Alkali-activated mortars and concretes have been gaining increased attention due to their potential for providing a more sustainable alternative to traditional ordinary Portland cement mixtures. In addition, the inclusion of high volumes of recycled materials in these traditional mortars and concretes has been shown to be particularly challenging. The compositions of the mixtures present in this paper were designed to make use of a hybrid alkali-activation model, as they were mostly composed of class F fly ash and calcium-rich precursors, namely, ordinary Portland cement and calcium hydroxide. Moreover, the viability of the addition of fine milled glass wastes and fine limestone powder, as a source of soluble silicates and as a filler, respectively, was also investigated. The optimization criterium for the design of fly ash-based alkali-activated mortar compositions was the maximization of both the compressive strength and environmental performance of the mortars. With this objective, two stages of optimization were conceived: one in which the inclusion of secondary precursors in ambient-cured mortar samples was implemented and, simultaneously, in which the compositions were tested for the determination of short-term compressive strength and another phase containing a deeper study on the effects of the addition of glass wastes on the compressive strength of mortar samples cured for 24 h at 80 °C and tested up to 28 days of curing. Furthermore, in both stages, the effects (on the compressive strength) of the inclusion of construction and demolition recycled aggregates were also investigated. The results show that a heat-cured fly ash-based mortar containing a 1% glass powder content (in relation to the binder weight) and a 10% replacement of natural aggregate for CDRA may display as much as a 28-day compressive strength of 31.4 MPa.
Keywords: alkali-activated mortar; fly ash; recycled aggregates; waste glass; compressive strength
Reducing the environmental footprint of many of the economic activities that enable human progress is a difficult task. The natural world contains delicate equilibriums, which are often incompatible with the scale and nature of human undertakings, and while humanity has been able to ignore this issue for thousands of years, the negative environmental impacts of certain activities of mankind are nowadays so pervasive that their effect can now be felt in virtually every aspect of human life. Given the scale at which raw natural resources are consumed, ecosystems are destroyed and pollution is generated by the construction sector, and despite being one of the fundamental tools of human development, this field is one of the primordial targets for intervention, when policies concerning the mitigation of the environmental impacts of human activity are designed. Two of the most efficient practices which can be implemented to improve the eco-efficiency of construction activities are, on one hand, the selection of materials produced with the help of more sustainable processes, and on the other, the adoption of the use of recycled materials. The urgent need for recycling measures has already been inscribed in the Europe 2020 Strategy for smart, sustainable and inclusive growth [[
The vast majority of concrete compositions (one of the most used materials in the world) is produced with the help of ordinary Portland cement (OPC). Reflecting the previously stated environmental concerns, there has been a substantial effort from the scientific community to provide solutions that allow for the inclusion of higher rates of waste materials in OPC-based concretes [[
One of the wastes which has been thoroughly used as a precursor in alkali-activated binder mortars and concretes both in scientific literature and practical applications is fly ash [[
However, the selection of the binders, and therefore production processes, for mortars and concretes, is not limited to a choice between traditional OPC-based mixtures, and alkali-activated compositions. In fact, there is a class of binders, termed "Portland-alkaline cements", which has been receiving growing attention from the scientific community. These binders contain blends of OPC and aluminosilicates (such as fly ash and/or blast furnace slag) and, by reducing the cement content of the mixtures, allow for the upgrade of the environmental performance of mortar or concrete compositions. In addition, this process also enables the possibility of using higher amounts of non-calcined clays (such as bentonite [[
The reduction of the sodium silicate content of an AAB composition is a measure which further improves the environmental sustainability of its fabrication as its production process is highly energy intensive and utilizes a great number of natural resources [[
One of the main advantages of the use of glass wastes is its abundance. In the EU, for example, and according to the EUROSTAT online database on waste generation [[
As already mentioned, the analysis of the properties of AAB-based mortars and concretes suggests that, when compared to OPC-based mixtures, these materials possess a higher tolerance for the incorporation of, on the one hand, higher rates, and on the other hand, a wider diversity of wastes [[
