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Article

Sustainable Expansive Agent from FGD Gypsum and CAC Used to Mitigate Shrinkage in Alkali-Activated Mortars and Promoter the Valorization of Industrial By-Products

by
Thais Marques da Silva Moura
* and
Janaíde Cavalcante Rocha
Laboratory of Waste Valorization and Sustainable Materials (ValoRes), Department of Civil Engineering, Federal University of Santa Catarina (UFSC), Florianópolis 88040-900, Brazil
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(19), 8617; https://doi.org/10.3390/su17198617
Submission received: 8 August 2025 / Revised: 5 September 2025 / Accepted: 22 September 2025 / Published: 25 September 2025
(This article belongs to the Special Issue Resource Sustainability: Sustainable Materials and Green Engineering)
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Abstract

Mineral expansive from FDG—flue-gas desulfurization—blended with calcium aluminate cement CAC was analyzed as mitigation shrinkage of alkali-activated residual mortars AAM. The AAM mortars were composed of red mud (RM) and bottom ash (BA), as precursors of a metakaolin MK-based system. MK replacement (0, 50, 70%) in alkaline solution (10M) and ratio 1:2 (binder/sand) was studied. Engineering properties were performed, and included mechanical strength, setting times, and dry shrinkage (HR 60%), as well as the microstructure formed at 7 d and 28 days. A total of 10% CAC-FGD dosage was the most efficient, reducing drying shrinkage by 23% and autogenous shrinkage by up to 30%. The findings showed that this addition also improved mechanical strength by approximately 16% at 28 days. Under the addition of CAC-FGD, the results suggest the presence of aluminosilicate gels of the (Na,C)-(A)-S-H type and the formation of ettringite, which are possibly responsible for ensuring good performance and a controlled expansion that, in turn, compensates for the shrinkage of the activated mortars.

1. Introduction

Concrete production impacts the climate, as it is responsible for 5 to 8% of total anthropogenic CO2 emissions [1]; 95% of this CO2 is generated during cement manufacturing, with half of it being released from the decarbonation of limestone during the cement production process [2]. This situation does not occur in the production cycle of alkali-activated materials due to the possibility of utilizing by-products from other processes, which would be generated regardless of their application as precursors for alkali-activated materials (AAMs) [3].
The development of alkali-activated materials is intrinsically associated with sustainable development, as the process enables the utilization of industrial by-products, such as metakaolin, fly ash, red mud, and bottom ash to produce a material with significant added value and performance characteristics that may be comparable to those of Portland cement. These materials exhibit satisfactory mechanical performance for technical applications, which, in some cases, even surpasses that of Portland cement [4]. Moreover, they demonstrate high thermal stability and corrosion resistance [5], contributing to good durability and, depending on the nature of the precursor, it can absorb the high LOI (loss on ignition) ashes which are not used in cements. However, the magnitude of shrinkage in these systems remains a critical issue for their industrial application, being significantly higher than that of Portland-cement-based materials [6,7,8]. In such systems, the shrinkage can reach −250 microstrain to −2500 microstrain [9,10].
Shrinkage, defined as the reduction in volume of cementitious pastes at constant temperature and in the absence of external loads, is an inherent characteristic of such materials. In Portland cement concrete, shrinkage mainly occurs due to capillary pressure caused by water loss, leading to volume reduction. In alkali-activated binders, shrinkage may result not only from water loss through chemical reaction and evaporation over time [11], but also from pore structure, which is influenced by various factors such as the type of alkali activator, water content, precursor material, and curing conditions [12,13].
The use of expansive cements emerges as a potential solution to mitigate shrinkage. Their expansion mechanism is primarily based on the early formation of ettringite (AFt). Recent studies have investigated the partial replacement of the precursor with calcium aluminate cement (CAC), highlighting its positive effects on mechanical performance, reactivity, and the formation of hydration products. Ref. [14] observed that CAC increased compressive strength, ranging from 19 to 65 MPa, with the maximum strength achieved at 24% CAC, attributed to the higher Al and Ca content in the geopolymeric gel. Ref. [15] reported that a 5% CAC addition resulted in a 3-day compressive strength of 34.4 MPa, compared to 8 MPa in mortar containing only fly ash. At 10%, CAC addition led to the formation of a Si-rich gel, while 20% favored the formation of a more stable Al-rich gel with higher mechanical strength. Nevertheless, there is a notable research gap regarding the binary combination of CAC and calcium sulfate (CAC-C$) as an expansive binder to counteract shrinkage in alkali-activated systems. In view of environmental protection, flue-gas desulfurization has been used to meet the emission limits concerning the SO2 from coal-fired power plants. Depending on operation conditions, the FGD is estimated as the second product generated in coal production and it has been related to use as a secondary source of raw material in gypsum applications (wallboards, cement set, gypsum plasterboards, cement manufacturing, and precast products, among others). Advantages concern the possibility of managing this product with the by-products generated at the same plant (fly ash, bottom ash) as a strategy for the recovering of resources from waste and as a path to a circular economy. In this way, we propose the use of FGD in CAC systems to promote an expansive mineral with a view to reducing shrinkage in alkali-activated materials (AAM).
The red mud RM and bottom ashes BA have shown promising potential as precursors in alkali-activated materials [16].
Each ton of alumina produced generates about 1.5 tons of red mud residue, with an estimated 90 million tons of this waste produced globally every year [17]. China, a major alumina producer, generates 0.8 to 1.5 tons of red mud for each ton of alumina produced. It is estimated that over 7 million tons of red mud are impounded annually in the country, and this large quantity has led to serious environmental problems [18]. Since RM cannot be used directly to produce functional materials due to its strong alkalinity (pH 10–13) [19], it not only occupies a considerable area and entails high maintenance costs but also inevitably causes solid and groundwater pollution. Although red mud can be discarded in tailings dams [20], this method presents high maintenance costs and contamination risks. In the event of dam failure, the environmental impact is severe, as red mud can leach and contaminate the soil or groundwater [21]. Consequently, the red mud reuse rate remains low due to its high annual production, high alkalinity (pH 10–13), and the presence of heavy metals [19]. Therefore, studies that develop materials with RM and make its use feasible are essential to mitigate its disposal in the environment, providing a sustainable destination for it.
The extensive use of coal-related energy has resulted in the massive production of bottom ash (BA), which accounts for about 20% of the solid waste produced worldwide. According to [22], this is because coal is the second most used primary energy source, accounting for 30% and trailing only petroleum. Despite its chemical similarity to fly ash, BA contains relatively large and irregularly shaped particles with pores and cavities, which results in lower reactivity [23]. The low reactivity of BA makes its application as a construction material difficult. As a result, BA is generally discarded in landfills or lagoons, causing serious environmental problems due to the contamination of soil and surface and groundwater from the leaching of toxic elements like lead (Pb), zinc (Zn), cadmium (Cd), and copper (Cu) present in the ash [11]. Ash can not only have a negative environmental effect, but according to [24], ash nanoparticles also pose a risk to human health. When properly processed, these materials can deliver favorable physical and mechanical properties [23,25], while also promoting a more sustainable approach to developing more sustainable materials. Therefore, to mitigate the BA problem, research has been conducted on the application of BA as a cementitious material [26,27].
According to [28], metakaolin (MK) is an excellent source of highly reactive silicon and aluminum, which makes it a key precursor in alkali-activated (AA) materials. Thanks to this high reactivity, metakaolin can be used either as a sole precursor or in conjunction with other less reactive materials. This approach allows for the development of AA binders that incorporate a greater amount of waste, contributing to its valorization and eliminating the need for Portland cement.
In this context, the present study aims to evaluate the mechanical properties and shrinkage behavior of alkali-activated systems produced from alternative precursors, and to investigate the application of a new expansive additive, CAC-FGD, as a shrinkage-reducing agent. The FGD gypsum, a by-product of flue-gas desulfurization, consisted mainly of calcium sulfate dihydrate (CaSO4·2H2O), with properties similar to those of natural gypsum. It was used with calcium aluminate cement (CAC)-based mixtures as a promising opportunity for the valorization of industrial waste within a sustainable engineering framework. All alkali-activated mortars were developed using metakaolin MK as the main aluminosilicate source and compared with waste precursors.

