Mechanical Behavior of Sustainable Concrete with Alkali-Activated Pumice as Cement Replacement for Walkway Slabs in Humid Tropical Climates

Abstract

Portland cement production is a major source of global CO₂ emissions due to its high energy consumption and calcination processes. This study proposes a sustainable alternative through the partial replacement of cement with alkali-activated pumice, a naturally occurring aluminosilicate material with high regional availability. Mixes with 0%, 10%, 20%, and 30% cement replacement were designed for pedestrian slabs exposed to humid tropical conditions. Compressive strength was evaluated using non-destructive testing over a period of 364 days, and carbonation was analyzed at different ages. The results show that mixes with up to 30% pumice maintain adequate strength levels for light-duty applications, although with a more gradual strength development. A significant reduction in carbonation depth was also observed, especially in the mix with the highest replacement level, suggesting greater durability in aggressive environments. These findings support the use of pumice as a viable and sustainable supplementary cementitious material in tropical regions, promoting low-impact construction practices.

1. Introduction

In the current context of the climate emergency, the construction industry faces a dual challenge: to maintain its role as a driver of urban development and infrastructure while also reducing its environmental footprint [1], particularly the carbon dioxide (CO₂) emissions associated with Portland cement manufacturing [2,3]. This material, widely used for its mechanical properties and versatility, accounts for approximately 5% of global industrial energy consumption [4] and between 5 and 8% of total CO₂ emissions due to energy-intensive processes and carbonate calcination during clinker production [5]. In response to this issue, the search for supplementary cementitious materials (SCMs) has become a key strategy in the transition toward more sustainable and resilient construction practices [6,7].

Among the proposed SCMs, industrial by-products such as fly ash [8], ground granulated blast-furnace slag (GGBFS) [9], and rice husk ash [10] have demonstrated improvements in both the mechanical and durability properties of concrete. However, access to these materials may be limited or economically unfeasible in many regions—particularly in developing countries—where transportation, logistics, and dependence on external markets hinder their widespread adoption. In this context, natural pozzolans—such as pumice (PM), a volcanic-origin material with high regional availability—emerge as an attractive alternative to reduce clinker consumption while utilizing underexploited local resources [11,12].

Pumice, as an amorphous aluminosilicate, possesses pozzolanic properties that can be activated in the presence of water and calcium, contributing to the formation of secondary cementitious products such as calcium silicate hydrate (C-S-H) [13]. Laboratory studies have reported improved sulfate resistance, reduced permeability, and significant reductions in carbon footprint when using pumice as a partial cement replacement [14,15,16,17]. However, there is a notable lack of research evaluating its performance under real exposure conditions, where environmental factors such as ambient humidity, seasonal rainfall, and thermal fluctuations can significantly influence hydration processes, strength development, and concrete degradation mechanisms.

In particular, humid tropical zones such as the central region of Veracruz, Mexico, pose specific challenges for concrete: high relative humidity, frequent heavy rainfall, and elevated temperatures. These conditions can accelerate processes such as carbonation [18], a phenomenon in which atmospheric CO₂ reacts with hydroxides in the cement paste, lowering the pH of the concrete and promoting reinforcement corrosion [19,20]. Concrete’s resistance to this degradation mechanism has become a key indicator for assessing durability in exposed infrastructure, and its analysis becomes even more relevant when using alternative materials whose long-term behavior is not yet fully characterized [21].

From an applied engineering perspective, the use of local materials such as pumice represents a solution aligned with the principles of a circular economy, cost reduction, and construction decarbonization. The utilization of such natural resources could foster more sustainable construction practices tailored to the specific geographic, economic, and climatic conditions of each region, reducing dependence on high-impact industrial inputs [22].

Despite the growing interest in green concrete and the development of new low-clinker formulations [23,24], there remains a limited number of studies analyzing the structural performance and durability of concrete with SCMs under real tropical weather exposure [25,26]. Much of the scientific literature focuses on results obtained under laboratory conditions, which—although rigorously controlled—do not always reflect in-service realities [27,28]. The disconnect between experimental research and environmental exposure conditions represents a gap that must be addressed to ensure the practical applicability of findings and to support the transition toward more resilient and sustainable construction systems.

