Alkali-Activated Hybrid Cements Based on Fly Ash and Construction and Demolition Wastes Using Sodium Sulfate and Sodium Carbonate

This article demonstrates the possibility of producing alkali-activated hybrid cements based on fly ash (FA), and construction and demolition wastes (concrete waste, COW; ceramic waste, CEW; and masonry waste, MAW) using sodium sulfate (Na2SO4) (2–6%) and sodium carbonate (Na2CO3) (5–10%) as activators. From a mixture of COW, CEW, and MAW in equal proportions (33.33%), a new precursor called CDW was generated. The precursors were mixed with ordinary Portland cement (OPC) (10–30%). Curing of the materials was performed at room temperature (25 °C). The hybrid cements activated with Na2SO4 reached compressive strengths of up to 31 MPa at 28 days of curing, and the hybrid cements activated with Na2CO3 yielded compressive strengths of up to 22 MPa. Based on their mechanical performance, the optimal mixtures were selected: FA/30OPC-4%Na2SO4, CDW/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, and CDW/30OPC-10%Na2CO3. At prolonged ages (180 days), these mixtures reached compressive strength values similar to those reported for pastes based on 100% OPC. A notable advantage is the reduction of the heat of the reaction, which can be reduced by up to 10 times relative to that reported for the hydration of Portland cement. These results show the feasibility of manufacturing alkaline-activated hybrid cements using alternative activators with a lower environmental impact.


Introduction
Alkali-activated cements and geopolymers are materials that, thanks to their good durability and mechanical, physical, and thermal properties, compete with materials such as Portland cement; red or kaolinitic clays; and many raw materials used to manufacture common or special ceramic materials, such as refractory ceramics, photoluminescent ceramics, and antibacterial ceramics. From an environmental point of view, this material can have a negative impact, mainly associated with the type of raw materials that compose it. For example, the use of silicates and hydroxides as activators, although they give rise to high mechanical strength and durability, are the activators that exhibit the highest CO 2 load per kg of material produced [1,2]. However, in relation to the production process, the alkaliactivated materials have advantages of low energy consumption and low generation of CO 2 emissions, as its production is carried out at temperatures below 100 • C.
Considering that the key factor associated with the life cycle of a product is its positive contribution to climate change, the relevant indicator is the amount of greenhouse gases emitted into the environment, which is calculated by means of kilograms of CO 2 equivalents (kg CO 2 eq) of the product, or its so-called carbon footprint. The main relevant environmentally oriented approaches to reduce the kg CO 2 eq associated with alkaline cements are the following: substituting alkaline activators such as sodium/potassium silicate and alkali hydroxides; use of supplementary raw materials such as industrial, with a high percentage of FA addition (60%) alkali-activated with sodium sulfate, showed better resistance performance, lower porosity, and reduced chloride penetration compared with the same type of concrete without activator [18]. Adesina et al. [1] recently reviewed the properties of cementitious materials activated with sodium sulfate, and stated that it is possible to achieve adequate strengths for structural applications by controlling the dosage of the activator, increasing the fineness of the precursor, or adding OPC or additives such as silica fume.
This article aims to investigate the compressive strength and microstructure of hybrid cements (10-30% OPC) based on different precursors such as FA, concrete waste (COW), and red ceramic waste (CEW) using sulfate (Na 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ) as activators. Additionally, a precursor obtained from the mixture COW + CEW + masonry waste (MAW) in equal proportions (33.33%), which was called CDW, was investigated. It is expected that the results of this study will be the basis for the future use of FA and CDW as precursors for the manufacture of hybrid cements activated with sodium sulfate and carbonate, which will generate a lower environmental impact.

