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Article

Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes

by
José Luis García Calvo
* and
Pedro Carballosa
Institute for Construction Sciences Eduardo Torroja, IETcc-CSIC, 28033 Madrid, Spain
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(17), 3128; https://doi.org/10.3390/buildings15173128
Submission received: 15 July 2025 / Revised: 20 August 2025 / Accepted: 25 August 2025 / Published: 1 September 2025
(This article belongs to the Special Issue Towards Sustainable Construction: New Trends in Building Renovation, Energy Efficiency and Innovative Materials)
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Abstract

Heat-curing processes are often used to ensure the production rate of precast concrete elements, as this process increases the early strength of the material. However, the increase in curing temperature can negatively affect the final mechanical properties since cracking, and especially high porosity, may occur under these conditions. In order to compensate for the expected loss in mechanical and durability-related properties, the cement content is typically increased. This solution raises the cost of the final product and reduces its sustainability. Thus, in this study, the development of expansive self-compacting concretes (SCCs) is proposed to achieve higher final mechanical properties without increasing cement contents. The mechanical properties, expansive performance, and porous microstructure have been evaluated under different curing regimes. The obtained results show that it is possible to obtain similar or even better mechanical performance in expansive concretes cured at high temperatures than in those cured in standard conditions, particularly when using ettringite-based expansive agents (EAs). Moreover, the use of limestone filler (LF) proved to be more suitable than the use of fly ashes in the working conditions evaluated in the present study. In this sense, the compressive strength at 28 days of SCC with LF and ettringite-based EAs is 4.3% higher than the one obtained under standard curing; moreover, the total porosity is reduced (5%), and the drying shrinkage is also limited. These aspects have not been previously reported in non-expansive heat-cured concretes and represent a unique opportunity to reduce the cement content and, therefore, the carbon footprint of precast concretes without reducing their mechanical properties. When using CaO-based EAs, the results are also better than those of non-expansive SCC, although the improvement is less pronounced than in the previous case.