Therefore, the present study aims to evaluate the mechanical performance and carbonation resistance of concrete mixtures with different percentages of cement replacement by alkali-activated pumice, manufactured and exposed under real tropical weathering conditions in Misantla, Veracruz. Non-destructive techniques were used to monitor compressive strength over a period of 364 days, and the evolution of the carbonation index was modeled based on experimental data. The findings provide insight into the technical feasibility of these mixtures for light-duty urban infrastructure applications while offering valuable criteria for the design of more sustainable concretes tailored to regions with similar climatic conditions.

2. Materials and Methods

2.1. Materials

In this study, Portland Composite Cement type CPC30R RS was used, in accordance with the Mexican standard NMX-C-414-ONNCCE [29]. This cement is characterized by its high early strength and suitability for structural applications. It was used as the base material for the control mix and as a reference for the modified mixtures.

The aggregates consisted of natural river sand (with a fineness modulus of 2.9) and crushed gravel (nominal maximum size of 19 mm), both sourced from local quarries and selected according to the requirements of the NMX-C-111-ONNCCE standard [30]. These materials were previously characterized to ensure appropriate gradation, cleanliness, and volumetric stability.

The mixing water used was potable, complying with the NMX-C-122-ONNCCE standard [31], and free of impurities that could interfere with cement hydration or pumice activation.

The supplementary cementitious material employed was natural pumice, obtained from deposits in the central region of the state of Veracruz, Mexico. This pumice is characterized by its high porosity, abundant amorphous silica, and particle size distribution, which makes it suitable for reactions in alkaline media.

The pumice used in this study was obtained from the central region of the state of Veracruz. Its mineralogical characterization was carried out using X-ray diffraction (XRD), confirming the predominant presence of amorphous silica—responsible for its high pozzolanic reactivity—as well as traces of crystalline quartz [32]. This reactivity promotes the formation of calcium silicate hydrate (C-S-H) gels during the cement hydration process.

Although no chemical analysis by X-ray fluorescence (XRF) is currently available, previous studies have reported that pumice also contains aluminum oxide (Al₂O₃) and alkali oxides such as potassium oxide (K₂O) and sodium oxide (Na₂O), which may positively influence the rheological behavior and durability of concrete [32]. Altogether, this mineralogical information supports its potential as a natural pozzolan.

To enhance its reactivity, the pumice was alkali-activated using a sodium hydroxide (NaOH) solution, chosen for its availability and relatively low cost. A concentration of 0.25 N was used, equivalent to 10 g/L, based on a standard normality of 40 g/L.

2.2. Experimental Methods

The concrete mixtures were designed according to the ACI 211.1 method [33], targeting a compressive strength of 25 MPa and ensuring suitable workability for manual placement and vibration. Four mixtures were established (Table 1):

The water-to-cementitious-material ratio was 0.62 and remained constant across all mixtures. The total cementitious content was kept uniform in all formulations (per m³). No chemical admixtures were used in order to isolate the effect of pumice (Table 2).

2.3. Sample Preparation

The concrete mixtures were prepared manually. To enhance its reactivity, pumice was alkali-activated as a two-part geopolymer, using a sodium hydroxide (NaOH) solution at a concentration of 0.25 N (equivalent to 10 g/L), based on a standard normality of 40 g/L. The activation process consisted of mixing the pumice with the solution, resting it for 24 h at room temperature (~25 °C), and subsequently drying it at 100 °C before incorporation into the concrete.

The specimen preparation followed the two-part geopolymer concrete method [34] illustrated in Figure 1, where the activating solution is prepared and conditioned separately and then added to the pumice, which is mixed with cement and both fine and coarse aggregates. Additional water was then added to improve the workability of the mixture.