One of the applications of AAB-based materials which demonstrates greater potential is the case of the utilization aggregates sourced from construction and demolition waste (CDW) in concrete compositions. According to the previously mentioned EUROSTAT dataset, which concerns the total waste generation of EU-27 countries in 2018 [[
Another well-known strategy for improving the microstructure of mortars or concretes is the inclusion of a filler. Once again, this presents an additional opportunity for the inclusion of recycled wastes in the mixture. Several waste materials have already been shown to adequately improve the performance of OPC-based concretes [[
The present work focuses on the utilization of a fine waste that can be sourced from commercial CDW recycling plants (amongst others) and which is mainly composed, not only of masonry or cementitious products, but also of materials like wood, plastic or asphalt, among others. Although an official designation for these recycled aggregates does not exist, in this investigation, and similarly to other existing literature on this subject [[
In summary, the experimental work carried out consists of the development of a methodology to elaborate an eco-efficient mixture displaying adequate compressive strength. The simultaneous utilization of precursors composed of fly ash, Portland cement, calcium hydroxide and fine milled waste glass is a new approach, in a field (hybrid cements) where literature is scarce. Moreover, the study of the inclusion of fine CDRA in the mortars is also important to access the potential of geopolymers to accommodate higher waste contents.
The research work was performed in two major phases, one in which the object of investigation was the short-term compressive strength of ambient-cured mortar samples and another in which the investigation focused on the determination of the 28-day compressive strength of heat-cured mortar samples. For this purpose, several 40 × 40 × 160 mm parallelepiped samples were produced and subsequently submitted to compression tests [[
The primary precursor of the mixtures which can be found in this work is low calcium (type F) fly ash. This material was originally sourced from a Portuguese coal-fired power plant in Sines, CIMPOR, which afterwards tested and supplied the material to this investigation. According to the testing performed, this material fulfills the requirements stated in the European standard concerning fly ash use in concrete applications [[
Another highly important material thoroughly used in this investigation is ordinary Portland cement. The presence of OPC as starter material in an AAB concrete, as previously mentioned, allows for the simultaneous generation of supplementary calcium-based gels and therefore aids in the development of the mechanical strength of the mixture. In this investigation, the authors made use of type I 42.5 R Portland cement, most of which was sourced from the CIMPOR production facility in Souselas, Coimbra, Portugal.
Furthermore, the addition of slaked lime (calcium hydroxide) was also evaluated as a further source of calcium and as an accelerator of the setting of the samples.
The viability of using glass wastes as a source of silica in alkali-activated mortars for the improvement of mechanical strength results was analyzed through the addition of crushed used soda-lime bottles up to a particle size of less than 500 µm. The main objective of this procedure was to provide soluble silicates to the binder, with the intention of reducing the sodium silicate content of the mortar and therefore enhancing its eco-efficiency.
As to the production process of the powder, before reaching the crushing stage, the bottles were thoroughly washed, and all the non-glass materials were removed. The crushing process was performed via the insertion of the bottles and the steel balls in a Los Angeles abrasion test machine and then performing standard abrasion test cycles of 500 revolutions at a speed of 31 revolutions per minute [[
To produce sodium hydroxide solutions, and in order to reduce the probability of the presence of impurities, pellets with 98–99% purity were dissolved in distilled water until the target concentration was reached.
As to the other alkaline activator used in this work, sodium silicate, its chemical composition is indicated in Table 2.
The fine recycled aggregates used in this work (Figure 2) were obtained from an operating commercial CDW recycling plant located in Figueira da Foz (RCD—Resíduos de Construção e Demolição). As to the recycling process implemented in this facility, prior to being introduced in the production line of the industrial plant, the raw CDW is not subjected to any sorting. The fine material resulting from the physical processes used by this company to separate the vast majority of the waste's contaminants is a somewhat heterogeneous aggregate which possesses a particle size within the 0/10 mm range and which is rarely recycled into OPC-based concretes.