2. Materials and Methods

2.1. Raw Materials—Expansive Cement

The expansive compound was formulated using calcium aluminate cement (CAC) of the Fondu type (Lafarge) and flue-gas desulfurization (FGD) gypsum as the calcium sulfate source. FGD gypsum was used as a source of calcium sulfate to produce the new expansive binder (CAC-FGD). This residue is a by-product of a coal-fired power plant in the state of Rio Grande do Sul (RS), Brazil. The specific gravity of cement CAC was 3.08 g/cm3 and a Blaine specific surface area of 5684.66 m2/kg. The FGD gypsum was pre-dried at 50 °C, ground in a ball mill for 1 h at 40 rpm and calcined in a muffle furnace at a heating rate of 10 °C/min for 4 h at 150 °C to obtain a hemihydrate form. The resulting material presented a specific gravity of 2.43 g/cm3 and a Blaine specific surface area of 3277.43 m2/kg.

2.2. Aluminosilicate Precursors

Metakaolin (MK), coal bottom ash (BA), and red mud (RM) were used as aluminosilicate sources for the alkali-activated (AA) binders. The alkaline activator was prepared using a commercial sodium silicate solution containing 26.5 wt% SiO2, 10.6 wt% Na2O, and 63.0 wt% H2O, with a density of 1.510 g/cm3, in combination with analytical-grade NaOH pellets. The MK, provided by the company Metacaulim do Brazil, was dried in an oven at 100 °C ± 5 °C before use. MK presented a specific gravity of 2.62 g/cm3 and a Blaine fineness of 1347.9 m2/kg. The bottom ash was collected from the Jorge Lacerda Thermoelectric Complex, Brazil. It was oven-dried at 105 °C. To obtain fine powdered material < 150 μm grinding was performed until all the ground material presented < 150 μm [29]. Grinding these materials provides increased strength and pozzolanic reaction, improving mechanical and microstructural properties [25]. The red mud was collected from an alumina industry located in the northern region of Brazil. RM was drying in an oven at 105 ± 5 °C for 48 h, then milled to a particle size of less than 75 µm. Before the use of RM, it was calcined at 600 °C for a period of 1 h. The treatment for the bottom ash was based on a previous study [30] while the red mud treatment was based on a study by [31]. These previous studies explained the relationship between grinding and reactivity in order to obtain pozzolans. It was noted that when red mud dries, it shrinks and becomes a very hard aggregate, which requires grinding before its calcination process.
According to [32], calcination positively affects the solubility of aluminosilicate materials in RM and is necessary before their use. The specific gravities of BA and RM were 2.27 g/cm3 and 2.79 g/cm3, and the values of specific surface areas (Blaine) were 503.78 m2/kg and 1252.51 m2/kg, respectively. Particle size distribution analysis by laser granulometry provided data on size D50 values of 3.07 µm (MK), 16.33 µm (BA), and 3.39 µm for RM.
The chemical composition of all materials was determined using X-ray Fluorescence Spectrometry (XRF) SHIMADZU equipment, model EDX-7000 HS (Shimadzu, Tokyo, Japan). Table 1 shows the results with the main oxides present in the samples.