Prior to any fresh-state testing, the temperature of the concrete was measured using an immersion thermometer according to NMX-C-159-ONNCCE-2013 [35]. The reading was taken within the first five minutes after sampling, with the instrument submerged at least 75 mm into the mixture. Subsequently, the slump test was conducted in accordance with NMX-C-156-ONNCCE-2010 [36], using an Abrams cone on a rigid, moist surface. Concrete was placed in three equal layers, each compacted with 25 rod strokes. Upon vertical removal of the mold, the difference between the mold height and the specimen height was measured to assess the mixture’s workability.

The 10 cm cubes were cast into two layers, each compacted with 25 rod penetrations and tapped 10 to 15 times with a rubber mallet to expel entrapped air. The surfaces were then leveled, and specimens were cured in storage tanks following the NMX-C-159-ONNCCE standard [37].

Additionally, concrete slabs measuring 30 cm × 100 cm × 15 cm were cast and exposed to environmental conditions in Misantla, Veracruz—a municipality characterized by a warm–humid climate with high relative humidity.

2.4. Tests Conducted

2.4.1. Compressive Strength

The compressive strength of the concrete slabs was evaluated using non-destructive rebound hammer tests, following the guidelines established by the Mexican standard NMX-C-131-ONNCCE-2014 [38]. Measurements began at 28 days of curing and continued up to 364 days, with monthly readings to analyze the evolution of the concrete’s mechanical performance in service.

During each measurement campaign, readings were recorded at 16 evenly distributed points across the surface of each slab to ensure representative coverage of the structural element. The monthly average value from these readings was considered the characteristic parameter of the superficial compressive strength for each evaluated mix.

The use of rebound hammer testing enabled continuous and non-invasive monitoring of strength gain over time without compromising the physical integrity of the slabs. This methodology facilitated the comparison of mechanical development among the different formulations under real exposure conditions.

Since the methodology was based on non-destructive tests, it was not possible to observe compression failure modes or to obtain destroyed samples. However, the execution of the protocol was visually documented through representative photographs of the hardened slabs and during the testing procedures (see Figure 2).

2.4.2. Carbonation Depth

Carbonation depth was determined according to the guidelines of the Mexican standard NMX-C-178-ONNCCE-2010 [39]. The test was applied to concrete cube specimens exposed to natural environmental conditions in Misantla, Veracruz (Mexico), a warm–humid climate region. Measurements were performed after 28, 112, 196, 280, and 364 days to evaluate the progression of the carbonation front over time.

For each age, representative cubes were selected and vertically sectioned to expose the internal concrete surface. A 1% phenolphthalein indicator solution in ethanol was applied to this section. This technique visually distinguishes the carbonated zone (colorless) from the non-carbonated zone (pink), based on the concrete’s pH.

Carbonation depth was measured at two points on each of the four sides of the cut surface, obtaining a total of eight measurements per specimen. The average of these readings was considered the characteristic depth for each exposure interval. This methodology allowed comparative evaluation of the different mixtures’ resistance to carbonation under real exposure conditions.

3. Results

Figure 2a,b present the environmental conditions (monthly temperature and precipitation) recorded during the exposure period. These variables influence the progression of the carbonation front and strength development. It can be observed that high relative humidity and recurrent precipitation may slow down carbonation, while tropical temperatures favor the advancement of pozzolanic reactions.

Figure 3 shows the evolution of mechanical strength, evaluated by non-destructive rebound hammer testing, as a function of precipitation (Figure 3a) and ambient temperature (Figure 3b) over a period of 364 days following the casting of pedestrian slabs exposed to outdoor conditions in Misantla, Veracruz, characterized by a warm, humid tropical climate.

In Figure 3a, an inverse relationship is observed between precipitation intensity and the development of concrete strength, especially during the early and middle stages of curing. Notably, the period between days 168 and 224 registered a precipitation peak exceeding 500 mm, coinciding with a slowdown or slight decrease in strength development, more pronounced in mixes with higher cement replacement percentages by pumice (P20 and P30). This phenomenon could be attributed to overexposure to moisture, which may interfere with pozzolanic reactions, particularly in systems with lower availability of calcium hydroxide. Despite this, all mixtures, including the 30% replacements, showed progressive strength gain throughout the year, demonstrating their potential for light-duty urban infrastructure applications.