To further illustrate this reality, an image of the contaminants that it was possible to remove from a sample of dried fine recycled aggregates after performing a visual inspection is presented in Figure 3.
Previously to being transported to the laboratory, the recycled aggregates in the plant were stored unsheltered in the plant. For this reason, to remove the humidity, the aggregates were dried at a temperature of 110 °C to constant mass and then sieved to obtain the fraction 0/8 mm for use in the production of the mortars.
In addition, sand of the fraction 0/4 mm was used as fine natural aggregate in the production of the mortars. The particle size distribution of both aggregates can be observed in Figure 4.
Finally, it is important to mention that, in specific compositions, the upgrade of the aggregates particle size distribution was also evaluated through the use of a filler composed of waste limestone powder.
A high-performance superplasticizer based on modified polycarboxylates, Sika ViscoCrete 3005 (supplied by Sika Portugal), was added to the mixtures in order to reduce the hydration needs of the mortars (especially of the recycled aggregates) and thus optimize the particle packing of the mortars.
The current section details the design principles subjacent to the conception of the eco-efficient mortars produced in the scope of this work. Also presented are the detailed material compositions of the mentioned mixtures.
This investigation was preceded by a long stage of preliminary compressive strength testing performed on experimental samples. In this stage, several factors, such as the optimal curing temperature, recycled aggregate replacement rate and several material proportions were established and thus this work presents only the fine tuning of the optimization of the design of mixtures and the compositions and not the justification for every design decision previously taken.
The main goal of the first testing campaign was the maximization of the compressive strength of ambient cured fly ash-based alkali-activated mortars containing waste glass and recycled aggregates. For this purpose, an initial set of three mortar mixtures was designed and the main material properties of these mixtures can be observed in Table 3.
As can be observed, the majority of the binder's weight was, in all mixtures, composed of fly ash. Moreover, ordinary Portland cement, slaked lime and waste glass powder were used as secondary precursors in an attempt at maximizing the compressive strength of the samples. Furthermore, following the knowledge acquired during the already mentioned preliminary testing stage (and to current available knowledge), 10% of the natural aggregates were replaced with CDRA. This replacement typically hinders the workability of the mixtures, and thus, the use of a superplasticizer was implemented (in a proportion of 1%, relative to binder weight). The remaining material ratios of the compositions, such as activator to binder ratio, sodium silicate to hydroxide ratio, hydroxide concentration and aggregate to binder ratio are situated within the usual range of fly ash-based geopolymers.
Following the first round of compression tests, the investigation focused on analysis of the effects of increasing the OPC content of the mixes on the compressive strength of the mortars and simultaneously the consequences of reducing the hydroxide concentration (from 16 M to 10 M) and reducing the activator to binder ratio (from 0.7 to 0.6) in mixtures in which the binder was composed of fly ash and OPC in equal parts (Table 4).
In the third step of the initial compressive strength optimization stage, the most important modification of the mortar design was the substantial increase of the silicate to hydroxide ratio (from 1 to 2). In addition, for one of the mixtures (MM8), the alkaline solution to binder ratio was reduced, while simultaneously, the CDRA were completely replaced by natural aggregates (Table 5).
Building on the results provided by the previous stage, a new phase of the work was devised. More specifically, it was deemed important to investigate the strength performance of heat-cured compositions as the inclusion of a heat-curing stage would allow for compositions containing lower cement contents and to potentially reduce the silicate content of the alkaline solution. This strategy also allowed for the study, in more detail, of the influence of the addition of waste glass powder on the mechanical performance of the mortars.