Materials and Experimental Methodology
As raw materials for the production of the alkaline activation hybrid systems, fly ash (FA), construction and demolition waste (COW, CEW, and MAW), and ordinary Portland cement (OPC) were used. The chemical composition of these materials, determined by X-ray fluorescence (XRF) using a MagiX-Pro PW-2440 spectrometer (Phillips PANalytical, Tollerton, USA), is given in Table 1. Approximately 88.98% of FA is composed of oxides of silica, alumina, and iron, and the unburned content (LOI) was 6.35%, so it could be classified as a type F, according to the ASTM C618 standard. The construction and demolition wastes are mainly composed of SiO 2 and Al 2 O 3 , and COW has a high CaO content. Of the three materials, the highest unburned content (15.94%) was observed for COW. A new precursor called CDW was generated from the mixture COW + CEW + MAW in equal proportions (33.33%), and its composition is also included in Table 1. The mean particle size (D(4,3)) of the precursors FA, COW, CEW, and CDW, analyzed by laser granulometry with a Mastersizer-2000 (Malvern Instruments equipment, Malvern, UK), was 36 µm, 57 µm, 40 µm, and 75 µm, respectively. The particle size distributions of these materials are presented in Figure 1.
The production of the alkaline activation materials from FA and the construction and demolition wastes (CEW, COW, and CDW) is summarized in Figure 2. The pastes were prepared in a Hobart mixer with a total mixing time of 5 min. The liquid/solid ratio (L/S) was 0.3. OPC was added in small proportions (10-30% by weight) to produce a "hybrid cement" (binder), which hardened and developed strength at room temperature. The fresh pastes were molded into 20 mm cubes and vibrated for 30 s on an electric vibrating table to remove the trapped air. Subsequently, the molds were covered with a polyethylene film and held in a laboratory environment for 24 h. After this time, the specimens were removed from the molds and placed in a final curing chamber (≈25 • C) with a relative humidity greater than 80% until reaching the test age. were removed from the molds and placed in a final curing chamber (≈25 °C) with a relative humidity greater than 80% until reaching the test age.  To optimize the mixing proportions of the hybrid cement, the effects of the alkaline activator content (Na2SO4-2, 4, and 6%; Na2CO3-5, 7.5, and 10%) and OPC content (10, 20, and 30%) were evaluated by assessing the compressive strength of each paste at curing ages of 3, 7, 28, and 90 days. From these results, the optimal proportions were selected. In the produced pastes, the setting time, the heat of the reaction, and the mechanical resistance to compression were determined at ages of up to 180 days of curing. Pastes without alkaline activator and based on 100% OPC (type GU) were used as reference materials. The compressive strength was evaluated with an INSTRON 3369 universal testing machine, which has a capacity of 50 kN force, at a speed of 1 mm/min. For each mixture, a minimum of three specimens were tested. The setting time was determined according to the procedure described in the ASTM C191 standard (method B), and the heat evolution (during alkaline activation) and the total heat of reaction (68 h) were determined in an I-Cal 8000 isothermal calorimeter (Calmetrix, Needham, MA, USA). were removed from the molds and placed in a final curing chamber (≈25 °C) with a relative humidity greater than 80% until reaching the test age.  To optimize the mixing proportions of the hybrid cement, the effects of the alkaline activator content (Na2SO4-2, 4, and 6%; Na2CO3-5, 7.5, and 10%) and OPC content (10, 20, and 30%) were evaluated by assessing the compressive strength of each paste at curing ages of 3, 7, 28, and 90 days. From these results, the optimal proportions were selected. In the produced pastes, the setting time, the heat of the reaction, and the mechanical resistance to compression were determined at ages of up to 180 days of curing. Pastes without alkaline activator and based on 100% OPC (type GU) were used as reference materials. The compressive strength was evaluated with an INSTRON 3369 universal testing machine, which has a capacity of 50 kN force, at a speed of 1 mm/min. For each mixture, a minimum of three specimens were tested. The setting time was determined according to the procedure described in the ASTM C191 standard (method B), and the heat evolution (during alkaline activation) and the total heat of reaction (68 h) were determined in an I-Cal 8000 isothermal calorimeter (Calmetrix, Needham, MA, USA). To optimize the mixing proportions of the hybrid cement, the effects of the alkaline activator content (Na 2 SO 4 -2, 4, and 6%; Na 2 CO 3 -5, 7.5, and 10%) and OPC content (10, 20, and 30%) were evaluated by assessing the compressive strength of each paste at curing ages of 3, 7, 28, and 90 days. From these results, the optimal proportions were selected. In the produced pastes, the setting time, the heat of the reaction, and the mechanical resistance to compression were determined at ages of up to 180 days of curing. Pastes without alkaline activator and based on 100% OPC (type GU) were used as reference materials. The compressive strength was evaluated with an INSTRON 3369 universal testing machine, which has a capacity of 50 kN force, at a speed of 1 mm/min. For each mixture, a minimum of three specimens were tested. The setting time was determined according to the procedure described in the ASTM C191 standard (method B), and the heat evolution (during alkaline activation) and the total heat of reaction (68 h) were determined in an I-Cal 8000 isothermal calorimeter (Calmetrix, Needham, MA, USA).