Figure 3b shows the correlation between mechanical strength and average monthly temperature recorded during the same period. A positive trend is evident, with strength increasing during the warmer months, particularly between days 112 and 196 when temperatures ranged from 27 °C to 33 °C. These thermal conditions favored the hydration and activation of pumice, especially relevant for mixes with high pozzolanic content that require elevated temperatures to develop their binding potential. In later stages (days 280 to 364), despite a slight decrease in temperature, strength was maintained or even slightly increased, reinforcing the long-term efficacy of using pumice as a supplementary cementitious material.

These results highlight the importance of considering the local climatic environment, in this case, the tropical regime of Misantla, for the design and curing of cement replacement mixes. While high precipitation may pose a challenge during early curing phases, the region’s typically elevated temperatures promote the development of adequate strength for light structural applications such as pedestrian slabs. Therefore, the implementation of activated pumice under tropical conditions is technically viable, sustainable, and replicable in regions with similar climatic characteristics.

The mechanical behavior of the control mix (P00) was modeled using a third-degree polynomial regression based on the average compressive strength values obtained from rebound hammer tests up to 364 days of exposure, as shown in Figure 4. The resulting model is expressed as

f(t) = 2 × 10⁻⁷ t³ − 0.0002 t² + 0.0411t + 22.786(1)

where f(t) represents the mechanical strength (MPa) as a function of time t (días). This model exhibits a coefficient of determination R² of 0.9506 and a correlation coefficient R of 0.9341, indicating an excellent fit between the experimental data and the fitted curve.

The cubic form of the model accurately captures the different stages of strength development: an initial phase of moderate gain, an acceleration during the mid-period (approximately between 84 and 196 days), and a gradual stabilization after 280 days. The initial strength was around 22.8 MPa and increased continuously, surpassing 30 MPa by the end of the monitoring period.

This behavior confirms the mechanical stability of the conventional mix under tropical conditions, serving as a reference to evaluate the effect of progressively increasing pumice replacements in mixes P10, P20, and P30.

For the P10 mix, which incorporates 10% cement replacement by pumice, a third-degree polynomial model was fitted to the experimental compressive strength data shown in Figure 5. This mix exhibited adequate mechanical behavior under tropical environmental conditions, with strength evolution represented by the following expression:

f(t) = 1 × 10⁻⁷ t³ − 1 × 10⁻⁵ t² + 0.0369t + 17.671(2)

where f(t) represents the mechanical strength (MPa) as a function of time t (days). This model exhibits a coefficient of determination R² of 0.9528 and a correlation coefficient R of 0.9371, which indicates an excellent fit with the rebound hammer data obtained.

The fitted curve shows progressive strength gain during the first two-thirds of the analyzed period, with a slight attenuation of the slope toward the last few months, as also observed in the data dispersion in Figure 5. The initial strength was approximately 17.7 MPa and it reached levels close to those required for pedestrian structures between 280 and 364 days, demonstrating that this mix maintained structurally functional behavior despite a significant reduction in clinker content.

The evolution of the mechanical strength of the P20 mix over time is shown in Figure 6. The experimental data were fitted using a third-degree polynomial model, expressed by the following equation:

f(t) = 3 × 10⁻⁸ t³ − 0.0001t² + 0.0865t + 15.302(3)

where f(t) corresponds to the mechanical strength in megapascals (MPa) and t to time in days. The model presents a coefficient of determination R² = 0.9784, indicating that 97.84% of the variability in mechanical strength is explained by the temporal variation described by the polynomial. Likewise, the correlation coefficient R = 0.9712 confirms a strong positive relationship between time and the increase in strength.

As shown in Figure 6, the initial strength at day zero was approximately 15.3 MPa, reflecting a significant base strength attributable to initial hydration and the alkaline activation of pumice. Subsequently, the strength increased steadily, reaching maximum values close to 38 MPa after around 360 days, demonstrating the ongoing geopolymeric reaction and progressive formation of hydration products that reinforce the concrete matrix.