As the addition of the glass waste implies the modification of the remaining material proportions, it was clear that a methodology would have to be devised in order to minimize the impact of the other material relationships on the final compressive strength. Consequently, and in light of the data provided by the initial testing stage, a reference composition was designed. This mortar was designed so that oven cured samples would achieve the highest compressive strengths possible, while minimizing the samples' OPC content. The composition of the reference sample can be seen in Table 6.
After designing the reference sample, two sets of mixtures based on variations of this composition were formulated:
- one set in which the addition of increasing amounts of waste glass (namely, 1%, 4% and 7% of the weight of the precursors) was compensated by a reduction in sodium silicate (plus proportionate adjustments in the fly ash, cement and aggregates content in order to maintain overall weight proportions);
- another group in which the same waste glass additions were matched by the increase in sodium hydroxide and decrease in sodium silicate (in this case, only cement and fly ash contents were impacted by the waste addition).
The variation in the relative mass (in relation to the total weight of the mixture) of each component of the mixtures designed according to the previously stated principles (group one and two of this section) is shown in Table 7.
For further clarification, the most important material ratios of the compositions resulting from the application of the first and second approach of the described method, respectively, are presented in Table 8 and Table 9.
All compositions were produced according to the specifications stated in the EN 196-1 standard [[
The resulting mortar was poured, in two equal layers, into 40 × 40 × 160 mm steel moulds. After the first layer was poured, a vibrating table was used to remove the air from inside the mixture. Then, the mold was filled by pouring the last layer of material and repeating the vibration process. Afterwards, the specimens were wrapped in plastic film and allowed to cure, either at room temperature or at 80 °C, for twenty-four hours, in an electric oven (methodology also followed by [[
No extra water was used in the production of the mortars.
Each 40 × 40 × 160 mm sample allowed for the compressive strength test to be performed three times, as the test machine was equipped with an adapter which transferred the loading from the plates to a 40 × 40 mm square. For logistical reasons, in each of the two stages of optimization, two different load testing machines were used (although the exact same procedures were used in both). The compressive strength test set-up used in the first stage of optimization is presented in Figure 5.
For each mixture, three cubic 40 mm × 40 mm × 40 mm samples were tested. The specimens were, as required by the European standard [[
As previously mentioned, the ten mixtures that belong to the initial stage were all tested up to 7 days. The average compressive strength for each of the curing periods and for each of the samples belonging to the first group of mortars, along with the 95% confidence interval for each sample and curing period (represented by the error bars), are presented in Figure 7.
As can be observed, even when 40 to 50% of the precursor's fly ash content is replaced by secondary precursor materials and, simultaneously, a high NaOH concentration (16 M) is utilized, the maximum average 7-day compressive strength obtained by the ambient cured mortars is inferior to 18 MPa. In addition, the utilization of a 7.5% cement content (in relation to the weight of the binder) results in a mortar displaying significantly lower average 7-day compressive strength (around 10 MPa).
The investigation now focused on mixtures containing a higher OPC content (50%) but lower silicate to hydroxide ratio (
The deterioration in strength results demonstrated by the compositions belonging to the second round of tests, in which every mixture has a low silicate to hydroxide ratio (
Finally, Figure 9 contains the strength results from the third round of mixes.
The results provided by these compositions show the positive effects of increasing the silicate to hydroxide ratio (from 1 to 2) as the range of the 7-day average compressive strength was the highest achieved thus far (in fact, even the worst-performing composition—MM7—displayed a strength of 23.1 MPa). In addition, similarly to what was observed in the previous round of testing, the reduction in alkaline solution did not lead to a reduction in strength (although a direct comparison between MM7 and MM8 cannot be made, as the recycled aggregates and limestone powder rate are also different in both mixtures). Nevertheless, the results clearly indicate that limestone powder additions are beneficial to the strength of mixtures. In reality, mixture MM9, which contains a 10% limestone powder for natural aggregates replacement, demonstrated a 7-day compressive strength of 28.1 MPa.