Visual inspection of the samples was performed to observe the surface changes generated throughout the exposure time. The structural and mineralogical changes in each of the samples were evaluated using the following techniques: -Fourier transfer infrared (FTIR) spectroscopy was performed using an R-100 spectrometer (Perkin Elmer, Shelton, CT, USA) in transmittance mode with a frequency between 4000 and 450 cm −1 . The samples were evaluated using compressed KBr pellets.
Molecules 2021, 26, 7572 5 of 14 -Scanning electron microscopy (SEM-EDS) was performed using a JSM 6490LV JEOL electron microscope (JEOL, Tokyo, Japan) with an acceleration voltage of 20 kV. The samples were evaluated in low-vacuum mode with a Link-Isis X-ray spectrometer (Oxford Instruments, Abingdon, UK) coupled to the microscope.

Results and Analysis
3.1. Characterization of Hybrid Cements Activated with Na 2 SO 4 and Na 2 CO 3 The effects of the Na 2 SO 4 /Na 2 CO 3 and OPC contents on the compressive strength of the alkaline-activated hybrid systems can be observed in Figures 3 and 4. In general, with higher cement contents (30%), higher compressive strength results were obtained at different curing ages, which coincides with the results reported by other authors [19]. The hybrid CDW/OPC systems activated with sodium sulfate (Figure 3d) demonstrated better mechanical performance than that observed for COW and CEW. The good behavior of CDW/OPC hybrid systems activated with Na2SO4 coincides with that reported by other researchers [17], who demonstrated that, the higher the OPC content, the greater the resistance. Furthermore, they report optimal percentages of Na2SO4 between 1 and 5%.
Therefore, for the following phases of the study, FA/30OPC-4%Na2SO4, CDW/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, and CDW/30OPC-10%Na2CO3, were selected as optimal mixtures.   Figure 5 shows the evolution of the compressive strength of the optimal cementitious materials mentioned above as a function of curing time (1-180 days) in comparison with the reference pastes, which correspond to the same type of mixture without activator and were based on 100% OPC. All the pastes tended to exhibit increased resistance with the evolution of curing time, highlighting that, in general, the mechanical performance of the OPC paste was superior to that of the pastes of the different alkaline-activated hybrid systems and the pastes without activation at short curing ages. However, it should be noted that this difference decreased from 28 to 180 days, a period in which the alkaline activation pastes FA/30OPC-4%Na2SO4, CDW/30OPC-4%Na2SO4, and FA/30OPC-10%Na2CO3 presented a higher strength gain than 100% OPC paste. The FA/30OPC-4%Na2SO4 paste exhibited the best performance, with a compressive strength value of 45 MPa, 22% better than the values for OPC pastes and systems without alkaline activation. This behavior agrees with that reported by Rashad et al. [5], Joseph et al. [19], Velandia et al. [23], and Zhao et al. [21], who obtained similar resistance increases for alkaline activation systems activated with sodium sulfate at long ages. The increases in resistance for the In the case of hybrid systems activated with Na 2 SO 4 (Figure 3), activation with 4% Na 2 SO 4 produced a positive effect on the compressive strength, coinciding with the activator percentages recommended by other authors [5,20]. An increase in Na 2 SO 4 content (6%) caused a decrease in the compressive strength of the hybrid systems at different curing ages, coinciding with the results reported by Zhao et al. [21]. This finding demonstrates the importance of studying the proportion of the activator in the mixture and determining the optimal content. In the case of sodium sulfate, the mechanical strength is attributable to the formation of ettringite, which contributes to greater densification of the material [15,19]. However, by increasing the proportion of the activator (>4%), the formation of an excess of ettringite likely causes microcracks, which can negatively affect the strength.