1. Introduction

The use of self-compacting concrete (SCC) is widespread in the precast concrete industry, primarily due to its ability to improve casting efficiency, enhance surface finish, and reduce labor and vibration requirements during placement [1,2]. To meet rapid production demands, heat-curing processes are commonly employed, as elevated temperatures accelerate early-age strength gain. However, this accelerated hydration often leads to undesirable side effects, including negative impacts on the long-term mechanical and durability properties of concrete [3,4]. As early as seven days after casting, rapid hydration under elevated temperatures can result in a less robust physical structure of hydration products, leading to increased porosity and less favorable pore distribution, ultimately compromising long-term strength. Studies have shown that heat curing may not increase total porosity but rather alter pore size distribution, favoring meso- and macropores, depending on the thermal treatment’s duration and intensity [4,5,6,7]. Furthermore, durability-related properties such as carbonation resistance, chloride ingress, water sorptivity, permeability, and surface resistivity are also affected by heat curing [8,9,10]. To mitigate such performance and durability concerns, Portland cement (PC) content is often increased in precast mixes; however, this approach raises both economic and environmental costs and does not align with current sustainability requirements. As an alternative, partial replacement of PC with supplementary cementitious materials (SCMs), such as limestone filler (LF) and fly ash (FA), has been proposed to counteract these adverse effects while reducing the CO2 footprint [1,11,12].
In order to decrease the cement content of heat-cured SCC, replacing 20% of PC content with filler was proposed in previous studies without significantly influencing the durability properties of the final product [6]. The main objective of the LF inclusion was to obtain a more compact and denser cementitious matrix. However, heat curing still influenced final strength values, increased sorptivity coefficients and total porosity, altered pore size distributions, and modified the composition of C-S-H gels (mainly in alumina and sulfate contents). As a consequence, heat curing decreases the resistance of reinforced concrete against carbonation and chloride penetration. Other approaches, such as the use of synthetic C-S-H, have aimed to improve early-age strength and reduce steam curing energy consumption, though they may increase shrinkage and affect durability [13,14,15]. However, synthetic C-S-H, in combination with SRPCA, can improve early strength, shorten setting time, and reduce shrinkage [16].
Despite these efforts, a knowledge gap remains in understanding how expansive agents (EAs) interact with SCMs under heat curing conditions, trying to improve both mechanical performance and durability. This study addresses that gap by evaluating the combined effects of EAs, LF, and FA in SCC systems subjected to heat curing. The present study proposes the use of expansive agents as a means to enhance both the durability and sustainability of the precast concrete industry. By compensating for shrinkage and reducing crack formation, expansive agents have the potential to improve service life and reduce the need for higher cement contents, thereby lowering the environmental impact of concrete production. This is particularly relevant when combined with partial cement replacements, such as LF and FA, which reduce carbon footprint and improve workability. Although LF has traditionally been regarded as inert, recent studies show that it can influence early hydration processes, especially in systems containing EAs [17,18,19,20]. Previous works have demonstrated that after heat curing, the synergistic effect between limestone powder and metakaolin promotes reactions of both metakaolin and PC, reducing long-term sorptivity and improving mechanical properties [12]. Moreover, FA in combination with granulated blast-furnace slag has been reported to increase the structural strength of heat-cured concretes [21]. In contrast to these combined approaches, the present study evaluates FA and LF separately, each in combination with EAs, to isolate their individual contributions.
Expansive agents have already been used successfully to mitigate shrinkage in various concrete applications, including joint-free pavements, monolithic elements, confined systems, such as concrete-filled steel tubes, and crack sealing [22,23,24,25,26,27,28]. The two most widely used types are CaO-based (type-G) and ettringite-based (type-K) agents. Type-G promotes portlandite formation, while type-K generates ettringite via calcium sulfoaluminate and calcium sulfate [23,27]. Expansion is induced by the crystallization pressure exerted by the oversaturation of hydration products in the hardened matrix. However, these mechanisms are not yet fully understood [25,29,30,31], and the efficacy of EAs is highly sensitive to curing conditions, mix composition, and the degree of restraint [23,24,25,26,27,28,29,30,31,32,33,34]. Moreover, recent studies have proposed the use of aluminum waste to develop expansive concretes with a lower carbon footprint [35].
Due to the specific characteristics of heat curing (high humidity and temperature), the performance of EAs under these conditions is expected to improve the mechanical properties of heat-cured concretes. However, each type of EA presents specific limitations: CaO-based agents hydrate rapidly and form highly soluble portlandite, reducing long-term expansion potential [26,36]; ettringite-based agents require high water content and adequate curing to be effective [25]. These sensitivities become even more critical in SCC systems, where the inclusion of chemical admixtures and SCMs may alter EA performance [34]. While prior research has demonstrated the feasibility of combining SCCs with EAs [32,37,38], their performance under heat curing regimes and their potential to enhance post-curing mechanical properties remain underexplored.
This study aims to address these gaps by exploring the synergistic effects of expansive agents, SCMs (FA and LF), and heat curing on expansive self-compacting concretes, with the goal of improving mechanical performance post-heat-curing—typically a weakness in conventional (non-expansive) concretes. Successful implementation of this approach could enable a reduction in PC content in the precast concrete industry, thereby enhancing its sustainability without compromising performance.

