Eco-Sustainable Cement: Natural Volcanic Tuffs’ Impact on Concrete Strength and Durability

Abstract

This study underscores the potential of utilizing natural volcanic tuffs (NVTs) as supplementary cementitious materials (SCMs) in alignment with global sustainability efforts aimed at mitigating the cement industry’s negative impacts on both the economy and the environment. Experimental investigations were conducted on concrete mixtures containing 10%, 20%, 30%, 40%, and 50% NVT as partial cement replacements to assess their influence on concrete’s mechanical and microstructural properties. Based on the findings, concrete samples with 10% NVT replacements exhibited increased flexural and compressive strengths of 35.6% and 5.6%, respectively, compared with ordinary concrete after 28 days. The depth of water penetration in the concrete samples was significantly reduced by the inclusion of NVT, with a maximum reduction of 56.5%. Microstructural analysis using scanning electron microscopy (SEM) revealed enhanced densification of the concrete microstructures, attributed to the high pozzolanic activity of NVT use in cement-based composites. Analysis of variance (ANOVA) revealed statistically significant relationships between NVT content and both the compressive and flexural strengths of the concrete samples. In conclusion, substituting 10% cement with NVT not only enhances the mechanical properties of concrete but also decreases the energy demand for cement production and reduces carbon dioxide (CO₂) emissions, thus contributing to more sustainable construction practices.

1. Introduction

Many countries are currently experiencing an increase in sustainability initiatives, resulting in the implementation of numerous programs aimed at sustainable development utilizing Supplementary Cementitious Materials (SCMs), which are natural pozzolanic materials [1]. This initiative seeks to mitigate the negative impacts of the cement industry on the economic and environmental sectors [2]. Cement production is notably energy-intensive and responsible for significant carbon dioxide emissions [3]. Consequently, partially replacing cement with SCMs can reduce both the energy consumption and the environmental carbon footprint [4]. The use of SCMs, including fly ash, slag, silica fume, limestone powder, and natural pozzolan, as partial cement replacements in concrete has demonstrated performance benefits in several recent studies [4,5,6].

Research efforts have increasingly focused on utilizing natural pozzolans, specifically Natural Volcanic Tuffs (NVTs), as additive materials or cement substitutes in cementitious composites [6]. NVTs are environmentally friendly, economically viable, and abundant in various geographical locations, such as Egypt, Saudi Arabia, and Jordan [7]. For instance, Jordan possesses substantial volcanic tuff reserves, estimated at around 800 million tons [8], though selective use is required due to their variable properties [9]. NVTs represent a cost-effective material that can improve the permeability, mechanical, and microstructural properties of concrete matrices when added in specific proportions [10]. Historically, the Romans utilized these materials to create strong hydraulic binders with lime [11], which combine with calcium hydroxide to produce a significant amount of cementitious calcium-silicate-hydrate (C-S-H), enhancing the characteristics of binders [12].

Several studies have highlighted the benefits of employing natural tuff to enhance various properties of concrete. Ababneh and Matalkah [13] emphasized the beneficial chemical composition of NVTs, in line with ASTM standards for natural pozzolan materials. They reported that the presence of SiO₂, Al₂O₃, and Fe₂O₃ in natural tuffs signify their potential as reliable supplementary materials, thereby facilitating pozzolanic processes and aiding in the development of concrete strength. Additionally, these materials enhance the durability and economic viability of concrete. Mohsen et al. [14] investigated the mechanical and microstructural properties of concrete composites containing natural pozzolana tuff as a cement replacement. They observed improvements in workability and strength with 10% and 20% pozzolana tuff replacements, attributed to the denser microstructure of the concrete matrix. Furthermore, they highlighted the financial benefits of using these natural materials in concrete production. Similarly, Edris et al. [15] noted a slight decrease in water absorption and ultrasonic pulse velocity with a higher tuff replacement, focusing on its strength-enhancing properties and positive influence on durability. Boumaza et al. [16] explored the potential of using NVTs as precursors for geopolymers, enhancing environmental friendliness. Fode et al. conducted a comprehensive review of the impact of NVTs from various volcanic sources on concrete characteristics. They found that many NVT types meet ASTM standards in terms of their chemical compositions and that their use improved concrete workability by prolonging setting time and by reducing bulk density, porosity, water absorption, and chloride ion migration, resulting in a denser microstructure. Moreover, adding 5–20% NVT significantly improved compressive, split tensile, and flexural strengths, with an optimal replacement level of 15% from volcanic sources [17].