The third-degree polynomial fit is particularly useful for capturing the nonlinearity in strength development, especially during the early days, when the rate of increase is more accelerated, and in later stages, where growth tends to stabilize. This model allows for a highly accurate prediction of strength at intermediate and long-term periods, which is crucial for the design and performance evaluation of rigid pavements and other civil works requiring durability under severe environmental conditions.

The evolution of mechanical strength for the evaluated mix is presented in Figure 7, where the experimental data were fitted to a third-degree polynomial model described by the following equation:

f(t) = 5 × 10⁻⁸ t³ − 1 × 10⁻⁵ t² + 0.0262t + 15.12(4)

where f(t) corresponds to the mechanical strength in megapascals (MPa) and t to time in days. The model presents a coefficient of determination R² = 0.9837, indicating that 98.37% of the variability in strength is explained by time, while the correlation coefficient R = 0.9783 confirms a strong positive correlation.

According to Figure 7, the initial strength is approximately 16.03 MPa, reflecting the early stage of hydration and alkaline activation of the material. During the analyzed period, the strength increased steadily, reaching a final value close to 20.5 MPa, evidencing gradual but sustained mechanical development.

This behavior suggests moderate hardening kinetics, which may be associated with the proportion and specific characteristics of the mix. The third-degree polynomial model captures the nonlinear evolution of strength, enabling reliable predictions for the long-term performance of the concrete.

Figure 8 shows the temporal behavior of the carbonation depth (in millimeters) for the four studied mixes with different percentages of cement replacement by activated pumice (0%, 10%, 20%, and 30%). The data evidence a progressive increase in carbonation depth as the exposure time advances, from 28 to 364 days.

It is observed that the control mix (P00) exhibits the highest carbonation depth at all ages, reaching 10.2 mm at 364 days, indicating greater susceptibility to carbon dioxide penetration. In contrast, the mix with 30% replacement (P30) shows the lowest depth, with 6.5 mm at the end of the period, demonstrating enhanced resistance to carbonation due to the incorporation of activated pumice.

The intermediate mixes P10 and P20 present intermediate values, with carbonation depths of 8.2 mm and 7.7 mm, respectively, at 364 days. This decreasing trend in carbonation depth with increasing replacement percentage confirms the effectiveness of the supplementary material in improving the durability of concrete against carbonation.

The development of carbonation depth follows consistent behavior across all mixes, with a faster increase during the first 112 days and a moderated rate of increment toward the end of the period, reflecting the kinetics of the chemical and physical reactions involved in CO₂ penetration.

These results are fundamental for assessing the service life and performance of concrete in aggressive environments, reaffirming that the partial replacement of cement with activated pumice helps reduce susceptibility to carbonation, thereby enhancing the structural durability of slabs and rigid pavements.

Figure 9 shows the projected evolution of the carbonation depth (average depth in mm) for the different mixes, calculated by fitting the carbonation coefficients obtained experimentally for each mix. The analysis is based on modeling the growth of carbonation over time, allowing for estimation of the period in which each mix reaches a critical value of 30 mm, corresponding to the cover thickness of the reinforcing steel used in structural applications.

As observed in the figure, the control mix P00 (without cement replacement) exhibits the highest carbonation rate, exceeding the critical cover before 15 years, indicating greater vulnerability to reinforcement corrosion. In contrast, mixes with partial cement replacement by activated pumice (P10, P20, and P30) show a slower progression of the carbonation front, delaying the time taken to reach a 30 mm depth.

In particular, the P30 mix shows the greatest delay in carbonation, with an estimated time to reach the critical cover exceeding 20 years, demonstrating the effectiveness of the supplementary material in enhancing the durability and protection of steel reinforcement in concrete structures.

This predictive approach based on carbonation coefficients and the evaluation of critical cover is fundamental for the design and maintenance planning of concrete infrastructures, ensuring prolonged service life and improved resistance to carbonation-induced degradation.