The strength results obtained in this stage of the investigation partly agree with the ones reported by [[
The disparity between the outcomes of both investigations is likely predominantly due to the difference in the comparable compositions of the mortars present in each investigation, namely, the recycled aggregates content (10% vs. 0%), lower sodium hydroxide concentrations (10 vs. 14M) and lower silicate to hydroxide ratio (2 vs. 2.5) used in the composition.
Furthermore, chemical, physical and microstructural differences of the fly ash used in each study may have also played a role in the contrasting strength results. In particular, the low value of the Blaine specific surface area of the fly ash utilized in the present work (257 m
The optimization of the waste glass content was performed through a set of mixtures in which mortars containing a wide range of material ratios were tested to determine the compressive strength. As previously detailed, the addition of the increased waste glass content of the mixtures was partly accommodated by either reducing the sodium silicate content while maintaining the sodium hydroxide amount and increasing the aggregates content or by also reducing sodium silicate but maintaining the aggregates and increasing the sodium hydroxide quantities (while keeping the remaining material proportions unchanged). This methodology was designed for the assessment of which of the alternatives would be most effective in minimizing the impacts of the replacement of the mixtures' binding materials (mostly fly ash) for glass wastes on the compressive strength of the compositions. The compressive strength results displayed by the mortars, for each of the two designed approaches, are presented in Figure 10.
As can clearly be observed via the analysis of the results, the modifications implemented for the composition of the reference sample were, with one exception, detrimental to the compressive strength of the mortars. These findings suggest that, for both approaches, the negative impacts of the cement/silicate reduction could not, with one exception, be offset by the changes promoted to the other components' contents. Notwithstanding, results from MM11 show that that a reduction in sodium silicate (−1% of the composition weight), fly ash (−0.56%) and cement (−0.19%) may, in these conditions, be compensated by the addition of a 1% waste glass content (and a 0.75% increase in the mixture's aggregate content). This composition displayed a 28-day compressive strength of 31.4 MPa.
To determine whether the factor 'glass addition' had a statistically significant effect on the 'compressive strength' variable, a one-way ANOVA test was conducted for each mean compressive strength and curing age period (short, medium and long term), the resulting p-values can be observed in Table 10.
The above-mentioned p-value results show, although for the short and medium-term curing age, a statistically significant relation between the glass addition factor and the compressive strength, as both p-values are situated below 0.05 (95% confidence level); the contrary can be stated in relation to the compressive strength values at 28 days, as the respective p-value (0.0309) suggests a statistically significant difference in the 28-day compressive strength with the increase in the mortars' waste glass content.
The range of compressive strength results demonstrated by the mortars is superior to the ones provided by the literature. As an example, [[
Another important conclusion that can be drawn from the analysis of the data is that the compressive strength development of heat-cured fly ash-based hybrid geopolymer (in which the activation model is based both on the activation of an alkaline solution and cement hydration) is highly unstable. In fact, the strength results are highly variable, with elevated dispersion of values both amongst the samples of the same mixtures and also for different time periods. As an example of this phenomenon, it is to be noted that the results frequently show [[
The explanation for this lack of stability in the strength development of the mortars is likely related to the complex chemical interactions (and competition) between the different types of gels which are typically formed in the activation of hybrid systems, as the utilization of high-calcium components, such as OPC or slaked lime, leads to the formation of mixed (C,N)-A-S-H or N-(C)-A-S-H-type gels [[
In relation to the environmental sustainability gains that these mixtures may offer, the evaluation can be performed in two distinct planes. From the standpoint of the reutilization of waste materials, the superiority of this mixture in comparison to OPC mortars is evident, as it allows for the recycling of, on one hand, glass wastes and, on the other, fine recycled aggregates from CDW, a product for which there is a lot of applicability in real-world industrial applications [[
This investigation focused on the design optimization of fly ash-based AAB mortars, based on the compressive strength performance of these mixtures. The study can be divided into two distinct stages: the first, in which ambient-cured compositions were taken to compressive strength failure after short curing periods (up to 7 days), and a second stage, conceived for testing heat-cured samples, tested at up to 28 days. The ultimate goal of the investigation was to optimize the eco-efficient performance of the compositions while simultaneously maximizing the compressive strength of the samples. To achieve this objective, the main priority of the first stage of the study was to avoid the energy intensive heat-curing period. Afterwards, the experimental work focused on the minimization of the cement content of the mixtures through the addition of waste glass powder. The most important conclusions drawn by this study are as follows:
- It is possible to obtain 7-day compressive strength values of about 28 MPa using ambient-cured fly ash-based mortars. Nevertheless, in the case ordinary Portland cement is used, large amounts of this material (50% of the binder) and alkaline activators must be used to produce mixtures with moderate potential for structural use.