2. Materials and Methods

Six different SCC mixes were designed and characterized; three of them were expansive concretes. Table 1 shows the nominal compositions of the six SCCs evaluated. A CEM I 42.5R according to [39] was used in all of them. Two sizes of siliceous aggregates were used: crushed siliceous gravels (4/12 mm) and siliceous sand (0/4 mm). A liquid polycarbolixate-based superplasticizer (density = 1.056 g/cm3) was used, and the fine content was increased using class F low-CaO fly ash (FA) or limestone filler (LF). Two expansive agents were used to fabricate the expansive SCC: the type-K agent (based on calcium sulfoaluminate) and the type-G agent (based on calcium oxide). A total of 10% in the weight of cement of the expansive agent was used in the expansive SCC. In previous studies made by the authors, higher dosages of expansive agent type-G promoted internal cracking in the concrete samples cured under water. For this reason, 10% of the cement content was selected as the most acceptable maximum type-G expansive agent content for the SCC of the present study. In this sense, to ensure comparability, the dosage of the type-K agent was also fixed at 10% of the cement content. Table 2 shows the chemical composition (%) of the cement and the EAs used.
The fresh state of the fabricated concrete mixes was assessed by conducting the slump flow test according to [40] standard. Density [41] and air content [42] were also determined in all cases.
Three different curing conditions were considered for each concrete mix as follows:
-
Standard curing: the samples were cured in a humidity chamber at 98 ± 2%RH and 20 ± 2 °C.
-
Dry during: the samples were maintained at 50 ± 5% of relative humidity (RH) and 20 ± 2 °C.
-
Heat curing: the samples were submitted to a heat curing process that simulated a typical process carried out in the fabrication of precast concretes. This heat curing process was already validated in previous studies [6]. Figure 1 shows the heat curing process followed. After this heat curing, the samples were maintained at 50±5% of relative humidity (RH) and 20 ± 2 °C.
In the hardened state, the physical–mechanical properties and the expansion characteristics were evaluated, as well as the pore structure characteristics. A total of Ø100 × 200 mm cylindrical concrete specimens were fabricated to assess the compressive strength evolution at 2 and 28 days according to [43]. Three samples for each age, concrete type, and condition were used. Smaller cylindrical specimens (Ø75 × 150 mm) were fabricated to evaluate the total porosity and the pore size distribution of the fabricated samples using a Mercury Intrusion Porosimeter (MIP, Micromeritics porosimeter Model 9320) at 2 and 28 days of curing. A sample of concrete of approximately 1cm3 was used according to [44] and [45].
Additionally, prismatic specimens were fabricated to measure the expansion under uniaxial restraining following a standardized method [46]. In this test, two square end steel plates connected by a steel bar were placed on each end of the prismatic molds before specimens of 254 × 76 × 76 mm were cast (six per concrete mix). For all cases, longitudinal expansion along the main axis of the prism was measured using a digital comparator with 0.002 mm accuracy. Two identical samples were tested for each age, concrete type, and condition. The expansion performance was measured for more than 80 days. During the first 15 days, the measurement was taken daily, while later, the measurement was taken weekly.

3. Results and Discussion

3.1. Fresh-State Characteristics of the Fabricated Concretes

Table 3 presents the properties measured in the fresh state for all the concrete mixes produced. In every case, concretes incorporating the expansive agents showed lower workability compared to the reference mixes, mainly the concretes with expansive agent type-K. Nevertheless, it was still possible to develop self-compacting concrete that met the required fresh-state performance using either type of expansive agent. Indeed, previous research has demonstrated that incorporating type-G or type-K agents does not compromise the ability to meet self-compacting criteria. However, the slump flow values can vary, depending on the specific agent–cement pairing used [32,37,38]. The lower slump flow obtained when using expansive agent type-K could be related to interactions between the anhydrous of this agent and the superplasticizers. In this sense, it is well known that the sulfate content of the binder influences the superplasticizer efficiency, and type-K agents clearly increase this content. Previous results showed that with increased dosage of sulfate added into cement paste, the adsorption-dispersing behavior of polycarboxylate superplasticizer decreases. Alkali sulfates (Na2SO4 and K2SO4) significantly affect both the adsorption behavior as well as the dispersing capability of this type of superplasticizer [47].
One slight difference between the expansive SCC and the reference SCC was the increased air content found in the former. In the two concrete types considered, with the FA of LF, the air content increase was greater when using expansive agent type-G; this aspect can be related to the higher slump flow measured in the SCC with the type-G agent with respect to that measured in the SCC with the type-K agent. In fact, the use of both EAs led to the formation of very fine bubbles during mixing. This effect appears to be mainly influenced by the presence of the superplasticizer. In earlier research, the authors also observed a rise in air content in SCC when expansive agents were added, depending on the type of cement used [19]. In this study, concretes made with an LF binder had higher air content in the fresh state compared to those with an FA binder, confirming that binder composition plays a significant role in this behavior.
Despite the increased air content in the expansive SCC, its density was quite similar to that of the reference SCC, possibly due to a packing effect promoted by the addition of the expansive agents. Moreover, no visible surface irregularities due to entrapped air were noticed in the hardened specimens. Previous studies by the authors have also demonstrated that this increase in air content in the fresh state does not compromise the long-term mechanical strength of the fabricated expansive SCC [19,22].