2.3. Mix Design and Methodology

In this study, MK was replaced with 50% and 70% BA and RM by weight to achieve greater valorization of these materials without applying thermal curing. The alkaline solution was prepared 24 h before the production of pastes. The solution of sodium silicate was mixed with the hydroxide solution, maintaining a proportion of 1:2 of hydroxide to silicate (NaOH: Na2SiO3) and molar concentration of 10 M for all mixtures. The solution-to-binder ratio was set at 0.65, a value chosen specifically from the MK mixture. This was necessary because MK absorbs more water than the other materials, which would otherwise negatively impact the workability of the mixture. Due to the excellent particle size distribution of RM and BA, the fluidity of the pastes improved. This is consistent with findings from [33]. Despite this, the workability of the mortars remained similar. A spreading analysis confirmed that this specific liquid-to-solid ratio (L/s) was ideal for particle wetting within the mortars. The binder-to-sand ratio was 1:2. The sand used was natural sand with a fineness modulus of 1.84 and its particle size distribution can be found in Figure 1, according to limits established at ASTM C778—Standard Specification for Standard Sand [34].
The CAC-FGD blend was prepared at a mass ratio of 70:30, optimized proportion determined previously. This blend was added to the AA mortar formulations at two levels of incorporation: 10 wt% and 20 wt% of the total binder by mass (aluminosilicates + CAC-FGD). The expansive agent was composed with the same ratio of 70:30 of CAC to FGD. Table 2 shows the mixture compositions. Three mortar mixes were produced: a Series 01 with reference mortar MK, a Series 02 with 50% of bottom ash BA and 70% BA in replacement of MK, and a Series 03 with 50% of red mud RM and 70% RM at the same level of MK replacement. All samples were tested at 0%, 10%, and 20% of expansive agent comprising (CAC-FGD).
Shrinkage measurements were carried out on mortars cast in steel prismatic moulds (dimensions of 25 mm × 25 mm × 225 mm) for each mortar formulation (N = 3). The shinkage was analyzed in two conditions: autogenous and drying conditions. In autogenous conditions, the samples were unmoulded and wrapped in film during the measurements. In drying conditions, the specimens were unmoulded after 24 h and immediately placed in a controlled chamber maintained at 60 ± 5% relative humidity (HR) for 28 days.
Compressive strength and flexural tensile strength were evaluated in accordance with the NBR 13279 [35]. For each test, three prismatic specimens (dimensions of 40 mm × 40 mm × 160 mm) were cast and tested at 7 and 28 days.
The reaction kinetics of the alkali-activated pastes were analyzed using isothermal conduction calorimetry at 23 °C for a period of 72 h. The tests were conducted with a TAM Air calorimeter. Approximately 7 g of sample paste was used, and the heat evolution was normalized by the total sample mass.
To stop the hydration process at the target age, samples were immersed in analytical-grade isopropyl alcohol for 48 h before being dried in an oven at 40 °C for 72 h. The samples were then ground into a fine powder with particles smaller than 75 µm for XRD and FTIR tests.
The XRD patterns were recorded over a 2θ range from 5° to 80°, using a step size of 0.020°. Phase identification was performed with HighScore software (Version 3.0) and the ICDD PDF-2 database. The test was conducted at 28 days.
Fourier-Transform Infrared Spectroscopy (FTIR) analysis was performed using an FT/IR-4200 spectrometer (JASCO Corporation, Tokyo, Japan.). At 28 days, the samples were ground and mixed with potassium bromide (KBr) at a ratio of 1 mg of sample to 300 mg of KBr. Potassium bromide (KBr) is used to prepare samples for FTIR analysis because it is transparent to infrared radiation. The sample powder is mixed with KBr, and then pressed into a transparent pellet. The spectra were collected over the wavenumber range of 4400 to 400 cm−1, with an accumulation of 52 scans per sample.

3. Results and Discussions

3.1. Isothermal Calorimetry

Alkali-activation involves different stages of reaction. These stages include the initial dissolution, the induction period, and the main geopolymerization or hydration reaction. For many AAM systems, the peak heat flow and the vast majority of total heat evolution happen within the first 72 h. This choice is supported by various studies. For example, while [36] used a 24 h test to analyze the exothermic peaks of a high-calcium fly ash geopolymer and found a good relationship between total heat and strength, other researchers opted for longer periods. On the other hand, ref. [37] monitored the heat evolution of early metakaolin geopolymerization for 72 h while varying the alkaline solution concentration. Furthermore, research on expansive materials also uses the same timeframe for analyzing isothermal conduction, as seen in the work of [38,39]. This confirms that the 72 h period is a common and accepted duration in this field of study.
The calorimetric curves obtained on pastes revealed a single heat flow peak in all samples, observed in the initial minutes of the reaction, which is attributed to the dissolution of the solid material (Figure 1). In some formulations, a secondary shoulder was identified, although not clearly defined as a second peak, which may indicate the early formation of gels and crystalline phases such as C-A-S-H and N-A-S-H [40]. Figure 2 presents the heat flow and cumulative heat profiles for the paste samples from the MK series.
In the MK series, the addition of 10% and 20% CAC-FGD led to a significant reduction in both heat flow and cumulative heat, as well as a delayed onset of the reaction. The sample containing 100% MK exhibited a more intense and earlier peak, indicating higher reactivity. These effects are attributed to the presence of Al2O3-rich phases in the CAC-FGD, which may slow down the dissolution of aluminates [41]. The cumulative heat after 72 h for pure MK was 126.9 J/g, approximately 20% higher than the formulations containing CAC-FGD.
In the MK-BA series (Figure 3a,b), the addition of 20% CAC-FGD promoted greater initial reactivity, while the incorporation of 10% of the additive increased the cumulative heat in the mixtures containing 70% BA, reaching 91.2 J/g at 72 h, higher than the reference sample without the expansive agent.
In the MK-RM series, a similar behavior was observed to that of the MK series, with a reduction in heat flow and slower reaction kinetics in formulations with CAC-FGD. However, the sample containing 70% RM (30MK70RM) exhibited a distinct response, with increased heat flow and cumulative heat, suggesting that the additive may act as a catalyst in systems with a high proportion of RM, as evidenced in Figure 4.
Overall, the addition of CAC-FGD tended to delay hydration kinetics, likely due to its aluminum-rich mineralogical composition, which directly influences the formation of reactive phases. According to [42], the modest increase in cumulative heat is associated with continuous dissolution and limited transformation of the dissolved species into gel phases.