- The implementation of a 24-h cycle of heat curing at a temperature of 80 °C allows an improved strength performance to be obtained with lower cement contents (below 25%).
- The utilization of glass waste powder revealed some potential in offsetting a decrease in the content of the most carbon intensive products of the mixtures (sodium silicate and cement). In fact, the mixture with the best 28-day compressive strength results (M11) possessed a 1% glass powder content (in relation to the binder weight) and displayed a 28-day compressive strength of 31.4 MPa.
- Although superplasticizers were used, the highest compressive strength results pertained to compositions with low workability.
- In most samples, the strength results of the fly ash-based mortars displayed high dispersion, and wide 95% confidence intervals were observed. This phenomenon was more prominent in heat-cured samples.
- A one-way ANOVA test showed that, for a 28-day curing period, the addition of glass wastes had a statistically significant effect on compressive strength.
Graph: Figure 1 Crushed soda-lime bottles glass (10× magnification; Φ < 500 µm).
Graph: Figure 2 Dried construction and demolition recycled aggregates.
Graph: Figure 3 Example of contaminants found in a random sample of CDRA.
Graph: Figure 4 Grading of the natural and recycled aggregates.
Graph: Figure 5 Compressive strength test set-up.
Graph: Figure 6 Fly ash-based AAB mortar specimen displaying conical semi-explosive failure.
Graph: Figure 7 Compressive strength results for the first round of the initial optimization stage.
Graph: Figure 8 Compressive strength results for the second round of the initial optimization stage.
Graph: Figure 9 Compressive strength results for the third round of the initial optimization stage.
Graph: Figure 10 Compressive strength results for the waste glass optimization stage. (a) First and (b) second approach.
Graph: materials-15-01204-g010b.tif
Table 1 Main fly ash properties.
Element Content (wt.%) SiO2 61.7 Al2O3 18.64 Fe2O3 6.81 CaO 1.45 Others 5.94 LOI 5.46 Reactive Silica 39.22 Part Diam. < 45 µm 62.9 Blaine Specific Surface (m2/kg) 257.0
Table 2 Chemical composition of the sodium silicate solution.
SiO2 (wt.%) Na2O (wt.%) Al2O3 (wt.%) H2O (wt.%) 27.3–28.3 8.2–8.6 <4.0 59.1–64.5
Table 3 Key material ratios of the first group of mortar mixtures.
Sample FA 1 (%) CM 2 (%) SL 3 (%) Gl 4 (%) S/H 5 H 6 (M) A/B 7 RA 8 (%) LP 9 (%) Ag:B 10 Sp/B 11 (%) MM1 60 7.5 12.5 20 2 10 0.7 10 10 3 1 MM2 60 25 15 0 2 16 0.7 10 10 3 1 MM3 50 25 15 10 2 16 0.7 10 10 3 1
Table 4 Key material ratios of the second group of mortar mixtures.