4. Discussion

The results obtained in this study confirm that the partial replacement of Portland cement with activated pumice represents a technically viable and environmentally sustainable alternative for concrete production exposed to humid tropical conditions. In particular, mixes with replacement levels of 10% and 20% (P10 and P20) maintained satisfactory mechanical performance throughout the 364-day period, even outperforming the control mix (P00) during certain intervals. This finding supports clinker content reduction without compromising structural functionality and aligns with a study by Martirena et al. [40], which evidenced improved strength with 20% natural pozzolan additions in Caribbean environments.

The influence of local climate on strength development was notable. Elevated temperatures favored accelerated strength gain during the first 200 days, consistent with results by Avet et al. [41], who demonstrated the higher reactivity of activated pozzolans under high thermal conditions. However, a slowdown in strength increase was observed between days 168 and 224, coinciding with a precipitation peak exceeding 500 mm. This behavior matches findings by Robayo-Salazar et al. [42], who also reported that overexposure to moisture dilutes Ca(OH)₂ and affects pozzolanic reactions in mixes with high replacement levels.

From the durability standpoint, the data show a systematic decrease in carbonation depth as pumice content increases, with P30 exhibiting the lowest advancement (6.5 mm at 364 days). This agrees with Mohsen et al. [43], who reported that the use of natural pozzolans significantly improves permeability and reduces carbonation depth by densifying the concrete matrix. According to projections, P30 could take over 20 years to reach the critical cover of 30 mm, compared to less than 15 years for P00, representing a significant improvement in service life and reinforcement protection.

The modeling of strength evolution using nonlinear regression models also yielded robust results. These third-degree polynomial models with R² ≥ 0.95 coincide with recent studies, such as Raju et al. [44], who employed artificial neural networks and random forests (RFs) to predict the strength of pozzolan-containing concretes, obtaining R² values between 0.96 and 0.98. This approach reinforces the applicability of advanced predictive models in analyzing the long-term mechanical behavior of concrete.

Although the results showed high coherence and reliability, future research is recommended to incorporate microstructural techniques such as XRD, DTA-TG, and SEM and to include accelerated tests (salt fog, wet–dry cycles, and accelerated carbonation) to evaluate concrete performance under more aggressive conditions.

Overall, these findings consolidate the use of activated pumice as a viable supplementary material in light urban infrastructure in tropical regions, highlighting its benefits in terms of the durability, environmental efficiency, and extended service life of exposed concrete.

Although a reduction in carbonation depth was observed and attributed to possible microstructural densification, this hypothesis is based on inferences from macroscopic behavior. No microscopic analyses (such as SEM or XRD of the hardened samples) were conducted, which represents a limitation of this study. Future work is expected to incorporate complementary techniques aimed at gaining deeper insight into the underlying hardening mechanisms. Moreover, since rebound hammer testing provides information limited to surface hardness, it will be necessary to validate these findings through destructive mechanical testing. To this end, the use of cylindrical specimens is planned to enable direct compressive and tensile strength assessments.

Furthermore, while this study focused primarily on carbonation as the main degradation mechanism, other deterioration processes—such as chloride ion penetration and thermal cycling—may also be relevant in tropical environments exposed to aggressive conditions. These factors will be addressed in future research to provide a more comprehensive understanding of durability under such conditions.

5. Conclusions

This study provides robust evidence of the technical, environmental, and functional viability of using alkali-activated pumice as a partial replacement for Portland cement in concrete exposed to real humid tropical conditions. By implementing a field-based experimental strategy, the research overcame limitations typically associated with laboratory-controlled studies, enabling a more realistic assessment of surface mechanical performance and carbonation resistance.

Concrete mixes incorporating up to 30% alkali-activated pumice exhibited adequate strength development, with favorable progression over time, particularly under the elevated ambient temperatures characteristic of tropical climates, which likely enhanced the pozzolanic reactivity of the material. In particular, mixtures with 10% and 20% replacement demonstrated optimal strength levels for light urban infrastructure, such as pedestrian pavements, establishing a technically viable and environmentally conscious alternative.