The compressive strengths of the hybrid systems activated with Na 2 CO 3 are shown in Figure 4. The strength gain of the FA/OPC systems at early ages (3 and 7 days) was high, and a gradual increase was observed with prolonged curing time. Similarly, increasing the proportion of the activator from 5% to 10% significantly improved the strength of all mixtures. This result indicates that increases in alkali content (Na 2 O) play a vital role in the development of mechanical strength. The improvement can be attributed to the fact that a higher Na 2 O content increases the pH of the system, which increases the dissolution of reactive SiO 2 and Al 2 O 3 [22]. This greater dissolution in turn increases the generation of reaction products and, consequently, increases resistance. In this study, compressive strengths of up to 35 MPa were reached at 90 days of curing; these results agree with those reported by Abdalqader et al. [22] and Li and Sun [10]. The inclusion of up to 30% OPC in the systems showed a satisfactory increase in the compressive strength, coinciding with that observed in the systems activated with sodium sulfate.
In general, from the results reported in Figures 3 and 4, the hybrid cementitious systems with a higher OPC content (30%) reached strengths of up to 37 MPa when subjected to alkaline activation with Na 2 SO 4 (4%) and 30 MPa when activated with Na 2 CO 3 (10%). These results correspond to those obtained at 90 days of curing. FA/OPC and CDW/OPC stood out among the different systems evaluated.
The increase in compressive strength with the curing age of the FA/OPC mixtures activated with Na 2 SO 4 and Na 2 CO 3 (Figures 3a and 4a) is higher than that of COW/OPC, CEW/OPC, and CDW/OPC. This difference can be attributed to the smaller particle size of FA and the higher proportion of amorphous phase present and generally coincides with the results reported by other studies [19].
The hybrid CDW/OPC systems activated with sodium sulfate (Figure 3d) demonstrated better mechanical performance than that observed for COW and CEW. The good behavior of CDW/OPC hybrid systems activated with Na 2 SO 4 coincides with that reported by other researchers [17], who demonstrated that, the higher the OPC content, the greater the resistance. Furthermore, they report optimal percentages of Na 2 SO 4 between 1 and 5%.