3.2. Tensile Flexural and Compressive Strength

Compressive strength was evaluated at 7 and 28 days, as well as flexural tensile strength.
The molar ratios of Si/Al, Si/Na, and Na/Al have a direct effect on the mechanical properties and strength of alkali-activated materials. Table 3 shows that the Si/Al molar ratios vary between 1.3 and 4.2
In the metakaolin (MK) sample group, the highest compressive and flexural strength was achieved with an Si/Al ratio of 2.56. In this group, strength tends to increase as the Si/Al ratio increases. Similarly, in the MKBA group, the 50MK50BA_10Exp mix, with an Si/Al ratio of 2.788, showed the highest strength at 7 and 28 days. Although the 30MK70BA mix had a higher Si/Al ratio of 4.2, its strength was lower, which contradicts the trend observed in the other groups.
According to [43], increasing the Si/Al ratio makes polysialatesiloxo (Si-O-Si) structures dominant in the geopolymer network. This results in greater strength compared to polysialate (Si-O-Al) structures. This finding is supported by studies from [18,44], who also found improved mechanical properties with an increased Si/Al ratio. Ref. [18] explains that a high Si/Al ratio promotes the formation of a more stable bond structure. This is because a continuous increase in Si raises the amount of dissolved elemental silicon, which in turn leads to the formation of more -Si-O-Si- bonds. The bonding energy of these bonds is stronger than that of -Si-O-Al- and -Al-O-Al- bonds. However, ref. [45] points out that very high Si/Al ratios can decrease solubility and gel formation, leading to an increase in unreacted metakaolin. These unreacted particles can act as defect sites, weakening the material and reducing compressive strength. Therefore, the strength of geopolymers can decrease at extremely high Si/Al ratios.
In the MK series, the addition of CAC-FGD did not enhance the strength development of mortars at any evaluated age. At 7 days, flexural strength increased by 5% and 15% with the incorporation of 10% and 20% CAC-FGD, respectively. However, at 28 days, mortars with the expansive agent showed reduced strength, with values lower than the reference mix (Figure 5).
In the MK-BA series, the behavior of the mortars differed from that observed in the MK-only series. Mortars containing 50% MK and 50% BA (50MK50BA) exhibited a compressive strength of 9 MPa at 7 days (Figure 6a). With the addition of 10% and 20% CAC-FGD, this value increased by 9% and 6%, respectively. At 28 days, the mix with 10% CAC-FGD maintained superior performance compared to the reference mix, while the others showed similar results. For mortars with 70% BA, flexural strength reached 7 MPa at 7 days and 9 MPa at 28 days. The addition of 10% CAC-FGD did not change this behavior, while 20% did not contribute to any improvement in strength.
In the MK-RM series, the addition of CAC-FGD resulted in increased flexural strength at all evaluated ages, with better performance observed in mortars with higher content (20% Exp). At 28 days, the incorporation of 20% CAC-FGD raised the strength to values above 9 MPa, representing increases of 16% for the 50MK50RM mortar and 50% for the 30MK70RM mixture.
The effect of CAC-FGD was more pronounced in mortars containing 70% mineral residue (RM). The difference in strength between 7 and 28 days was minimal, indicating that most of the mechanical development occurs at early ages (Figure 6b). This behavior can be attributed to the rapid formation of the N-A-S-H geopolymer gel, which is responsible for the early strength development [46].
The compressive strength of the mix containing 10% CAC-FGD was higher at 7 days and comparable at 28 days to the mix without CAC-FGD in the MK group (Figure 7). The rapid dissolution of hemihydrate leads to early reactions [47], which can negatively impact the material’s early mechanical strength. However, strength development continues at later ages [48].
In the 50MK–50BA group, the addition of CAC-FGD resulted in a reduction in strength during the first 7 days. However, at 28 days, the strength of the mortars with 10% expansive agent exceeded the strength of the reference mortars, as shown in Figure 8a. A similar result was reported by [49], who also observed increased strength in mortars containing fly ash when 10% FGD was added. According to the authors, the strength gain is attributed to the increased availability of Ca2+ ions, which reacts with silicate to form additional C–S–H, and also interacts with aluminosilicate groups, reinforcing the aluminosilicate network. These phases contribute to the mechanical development of the mortars.
BA and RM, both being low-reactivity materials, are difficult to activate. For this reason, they require thermal curing above 40 °C, the use of an additional precursor, or a high-molarity activator (above 12 M). In a study by [50], pastes made with red mud (RM) and fly ash (FA) achieved strengths of up to 10.6 MPa when cured at temperatures up to 200 °C. In the same study, when 26.3% of the red mud was replaced with fly ash and silica fume was added, the compressive strength reached 20 MPa. Similarly, the research by [51] developed an alkali-activated material using only red mud. They observed that, even with thermal curing at 60 °C and a 12 M NaOH activator, the maximum strength was only 3.6 MPa. In this current research, it is possible to see that up to 70% of RM was used without the need for thermal curing or high molarity. With the addition of a new expansive cement, which contributed to the material’s activation, the compressive strength reached over 25 MPa, allowing for a much greater use of this material.
In the MK-RM series, the expansive agent at both dosages had a positive effect on compressive strength at all ages evaluated. The increase in strength at 28 days ranged from 13% to 50%, depending on the CAC-FGD content used (Figure 8b). According to [52], this effect may be related to differences in the alkaline reaction products formed in these three systems.
Lower strength at low Si/Al ratios (generally less than 2) is attributed to the formation of a weak and porous structure. Ref. [43] explains that this happens due to an insufficient amount of soluble silicate to complete the geopolymerization process. The research by [53] on red mud supports this observation. They achieved the highest strength (45 MPa) with a silica modulus (Si/Na) activator of 1.8, which corresponds to an optimal Si/Al ratio of 3.36. In the present study, the silica modulus was 1.0, and the Si/Al ratio for the red mud and metakaolin (30MK70RM) mixes were 1.3–1.8, which may explain the low mechanical strength of these mixtures. The H2O/Na2O molar ratio of the mortars was a fixed value of 11.47. Ref. [43] states that high ratios (14–18) significantly reduce strength because excess water does not bind to the geopolymer network. This free water can evaporate, creating large voids that compromise the structure. To optimize strength, the paste’s viscosity and the amount of initial mixing water should be as low as possible, with the ideal H2O/Na2O molar ratio ranging from 10 to 12.5.
Previous studies using calcium aluminate cement (CAC) or flue-gas desulfurization (FGD) gypsum have demonstrated the effect of these materials on alkaline activation. Ref. [15] found that samples with 10% CAC resulted in the formation of a Si-rich gel, while samples with 20% CAC formed a more stable and higher-strength Al-rich gel. The authors concluded that CAC has a positive influence on the development of strength in mortars due to its high alumina content. Mixtures containing 10% CAC achieved a 3-day strength of 34 MPa. Similarly, ref. [54] observed positive changes in the strength and microstructure of low-calcium fly ash by incorporating up to 7.5% of CAC into the mixture, leading to the formation of C-A-S-H gel. FGD gypsum also improves mechanical strength when used in alkaline activation mixtures. The strength of the material containing FGD increased by 22% at 28 days. This is attributed to the CaO in the FGD, which can react with the SiO2 and Al(OH)3 from the precursor to form C–S–H and/or C–A–S–H [55].
The approach of previous studies has been to use CAC and FGD gypsum individually to explore the increase in material reactivity and mechanical strength. However, no studies were found that used the combination of CAC-FGD for the activation of low-calcium materials with a focus on shrinkage mitigation, which consequently improves mechanical strength. This makes the present research innovative. This study found that the strength of BA and RM, two low-calcium materials that are difficult to activate, increased by about 10–50% with the use of 10% of the binary CAC-FGD material, corroborating previous research. Furthermore, this research achieved not only mechanical development but also the high-volume activation of BA and RM and the reduction in shrinkage.

