity of three.
Upon the completion of the curing and drying period, the specimens were analyzed using scanning electron microscopy (SEM), specifically the FEI Teneo model, to investigate potential microstructural changes, including the occurrence of microcracks.
After 28 days of curing and 28 days of drying, three specimens of each mortar were subjected to compression tests, in accordance with the EN 196-1 standard, using EM2 (Microtest, Madrid, Spain) universal testing machine. Moreover, the oxidation front was measured in the split mortars.
The valuation of the mechanical performance of the mortars was completed with a wear test. A Böhme machine (Ibertest, Madrid, Spain) was used to perform the abrasion test of the mortars in accordance with UNE-EN 13892-3. The tests were carried out at room temperature. Brown fused alumina (Al2O3), with an applied load of 294 N, was used as abrasive material. Three specimens of each mortar were evaluated. In accordance with the standard, height loss was quantified at six points per specimen after undergoing four test cycles (each of 22 revolutions), in which the specimen was progressively rotated 90° after the end of each cycle. A total of 16 cycles were assessed, monitoring the height loss.
After curing and drying of the mortars, they were subjected to simultaneous DTA/TG (SETSYS Evolution, Setaram Instrumentation, Caluire, France) to analyze the formed phases. During these measurements, powdered samples (30 mg) were heated at a constant rate of 10 °C/min in air until reaching 700 °C.
The pH of both cured mortars and dried mortars was assessed on the surface of samples and in their inner part. The pH of the flat surface was directly measured using a pH electrode (EXTECH PH100, EXTECH, Nashua, NH, USA). For the inner parts, the methodology outlined in ASTM D 4972-01 was followed. Powder samples were obtained from the center of broken compression samples and dissolved in distilled water (5 g of solid material in 5 mL of water). After 1 h, the pH of the suspension was measured using the same pH electrode. In the case of dried mortars, the pH of the oxidated region was also assessed following the same procedure. Measurements were always conducted in triplicate.
Experimental methods developed for chloride diffusion evaluation can be categorized into two groups: natural diffusion tests and migration tests. Natural diffusion tests are time-consuming, but they are closer to real applications [43]. Electrical methods try to accelerate the process by applying electrical fields, but confusing information can be obtained in materials with high hydroxyl concentrations, as these anions also migrate, interfering with the chloride transport. Hence, a chloride transport test that does not involve current was chosen in this study. To obtain information about chloride transport into the mortars, cylindrical samples of 60 mm high and 60 mm diameter were manufactured to carry out the chloride transport test. For this test, the surface of cured samples (except the circular bases) was covered with an isolating epoxy resin. Then, the samples were immersed in 3 M NaCl solution at 23 ± 2 °C for 35 days (Figure 1a), following the procedure described in the ASTM C1556-04 standard. After the exposure, the cylindrical specimens were broken, and powders were extracted at different distances from the non-coated circular surface of the mortar through which the chlorides had unidirectionally penetrated from the solution (Figure 1b). The amount of water-soluble chlorides in the samples obtained from the different mortars at different depths were determined according to the UNE 83,989 standard. Twelve milliliters of distilled water were added to 1.2 g of powder, and they were mixed under constant stirring at 1200 rpm. The chloride concentrations were determined with the chloride selective electrode Imacimus 2 (Figure 1c, NT Sensors, Vila-seca, Spain). The sensor combined a chloride selective electrode and a pH electrode, being previously calibrated, taking into account both parameters to improve accuracy. This study was carried out in quadruplicate.
In the analysis of the experimental data carried out hereafter, the criterion that results whose experimental errors bars overlap are statistically similar is followed. This approach allows identifying differences in the average values that are, in fact, due to experimental dispersion.