3.2. Modifications Promoted in the Mechanical Properties of the Heat-Cured Concretes Due to the Inclusion of the Expansive Agents

Figure 2 presents the compressive strength values obtained in the concrete samples after 2 and 28 days of curing. Two different curing conditions were followed in this case, standard curing and heat curing, followed by dry curing.
According to the obtained results, three main aspects can be highlighted. First of all, the inclusion of both expansive agents increases the compressive strength in all cases. However, this increase would be mainly related to the higher binder content used in the expansive SCC, considering the binder as the cement plus FA or LF plus the expansive agent.
The second aspect to be highlighted is that the compressive strength values obtained for the SCC with type-K agents exceed the values of the corresponding SCC with type-G agents in all cases. Similar performance was reported in previous works using standard curing in restraining conditions [32]. This phenomenon was related to the fact that the ettringite nodules formed in expansive restraining conditions must be formed within pores, thus decreasing the total porosity of the concrete, with a subsequent strength increase. However, later studies showed that the ettringite formed by the blowing agents does not preferentially grow in pores, but in situ precipitation occurs [29,30,31,48]. In any event, the hydration of EAs leads to volume expansion of concrete, and it also makes some contribution to the strength of concrete, especially in the restrained conditions [49]. In these conditions, the lack of free space could induce the precipitation of expansive hydrates in the cracks promoted by the expansions. During the heat curing, the concrete mixes are in restrained conditions (in the steel molds), so this can partially explain the obtained results.
The third and most important result obtained is the higher compressive strength value obtained in some cases in the expansive SCC with the type-K agent after heat curing with respect to the values obtained under standard curing. This phenomenon can be observed in Figure 2 and in Table 4, where the decrease (or increase) in compressive strength when using heat curing with respect to standard curing is shown. Table 4 defines the percentage loss or gain in compressive strength of each concrete mix after heat curing, compared to that shown after standard curing at the same age. While in almost all cases, as is usually the case in precast concretes subjected to heat curing, the compressive strength after heat curing is lower than after standard curing at the same age. There are two exceptions in the SCC with K-type expansive: after two days in SCC with FA and after 28 days in SCC with LF. In fact, it is very remarkable that the decrease in compressive strength detected in the SCC with the type-K agent, when it occurs, is very low compared to the one measured in the reference SCC. The decrease in this parameter is also lower than in the corresponding reference case when using expansive agent type-G; however, in this last case, the difference is lower than when the expansive type-K is used. Therefore, it is evident that the use of EAs, especially type-K, in precast concretes subjected to heat curing can limit or even eliminate the problem of loss of strength promoted by heat curing. According to the obtained results, the concrete composition influences the efficiency of the expansive agent with respect to the issue evaluated in this manuscript.
The better performance of expansive SCC under heat curing compared to conventional (non-expansive) concretes could be related to modifications in the expansion regimes or to modifications in the porous microstructure. Both aspects are evaluated in the following sections.