Therefore, for the following phases of the study, FA/30OPC-4%Na 2 SO 4 , CDW/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , and CDW/30OPC-10%Na 2 CO 3 , were selected as optimal mixtures. Figure 5 shows the evolution of the compressive strength of the optimal cementitious materials mentioned above as a function of curing time (1-180 days) in comparison with the reference pastes, which correspond to the same type of mixture without activator and were based on 100% OPC. All the pastes tended to exhibit increased resistance with the evolution of curing time, highlighting that, in general, the mechanical performance of the OPC paste was superior to that of the pastes of the different alkaline-activated hybrid systems and the pastes without activation at short curing ages. However, it should be noted that this difference decreased from 28 to 180 days, a period in which the alkaline activation pastes FA/30OPC-4%Na 2 SO 4 , CDW/30OPC-4%Na 2 SO 4 , and FA/30OPC-10%Na 2 CO 3 presented a higher strength gain than 100% OPC paste. The FA/30OPC-4%Na 2 SO 4 paste exhibited the best performance, with a compressive strength value of 45 MPa, 22% better than the values for OPC pastes and systems without alkaline activation. This behavior agrees with that reported by Rashad et al. [5], Joseph et al. [19], Velandia et al. [23], and Zhao et al. [21], who obtained similar resistance increases for alkaline activation systems activated with sodium sulfate at long ages. The increases in resistance for the different alkaline activation systems are related to the densification of the matrix and the greater formation of "hydrated sodium-calcium aluminosilicates" or gels, (N,C)-ASH, CASH, carbonates, and ettringite with prolonged curing time [14,22]. The samples activated with Na 2 CO 3 show slow compressive strength development. The strength at early ages is low; however, at later ages, an increase is observed ( Figure 5). This increase is associated with the formation of calcium carbonate, which densifies the structure [4]. different alkaline activation systems are related to the densification of the matrix and the greater formation of "hydrated sodium-calcium aluminosilicates" or gels, (N,C)-ASH, CASH, carbonates, and ettringite with prolonged curing time [14,22]. The samples activated with Na2CO3 show slow compressive strength development. The strength at early ages is low; however, at later ages, an increase is observed ( Figure 5). This increase is associated with the formation of calcium carbonate, which densifies the structure [4].  Figure 6 presents the calorimetric curves of the alkaline activation systems FA/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, CDW/30OPC-4%Na2SO4, and CDW/30OPC-10%Na2CO3, compared to 100% OPC paste (100% OPC + H2O).  Figure 6 presents the calorimetric curves of the alkaline activation systems FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 , compared to 100% OPC paste (100% OPC + H 2 O).

Reaction Monitoring and Characterization of Reaction Products
For the different alkaline-activated hybrid systems, there is a first peak that, in general, occurs in less than 10 min. The intensity of the peak varies depending on the activation conditions, but the maximum value for the systems activated with Na 2 SO 4 is 8 J/gh, and that for the systems activated with Na 2 CO 3 is 50 J/gh. For the systems activated with Na 2 SO 4 , the acceleration phase (~5-15 h, depending on the system) occurs at a longer time than that observed for OPC hydration (~2-10 h), mainly owing to the dilution effect [15]. For the FA/30OPC-10%Na 2 CO 3 system, a single peak is observed, which could be related to the greater alkalinity of the system and the greater reactivity of FA, as a result of which the preinduction, induction, and acceleration peaks are not separately identifiable because dissolution and precipitation reactions develop simultaneously for this system [24]. For the CDW/30OPC-Na 2 SO 4 system, the acceleration peak is observed, which is attributed to the formation of reaction products of CASH/CSH and (Ca, Na)-ASH. The heat evolution rate for the CDW/30OPC-10%Na 2 CO 3 system during the acceleration period is 13 J/gh, which is higher than that for the systems activated with Na 2 SO 4 . This result could be related to the greater alkalinity of the medium. Similarly, a reduction in the time of the acceleration peak is observed, which could be related to the greater reactivity of the system.