3. Results and Discussion

3.1. Dimensional Stability, Porosity and Phases Formed

The results of the dimensional variation in the hybrid mortars (Figure 2) indicate that all materials experience global shrinkages during the curing and drying processes. For the five mortars under study, the shrinkages suffered during the drying were 40% RH higher than those previously experienced during the curing at high RH. While curing at 99% RH, the pores of the mortars retain water, which favors the hydration and formation of cementitious products [25]. However, when mortars are subsequently exposed to a drier environment (40% RH), water evaporates from the pores, generating internal stresses that cause shrinkage [44]. The drying shrinkages are meaningful for approximately the first 20 days at 40% RH, which suggests that the greatest loss of water in the systems occurs during this period, before equilibrium with the environment is reached.
Although OPC shows slightly better dimensional stability during curing than the hybrid materials, the mortar manufactured from commercial cement undergoes more significant shrinkage than the ecological alternatives studied during the drying process. Moreover, it must be stressed that the studied hybrid mortars show global shrinkages during the curing and drying periods that are very similar to those suffered by OPC. This is a clear advantage of the hybrid mortars as opposed to other traditional cement alternatives manufactured from slag such as AAMs [45].
During curing, hybrid mortars activated with Na2CO3 undergo an initial expansion followed by stabilization. This expansive reaction can be related to the well-known expansive reaction of the carbonates in alkali media [46], and this must be borne in mind as it can affect the final properties of the material. The early expansion observed in HC mortars (Figure 2) is attributed to the interaction between Na2CO3 and dissolved calcium species, which promotes the precipitation of calcium carbonate and other intermediate carbonate phases under high pH conditions. These reactions can lead to localized supersaturation and transient CO2 gas release, generating internal stresses during the initial curing stage. Although the systems do not contain carbonate aggregates (thus excluding a classical alkali–carbonate reaction, ACR), the expansive behavior shares some characteristics with ACR, in the sense that it involves carbonate phase instability in alkaline environments. Although the formation of ettringite could also generate expansions in cementitious systems, the absence of an external sulfate source, the low SO3 content in the slag precursor, and the clear correlation between the use of Na2CO3 and the observed initial expansion suggest that carbonation reactions are the most probable mechanism responsible for the initial expansion observed in HC mortars under the conditions of this study.
Curing shrinkage values of hybrid mortars can be compared with those of AAMs based on slag. NaOH and waterglass as activators of AAMs provoke important shrinkages stresses in the materials, that mortars do not withstand [9]. After the curing period at high RH, length increases of 0.01–0.02% (depending on the activator and its amount) have been monitored and related to the extensive microcracking caused by the huge stresses these AAMs suffer during curing. The global dimensional changes taking place after curing (Figure 2) correspond to shrinkage in hybrid mortars; so, if microcracking appears, it must take place in a more reduced amount than in slag-based AAMs, and it does not determine the global length change.
The porosity volume in all the mortars under study was determined by the Archimedes method, and the obtained results are plotted in Figure 3. The hybrid mortars show higher porosity values after the drying process than just after the curing. This can be understood bearing in mind that, after curing, some water remains in the pores of the mortars blocking the He penetration. Hence, the He-pycnometer measurements are unable to distinguish between dense mortar and pores with remaining water. An intensive, prolonged drying at 40% RH completely eliminates water from the pores, erasing this interference and increasing the measured porosity values.
The porosity of slag-based AAMs activated with NaOH or water-glass after curing and measured in our laboratory following the same procedure [9] ranges between 5 and 2%, which is similar to the values measured for HN mortars and lower than the values determined for the HC binders (Figure 3).
The drying effect in the porosity is clearly evident for the HN mortars (Figure 3), but seems masked for HC, especially for the HC-6%. The expansion process taking place at the beginning of the curing (Figure 2) can have generated microcracks that affect the porosity obtained for HC and become evident after drying or that can even grow from the stresses caused by water evaporation.
On the other hand, during the drying step of OPC, two competitive processes affect the porosity: hydration and drying. The curing kinetics of OPC, which lacks activators in its formulation, is slower than that of hybrid mortars. The OPC mortar curing process lasts more than the 28 days that the samples are exposed to 99% RH [47], and continues at 40% RH, as long as water remains in the pores [44]. In Figure 3, it can be confirmed that OPC decreases its porosity during the drying process by hydration. For OPC, the measured porosity values are lower after the drying step than just after curing, as the effect of curing dominates over the effect of a complete water removing from the pores in the porosity results for this traditional cement.
Additional information about the pore structure of the dried mortars has been obtained with Hg intrusion porosimetry (Figure 4). The global porosity values obtained with this technique (Figure 4a) show a trend similar to that of the porosity values obtained with the Archimedes method after the same exposure condition (Figure 3). The results obtained confirm that the use of Na2CO3 is more effective than the use of NaNO3 (for identical Na amounts) to reduce the porosity volume of hybrid mortar (Figure 3 and Figure 4a), but it must be stressed that both activators allow mortars denser than OPC to be obtained.
In spite of the expansive reactions taking place at short curing times (Figure 2), the HC mortars show low porosity. As occurs with sulfates, carbonates affect the curing process of the binders [48], favoring the fast development of denser and more cohesive gels than those generated by OPC hydration. Increasing the activator concentration seems to have a positive effect on the porosity reduction in HC after the drying process, which is coherent with the results obtained for mortar activated sulfates at similar Na concentrations [42].