Sample FA 1 (%) CM 2 (%) S/H 3 H 4 (M) A/B 5 RA 6 (%) LP 7 (%) Ag:B 8 (%) Sp/B 9 (%) MM4 50 50 1 10 0.7 10 10 3 1 MM5 50 50 1 16 0.7 10 10 3 1 MM6 50 50 1 10 0.6 10 10 3 1
Table 5 Key material ratios of the third group of mortar mixtures.
Sample FA 1 (%) CM 2 (%) S/H 3 H 4 (M) A/B 5 RA 6 (%) LP 7 (%) Ag:B 8 (%) Sp/B 9 (%) MM7 50 50 2 16 0.7 10 0 3 1 MM8 50 50 2 16 0.6 0 10 3 1 MM9 50 50 2 16 0.7 10 10 3 1
Table 6 Composition of the reference sample.
Sample FA 1 (g) C 2 G 3 H 4 (g) S 5 A 6 MM10 281.3 93.8 0.0 100.0 200.0 1012.5
Table 7 Variation in the relative weight of the components of the mortars in relation to the reference sample.
Sample ΔFA 1 (%) ΔC 2 ΔG 3 ΔH 4 (%) ΔS 5 ΔA 6 MM11 −0.56 −0.19 1.0 0 −1.0 0.75 MM12 −2.81 −0.94 4.0 0 −1.0 0.75 MM13 −5.06 −1.69 7.0 0 −1.0 0.75 MM14 −0.75 −0.25 1.0 1.0 −1.0 0 MM15 −3.00 −1.00 4.0 1.0 −1.0 0 MM16 −5.25 −1.75 7.0 1.0 −1.0 0
Table 8 Key material ratios of the mortar mixtures—first approach.
Sample FA 1 (%) CM 2 (%) Gl 3 (%) S/H 4 H 5 (M) A/B 6 RA 7 (%) Ag:B 8 T 9 (°C) MM10 75.0 25.0 0 2 10 0.8 10 3 80 MM11 71.4 23.8 1 2 10 0.7 10 3 80 MM12 60.8 20.3 4 2 10 0.7 10 3 80 MM13 50.1 16.7 7 2 10 0.7 10 3 80
Table 9 Key material ratios of the mortar mixtures—second approach.
Sample FA 1 (%) CM 2 (%) Gl 3 (%) S/H 4 H 5 (M) A/B 6 RA7 (%) Ag:B 8 T 9 (°C) MM10 75.0 25.0 0 2 10 0.8 10 3 80 MM14 71.4 23.8 1 2 10 0.8 10 3 80 MM15 60.6 20.2 4 2 10 0.8 10 3 80 MM16 49.8 16.6 7 2 10 0.8 10 3 80
Table 10 The p-values of the F-tests performed with the values of the compressive strength for different curing periods.
Curing Period Compressive Strength Short-term (7/8 days) Medium-term (14 days) Long-term (28 days)
Conceptualization: S.M., S.L. and F.P.-T.; methodology: S.M., S.L., A.V.L. and F.P.-T.; investigation: S.M.; formal analysis: S.M., S.L., A.V.L. and F.P.-T.; data curation: S.M. and A.V.L.; writing—original draft preparation: S.M.; writing—review and editing: S.M. and S.L., A.V.L.; supervision: S.L. and F.P.-T. All authors have read and agreed to the published version of the manuscript.
This research was funded by FCT—Fundação para a Ciência e Tecnologia, grant number SFRH/BD/111813/2015.
Not applicable.
Not applicable.
Data are contained within the article or are available on request from the corresponding author.
The authors declare no conflict of interest.
The authors would like to thank CEMMPRE (project with reference UIDB/00285/2020) and C-TAC research centres, the Foundation for Science and Technology (FCT) for the financial support and also CIMPOR S.A., RCD—Resíduos de Construção e Demolição S.A.—and Sika Portugal–Produtos Construção e Indústria for the products supplied.
By Sérgio Miraldo; Sérgio Lopes; Adelino V. Lopes and Fernando Pacheco-Torgal
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