However, challenges in using NVTs have been observed. Sarireh et al. [18] expressed concerns about fresh concrete’s characteristics and workability, noting that there is a decrease in workability with increased tuff content, which could impede compaction and structure, presenting practical challenges in construction applications. Al-Zboon et al. [19] conducted an experimental study on using volcanic tuff in concrete and found a balance between density and mechanical strength, with varying levels of strength improvement or reduction compared with ordinary concrete mixes. Al-Chaar and Alkadi [20] emphasized the need for compliance with standards, highlighting the importance of enhancing volcanic tuff compositions to meet industrial standards. Despite their pozzolanic activity, the variable chemical and physical characteristics of volcanic tuffs necessitate careful consideration for their use as substitutes for fly ash (FA) or silica fumes (SF). Standards such as ASTM C618-17 [21] must be adhered to, and careful formulation and evaluation protocols are essential to harness the potential of NVT materials while ensuring compliance with established standards. Therefore, this study will highlight the scarcity of natural pozzolanic materials as viable alternatives to FA, particularly in regions where FA is unavailable or of inconsistent quality due to emerging environmental regulations. We will emphasize the importance of identifying and validating natural alternatives, such as volcanic tuff, that can match or exceed the performance characteristics of fly ash in concrete production.

The widespread availability and low extraction cost of natural volcanic tuff, combined with its role in reducing the environmental impact of concrete production, underscore the need for further research to verify its effectiveness in various construction contexts. Volcanic tuff has the potential to be a sustainable alternative to traditional concrete production methods if its use is carefully controlled and customized to meet project-specific requirements. This study’s novelty lies in determining the benefits and challenges of using natural volcanic tuff as a cement substitute in ordinary concrete production and in identifying the optimal dosage for concrete mixes. Six concrete mixtures containing different percentages (0%, 10%, 20%, 30%, 40%, and 50%) of volcanic tuff as partial cement substitutes were prepared. The workability of fresh concrete was measured and compared with normal concrete, and the permeability, microstructural, and mechanical properties were tested after 28 days. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) analysis were used to examine the microstructure and elemental composition of broken samples. The findings were analyzed using the statistical method of variance (ANOVA) analysis. The study will greatly add to the literature based on the overall behavior of the NVT concrete mixes and recommend future research focused on enhancing and modifying the compositions and physical properties of this environmentally friendly pozzolanic material to attain better ratios of replacement of cement in concrete mixes.

2. Experimental Program

2.1. Materials

• Natural volcanic tuff (NVT): This research utilized NVT samples from natural pozzolanic deposits in the northern regions of Jordan. The collection process complied with established standard methods and procedures for obtaining field samples. The collected volcanic tuff samples underwent cleaning, drying, and crushing at Al Rajhi Laboratories for Control and Quality to enhance their properties and to reduce their particle size. The powdered volcanic tuff, with particle sizes ranging from 0 to 45 μm, was sieved through a No. 325 sieve according to ASTM C618 specifications [21], before being used as a partial replacement for Portland cement. Figure 1 and Figure 2 show the SEM image at a 10 µm scale and the particle size distribution for the NVT samples, respectively. Tuff particle size distribution (PSD) was compared with cement particles. This comparison is crucial as PSD significantly influences the material’s properties, including its workability, strength, and durability in construction applications. The PSD of NVT particles showed a characteristic curve compared with cement particles, which is attributed to the fact that Tuff particles, as volcanic materials, typically exhibit a wider range of particle sizes due to their heterogeneous composition. In contrast, cement particles manufactured under controlled conditions display a more uniform and narrower PSD.

  Table 1. Chemical and physical properties of volcanic tuffs.