There are no dramatic differences in pore size distribution between OPC and the hybrid mortars (Figure 4). Most of the pore volume in the five mortars studied is formed by pores of small diameter. This fraction of pores that can significantly contribute to the physisorption of chloride on pore walls [49] is smallest for the HC mortars (Figure 4b). The volume of big pores (Figure 4b), which can be the most dangerous for compromising the mechanical properties or enhancing chloride diffusion, do not show significant changes in the different materials. It is worth mentioning that HN-4% have a more refined porosity than those manufactured with 8%, which could seem surprising at first sight. Moreover, it must also be stressed that HC-6% have a volume of large pores similar to or higher than these determined in more porous materials such as OPC, and higher than HC-3% (Figure 4b). This last fact can be coherent with cracks generated during the beginning of the curing (Figure 2), which can also progress during the drying step.
The analysis of hysteresis loops during the Hg intrusion measurement (due to intrusion and extrusion of Hg in the pores) informs that the five materials under study present predominantly inkbottle pore shapes, together with ultimate mercury entrapment after the extrusion of mercury (see Supplementary Materials, Figure S1). The use of other techniques such as computed tomography could allow a better understanding of porosity in construction materials [50].
On the other hand, it is clear that the effect of nitrates in the porosity is different from that observed for carbonates, at least for the highest activator concentration (Figure 4). It has been reported that the addition of nitrates in traditional cements increases the initial curing heats [38], which in turn favors the hydration of the slag present in the hybrid binder [51]. The nitrate additions also boost the formation of large amounts of needle-type hydrates, such as nitrate–AFt compounds, in traditional binders [38], favoring their early strength development. The quickly formed hydrates partially fill the pores and explain the pore reduction monitored even after short curing periods [52]. However, it has also been demonstrated that the addition of calcium nitrates can interfere in the usual hydration process of the traditional cements, decreasing the formation of C-S-H gel and portlandite [38], especially when the nitrates are added in relevant amounts. This addition can have either a predominately positive or negative effect, essentially depending on its concentration. Hence, bearing in mind the obtained results (Figure 4), the addition of nitrates in 8% concentrations to the precursor under study seems to be high enough to negatively affect the expected beneficial effect of the CEM I in the mix (Table 3). The reaction of nitrates with the clinker can negatively affect the formation of gels relevant for the decrease in the porosity in the long term, as has been confirmed (Figure 3 and Figure 4a).
A SEM study was carried out to evaluate the possible occurrence of microcracks in the dried mortars. Figure 5 summarizes the most significant observations. The presence of cracks is frequent in alternative mortars, such as AAMs [9] or hybrids [53]. These cracks have been associated with high stresses related to the very refined porosity generated by the heavy activation employed to achieve the high mechanical properties [9].
Cracks or microcracks can act as preferential pathways for chloride diffusion and may significantly affect the corrosion behavior of the embedded metal reinforcements [23]. The mortars formulated in the present work, even after the drying step, generally show homogenous non-cracked microstructures. The detailed study carried out confirms that HN (Figure 5d,e) and OPC (Figure 5a) mortars can be considered microcrack-free after the curing and drying processes.
On the other hand, a reduced number of microcracks have been identified in some observed sections of HC mortars (Figure 5b,c), with their presence being clearly more relevant in HC-6%. These cracks in HC mortars can have different origins. The first reason can rely on the contribution of the expansion process taking place at the beginning of the curing for HC (Figure 2). The expansive reactions shown, related to alkali–carbonate reaction [46] can generate small microstructural defects. Second, carbonates have proven to be a more effective activator than nitrates, as the former is able to cause a greater decrease in the porosities than the latter in similar concentrations (Figure 3 and Figure 4). This denser microstructure will present lower water permeability and can lead to higher pore pressure and stress concentrations during the drying step of the tests [54]. The stresses in the pores due to drying can easily increase the size of these microcracks in HC mortars and make them more relevant. Thus, drying periods can potentially favor the nucleation and/or growth of cracks. Previously, other authors [15,28,31] have already shown that the formation of carbonated-based hydrates or dense microstructure early on can induce internal pressure or restraint shrinkage phenomena that promote microcracking. The higher incidence of microcracks in HC-6% mortars (Figure 5) correlates with the higher activator content and the more intense expansion detected, reinforcing the hypothesis that these early chemical processes contribute directly to microstructural damage.
To understand the porosity results of HN (Figure 3 and Figure 4), DTA analyses were carried out to evaluate the effect of nitrates on the formation of certain hydration products, and confirm the formation of nitrate-Aft reported in previous bibliography [38,52] in hybrid mortars. The obtained DTA results can be seen in Figure 6. All the studied mortars show an endothermic peak of about 100–150 °C, which corresponds to the loss of free water or partial dehydration of hydrated aluminosilicates (mainly CSH decomposition [55] and/or Aft phases) [56]. OPC also shows another endothermic peak at 400–500 °C, which corresponds to CH decomposition [56]. This peak has much lower intensity for hybrid mortars, confirming their lower alkaline reserve, which is clearly related to their reduced initial CaO content and its consumption in the formation of hydration products other than portlandite.
An important fact that can be observed in Figure 6 is the presence of two endothermic peaks for HN between 250 °C and 400 °C. Both peaks are related to the formation of hydrotalcites. Hydrotalcites are laminar double hydroxides (LDH), and the NO3-AFm that can be formed with the addition of nitrates to cement has this structure [56]. The endothermic peak at approximately 325–330 °C can be related to decomposition of interlayer carbonates of hydrotalcites [57], while the other peak is related to the dihydroxylation of lamellar hydroxides (e.g., NO3/NO2-AFm) [56].