The total heat released for the hybrid systems FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 is compared with that of 100% OPC in Table 2. Alkaline activation systems activated with Na 2 SO 4 are much more reactive than hybrid systems activated with Na 2 CO 3 , although the total heat released for the alkaline activation systems is much lower than that of 100% OPC. In general, the greatest heat release in these systems occurs in the first hours and is rapidly attenuated or stabilized. This behavior differs from the hydration process of OPC, where the release of heat increases over time. Table 2 shows that the systems activated with Na 2 CO 3 present a notable reduction in the initial and final setting times compared with those observed for the hybrid system activated with Na 2 SO 4 . This result could be related to the greater alkalinity of Na 2 CO 3 (pH = 12.16) than Na 2 SO 4 (pH =~8), which accelerates the alkaline activation reactions. The final setting time for the FA/30OPC-10%Na 2 CO 3 system is reduced by 20% when compared with the FA/30OPC-4%Na 2 SO 4 system. In the case of the CDW/hybrid systems, 30OPC-10%Na 2 CO 3 shows a reduction of 80% when compared with CDW/30OPC-4%Na 2 SO 4 . For the different alkaline-activated hybrid systems, there is a first peak that, in general, occurs in less than 10 min. The intensity of the peak varies depending on the activation conditions, but the maximum value for the systems activated with Na2SO4 is 8 J/gh, and that for the systems activated with Na2CO3 is 50 J/gh. For the systems activated with Na2SO4, the acceleration phase (~5-15 h, depending on the system) occurs at a longer time than that observed for OPC hydration (~2-10 h), mainly owing to the dilution effect [15]. For the FA/30OPC-10%Na2CO3 system, a single peak is observed, which could be related to the greater alkalinity of the system and the greater reactivity of FA, as a result of which the preinduction, induction, and acceleration peaks are not separately identifiable because dissolution and precipitation reactions develop simultaneously for this system [24]. For the CDW/30OPC-Na2SO4 system, the acceleration peak is observed, which is attributed to the formation of reaction products of CASH/CSH and (Ca, Na)-ASH. The heat evolution rate for the CDW/30OPC-10%Na2CO3 system during the acceleration period is 13 J/gh, which is higher than that for the systems activated with Na2SO4. This result could be related to the greater alkalinity of the medium. Similarly, a reduction in the time of the acceleration peak is observed, which could be related to the greater reactivity of the system.
The total heat released for the hybrid systems FA/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, CDW/30OPC-4%Na2SO4, and CDW/30OPC-10%Na2CO3 is compared with that of 100% OPC in Table 2. Alkaline activation systems activated with Na2SO4 are much more reactive than hybrid systems activated with Na2CO3, although the total heat released for the alkaline activation systems is much lower than that of 100% OPC. In general, the greatest heat release in these systems occurs in the first hours and is rapidly attenuated or  The FTIR results for the hybrid systems FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 at 180 days of curing are shown in Figure 7. The broad band centered at ±3445 cm −1 is associated with stretching of O-H bonds, which indicates the presence of adsorbed water [25,26]. The bands located at 1653 cm −1 in the materials activated with Na 2 SO 4 and Na 2 CO 3 correspond to the bending vibration of the water molecules linked to the inorganic structure and the presence of -OH groups [17,25,27,28]. A greater intensity of this band has been associated with greater amounts of reaction products of CSH, CASH, and (N, C)-ASH types [17].   Figure 8 presents the SEM-EDS microstructural analysis of the FA/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, CDW/30OPC-4%Na2SO4, and CDW/30OPC-10%Na2CO3 hybrid systems at 180 days of curing. Through this technique, it was possible to study the elemental chemical composition of the different systems and create ternary diagrams of SiO2-Al2O3-CaO ( Figure 9). These diagrams represent a total of 24 EDS points analyzed in each of the systems (FA/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, CDW/30OPC-4%Na2SO4, and CDW/30OPC-10%Na2CO3). The results for the different hybrid alkaline activation systems are grouped into "low calcium compositions" and "medium-high calcium compositions". Some authors have correlated representative areas of the SiO2-Al2O3-CaO ternary diagram with reaction product type, (N,C)-ASH (poor in Ca 2+ ), CSH, and CASH (rich in Ca 2+ ) for alkali-activated materials and hybrid cements based on other precursors, such as FA, GBFS, and/or natural pozzolans [30,[35][36][37][38]. In the systems activated with Na 2 SO 4 and Na 2 CO 3 , two bands appear at 1090 and 970 cm −1 , and these bands indicate the substitution of SiO 4 species by AlO 4 [27,28] due to the formation of new products; CSH gel (normally detected at approximately 