  Chemical Properties
  Compound	SiO2	Al2O3	CaO	Fe2O3	TiO2	MgO	K2O	Na2O
  Content (%)	47.85	14.38	9.54	12.99	2.11	9.63	1.14	0.49
  Physical Properties
  Property	Moisture content		Loss on ignition			Fineness
  Result (%)	0.4–1.2		0.8–1.2			1.7–3.3

  presents the chemical and physical properties of the NVT. The chemical compositions were analyzed using X-ray fluorescence, revealing that the total amount of SiO₂ and Al₂O₃ in the NVT sample exceeded 60%. The SiO₂ was predominantly found in the quartz phase, a typical phase in volcanic tuffs, which is consistent with a high SiO₂ content, while the relatively low CaO levels indicate that calcium-bearing phases, such as calcite (CaCO₃) or plagioclase feldspar (which can contain Ca), are present but not dominant.

• Cement: Portland cement [CEM II/B with EN 197-1:2011] from the Manaseer Group in Jordan was utilized in this study.

  Table 2. Chemical and physical properties of Portland cement.

  Chemical Properties
  Compound	SiO2	Al2O3	CaO	Fe2O3	SO3	MgO	K2O	Na2O
  Content (%)	24.0 ± 0.5	6.5 ± 0.5	52.5 ± 1.0	6.5 ± 0.5	2.11	3.2 ± 0.5	0.8 ± 0.1	0.95 ± 0.1
  Physical Properties
  Property	2 Days compressive strength (MPa)		28 Days compressive strength (MPa)		Water/Cement ratio (%)		Fineness (%)
  Result	24.0		49.0		29.6		2.5

  presents the physical and chemical properties of the cement.

• Aggregate: Coarse aggregates (dolomite limestone) and fine aggregates (natural sand) that conformed to ASTM C-33 standards were used. The physical properties (

  Table 3. Physical properties of aggregates.

  	Unit	Coarse Aggregate	Fine Aggregate
  Maximum Size	mm	20.000	4.750
  Minimum Size	mm	5.000	0.150
  Bulk Specific Gravity (SSD)	-	2.518	2.646
  Bulk Specific Gravity (dry)	-	2.476	2.630
  Apparent Specific Gravity (SSD)	-	2.584	2.672
  Absorption	%	1.707	0.604
  Fineness Modulus	-	-	2.444

  ) of these materials were determined in the laboratories of Al-Ahliyya Amman University.

• Superplasticizer: A polycarboxylate-ether superplasticizer, provided by BASF Construction Chemicals, UAE LLC and known commercially as Master Glenium 51, was used. The properties of such a superplasticizer are presented in

  Table 4. Physical properties of superplasticizer (Master Glenium 51).

  Appearance	Specific Gravity@ 20 °C	pH-Value	Alkali Content %	Chloride Content (%)	Air Content
  Whitish to straw Liquid	1.10 ± 0.03 g/cm3	6.0 ± 1	≤5.00 by mass	≤0.10 by mass	Fulfilled

  .

2.2. Mixture and Sample Preparation

The preparation of the concrete mixtures was conducted in two phases, as outlined below:

Phase 1: Dry materials, including cement, fine and coarse aggregates, and NVTs used as a cement replacement, were mixed in dry condition for 15 min according to the proportions specified in

Table 5. Proportions of concrete mixtures.

Batch #	Cement (kg)	Fine Aggregate (kg)	Coarse Aggregate (kg)	Natural Volcanic Tuff (kg)	Water (kg)	Superplasticizer (g)
Batch 1 (control)	20	46	50	0	10	200
Batch 2 (10%)	18	46	50	2	10	200
Batch 3 (20%)	16	46	50	4	10	200
Batch 4 (30%)	14	46	50	6	10	200
Batch 5 (40%)	12	46	50	8	10	200
Batch 6 (50%)	10	46	50	10	10	200

, which represent the proportions of the mix design per cubic meter.