3.2. Mechanical Properties

The compression strength of the hybrid mortars after curing and drying is shown in Figure 7. Drying always improves the mechanical properties of the mortars, as the presence of water inside the pores stiffens and embrittles the materials. In general, a clear inverse correlation can be observed between compressive strength (Figure 7) and porosity (Figure 3 and Figure 4).
The HC-6% has the highest compression strength of the mortars under study (Figure 7) due to its reduced porosity (Figure 3 and Figure 4) achieved thanks to its strong activation. The compression strengths of this mortar could be even higher if small microcracks as those shown in Figure 5c were not generated in its microstructure. Moreover, the influence of microcracks in HC-3% (Figure 5b) can explain that this compression strength is lower than expected because of the porosity of this mortar (Figure 3 and Figure 4a).
On the other hand, as for HN mortars, the fragile nature of the reaction products, whose precipitation is favored by the nitrates, negatively affects optimal achievement of mechanical properties (Figure 7). This effect is especially evident when nitrates are added in high amounts (HN-8%). The presence of NO3-AFm has been previously reported as negative for the mechanical properties [58,59]. The lower strength of HN mortars can rely on the hydrotalcite phases shown by DTA (Figure 6), as well as on the quick consumption of Portland cement to form NO3-AFm. As Portland cement is present in a limited amount (Table 3), the formation of hydration products with good mechanical properties such as C-H-S [38] or other gels is hindered in HN mortars with increasing amounts of nitrates.
The mechanical properties of AAM mortars activated with highly alkaline water-glass can be much higher than those of PC and studied hybrid mortars [9], due to the more refined porosity of the AAM [60].
Additionally, the wear performance of the different mortars was evaluated, as it can be a key-factor for certain applications [32,42]. This aspect was evaluated through the Böhme test described in Section 2.2. In Figure 8, it can be seen that none of the new hybrid mortars can improve the wear resistance of the OPC mortar either as-cured or dried. After the 28 days of curing (Figure 8a), the surfaces of the hybrid mortars are clearly less wear-resistant than that of the OPC. The drying step seems to slightly improve the wear performance of the mortars (Figure 8b). The characteristics of Böhme wear tests make the differences found for the same mortar after the two exposure conditions much less relevant than those found in the compression tests (Figure 7). Part of the water of the as-cured samples should be progressively erased from the pores as the abrasion progresses into the mortars. Hence, the detachment of mortar particles that cause the wear takes place in mortar regions that are already water-poor.
The relative wear resistances of the mortars have been previously related to their porosity [42]. The sand hardness and its adherence to the binder are key factors for determining the wear resistance of the mortars, and the adhesion is clearly related to binder porosity. However, for the mortars under study, the relationship is clearly more complex, and the sand–binder bonding is also influenced by other factors.
The microcracks observed in the HC (Figure 5b,c), which appear more easily on the surface of the samples, can explain that the wear performance of the mortars activated with carbonate was not better than that of the OPC despite their lower porosity (Figure 3 and Figure 4). The microcracks can favor sand detachment from the surface and the subsequent effect of the detached hard particles as an additional abrasive agent.
For HN mortars, hydrates with low-mechanical properties (Figure 6), whose precipitation the nitrate addition favors [38,52], can explain this worse wear performance. The fast precipitation of the nitrate-AFm during the hydration consumes calcium and limits the formation of gels with good mechanical properties such as C-S-H or C-(A)-S-H [38]. Those gels are capable of creating good bonding with sand particles. The anisotropic nature of nitrate-AFm and its layered structure with weak chemical bonds in certain directions can also negatively affect the wear performance of mortars that comprises this phase. All these factors can lead to a weaker sand–binder interface, thus affecting wear performance.
The wear resistance of AAM can be higher due to the lower porosity of these materials and better sand–binder bonding in comparison with hybrid mortars [42].