970 cm −1 ) [28] and gels such as C-(A)-SH have been associated with peaks at wavenumbers of 1090, 1085, and 1005 cm −1 [17]. In addition, this main band is wide and could indicate the coexistence of several types of gels, such as C-(A)-SH, CSH, and (N),C-(A)-SH [29]. Calcium carbonates are present in different alkaline activation systems, as indicated by the band at approximately 1430 cm −1 , which is associated with the asymmetric tension of O-C-O bonds in CO 3 −2 groups [30], and a band at approximately 850 cm −1 that indicates the stretching vibration of C-O groups [31][32][33]. The alkaline activation systems FA/30OPC-10%Na 2 CO 3 and CDW/30OPC-10%Na 2 CO 3 exhibit greater intensity than that observed for the systems activated with Na 2 SO 4 . The band at approximately 720 cm −1 is attributed to the presence of mullite present in the raw materials (FA and CDW) [34]. The band identified at 1090 cm −1 for the samples activated with sodium sulfate indicates the presence of anhydrous calcium sulfate (CaSO 4 ); other bands associated with the presence of semihydrated calcium sulfates located at 660 cm −1 [26] were not observed, possibly owing to the overlap of the bands associated with aluminate species. Figure 8 presents the SEM-EDS microstructural analysis of the FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 hybrid systems at 180 days of curing. Through this technique, it was possible to study the elemental chemical composition of the different systems and create ternary diagrams of SiO 2 -Al 2 O 3 -CaO (Figure 9). These diagrams represent a total of 24 EDS points analyzed in each of the systems (FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 ). The results for the different hybrid alkaline activation systems are grouped into "low calcium compositions" and "medium-high calcium compositions". Some authors have correlated representative areas of the SiO 2 -Al 2 O 3 -CaO ternary diagram with reaction product type, (N,C)-ASH (poor in Ca 2+ ), CSH, and CASH (rich in Ca 2+ ) for alkali-activated materials and hybrid cements based on other precursors, such as FA, GBFS, and/or natural pozzolans [30,[35][36][37][38].   Elemental EDS color mapping was performed on a specific area of the FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 hybrid systems ( Figure 10 displays systems activated with Na 2 SO 4, and Figure 11 displays systems activated with Na 2 CO 3 ). The results showed a homogeneous distribution of silica (Si: blue), alumina (Al: green), sodium (Na: turquoise), and calcium (Ca: yellow) in the pastes, corroborating the formation and/or coexistence of NASH, CSH, CASH, and (N,C)-ASH gels resulting from chemical interactions between the precursors (FA and CDW), OPC, and the alkaline activator (Na 2 SO 4 or Na 2 CO 3 ).

Conclusions
In this study, the effects of Na2SO4 and Na2CO3 on the alkaline activation of hybrid cements based on fly ash and construction and demolition wastes were investigated. The proportions of activators and OPC in the mixtures were studied, and the compressive strength at ages of up to 180 days, the hydration process, and the microstructure were evaluated. From the results, the following conclusions can be drawn.
FA has greater reactivity than construction and demolition waste in the presence of alkaline media (Na2SO4 and Na2CO3), where an optimal proportion of activating solution yields alkaline-activated materials with a mechanical strength of 25 MPa for hybrid cements based on FA with Na2CO3 activation and 32 MPa for systems activated with Na2SO4 at 28 days of curing, room temperature (24 °C), and relative humidity >90%.
In hybrid alkaline activation systems based on construction and demolition waste, better performance was obtained in those based on the mixture of CDW/OPC, which may be related to the positive synergistic effect promoted by the combination of COW powder, CEW, and MAW.
In general, the optimal percentage of Na2SO4 activator was 4%, and in the case of the Na2CO3 activator, the best mechanical behavior was obtained when using 10%. Regarding the OPC content, the greatest strength increases were achieved with 30% OPC.
The isothermal calorimetry results showed that the heat of the reaction of the hybrid cements activated with Na2SO4 is higher than that of the hybrid cements activated with Na2CO3. This behavior indicates that the systems activated with Na2SO4 present a greater degree of reaction. However, in general, the heat of reaction of the activated systems is lower than the total heat released in the hydration of the 100% OPC pastes. The SEM-EDS microstructural analysis performed on the hybrid pastes FA/30OPC-4%Na2SO4,

Conclusions
In this study, the effects of Na2SO4 and Na2CO3 on the alkaline activation of hybrid cements based on fly ash and construction and demolition wastes were investigated. The proportions of activators and OPC in the mixtures were studied, and the compressive strength at ages of up to 180 days, the hydration process, and the microstructure were evaluated. From the results, the following conclusions can be drawn.