3.3. Autogenous Shrinkage

Some materials have already been tested to reduce shrinkage in alkali-activated systems. Ref. [56] used a superabsorbent polymer (SAP) and observed that it effectively mitigated autogenous shrinkage in metakaolin and Portland cement mortars. With just 0.3% of SAP, shrinkage was reduced by 53% compared to the reference mixture. These results were corroborated by [57], who also used a polymer-based shrinkage-reducing admixture. Ref. [58] added commercial calcium sulfoaluminate (CSA) cement in three different percentages based on the binder quantity (slag + fly ash). They concluded that the expansive mechanism occurred through the generation of expansive hydrates like ettringite, which contributed to a 35% reduction in autogenous shrinkage.
The combination of CAC-FGD as an expansive cement to reduce both drying and autogenous shrinkage has not yet been explored. CAC differs from CSA by not being inherently expansive. However, when combined with a source of calcium sulfate, CAC exhibits characteristics similar to CSA by forming expansive hydrates.
For the mortars in the MK group, the highest autogenous shrinkage deformation occurred within the first 7 days. In the mix with only MK, the highest shrinkage was observed at 21 days (Figure 9). All mixes showed shrinkage behavior over the 28-day period. However, the results indicate that the expansive agent reduced autogenous shrinkage at 28 days, with a decrease of 30% and 21% for mortars containing 10% and 20% CAC-FGD, respectively. Ref. [58] used a Polyol-based shrinkage-reducing admixture in metakaolin (MK) mortar and achieved a 12% reduction in shrinkage. The mechanism for this reduction was pore refinement, which is different from the CAC-FGD system, where shrinkage is mitigated through ettringite expansion.
Figure 10a shows the autogenous shrinkage results for the MK-BA mixtures. In 50MK50BA mortars, the 20% CAC-FGD dosage resulted in expansion starting from the third day. The 20% addition of CAC-FGD led to a 60% reduction in autogenous shrinkage at 28 days for the 50MK50BA mortars.
The highest shrinkage rate was recorded for the reference 50MK–50BA mix, reaching 634.506 µm/m at 28 days. In the 30MK70BA mortars at 10% and 20% CAC-FGD contents they both showed similar performance starting at day 21, reducing shrinkage at 28 days by 35% and 47%, respectively.
Mortars from the 50MK50RM group with the expansive agent showed an expansive tendency over the 28-day period. For this group, the addition of 10% and 20% CAC-FGD reduced shrinkage by 77% and 45%, respectively. In the 30MK70RM mortars, the use of CAC-FGD had the most pronounced effect, reducing autogenous shrinkage by more than 95% (Figure 10b).
The shrinkage-compensating mechanism has been mainly attributed to the formation of portlandite (CH) and ettringite (AFt) [59]. Ref. [60] used lime and anhydrite as expansive agents in alkali-activated mortars and also attributed the shrinkage reduction to the generation of CH in the pastes, which helped compensate for shrinkage in the AA composites. Similarly, ref. [61] stated that the reduction in shrinkage in alkali-activated materials (AAMs) originates from the formation of CH and ettringite. These crystalline phases (i.e., CH and AFt) were closely integrated into the paste matrix and contributed to the inhibition of deformation in the alkali-activated gel. Although the authors used different shrinkage-reducing materials, their results support the findings of this research and demonstrate that the CAC-FGD combination can be used as an expansive cement to mitigate shrinkage in alkali-activated materials.

3.4. Drying Shrinkage

It was observed that the use of an expansive agent in the metakaolin (MK) group did not reduce shrinkage over the 28-day analysis period (Figure 11). Ref. [9] reported that shrinkage in alkali-activated mortars (AAM) exposed to dry conditions with 50% (RH) was three times greater than in humid or submerged environments, due to the rapid evaporation of water from the pore structure and the resulting capillary pressure. Ref. [62] attributed the increased shrinkage in MK-based mortars to free energy effects, highlighting the more connected pore structure and accelerated water loss under dry conditions.
Furthermore, the high shrinkage observed in MK-based systems is associated with their high water demand, which is attributed to the largest specific surface area and particle shape of metakaolin, factors that also affect the physical and mechanical properties [62]. It is also possible that the expansive agent interacted unexpectedly with other components of the mixture, inhibiting its expansive action or even causing adverse effects. Ref. [63] observed that increasing the gypsum content in AAMs led to higher drying shrinkage due to an increase in internal moisture.
The rate of moisture loss in cementitious materials under a given RH depends on the pore structure and the physicochemical properties of the pore solution [64]. According to [15], calcium aluminate cement (CAC) is an effective source of reactive aluminum, and its components (Al and Ca) can be incorporated into the C–A–S–H gel. In MK-based mixtures, CAC-FGD may have promoted the formation of a different type of gel, increasing internal stress and consequently contributing to raise the shrinkage.
Partial replacement of MK with BA at 50% significantly reduced drying shrinkage in mortars, with an average decrease of 24%. This substitution lowers the overall reactivity of the solid precursors, delaying the polymerization process and promoting the densification of binding gels prior to hardening. As a consequence, a more compact N–A–S–H gel phase with a lower Al/Si ratio is formed [65]. Although reduced effective porosity increases capillary tension, contributing to autogenous shrinkage, it also limits water evaporation from the pore network, thereby decreasing drying shrinkage.
The addition of the expansive agent CAC-FGD further enhanced shrinkage reduction. Figure 12 shows that in 50MK50BA mortars, CAC-FGD at 10% and 20% reduced drying shrinkage by 51% and 7%, respectively. Ref. [39] achieved a reduction in the shrinkage of coal gangue mortars by using an expansive cement. According to the authors, calcium sulfoaluminate (CSA) compensated for shrinkage through the formation of the crystalline phases ettringite and hydrotalcite. This compensation was also a result of an increase in surface area, which influenced the pore structure by raising the average pore size and, consequently, reducing drying shrinkage. Ref. [66] also attributed the shrinkage-compensating potential to the high formation of ettringite and the presence of calcium hydroxide. These products were generated from the expansive CSA cement. Both studies used a cement that is already produced with expansive characteristics and is well-established and commonly used. However, this research shows that it is possible to develop a new expansive binder by using an industrial by-product in its formulation.
In 30MK70BA mortars, 10% CAC-FGD was the most effective, achieving a reduction of approximately 18%. This reduction may be attributed to the reaction between sulfate ions and alumina in the ashes, forming a dense aluminosilicate network [54]. The resulting uniform microstructure limits water loss and mitigates internal stresses, thereby minimizing shrinkage.
Partial replacement of MK with RM also contributed to shrinkage reduction, with decreases of up to 50% compared to pure MK mortars. This effect is possibly due to red mud’s role in refining the pore structure [67] and the formation of additional hydration products [68]. In the MK-RM system (Figure 13), the addition of 10% CAC-FGD resulted in a positive impact on shrinkage, with the 50MK50RM mixture showing a 23% compensation. However, increasing the CAC-FGD content to 20%, although potentially beneficial to mechanical strength (inferred), resulted in higher level of drying shrinkage. The 30MK70RM mixture without the expansive agent exhibited the highest shrinkage, reaching 4.534 mm/mm at 28 days. Nevertheless, the addition of 10% and 20% CAC-FGD effectively reduced shrinkage by 42% and 40%, respectively, confirming that the expansive agent has a potential to mitigate shrinkage, an effect generally associated with the volumetric stability promoted by mineral additives [67].