3.3. Chlorides and Sulfide Oxidation Profiles and pH

Static exposure of the mortars to chloride-containing solutions has been carried out following a standardized test. The chlorides that penetrated into HC and HN mortars after the curing and drying processes at different thickness (Figure 1) have been quantified after being submitted to a non-electrically forced, standardized immersion test, and compared with the amount penetrated into the OPC mortar. It can be verified in Figure 9 that the HN mortars show the lowest chloride ingresses. The average chloride concentrations obtained for HN mortars are the lowest, which informs that the fragile phases precipitated in the pores due to nitrate addition (Figure 7 and Figure 8) seem able to decrease chloride penetration.
The LDH are well known for their excellent abilities for binding chlorides due to ion-exchange properties [61]. Anions present in the interlayer of LDHs can be exchanged with anions present in the environment. The exchange capability of different synthetic LDH/Afm phases with chlorides has been recently shown [62]. These aspects have also already been shown in traditional [58,63], alkali activated slag binders [62], and new environmentally friendly cementitious binders [64]. The formation of nitrate-AFMs (Figure 6) with a LDH structure in HN mortars explains the ability of HN to control the chloride ingress (Figure 9) in spite of the fact that their pore volume is not outstandingly reduced (Figure 3 and Figure 4a). The addition of synthetic nitrate-LDHs to slag/fly ash binders results in improved chloride resistance [65], although LDH dosage can be critical. Nitrate anions can probably be located in the interlayer of these phases, being able to be exchanged with chlorides during the diffusion test, explaining the better performance of HN compared to OPC. The lower porosity of the HN-4% compared to the HN-8% (Figure 3 and Figure 4), which explains the better mechanical performance of HN-4% in comparison with HN-8%, along with the information obtained up to now about the chloride binding abilities of the formed phases, makes the HN-4% the most interesting ecological alternative for reinforced structures exposed to chlorides among the ones explored in the present investigation.
Regarding the dried HC mortars, the chloride penetration is higher than for OPC (Figure 9) in spite of their low porosity (Figure 4). However, the presence of microcracks in HC mortars (Figure 5b,c) but not in OPC (Figure 5a) is the fact that can allow to understand that the chloride penetration is higher than for OPC (Figure 9). It is well-known that microcracks can act as preferential diffusion paths for chlorides [66]. HC mortars also have the smallest fraction of pores with small diameter (Figure 4b), which are the most effective ones for delaying chloride diffusion by physisorption. So, the HC pore distribution and microstructure can favor chloride diffusion.
On the other hand, for tidal regions or structures exposed to drying and wetting cycles in chloride-containing environments, the porous structure of the binder can affect the results regarding chloride transport obtained in static conditions [67], and this is a point that deserves further studies.
Another factor that could affect the corrosion performance of embedded steel bars in hybrid mortars like the ones considered in this study, is the sulfur content (Table 1) and its chemical form. The use of slags for the hybrid mortar manufacturing adds sulfides into the binder that, with time, could become oxidized due to air exposure [68]. The oxidation of this compound occurs in the presence of water and oxygen, and transforms sulfides into SO42−, which are assumed to be present as iron salts and give the binder a greenish color. Previous literature [69] proposes that the oxidation of sulfides in concrete takes place as reaction 1 describes:
Fe1−xS (s) + (2 − x/2) O2 + 2x OH− → (1 − x) Fe2+ + SO42− + x H2O
This reaction changes the color of the mortar, as can be seen in Table 4. The interphase observed in mortars with slag between the outer gray region, where the sulfurs have been transformed into sulfates, and the inner green region, where the sulfurs remain unreacted, is called the “oxidation front”. The measured values for the progress of the oxidation front after drying, as well as the aspect of the cross sections of the samples, are also shown in Table 4.