Phase 2: The aqueous solution containing the superplasticizer was combined with the dry materials and mixed in a rotary mixer for 15 min, for a total mixing time of 30 min. Mixing for 30 min ensures uniform distribution of all components, including the superplasticizer, within the dry material. Proper mixing is critical to achieving consistent workability and the wettability and strength required. Superplasticizer is also used to enhance the workability of the paste without adding more water. It helps reduce the water–cement ratio, resulting in increased strength and durability. The ACI 211.4R-93 standard was used to determine the proportions of the concrete mix, with the mix design targeting a 28-day compressive strength of 50 MPa.

Upon completion of the mixing process, the concrete mixture was placed in steel molds. The samples were taken out from the molds after 24 h and cured in a water bath with a 90% humidity level and 25 °C constant temperature as illustrated in Figure 3.

2.3. Experimental Tests

2.3.1. Slump Test

The workability of the concrete mixes was assessed by the slump test according to ASTM C 143 [22]. Fresh concrete was added in three equal layers, with each layer compacted 25 times. The height of the fresh concrete was then measured after the cone was lifted. The workability of each batch of fresh concrete was recorded before placing it in the molds.

2.3.2. Mechanical Properties

Compressive strength tests were carried out as per ASTM C39 standards [23]. Three cylinders with diameter of 150 mm and length of 300 mm were prepared from each mix and tested after 28 days using a strength testing machine. Additionally, flexural strength was assessed by testing three 150 mm × 150 mm × 550 mm simple beams in accordance with ASTM C78 standards [24].

2.3.3. Water Penetration Depth

The water penetration resistance of concrete under pressure was measured after 28 days, following DIN-1048 (Part 5) (DIN 1991) [25]. The underside of three cubic concrete samples (150 mm × 150 mm × 150 mm) from each batch was subjected to 0.5 N/mm² steady water pressure for three days.

2.3.4. Microstructural Analysis

Microstructural analysis was performed on fractured concrete samples to understand the impact of NVT’s use on the concrete properties. This was done using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX). To achieve high-resolution images, gold–palladium was used to coat the sample surfaces in order to eliminate any surplus charges after being dried in a vacuum chamber. The FEI-Nova Nano SEM microscope (commercial name) was used for scanning and imaging. Additionally, EDX analysis was utilized to investigate the elemental composition of all samples.

2.3.5. ANOVA Analysis Statistical Test

In order to investigate the effect of various NVT concentrations on the compressive and flexural strengths of the concrete mixes, the analysis of variance (ANOVA) was utilized. ANOVA is a robust statistical technique used to determine if there is a statistically significant difference between two or more groups’ means. When there are three or more treatments or groups, the ANOVA F-test is used to statistically determine the equality of the groups’ means. In this procedure, the variance between the groups’ means as well as the variance within each group are computed, and the F-value (presenting the ratio between both variances) is calculated. If such a value exceeds the critical value obtained from the F-distribution table for a specific degree of freedom, then the null hypothesis (no significant difference between the groups’ means) is rejected. This implies that there is a significant statistical difference between the groups’ means.

3. Results and Discussion

3.1. Workability

One of the most significant findings from this experimental study is the impact of natural volcanic tuff (NVT) on the workability of fresh concrete mixtures. Concrete workability is determined by its filling ability and stability. The filling capacity refers to the ability of the mix to cover all areas without external agitation, while the passing capacity refers to the mixture’s flowability. The results of slump tests conducted on all fresh concrete mixtures, to assess the impact of NVT as a partial cement substitute on concrete workability, are shown in Figure 4.

The slump rate of concrete mixtures prepared with 10% NVT increased by 11.1% compared with the control mixture (0% NVT). However, the slump rates decreased by 11.1% and 55.6% for mixtures with 20% and 30% NVT, respectively. In contrast, when cement was replaced with 40% and 50% NVT, the slump rates increased by 55.6% and 66.7%, respectively, compared with the control mixture. Thus, it was observed that increasing NVT content by up to 30% significantly reduces the water demand in the fresh concrete mixture, leading to a reduced slump and an increased stability. Beyond this percentage, additional NVT does not provide significant benefits in terms of water demand.