3.5. FTIR Analysis

The chemical bond bands in wavenumber range 4000 to 400 cm−1 referred to the 15 mortar AAM with 0, 10, and 20% of expansive agent with mortar mixes analyzed at 28 days, and the FTIR-spectra are presented in Figure 14.
Band (1), around 3500 cm−1 and present in all samples, is associated with O–H bond vibrations [69]. Samples without CAC-FGD exhibit distinct spectra compared to those with the expansive agent, due to differences in the either chemically bound or physically adsorbed water, which is incorporated into the reaction products [70]. In cement-based systems, water participates in the formation of hydrates (C3AH6, AH3), whereas in alkali-activated (AA) systems it is primarily adsorbed onto the surface of particles or retained within aluminosilicate gels.
Band (2), located between 1750 and 1500 cm−1, corresponds to the angular deformation of the H–O–H bond, indicating the presence of crystalline water in the alkali-activated matrix [31,33]. Band (3), near 1450 cm−1, appears in samples containing CAC-FGD and is attributed to O–C–O stretching, suggesting carbonation. The reduction in this band with lower CAC-FGD content indicates decreased carbon incorporation [39,71].
Band (4), located around 1000 cm−1, is related to Si–O–T (T = Si or Al) vibrations, characteristic of N–A–S–H gel [26]. Samples with CAC-FGD showed increased intensity and a shift to lower wavenumbers, suggesting higher Al content and enhanced formation of aluminosilicate gel [69,72].
Bands between 800 and 647 cm−1 are associated with SiO6 bending modes (quartz), while those between 560 and 440 cm−1 correspond to Si–O–Al and Si–O–Si bonds in silica tetrahedra [73,74].

3.6. XRD Analysis

The X-ray diffraction (XRD) analysis of MK mortars is shown in Figure 15. A reduction in the amorphous halo was observed, indicating increased crystallinity depending on the curing method. The main minerals identified were quartz and goosecreekite (CaAl2Si6O16·5H2O), consistent with previous studies [75,76]. Prehnite and gismondine, linked to C–A–S–H gel formation, were also detected. Gismondine, often found in low-calcium aluminosilicate systems [77], may contribute to improved compressive strength, as suggested by the authors [78].
In MK samples with 10% and 20% CAC-FGD new phases, ettringite, gibbsite, and thenardite were identified. Ettringite and gibbsite are known as hydration products of expansive cements and can offset internal stress-induced shrinkage. Thenardite, formed by the reaction of sulfate (SO42−) with sodium (Na+), indicates excess sodium ions in the pore solution [79]. These results are in agreement with the research by [55], who used FGD gypsum to activate fly ash and observed an increase in strength. In their study, the formation of ettringite and C-S-H was also identified, which led to a reduction in shrinkage. According to the authors, the formation of ettringite and sodium sulfate (Na2SO4) is the reason for the low drying shrinkage.
Figure 16 shows 50MK50BA samples results that thenardite, gismondine, and calcium-aluminum phases were present. When CAC-FGD was used in this series, tricalcium aluminate, ettringite, and calcite (CaCO3) were detected. These result from gypsum introduction [80] and the hydration of calcium oxide into calcium hydroxide and carbonate in the presence of water, alkaline activators, and CO2 [60], possibly contributing to reduced mechanical strength.
Ref. [81] noted that alkali activation of RM enriches the binder with iron and copper, mainly as hematite and hydrotalcite, which do not directly participate in the reaction. These results corroborated the study [82], in which it was observed that quartz and hematite may act as fillers but can reduce mechanical performance. The alkali-activated MKRM system primarily forms sodium aluminosilicate hydrate (N–A–S–H). The incorporation of CAC-FGD into the MKLV matrix led to the formation of new crystalline phases, such as aluminum silicate hydrates, identified as zeolitic structures (Figure 17).