From a durability standpoint, pumice incorporation contributed to a significant reduction in carbonation depth compared to the control mix. The P30 formulation displayed the highest resistance to carbonation, indicating that the dense matrix formed by the alkali activation process may mitigate the ingress of CO₂. Although the study did not include destructive core testing or microstructural analysis of the hardened material, visual evidence and field data support the observed trends. Future studies should incorporate these methodologies to further clarify the underlying mechanisms.

The inclusion of climatic data—specifically temperature, humidity, and precipitation—proved essential to contextualize the behavior of the mixtures under real tropical weathering. Despite the high relative humidity, the carbonation rates remained controlled, particularly in the mixes containing pumice, suggesting long-term performance advantages under these conditions.

Mathematical modeling of strength development using time-based regression analysis demonstrated a reliable capacity to predict concrete behavior over extended periods, offering a practical tool for engineers and designers involved in the planning of durable urban infrastructure. This approach is especially relevant in contexts where real-time long-term testing is not feasible.

However, this study is not without limitations. The use of non-destructive testing methods restricted the depth of mechanical and microstructural interpretation. Additionally, only carbonation resistance was evaluated as a deterioration mechanism. To address these constraints, future work should consider the following:

-. Core sampling for compressive and tensile strength testing;

-. Microstructural analysis of hardened materials using XRD, SEM, and thermal analysis (TG/DTG);

-. Assessment of other durability factors, such as resistance to chloride ingress, freeze–thaw cycles, and wet–dry cycling.

Furthermore, the potential synergy between alkali-activated pumice and other supplementary cementitious materials or industrial by-products (e.g., fly ash, slag, brick dust) offers a promising research direction to enhance the performance and sustainability of low-carbon concretes.

Finally, the use of a naturally occurring, abundant, and low-carbon material such as pumice aligns with the principles of the circular economy and contributes to the development of more sustainable construction practices. Its implementation may be particularly impactful in developing countries, where cost-effective and environmentally responsible materials are urgently needed to support resilient infrastructure and climate change mitigation strategies.

Author Contributions

Conceptualization, writing—review and editing, O.M.-V. and P.J.L.-G.; validation, data curation, B.S.T.-G. and J.S.-L.; methodology and supervision S.A.Z.-C. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study.

Acknowledgments

The authors would like to thank the Instituto Tecnológico Superior de Misantla for providing access to the civil engineering laboratory facilities for testing, as well as for offering a space for the placement and monitoring of concrete slabs.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

CO2	Carbon dioxide
DTA-TG	Differential Thermal Analysis—Thermogravimetry
MPa	Megapascals
PC	Portland Cement
PM	Pumice
SCM	Supplementary Cementitious Materials
SEM	Scanning Electron Microscopy
XRD	X-Ray Diffraction

Footnotes

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Figures and Tables

Figure 1 Concrete preparation procedure using the two-part geopolymer method.

Figure 2 Non-destructive evaluation of surface compressive strength.

Figure 3 Correlation between mechanical resistance and environmental conditions during curing: (a) relationship between resistance and precipitation; (b) relationship between resistance and temperature. Time is expressed in days.

Figure 4 Development of surface compressive strength of concrete for the P00 mix. The data presented are averages.

Figure 5 Development of surface compressive strength of concrete for the P10 mix. Data presented are averages.

Figure 6 Development of surface compressive strength of concrete for the P20 mix. Data presented are averages.

Figure 7 Development of surface compressive strength of concrete for the P30 mix. Data presented are averages.

Figure 8 Carbonation depth. Data presented are averages.

Figure 9 Projection of carbonation progression and its impact on steel reinforcement cover.

Table 1

Designed blends for experiments.

Run	Nomenclature	% of PM	% of Alkaline Solution Concentration
1	P00	00	0.00 N
2	P10	10	0.25 N
3	P20	20	0.25 N
4	P30	30	0.25 N

Table 2

Material dosage for concrete mixtures.

Material	Mix Concrete
	P00	P10	P20	P30
Water	205	205	205	205
Gravel	923	923	923	923
Sand	648	648	648	648
Cement	331	298	265	232
Pumice	0	33	66	99

Note: Method based on ACI 211.1; values expressed in kilograms per cubic meter (kg/m³).

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