This behavior can be attributed to the fact that volcanic tuff typically consists of fine particles with a porous structure, which allows it to absorb and retain water more effectively than traditional cement particles [26]. When used in appropriate amounts, such as 30%, this absorption can help regulate the moisture content within the mixture, reducing the overall water demand. Additionally, the pozzolanic reaction of the volcanic tuff with the calcium hydroxide produced during cement hydration may create secondary C-S-H (calcium-silicate-hydrate) gel, which can enhance the cohesion of the mixture and reduce the need for additional water to achieve workability [27]. However, at higher replacement levels beyond 30%, fine particles of volcanic tuff can act as fillers, improving the particle packing density and reducing internal friction. Thus, water demand decreases because the available water effectively coats these fine particles, resulting in a more fluid mix. This exciting potential for a more fluid mix can significantly enhance the workability of the concrete mixture [28].

Celik et al. [29] and Xu et al. [30] noted that using NVT in concrete enhances its flowability. However, increasing the fineness of NVT reduces the flowability of the fresh cement composite. This occurs because finer NVT particles, with increased surface area, begin to replace cement particles, thereby decreasing workability. Consequently, using alternative fine materials in excess of 30% of the cement content can reduce concrete’s workability.

Overall, the results of using volcanic tuff on rheological properties are consistent with various fields. A high slump indicates a more fluid mix, which is easier to cast and compact with, but may lead to segregation if it is too high. A low slump indicates a hardness mix, which may be more challenging but desirable for specific applications where form stability is critical. Therefore, the “best” slump value depends on the desired workability and application of the concrete. Lower slump values are not necessarily better; rather, the slump should be appropriate for the concrete’s intended use.

3.2. Mechanical Properties

Regarding the mechanical properties of the concrete samples,

Table 6. Impact of NVT on mechanical properties of concrete after 28 days of curing.

Concrete Mechanical Properties
	Batch No.	Compressive Strength (MPa)	Change %	Flexural Strength (MPa)	Change %
1	Batch 1 (control)	49.9	-	4.6	-
2	Batch 2 (10%)	52.8	5.6%	6.2	35.6%
3	Batch 3 (20%)	43.2	−13.5%	5.4	18.7%
4	Batch 4 (30%)	38.7	−22.4%	4.4	−2.4%
5	Batch 5 (40%)	33.5	−32.8%	4.4	−2.9%
6	Batch 6 (50%)	37.9	−23.9%	3.5	−23.7%

presents the experimental results for all batches after 28 days of curing, along with the percentage variation in compressive and flexural strengths compared with the control concrete samples (0% NVT).

3.2.1. Compressive Strength

Figure 5 illustrates the compressive strength results for the tested samples and the effect of replacing cement with natural volcanic tuff (NVT). The results indicated that the compressive strength increased by up to 5.6% after 28 days of curing in samples containing 10% NVT. This improvement is attributed to several factors, including the spherical shape of NVT particles, which fill the voids and pores in the concrete matrix and strengthen the interfacial transitional zone between the cement and aggregate particles, thereby creating a dense and strong microstructure. Additionally, the presence of silicon oxide (SiO₂ and Al₂O₃) in the tuff reacts with calcium hydroxide (Ca(OH)₂) to gradually form additional calcium-silicate-hydrate (C-S-H) bonds over time [12].

However, at higher replacement levels, NVT had a negative impact on strength. The compressive strength decreased by 13.5%, 22.4%, 32.8%, and 23.9% when the cement replacement levels were increased to 20%, 30%, 40%, and 50%, respectively. According to the Energy-Dispersive X-ray (EDX) analysis, which shows the chemical compositions for all batches after 28 days, it was observed that increasing the volcanic tuff content in the mixtures resulted in a relative decrease in SiO₂ content in the matrices. This reduction in SiO₂ content diminished the occurrence of pozzolanic reactions which produce hydration products that enhance the concrete’s structure, thus leading to a decrease in strength. Furthermore, as Pekmezci and Akyüz [27] reported, using NVT materials as a cement substitute, especially natural ones, often increases the water required for mixing concrete and slows down strength development. Consequently, in structural applications, the inclusion of NVT in concrete mixtures is typically limited to 30% or less.