3.7. Potential Applications of the Alkali-Activated Materials

Due to their excellent mechanical properties and sustainability benefits, alkali-activated materials (AAMs) have significant potential for various applications in the construction industry. Their specific use is largely determined by compressive strength, a property that can be optimized by adjusting key parameters such as the molar ratios of Si/Al and H2O/Na2O. Based on compressive strength and other performance indicators, AAMs show promising potential for a wide range of both structural and non-structural applications. Several studies have reported that these materials achieve compressive strengths comparable to or even higher than ordinary Portland cement (OPC), making them suitable for structural components like precast elements, paving units, and structural walls [4,16].
In addition, the high chemical resistance and thermal stability of AAMs, as demonstrated by [5], suggest their use in aggressive environments such as marine structures, sewage systems, and industrial floors exposed to chemicals. The excellent durability of these materials, which includes resistance to chloride and sulfate attack [83], further strengthens their long-term performance under adverse conditions. It is important to note that the ability to achieve these properties without the need for thermal curing expands their applicability for in situ construction, with a lower energy demand and greater feasibility for large-scale use [2].
The versatility of AAMs is confirmed by various innovative applications, many of which explore different precursors to optimize the material’s properties. For instance, ref. [84] demonstrated that alkali-activated cements are suitable for deepwater storage wells under downhole stress conditions and for carbon capture, thanks to their good acid resistance, high mechanical strength, durability, and low permeability. The production of masonry bricks from industrial waste is a sustainable alternative to conventional bricks. Ref. [85] demonstrated the feasibility of producing structural bricks with alkali-activated red mud. According to the authors, it is possible to use up to 50% red mud in combination with slag to obtain bricks with excellent compressive strength.
Metakaolin (MK) is another notable precursor that can provide high strengths (>40 MPa) [86], meeting the requirements for producing various construction materials like building blocks, paving materials, decorative exterior materials, and concrete masonry units. According to [87], MK-based geopolymers can therefore be recommended for the construction industry as a sustainable and eco-friendly building material. Despite extensive research on alkali-activated materials and their properties, the practical applications of these materials are still limited. Therefore, this research emphasizes the need to investigate other physical properties, in addition to mechanical strength, of alkali-activated materials with different industrial by-products, demonstrating the possibility of their practical use and ensuring a more sustainable production process.

4. Conclusions

The incorporation of CAC-FGD into alkali-activated material (AAM) systems exhibited distinct effects, strongly influenced by the composition of the mixtures analyzed. It was observed that although CAC-FGD tends towards delaying the activation reaction in the MK series, it could enhance it in specific formulations, particularly those with a high content (0% BA). In terms of mechanical performance, the addition of 10% CAC-FGD led to significant increases in flexural strength (up to 16%) and compressive strength (10% in 50MK50BA and up to 50% in 30MK70RM with 20% CAC-FGD).
Regarding drying shrinkage, the addition of CAC-FGD was not effective in the pure MK series. However, the partial replacement of MK with BA and RM proved to be an effective strategy for reducing shrinkage in this series. CAC-FGD showed positive performance in the MK-BA and MK-RM series, promoting shrinkage reductions of up to 23%.
The formation of phases such as ettringite, gibbsite, and others was observed in specific mixtures, highlighting the active role of CAC-FGD in the microstructure of the alkali-activated materials.
Thus, the use of CAC-FGD as an expansive agent in AAM mortars is technically feasible, especially when combined with industrial waste such as BA and RM. The 10% dosage stood out as a promising solution for improving mechanical performance and reducing shrinkage, while also contributing to the reuse of industrial by-products. This study demonstrates the potential of this approach as a strategy for developing construction materials with enhanced performance and lower environmental impact, in line with sustainability principles.

Author Contributions

Conceptualization: T.M.d.S.M. and J.C.R.; Data curation, T.M.d.S.M.; Formal analysis, T.M.d.S.M.; Investigation, T.M.d.S.M.; Methodology, T.M.d.S.M. and J.C.R.; Project administration, J.C.R.; Resources, J.C.R.; Supervision, J.C.R.; Validation, J.C.R.; Writing—original draft, T.M.d.S.M.; Writing—review and editing, J.C.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

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.