3.3. Modifications Promoted in the Expansive Performance of the Heat-Cured Concretes Due to the Inclusion of the Expansive Agents

The expansion performance of the fabricated SCC was evaluated for different curing regimes in order to analyze the influence of the external conditions on the efficacy of each expansive agent for each concrete type. The three curing regimes considered were as follows:
-
Standard curing: the samples were cured in a humidity chamber at 98 ± 2%RH and 20 ± 2 °C.
-
Dry during: the samples were maintained at 50 ± 5% of relative humidity (RH) and 20 ± 2 °C.
-
Heat curing and, after this heat curing, the samples were maintained at 50 ± 5% of relative humidity (RH) and 20 ± 2 °C.
Figure 3 shows the results of the SCC with fly ashes, while Figure 4 shows the results of the SCC with limestone filler. Both figures reveal that, as expected, the concrete composition and the curing conditions significantly influence the expansion behavior. In all cases, under water curing, once the peak expansion is achieved, it remains stable throughout the testing period. Additionally, the maximum expansion in concretes with the type-G agent occurs earlier than in those with the type-K agent, consistent with the faster hydration kinetics reported for type-G systems [26,36]. In contrast, for the other two curing methods, also as expected, a reduction in expansion is observed over time. In this sense, a difference is detected regarding the expansive agent used. In the SCC with the type-G expansive agent, the final shrinkage is higher in the samples subjected to heat curing than in the samples under dry curing; however, in the SCC with the type-K expansive agent, similar values are obtained in the concrete with limestone filler, and the final shrinkage is even lower after heat curing when fly ashes are added.
Previous studies already detected that under dry curing, the shrinkage was higher in concretes containing the type-K expansive agent than with the type-G one [25,32,50]. These results indicate that water availability is more important for the expansion promoted by type-K agents. Nevertheless, the lower shrinkage of the SCC with the type-K agent after heat curing compared to the values obtained in dry conditions must be related to the better mechanical performance exhibited by this type of concrete after heat curing with respect to the performance shown by SCC with the type-G agent. In this sense, it is evident that heat curing, at least with the characteristic tried in this study, influences SCC with the type-G expansive agent more significantly than with the type-K one. Thus, heat curing should promote more modifications in the hydration processes of the type-G expansive agent than in the hydration processes of the type-K one. In fact, since heat curing is known to accelerate the hydration of cementitious materials and to limit their complete hydration [2,3,4,5,6,7,8,9,10], the different hydration rate of the type-G expansive agent and the type-K agent, being faster in the former, must be related to the obtained results.
Regarding the performance of concretes incorporating the same expansive agent but different SCMs, higher final shrinkage is detected when using FA than when using LF. A recent study has demonstrated that concrete with high FA content was more sensitive to curing temperature compared to ordinary PC [51]. Moreover, it is well known that the reactivity of the two SCMs is different. The LF has a nucleation effect [19,21,52] that can accelerate the early hydration of the EAs. In contrast, FA has a long-term effect [53]. If the LF accelerates the hydration reactions, it is possible that, after heat curing, there is less free water available to be evaporated, thus limiting the drying shrinkage. This would largely explain the higher drying shrinkage observed in FA concretes. Therefore, it seems that the synergy between LF and PC in expansive SCC cured at high temperatures is more suitable for obtaining better properties, i.e., higher mechanical strength and lower shrinkage, than the synergy between FA and PC. Eventually, for the specific case of the expansive SCC based on the type-K agent, the different shrinkage behaviors of FA- and LF-based mixes could be related to a synergistic effect between LF and the type-K agent, where LF supports more stable ettringite formation. Previous studies in conventional concrete under standard curing have demonstrated that LF prevents the destabilization of ettringite since LF could react with the aluminates to form monocarbonate [18,54,55]. Thus, it is possible that this phenomenon also occurs in expansive concretes under high temperature, although this aspect must be evaluated in depth.

3.4. Modifications Promoted in the Porosity Properties of the Heat-Cured Concretes Due to the Inclusion of the Expansive Agents