These results are consistent with the previous literature, as Edris et al. [15] concluded that replacing cement with 10% volcanic tuff will improve the compressive strength of concrete by 38.2% compared with the reference concrete. This is due to a percentage of silicon oxide (SiO₂ and Al₂O₃), which reacts with non-aqueous calcium hydroxide (Ca(OH)₂) and creates an additional bond of (C-S-H). Likewise, a study conducted by Aburumman et al. [11] shows that replacing cement with 10% volcanic tuff achieves the best improvement results in the compressive strength of concrete samples.

3.2.2. Flexural Strength

The findings on the flexural strengths of (500 × 150 × 150 mm) beam samples for concrete mixes containing 10%, 20%, 30%, 40%, and 50% NVT are presented in Figure 6. Compared with the control batch, low replacement levels of 10% and 20% natural tuff powder resulted in significant increases in flexural strengths of approximately 35.6% and 18.7%, respectively. These NVT-containing mixes exhibited a higher early-age flexural strength than those made with only Portland cement. This improvement can be attributed to the pozzolanic reactions that produce calcium-silicate-hydrates, which contribute favorably to a concrete’s strength. Additionally, the enhancement in flexural strength in concrete with a minimal pozzolanic content is due to the continuity of pozzolanic reactions, which strengthens the bond between aggregates and paste.

Conversely, decreases in flexural strength of 2.4%, 2.9%, and 23.7% were observed for mixes containing 30%, 40%, and 50% NVT, respectively. These results are consistent with the findings of Edris et al. [15], who suggested that this significant decrease in flexural strength with higher NVT content is attributed to changes in the microstructure, such as increased porosity or weaker interfacial bonding between the aggregates and the cement matrix due to the reduced cement content and significant retard of hydration, thus consequently decreasing the amount of C3S and C2S.

3.3. Depth of Water Penetration

This test was conducted on 150 mm concrete cubes after 28 days of curing. The results, as shown in Figure 7, indicate that the water penetration depths in concrete samples decreased by 26%, 43%, 22%, 43%, and 57% for batches containing 10%, 20%, 30%, 40%, and 50% NVT, respectively, compared with the control concrete.

The inclusion of volcanic tuff particles in concrete matrices enhances the pore microstructure and reduces permeability. The presence of NVT content increases the amounts of SiO₂, Al₂O₃, and Fe₂O₃ compounds, which react with Ca(OH)₂ to produce calcium-silicate-hydrate (C-S-H) and calcium-hydrate (C-H). These reactions reduce the micropore concrete structure which leads to a denser microstructure. Moreover, the hydration products improve the properties of the transition zone, resulting in a denser concrete matrix. NVT enhances the concrete’s microstructure by replacing C-H crystals with C-S-H gel, leading to a thinner transition zone and a lower probability of micro-cracking, thereby reducing concrete permeability [31]. This improvement is primarily due to the finer particle size of pozzolans compared with cement particles, which increases concrete density and enhances its microstructure [14,32]. Additionally, using NVT in cement composites improves the densification of cement slurries, effectively preventing water penetration [33]. Minimizing water absorption is crucial for enhancing concrete’s durability, particularly in water-exposed construction projects such as dams, bridges, and culverts [17]. In summary, the high pozzolanic activity of natural pozzolana tuff powder in cement-based composites results from the combined effects of nucleation, filling, and pozzolanic reactions, with the filling and nucleation effects being the dominant factors, as illustrated in Figure 8.

In addition, water penetration results can be a valuable indicator of the pore structure and porosity of cementitious composites. The depth of water penetration reflects the connectivity and distribution of pores within the material. Deeper penetration indicates a more interconnected pore structure, allowing water to move more quickly through the material and, thus, a coarser pore structure or the presence of larger capillaries. Conversely, lower penetration indicates less porosity and, thus, a denser material with fewer and smaller pores. By comparing water penetration results with tuff particles, pore structure and porosity changes can be concluded. Mixtures that exhibit low water penetration may acquire an improved pore structure due to enhanced hydration, which improves the durability and compressive strength of the mixes.