Acknowledgments

The authors acknowledge the financial support of the Brazilian agencies CNPq (National Council for Scientific and Technological Development) and CAPES (Coordination for the Improvement of Higher Education Personne) finance code 001. The authors are also grateful to LDRX-UFSC for assistance in X-ray diffraction operation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Sand particle size distribution.
Figure 1. Sand particle size distribution.
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Figure 2. Heat flow curves and total accumulated heat of MK alkali-activated pastes.
Figure 2. Heat flow curves and total accumulated heat of MK alkali-activated pastes.
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Figure 3. Heat flow curves and total accumulated heat of 50MK50BA (a) and 30MK70BA (b) alkali-activated pastes.
Figure 3. Heat flow curves and total accumulated heat of 50MK50BA (a) and 30MK70BA (b) alkali-activated pastes.
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Figure 4. Heat flow curves and total accumulated heat of 50MK50RM (a) and 30MK70RM (b) alkali-activated pastes.
Figure 4. Heat flow curves and total accumulated heat of 50MK50RM (a) and 30MK70RM (b) alkali-activated pastes.
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Figure 5. Flexural strength of mortar from the MK series.
Figure 5. Flexural strength of mortar from the MK series.
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Figure 6. Flexural strength of mortar from the MK-BA (a) and MK-RM (b) series.
Figure 6. Flexural strength of mortar from the MK-BA (a) and MK-RM (b) series.
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Figure 7. Compressive strength of mortar from the MK series.
Figure 7. Compressive strength of mortar from the MK series.
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Figure 8. Compressive strength of mortar from the MK-BA (a) and MK-RM (b) series.
Figure 8. Compressive strength of mortar from the MK-BA (a) and MK-RM (b) series.
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Figure 9. Autogenous shrinkage of MK mortar with and without CAC-FGD.
Figure 9. Autogenous shrinkage of MK mortar with and without CAC-FGD.
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Figure 10. Autogenous shrinkage of MK-BA (a) and MK-RM (b) mortars with and without CAC-FGD.
Figure 10. Autogenous shrinkage of MK-BA (a) and MK-RM (b) mortars with and without CAC-FGD.
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Figure 11. Drying shrinkage of MK mortar with and without CAC-FGD.
Figure 11. Drying shrinkage of MK mortar with and without CAC-FGD.
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Figure 12. Drying shrinkage of MK-BA mortars with 0, 10, and 20% expansive agent (Exp) formed by CAC-FGD.
Figure 12. Drying shrinkage of MK-BA mortars with 0, 10, and 20% expansive agent (Exp) formed by CAC-FGD.
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Figure 13. Drying shrinkage of MK-RM mortars with and without CAC-FGD.
Figure 13. Drying shrinkage of MK-RM mortars with and without CAC-FGD.
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Figure 14. FTIR analysis of evaluated (a) mortar series with metakaolin MK, (b) mortar series with bottom ashes MK-BA, and (c) mortar series with red mud MK-RM.
Figure 14. FTIR analysis of evaluated (a) mortar series with metakaolin MK, (b) mortar series with bottom ashes MK-BA, and (c) mortar series with red mud MK-RM.
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Figure 15. X-ray diffraction patterns MK activated alkali series. (Legend: 1—Quartz-PDF: 00-046-1045; 2—Grossite-PDF: 00-046-1475; 3—Gibbsite-PDF: 00-033-0018; 4—Gismondine-PDF: 00-020-0452; 5—Calcite-PDF: 00-005-0586; 6—Thernadite-PDF: 00-037-1465; 7—Etringite-PDF: 00-031-0251).
Figure 15. X-ray diffraction patterns MK activated alkali series. (Legend: 1—Quartz-PDF: 00-046-1045; 2—Grossite-PDF: 00-046-1475; 3—Gibbsite-PDF: 00-033-0018; 4—Gismondine-PDF: 00-020-0452; 5—Calcite-PDF: 00-005-0586; 6—Thernadite-PDF: 00-037-1465; 7—Etringite-PDF: 00-031-0251).
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Figure 16. X-ray diffraction patterns MK-BA activated alkali series. (Legend: 1—Quartz-PDF: 00-046-1045; 2—Gismondine-PDF: 00-020-0452; 3—Gibbsite-PDF: 00-033-0018; 4—Thernadite-PDF: 00-037-1465; 5—Etringite-PDF: 00-031-0251; 6—Calcium aluminum oxide-PDF: 00-032-0148; 7—Tricalcium aluminate-PDF: 00-038-1429; 8—Goosecreekite (Zeolite)-PDF: 00-035-0469).
Figure 16. X-ray diffraction patterns MK-BA activated alkali series. (Legend: 1—Quartz-PDF: 00-046-1045; 2—Gismondine-PDF: 00-020-0452; 3—Gibbsite-PDF: 00-033-0018; 4—Thernadite-PDF: 00-037-1465; 5—Etringite-PDF: 00-031-0251; 6—Calcium aluminum oxide-PDF: 00-032-0148; 7—Tricalcium aluminate-PDF: 00-038-1429; 8—Goosecreekite (Zeolite)-PDF: 00-035-0469).
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Figure 17. X-ray diffraction patterns MK-RM activated alkali series. (Legend: 1—Quartz-PDF: 00-046-1045; 2—Gismondine-PDF: 00-020-0452; 3—Magnetite-PDF: 00-19-0629; 4—Zeolite A; 5—Grossite-PDF: 00-046-1475; 6—Hematite-PDF: 01-085-0599; 7—Calcite-PDF: 00-005-0586).
Figure 17. X-ray diffraction patterns MK-RM activated alkali series. (Legend: 1—Quartz-PDF: 00-046-1045; 2—Gismondine-PDF: 00-020-0452; 3—Magnetite-PDF: 00-19-0629; 4—Zeolite A; 5—Grossite-PDF: 00-046-1475; 6—Hematite-PDF: 01-085-0599; 7—Calcite-PDF: 00-005-0586).
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Table 1. Chemical composition of FGD, CAC, MK, BA, and RM (% main oxides).
Table 1. Chemical composition of FGD, CAC, MK, BA, and RM (% main oxides).
Raw MaterialSiO2Al2O3CaOFe2O3K2OTiO2SO3ZrO2OthersLOI
CAC4.6256.3932.310.670.210.99-0.320.354.15
FGD (Hemi)6.31-35.190.620.350.0846.97-0.0710.4
MK55.1536.570.221.841.571.45-0.090.142.96
BA56.5124.231.405.933.161.470.370.130.266.5
RM24.4418.961.5341.610.111.470.540.890.496.14
Table 2. Alkali-activated systems and mix composition with CAC-FGD.
Table 2. Alkali-activated systems and mix composition with CAC-FGD.
Mixby Weight (g) (per 100 g of Precursor)
MKBARMCACFGD* Sand** L/S Ratio
MK100----2.00.65
MK_10Exp100--73
MK_20Exp100--146
50MK50BA5050---
50MK50BA_10Exp5050-73
50MK50BA_20Exp5050-146
30MK70BA3070---
30MK70BA_10Exp3070-73
30MK70BA_20Exp3070-146
50MK50RM50-50--
50MK50RM_10Exp50-5073
50MK50RM_20Exp50-50146
30MK70RM30-70--
30MK70RM_10Exp30-7073
30MK70RM_20Exp30-70146
Note: ** L/S = Liquid/Solid ratio by mass; * Sand: 1:2 (by mass).
Table 3. Molar ratios range of alkali-activated mortars compositions.
Table 3. Molar ratios range of alkali-activated mortars compositions.
MixesSi/AlNa/AlH2O/NaSiO2/Na2O
MK2.5630.62511.4740.95
MK_10Exp2.3350.565
MK_20Exp2.1470.514
50MK50BA2.5950.752
50MK50BA_10Exp2.7880.666
50MK50BA_20Exp2.5230.597
30MK70BA4.2140.819
30MK70BA_10Exp3.7190.717
30MK70BA_20Exp3.3340.638
50MK50LRM2.4360.824
50MK50RM_10Exp2.1600.721
50MK50RM_20Exp1.9450.641
30MK70RM1.5900.944
30MK70RM_10Exp1.3750.811
30MK70RM_20Exp1.8350.712
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Marques da Silva Moura, T.; Cavalcante Rocha, J. Sustainable Expansive Agent from FGD Gypsum and CAC Used to Mitigate Shrinkage in Alkali-Activated Mortars and Promoter the Valorization of Industrial By-Products. Sustainability 2025, 17, 8617. https://doi.org/10.3390/su17198617

AMA Style

Marques da Silva Moura T, Cavalcante Rocha J. Sustainable Expansive Agent from FGD Gypsum and CAC Used to Mitigate Shrinkage in Alkali-Activated Mortars and Promoter the Valorization of Industrial By-Products. Sustainability. 2025; 17(19):8617. https://doi.org/10.3390/su17198617

Chicago/Turabian Style

Marques da Silva Moura, Thais, and Janaíde Cavalcante Rocha. 2025. "Sustainable Expansive Agent from FGD Gypsum and CAC Used to Mitigate Shrinkage in Alkali-Activated Mortars and Promoter the Valorization of Industrial By-Products" Sustainability 17, no. 19: 8617. https://doi.org/10.3390/su17198617

APA Style

Marques da Silva Moura, T., & Cavalcante Rocha, J. (2025). Sustainable Expansive Agent from FGD Gypsum and CAC Used to Mitigate Shrinkage in Alkali-Activated Mortars and Promoter the Valorization of Industrial By-Products. Sustainability, 17(19), 8617. https://doi.org/10.3390/su17198617

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