The oxidation rate depends strongly on the alkalinity of the pore solutions, which can be considered quite similar for the mortars under study (as will be shown hereafter), and on molecular oxygen availability [70]. This last factor is clearly related to the porosity (Figure 4) and eventual presence of defects (Figure 5) in the mortars. The drying conditions implemented in this study after curing have allowed the oxidation front to quickly progress (Table 4). The data in Table 4 show that the microstructure of HN-4% limits the progress of the oxidation front to a somewhat greater extent than the other hybrid mortars under study. The slowest progress of the oxidation front in HN-4% (Table 4) can be related to the crack absence (Figure 5d) and refined porosity (Figure 4b) in this mortar that can limit the O2 access to the bulk material during the exposure at 40% RH. The other low porous hybrid mortars studied (HC) have a microcracked structure (Figure 5b,c), and microcracks can act as preferential diffusion path favoring the progress of the oxidation front in the mortars.
Reaction 1 implies that the pH could decrease due to the hydroxyl ions consumption that this reaction entails, as this is a key point for durability if reinforced structures are considered [71,72]. So, the slightly slower progress of the oxidation front observed for HN-4% could perhaps be another positive point for this formulation.
The alkalinity of the mortars is a key factor for determining the passivity of the embedded steel. Low pH values are related to worse corrosion performance, leading pHs typical of carbonated concretes to general attack [71]. All the pHs measured in the different regions of the mortars under study are high enough to guarantee the passivity of the steel in the absence of aggressive ions in the pore solution. Moreover, in the presence of corrosive ions such as chlorides, the probability of corrosion of the steel for a given chloride concentration is related to the ratio [Cl−]/[OH−] [72].
Figure 10 shows the pH values of the HN and HC after the curing and drying processes and compares them with those of OPC in similar conditions. Figure 10a shows that the pH values of the bulk of the mortar samples after curing are clearly higher than 12.6, which is the pH of what the pore solution in equilibrium with the portlandite should have, so other more soluble hydroxides are present [28]. Those pH values are clearly in the range measured in as-cured hybrid binders by other authors, using different activators. Citrate, oxalate and silicate promote pH values above 13 [15] in fly ash-based hybrid binders, while sulfate activation in slag-based ones give values between 12.6 and 13 [23]. Carbonate activation of fly ash hybrid binders reports pH values up to 13.4 [15]. The pH of the outermost surfaces of the samples is clearly lower than in their bulk, due to the influence of a very thin carbonation layer that appears due to the reaction with the environmental CO2 [28].
The drying process (Figure 10b) hardly affects the pHs of the surfaces, as carbonation cannot progress due to the fast water elimination in this region. The pH values of the non-oxidized bulk region in the hybrid mortars also remain similar to that of the bulk before the drying step (Figure 10a). Moreover, the oxidized regions (whose thicknesses were negligible just after curing) are thick enough after drying to allow samples to be obtained for determining the pH value. In Figure 10b, it can be seen that the pH values are somewhat lower in the oxidated region than in the non-oxidated one, as reaction 1 suggested. However, if the pH of OPC mortar is measured at a certain distance from the surface, similar to that of the oxidized region in dried hybrid mortars, it can be observed that slightly lower pH values than those in the center of the sample can be measured. Hence, the CO2 from the air is able to cause a pH decrease of a similar order of magnitude at that distance from the surface during the test carried out. The most porous HN-8% (Figure 4a) seems to undergo the most marked pH decrease (Figure 10).
In summary, the quite similar pH values obtained for OPC and hybrid mortars (Figure 9), together with the similar chloride diffusion resistances found (or even somewhat better if HN mortars are considered) (Figure 8), suggest that the hybrid mortars can be a good option for carbon steel-reinforced structures if a negative interference of sulfur oxidation reaction is definitively discarded in further research, as longer exposures to dry environments would allow the progress of the oxidation front. Obviously, the potential negative effect that the limited alkaline of the hybrid mortars (Figure 6) could have on the development of the corrosive attack of the carbon steel bars in structures exposed in certain conditions is another point that merits further research.
On the other hand, the studied hybrid formulations cannot be adequate for applications that can suffer intense wear. Moreover, the exposure to continuous or cyclic mechanical stresses [73] would allow the progress of the cracks. On the other hand, there are other factors that deserve further research: exposure to dry–wet cycles could cause the progress of the microcracks due to the cyclic drying stresses in the pores [74], or detailed chemical analysis about how aging can affect the phases comprised in the binders.