At low replacement levels (around 10–20%), NVT content may reduce the overall porosity of concrete by filling voids and contributing to a denser microstructure due to its pozzolanic activity. Reaction with calcium hydroxide forms additional calcium-silicate-hydrate (C-S-H) bonds, which can reduce porosity and improve compressive strength. However, at higher replacement levels, the inherent porosity of the tuff particles may dominate, resulting in increased overall porosity and a corresponding decrease in compressive strength. This is particularly the case if the tuff particles are not well incorporated or do not fully interact with the cement matrix.

3.4. Microstructural and EDX Analysis

Microscopic analysis of samples tested after 28 days revealed that the pozzolanic tuff materials used in this study, despite their high SiO₂ content, exhibited a high crystallinity and were unable to undergo secondary hydration reactions. This explains why the strength properties of samples containing only 10% and 20% of natural tuff were improved. As shown in Figure 9a,b, the batches containing up to 10% and 20% NVT demonstrated an enhanced microstructure compared with the control mix due to the increased formation of C-S-H gel from the reaction between SiO₂ and Ca(OH)₂ produced by primary hydration. The formation of C-S-H gel creates a dense structural matrix, a thick microstructure, and relatively few pores [34]. Wei and Gencturk [35] attributed the enhanced strength and stability of concrete to the consumption of free lime during pozzolanic reactions, resulting in the formation of secondary C-S-H.

However, EDX analysis after 28 days indicated a higher Ca/Si ratio in batches with high cement replacement levels by NVTs (Figure 9c–e), suggesting a weaker volcanic pozzolanic activity [36]. As noted in Table 1, the most common components in natural tuffs (calculated as oxides) are SiO₂, Al₂O₃, and Fe₂O₃, typically constituting more than 70% of the main components. Thus, increasing NVT content in the batches leads to a decrease in Ca(OH)₂ content produced from primary hydration. Since Ca(OH)₂ is necessary for secondary reactions to generate a substantial amount of C-S-H gel, which is largely responsible for enhancing microstructure density, the weak pozzolanic activity resulted in the agglomeration of these elements in the concrete matrix. This weakened the concrete’s structure and generally reduced its strength properties.

Ahmed et al. [37] also reported that one of the key factors in increasing pozzolanic activity is the presence of SiO₂ and Al₂O₃. However, the presence of other elements such as Fe₂O₃, MgO, and K₂O might reduce pozzolanic activity. To increase the strength of pozzolans, and consequently, cement compounds, minerals containing SiO₂ should be finely ground to enhance their ability to bind with Ca(OH)₂ and form calcium-silicate-hydrate (C-S-H) in a shorter time, as well as to increase their ability to fill fine pores.

Overall, natural volcanic tuffs have a significant potential for pozzolanic activity from a research and experimental standpoint. When ground to adequate fineness, they can be effectively used as supplementary cementitious materials (SCMs) in cement-based composites.

3.5. ANOVA Analysis Results

The compressive and flexural strength results of all NVT concrete samples are summarized in

Table 7. Compressive strength results of the concrete samples.

0% NVT	10% NVT	20% NVT	30% NVT	40% NVT	50% NVT
50.80	53.96	43.71	39.45	34.72	37.83
49.20	52.45	43.05	36.92	33.71	36.23
49.70	51.60	42.69	39.77	32.20	39.85

and

Table 8. Flexural strength results of the concrete samples.

0% NVT	10% NVT	20% NVT	30% NVT	40% NVT	50% NVT
4.38	6.47	5.28	4.32	4.54	3.17
4.50	5.81	5.45	4.57	4.41	3.76
4.76	6.22	5.47	4.44	4.30	3.47

, respectively.

Various key parameters are involved in the ANOVA analysis as follows: the Sum of Squares (SS) representing the variation between and within groups; the Degree of Freedom (df) indicating the number of independents; the F- and p-values; and the Mean Square (MS). The latter is the ratio between the Sum of Squares to the Degree of Freedom, while the F-value is the ratio of the variance between groups to the variance within groups. Meanwhile, the p-value indicates the probability of getting such results fortuitously. A p-value of less than 0.05 indicates that the observed differences between groups are statistically significant.

Table 9. Compressive strength ANOVA analysis results.