Figure 5 shows the total porosity (%) values of the fabricated concretes at different curing conditions and at different ages. Based on the results obtained, no major differences are observed between the total porosity values of the different concretes in each of the curing regimes considered. Moreover, there is no clear relationship between the total porosity values and the compressive strength values either. In fact, in some cases, the total porosity value is even lower in the reference concrete that had lower compressive strength than the expansive concretes (see Figure 4). However, it is possible to point out a phenomenon detected in some cases in expansive SCC with the type-K agent that does not occur in the reference SCC. In the samples with FA, after two days, the porosity is lower in the heat-cured concrete and higher in the standard-cured concrete. A similar phenomenon occurs for the SCC with limestone filler after 28 days of curing. In both cases, the compressive strength was higher in the accelerated-cured concrete, so it is evident that there must be some kind of relationship between these two parameters. Furthermore, this phenomenon never occurs neither in the reference SCC considered in this study nor in the results reported in previous publications [1,2,3,4,5,6,7,8,9,10]. Nevertheless, it is also true that, after two days, the concrete with limestone filler and expansive agent type-G also shows higher total porosity under standard curing than under heat curing, yet no higher compressive strength was observed in the heat-cured samples. Therefore, there must be other parameters involved.
Figure 6 shows the pore size distribution of the SCC with fly ashes, while Figure 7 shows the results related to the SCC with limestone filler. As with the total porosity results, the pore size distribution alone does not explain the compressive strength results either. While total porosity provides a general measure of void content, the size and connectivity of pores are more critical in determining mechanical performance. Studies show that small, well-distributed pores can have less impact on strength than larger, interconnected pores, which act as stress concentrators and crack initiators [56]. Moreover, in expansive systems, the formation of ettringite or portlandite can refine the pore structure without necessarily reducing total porosity. In fact, the presence of SCMs alters the hydration kinetics and the type of gels formed. For example, FA contributes to secondary pozzolanic reactions, producing additional C-S-H gel that densifies the matrix, even if porosity remains relatively high [57]. Similarly, LF can enhance early-age reactions, especially in the presence of expansive agents, leading to a more cohesive microstructure [56]. Furthermore, expansive agents (e.g., CaO-based or ettringite-based) introduce internal stress and microstructural changes that can offset the negative effects of porosity. The expansion process can close microcracks and densify the interfacial transition zone (ITZ), improving strength independently of porosity levels [57]. Eventually, heat curing accelerates hydration, often leading to coarser pore structures. However, in systems with SCMs and EAs, the early formation of ettringite and C-S-H gels can compensate for this by enhancing matrix integrity. Thus, two concretes with similar porosity may exhibit different strengths depending on the nature and distribution of hydration products [58].
In any case, once again, better performance of the concretes with the type-K expansive agent in heat curing compared to the rest of the concretes evaluated is detected. On the one hand, in the heat-cured concretes, those with the more refined porous structure are the ones fabricated with expansive agent type-K (except after two days in the SCC with FA, where it is the concrete with the type-G expansive agent that presents a smaller average pore size). On the other hand, the only case in which the porous structure is more refined after heat curing than after standard curing occurs in a concrete with a K-type expansive agent (FA-K at 28 days).
Both aspects are indicative that the inclusion of a K-type expansive agent in the heat curing process generates certain benefits at the microstructural level that translate into a lower drop in mechanical strength with respect to the same concrete cured under high humidity conditions. With the type-G expansive agent, certain benefits are also observed, but on a smaller scale than in the previous case, which is consistent with both the compressive strength results obtained and the expansion rates measured under different environmental conditions. Previous studies have shown that an increase in porosity, but with further pore refinement, can lead to improvements in durable properties. This was demonstrated in non-expansive SCC to accelerate curing, in which LF was included to replace part of the PC [6]. In any case, the fact that type-K expansive agents outperform type-G in the evaluated conditions should be related to two main microstructural aspects, considering the SCM used in this study. On one hand, as previously mentioned, LF prevents the destabilization of ettringite since LF could react with the aluminates to form monocarbonate [18,54,55]. On the other hand, fly ash has also been shown to increase the stabilization of the ettringite by reducing the availability of reactive aluminates and sulfates and by modifying the pore solution chemistry. In particular, Class F fly ash, as the one used in the present study, is more effective than Class C in stabilizing ettringite due to its lower calcium content and higher pozzolanic activity [59]. In this sense, previous studies have shown that optimized steam curing regimes (e.g., controlled ramp-up and holding times) combined with FA or LF can significantly improve the mechanical properties and durability of concrete. For example, a 3 h static resting time followed by 6 h at constant temperature was found to enhance compressive strength and reduce porosity in steam-cured concrete with FA and slag [22].
The good performance of expansive SCC with agent type-K in high temperature curing, typical of the precast concrete industry, opens the possibility of a reduction in the cement content of these concretes, with a lower decrease in mechanical properties than that detected in conventional (non-expansive) concretes. Logically, the use of a lower cement content in these construction elements will reduce the carbon footprint of this process.
In any case, this study should be extended with microstructural studies that explain in depth the better performance of concretes with the type-K expansive agent since it is evident that the hydration mechanisms of this type of agent, studied in previous works [25,29,30,31], must be more suitable under high temperature curing than those of the type-G expansive agent. The microstructural mechanisms that cause expansion with one or another expansive agent are clearly different, and, according to the results obtained in this study, it is evident that those that occur with the K-type agent are less affected (or even favored) by the high-temperature curing carried out in this study.