4. Conclusions

After the study was completed, the following relevant conclusions were drawn about the activation with carbonates and nitrates of innovative hybrid mortars with ecological interest:
  • The activation of hybrid mortars is more effective when using carbonates than when using nitrates, if porosity reduction or compression strength are considered. After drying, the hybrid mortars activated with 6% of carbonate are less porous and more compression resistant than OPC.
  • Microcracks tend to be generated in the hybrid mortar microstructure when carbonate is used for activation.
  • Precipitation of nitrate-AFm hinders chloride diffusion into the mortars.
  • Mortar activation with moderate amounts of nitrates (4% wt.) promotes higher resistance to chloride penetration than that of the OPC used as reference, and reasonable high mechanical properties.
  • The wear performances of the hybrid mortars under study are worse than that of OPC. This weakness, if it is not overcome in future formulations, can limit certain eventual applications.
  • pH values of hybrid mortars are highly alkaline, as those of OPC. However, the long-term effect of their lower alkaline reserve warrants further study.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/app15148055/s1, Figure S1. Examples of intrusion-extrusion curves obtained during mercury intrusion porosimetry tests for the 5 studied materials.

Author Contributions

Conceptualization, A.B.; methodology, A.B., M.T.-C. and A.C.-H.; validation, A.B. and F.V.; formal analysis, A.C.-H.; investigation, A.C.-H.; resources, A.B. and M.T.-C.; data curation, A.C.-H.; writing—original draft preparation, A.C.-H. and A.B.; writing—review and editing, A.C.-H., A.B., F.V. and M.T.-C.; supervision, A.B. and F.V.; project administration, F.V. and M.T.-C.; funding acquisition, A.B. and M.T.-C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by PID2021-125810OB-C22 project “Development of eco-efficient cementitious materials with low-impact and high durability” (EcoCeMat), financed by MCIN/AEI/10.13039/501100011033/ and FEDER “Una manera de hacer Europa”.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AAMAlkali-Activated Materials
ACRAlkali-Carbonate Reaction
AFmAluminate Ferrite Monosulfate
AFtAluminate Ferrite Trisulfate
ASTMAmerican Society for Testing and Materials
C-A-S-HCalcium Aluminum Silicate Hydrate
C-S-HCalcium Silicate Hydrate
CHCalcium Hydroxide (Portlandite)
DTADifferential Thermal Analysis
HCHybrid Cement activated with Carbonate
HNHybrid Cement activated with Nitrate
LDHLayered Double Hydroxide
N-A-S-HSodium Aluminum Silicate Hydrate
OPCOrdinary Portland Cement
PCPortland Cement
RHRelative Humidity
SEMScanning Electron Microscopy
TGThermogravimetry
UNE-ENEuropean Standard–Spanish Implementation
XRFX-ray Fluorescence
s/bSand-to-Binder Ratio