FA has greater reactivity than construction and demolition waste in the presence of alkaline media (Na2SO4 and Na2CO3), where an optimal proportion of activating solution yields alkaline-activated materials with a mechanical strength of 25 MPa for hybrid cements based on FA with Na2CO3 activation and 32 MPa for systems activated with Na2SO4 at 28 days of curing, room temperature (24 °C), and relative humidity >90%.
In hybrid alkaline activation systems based on construction and demolition waste, better performance was obtained in those based on the mixture of CDW/OPC, which may be related to the positive synergistic effect promoted by the combination of COW powder, CEW, and MAW.
In general, the optimal percentage of Na2SO4 activator was 4%, and in the case of the Na2CO3 activator, the best mechanical behavior was obtained when using 10%. Regarding the OPC content, the greatest strength increases were achieved with 30% OPC.
The isothermal calorimetry results showed that the heat of the reaction of the hybrid cements activated with Na2SO4 is higher than that of the hybrid cements activated with Na2CO3. This behavior indicates that the systems activated with Na2SO4 present a greater degree of reaction. However, in general, the heat of reaction of the activated systems is lower than the total heat released in the hydration of the 100% OPC pastes. The SEM-EDS microstructural analysis performed on the hybrid pastes FA/30OPC-4%Na2SO4,

Conclusions
In this study, the effects of Na 2 SO 4 and Na 2 CO 3 on the alkaline activation of hybrid cements based on fly ash and construction and demolition wastes were investigated. The proportions of activators and OPC in the mixtures were studied, and the compressive strength at ages of up to 180 days, the hydration process, and the microstructure were evaluated. From the results, the following conclusions can be drawn.
FA has greater reactivity than construction and demolition waste in the presence of alkaline media (Na 2 SO 4 and Na 2 CO 3 ), where an optimal proportion of activating solution yields alkaline-activated materials with a mechanical strength of 25 MPa for hybrid cements based on FA with Na 2 CO 3 activation and 32 MPa for systems activated with Na 2 SO 4 at 28 days of curing, room temperature (24 • C), and relative humidity >90%.
In hybrid alkaline activation systems based on construction and demolition waste, better performance was obtained in those based on the mixture of CDW/OPC, which may be related to the positive synergistic effect promoted by the combination of COW powder, CEW, and MAW.
In general, the optimal percentage of Na 2 SO 4 activator was 4%, and in the case of the Na 2 CO 3 activator, the best mechanical behavior was obtained when using 10%. Regarding the OPC content, the greatest strength increases were achieved with 30% OPC.
The isothermal calorimetry results showed that the heat of the reaction of the hybrid cements activated with Na 2 SO 4 is higher than that of the hybrid cements activated with Na 2 CO 3. This behavior indicates that the systems activated with Na 2 SO 4 present a greater degree of reaction. However, in general, the heat of reaction of the activated systems is lower than the total heat released in the hydration of the 100% OPC pastes. The SEM-EDS microstructural analysis performed on the hybrid pastes FA/30OPC-4%Na 2 SO 4 , FA/30OPC-10%Na 2 CO 3 , CDW/30OPC-4%Na 2 SO 4 , and CDW/30OPC-10%Na 2 CO 3 demonstrated the formation of reaction products of the type (N,C)-ASH (low in Ca 2+ ), CSH, and CASH (rich in Ca 2+ ) in the cementing phase.
In general, these results indicate the viability of producing cementitious pastes activated with sodium sulfate and sodium carbonate from precursors such as FA and CDW, leading to products with greater technological and potentially environmental sustainability.