4. Conclusions

The use of expansive agents in precast SCC subjected to heat curing effectively mitigates the reduction in mechanical properties typically observed in heat-cured concretes after 28 days. In this sense, the drop in strength of the SCC with EAs after heat curing with respect to that obtained after standard curing is much lower than that detected in the non-expansive SCC. This finding is highly relevant for the precast concrete industry, as it enables the production of durable, high-performance elements without the need to substantially increase cement content—thus contributing to both economic efficiency and environmental sustainability.
In the concretes developed in this study, FA and LF have been used to partially replace cement and, therefore, reduce the carbon footprint of the resulting SCC. In this sense, the combination of LF with the EAs seems more appropriate to improve the properties of the SCC after the heat curing. In fact, the use of LF in the expansive SCC limits the drying shrinkage obtained after the heat curing. Furthermore, after 28 days, the concrete with limestone filler and expansive agent type-K subjected to heat curing shows higher compressive strength (>4.3%) than the same concrete subjected to standard curing.
The evaluation of the porosity structure of the fabricated concretes mark the better performance of the expansive agent type-K in the heat curing considered in this study with the obtaining of a more refined porous structure than when using expansive agent type-G and a total porosity value 5% lower than that obtained after 28 days of standard curing in the same concrete mix. However, it is important to emphasize that porosity alone does not fully explain the mechanical behavior. In this context, the distribution, connectivity, and chemical nature of pores, as well as the composition and morphology of hydration products, play a decisive role in strength development. For example, the formation of ettringite and secondary C-S-H gels in the presence of SCMs and EAs can densify the matrix and improve strength, even when total porosity remains relatively high.
Despite the promising results, detailed microstructural analyses are needed to better understand the hydration mechanisms of expansive agents in combination with FA and LF under heat curing. Future research should also explore the following:
–
The long-term durability of these systems.
–
The kinetics of ettringite formation and stabilization in the presence of SCMs under heat curing regimes.
–
The optimization of curing regimes to balance early strength gain and long-term performance.
These investigations will be essential to fully validate the industrial applicability of expansive SCC systems and to guide mix design strategies for sustainable precast concrete production.

Author Contributions

Conceptualization, J.L.G.C.; methodology, J.L.G.C. and P.C.; validation, J.L.G.C. and P.C.; formal analysis, J.L.G.C. and P.C.; investigation, J.L.G.C. and P.C.; resources, J.L.G.C.; data curation, J.L.G.C. and P.C.; writing—original draft preparation, J.L.G.C.; writing—review and editing, P.C.; project administration, J.L.G.C.; funding acquisition, J.L.G.C. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the Spanish Ministry of Science and Innovation for the financial support given to carry out this research as part of projects BIA2015-64363-R. They would similarly like to express their gratitude to Cementos Portland Valderribas and IMCD for supplying the cement and expansive agents used in this study.