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Figure 1. Schematic of the chloride diffusion test and chloride ion determination used for hybrid mortars: (a) NaCl immersion test; (b) depth profiling of the specimen for sample extraction; (c) chloride determination by means of a selective electrode.
Figure 1. Schematic of the chloride diffusion test and chloride ion determination used for hybrid mortars: (a) NaCl immersion test; (b) depth profiling of the specimen for sample extraction; (c) chloride determination by means of a selective electrode.
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Figure 2. Dimensional variation in hybrid mortars during their curing and drying periods.
Figure 2. Dimensional variation in hybrid mortars during their curing and drying periods.
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Figure 3. Porosity values determined by the Archimedes method. Striped bars plot the porosity values of the mortars just after curing, while full color bars plot the porosity of the mortars after the drying step.
Figure 3. Porosity values determined by the Archimedes method. Striped bars plot the porosity values of the mortars just after curing, while full color bars plot the porosity of the mortars after the drying step.
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Figure 4. Mercury intrusion porosimetry results obtained for the cured and dried mortars. (a) Total porosity and percentage of pores with diameters in different ranges; (b) % of mortar volume occupied by pores with different diameters.
Figure 4. Mercury intrusion porosimetry results obtained for the cured and dried mortars. (a) Total porosity and percentage of pores with diameters in different ranges; (b) % of mortar volume occupied by pores with different diameters.
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Figure 5. SEM images of dried hybrid mortars: (a) OPC; (b) HC-3%Na2CO3; (c) HC-6%Na2CO3; (d) HN-4%NaNO3; (e) HN-8%NaNO3.
Figure 5. SEM images of dried hybrid mortars: (a) OPC; (b) HC-3%Na2CO3; (c) HC-6%Na2CO3; (d) HN-4%NaNO3; (e) HN-8%NaNO3.
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Figure 6. (a) DTA for the five mortars under study; (b) detail of temperature range where the peaks corresponding to the decomposition of phases formed due to nitrate addition can be observed.
Figure 6. (a) DTA for the five mortars under study; (b) detail of temperature range where the peaks corresponding to the decomposition of phases formed due to nitrate addition can be observed.
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Figure 7. Compression strength of the samples under study. Striped bars plot the strength values of the mortars just after curing, while full color bars plot the strength of the mortars after the drying step.
Figure 7. Compression strength of the samples under study. Striped bars plot the strength values of the mortars just after curing, while full color bars plot the strength of the mortars after the drying step.
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Figure 8. Height losses suffered by the mortars under study during the abrasive cycles of the Böhme test: (a) as-cured mortars; (b) mortars cured and dried at 40% RH.
Figure 8. Height losses suffered by the mortars under study during the abrasive cycles of the Böhme test: (a) as-cured mortars; (b) mortars cured and dried at 40% RH.
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Figure 9. Chloride concentration determined after non-electrically forced diffusion test in cured and dried at 40% RH mortars.
Figure 9. Chloride concentration determined after non-electrically forced diffusion test in cured and dried at 40% RH mortars.
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Figure 10. pH values for hybrid mortars as determined using a pH electrode: (a) after the curing process and (b) after the curing and drying processes at 40% RH. Striped bars plot the pH values of the mortars measured at the surface, the full color bars plot the pH at the mortar center, and the checkered bars plot the pH values measured at 3–4 mm from the surface.
Figure 10. pH values for hybrid mortars as determined using a pH electrode: (a) after the curing process and (b) after the curing and drying processes at 40% RH. Striped bars plot the pH values of the mortars measured at the surface, the full color bars plot the pH at the mortar center, and the checkered bars plot the pH values measured at 3–4 mm from the surface.
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Table 1. Chemical composition of the precursors used for manufacturing the binders under study determined by XRF.
Table 1. Chemical composition of the precursors used for manufacturing the binders under study determined by XRF.
Composition (% by Weight)
MgOAl2O3SiO2SO3K2OCaOTiO2Fe2O3
Type I PC1.34.717.84.61.467.30.22.6
Type II/B-M PC2.26.219.44.11.661.70.84.0
Slag10.910.331.91.40.544.10.60.3
Table 2. Particle size of the precursors used for the manufacture of the binders, determined by dynamic light scattering.
Table 2. Particle size of the precursors used for the manufacture of the binders, determined by dynamic light scattering.
Particle Size (μm)
d10d50d90
Type I PC3.215.940.7
Type II/B-M PC3.514.143.0
Slag3.711.228.5
Table 3. Dosage and workability of the mortars under study.
Table 3. Dosage and workability of the mortars under study.
LabelPrecursorsActivatorNa/Ca mol RatioWorkability (cm)w/bs/b
OPC100% Type II/B-M PC----------17.0 ± 0.10.503/1
HC-3%78.5% slag + 18.5% Type I PC3% Na2CO30.0616.5 ± 0.10.47
HC-6%77% slag + 17% Type I PC6% Na2CO30.1216.6 ± 0.10.47
HN-4% 78% slag + 18% Type I PC4% NaNO30.0616.8 ± 0.10.45
HN-8%76% slag + 16% Type I PC8% NaNO30.1318.9 ± 0.10.45
Table 4. Thickness of the oxidation layer in hybrid mortars after drying and images of the cross sections of the samples of the different hybrid mortars.
Table 4. Thickness of the oxidation layer in hybrid mortars after drying and images of the cross sections of the samples of the different hybrid mortars.
HC-3%HC-6%HN-4%HN-8%
Cross-sectional view of the mortar samplesApplsci 15 08055 i001Applsci 15 08055 i002Applsci 15 08055 i003Applsci 15 08055 i004
Oxidation depth (mm)9.5 ± 0.69.6 ± 0.58.6 ± 0.59.5 ± 0.8
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MDPI and ACS Style

Cruz-Hernández, A.; Velasco, F.; Torres-Carrasco, M.; Bautista, A. Hybrid Mortars Activated with Alternative Steel-Compatible Salts: Impact on Chloride Diffusion and Durability. Appl. Sci. 2025, 15, 8055. https://doi.org/10.3390/app15148055

AMA Style

Cruz-Hernández A, Velasco F, Torres-Carrasco M, Bautista A. Hybrid Mortars Activated with Alternative Steel-Compatible Salts: Impact on Chloride Diffusion and Durability. Applied Sciences. 2025; 15(14):8055. https://doi.org/10.3390/app15148055

Chicago/Turabian Style

Cruz-Hernández, Angily, Francisco Velasco, Manuel Torres-Carrasco, and Asunción Bautista. 2025. "Hybrid Mortars Activated with Alternative Steel-Compatible Salts: Impact on Chloride Diffusion and Durability" Applied Sciences 15, no. 14: 8055. https://doi.org/10.3390/app15148055

APA Style

Cruz-Hernández, A., Velasco, F., Torres-Carrasco, M., & Bautista, A. (2025). Hybrid Mortars Activated with Alternative Steel-Compatible Salts: Impact on Chloride Diffusion and Durability. Applied Sciences, 15(14), 8055. https://doi.org/10.3390/app15148055

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