Acknowledgments

The authors would like to thank Alfredo Fernández-Escandón and Juan Carlos Porras from IETcc-CSIC for their collaboration in this work.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
SCCSelf-compacting concrete
PCPortland cement
SCMSupplementary cementitious material
LFLimestone filler
FAFly ash
EAExpansive agent

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Figure 1. Heat curing process followed in the concrete samples submitted to heat curing.
Figure 1. Heat curing process followed in the concrete samples submitted to heat curing.
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Figure 2. Compressive strength values obtained with standard and heat curing. HC, heat curing; SC, standard curing.
Figure 2. Compressive strength values obtained with standard and heat curing. HC, heat curing; SC, standard curing.
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Figure 3. Expansive performance of the SCC with FA under the three different curing regimes considered. SC, standard curing; DC, dry curing; HC, heat curing.
Figure 3. Expansive performance of the SCC with FA under the three different curing regimes considered. SC, standard curing; DC, dry curing; HC, heat curing.
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Figure 4. Expansive performance of the SCC with LF under the three different curing regimes considered. SC, standard curing; DC, dry curing; HC, heat curing.
Figure 4. Expansive performance of the SCC with LF under the three different curing regimes considered. SC, standard curing; DC, dry curing; HC, heat curing.
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Figure 5. Total porosity values of the SCC at different ages and curing regimes.
Figure 5. Total porosity values of the SCC at different ages and curing regimes.
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Figure 6. Pore size distribution of the SCC with FA at different ages and curing regimes. Up, 2 days; down, 28 days.
Figure 6. Pore size distribution of the SCC with FA at different ages and curing regimes. Up, 2 days; down, 28 days.
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Figure 7. Pore size distribution of the SCC with LF at different ages and curing regimes. Up, 2 days; down, 28 days.
Figure 7. Pore size distribution of the SCC with LF at different ages and curing regimes. Up, 2 days; down, 28 days.
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Table 1. Nominal composition (kg/m3) of the SCC designed.
Table 1. Nominal composition (kg/m3) of the SCC designed.
FA-RefFA+KFA+GLF-RefLF+KLF+G
Water195195195195195195
CEM I 42.5R385385385385385385
FA115115115---
LF---115115115
Expansive agent type-K-38.5--38.5-
Expansive agent type-G--38.5--38.5
Coarse aggregate (4/12 mm)681667667695680680
Sand (0/4 mm)962942942980961961
Superplasticizer4.64.64.64.64.64.6
Table 2. Chemical composition (%) of the cement and the expansive agent.
Table 2. Chemical composition (%) of the cement and the expansive agent.
CaOSiO2Al2O3SO3Fe2O3MgO
CEM I 42.5R60.317.44.683.175.081.78
Expansive agent type-K54.01.8813.626.50.491.33
Expansive agent type-G95.61.97--0.190.69
Table 3. Characteristics of the fresh state of the fabricated SCC.
Table 3. Characteristics of the fresh state of the fabricated SCC.
FA-RefFA+KFA+GLF-RefLF+KLF+G
Slump flow (mm)700645670645580650
Density (kg/m3)2.292.332.312.312.332.31
Air content (%)2.82.93.22.93.23.5
Table 4. Percent decrease (or increase) in compressive strength after heat curing compared to standard curing.
Table 4. Percent decrease (or increase) in compressive strength after heat curing compared to standard curing.
FA-RefFA+KFA+GLF-RefLF+KLF+G
2 days−13.2%+6.4%−12.9%−22.2%−5.9%−18.3%
28 days−22.8%−7.9%−12.1%−20.9%+4.3%−10.2%
The values in bold indicate higher compressive strength after heat curing than after standard curing.
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García Calvo, J.L.; Carballosa, P. Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes. Buildings 2025, 15, 3128. https://doi.org/10.3390/buildings15173128

AMA Style

García Calvo JL, Carballosa P. Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes. Buildings. 2025; 15(17):3128. https://doi.org/10.3390/buildings15173128

Chicago/Turabian Style

García Calvo, José Luis, and Pedro Carballosa. 2025. "Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes" Buildings 15, no. 17: 3128. https://doi.org/10.3390/buildings15173128

APA Style

García Calvo, J. L., & Carballosa, P. (2025). Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes. Buildings, 15(17), 3128. https://doi.org/10.3390/buildings15173128

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