Source of Variation	SS	df	MS	F	p-Value	F Critical
Between Groups	820.63	5	164.17	101.49	2.16 × 10−9	3.11
Within Groups	19.41	12	1.62
Total	840.04	17

provides a summary of the ANOVA analysis of the compressive strength results, revealing a statistically significant relationship between the NVT content ratio and compressive strength. The F-value is reported as 101.49, which is greater than the critical F-value of 3.11, and the p-value is shown as 2.16 × 10⁻⁹, indicating it is less than 0.05. These values imply that there are significant differences in compressive strengths among the various concrete samples.

Similarly,

Table 10. Flexural strength ANOVA analysis results.

Source of Variation	SS	df	MS	F	p-Value	F Critical
Between Groups	12.97	5	2.59	56.22	6.55 × 10−8	3.11
Within Groups	0.55	12	0.05
Total	13.52	17

presents the ANOVA analysis of the flexural strength results and demonstrates a statistically significant association between the NVT content ratio and flexural strength. The F-value is reported as 56.22, surpassing the critical F-value of 3.11, and the p-value is indicated as 6.55 × 10⁻⁸, which is less than 0.05. These values suggest that the flexural strengths of the different concrete samples are significantly different from one another.

3.6. Environmental Benefits of Alternative Binders in the Construction Sector

The experimental investigation conducted in this study provides additional evidence supporting the advantageous utilization of natural volcanic tuffs (NVTs) as supplementary cementitious materials (SCMs) in concrete mixes. The use of NVT in cementitious composites has resulted in an improvement in both the mechanical and microstructural properties of the concrete mixes, as detailed in the preceding sections of this research. Furthermore, utilizing an eco-friendly supplementary cementitious material as a partial substitute to the conventional cement offers a myriad of benefits.

Figure 10 provides a concise overview of carbon dioxide (CO₂) emissions throughout the concrete production process. (Substituting 10% of the cement with NVT decreases the energy demand for cement production, thereby reducing CO₂ emissions. Notably, Portland cement production is a major contributor to CO₂ emissions in the construction sector, accounting for approximately 8% of global emissions [38]. Although limited studies have evaluated the environmental impact of NVT, existing examples demonstrate reduced emissions (e.g., 20% volcanic ash mortar) and lower embodied energy (e.g., 50% Portland cement replacement with NVT). This reduction is particularly significant considering that the production of one ton of cement typically results in the emission of an equivalent ton of CO₂ [39].

In conclusion, the integration of NVT into concrete mixtures emerges as a promising strategy for enhancing sustainability within construction practices. Beyond improving the properties of construction mixes, this approach minimizes undesirable environmental impacts and raises ecological responsibility.

Also, volcanic tuff, as a natural pozzolan, offers a promising alternative to FA (a byproduct of coal combustion) which has long been used as a supplementary cementitious material (SCM) for its pozzolanic properties, enhancing the strength and durability of concrete. However, with tightening environmental and safety regulations, the availability and quality of fly ash have become increasingly uncertain. So, volcanic tuff presents a viable solution to address these challenges.

4. Conclusions

The primary objective of this experimental study is to investigate the effects of substituting Portland cement with natural volcanic tuffs (NVTs), specifically focusing on the applicability of NVTs in the construction field. Through meticulous analysis and interpretation of the findings, the following conclusions are warranted at this time:

• The results showed that substituting 10% of the cement with NVT is the optimum dosage to enhance the concrete’s mechanical and microstructural properties.
• Substituting Portland cement with NVT significantly improved the fresh concrete’s slump by 55% (when using 30% NVT) compared with the control mixture (0% NVT). This underscores the critical role of these pozzolanic materials in enhancing the properties of fresh concrete.
• After 28 days, the flexural and compressive strengths of the concrete mix with 10% NVT increased by 5.6% and 35.6%, respectively, compared with the control mixes.
• Volcanic tuff particles in concrete matrices improved the pore microstructure and reduced the water penetration depths in the concrete samples by 26%, 43%, 22%, 43%, and 57% for batches containing 10%, 20%, 30%, 40%, and 50% NVT, respectively.
• Microstructural examination revealed an improvement in the densification of the concrete mixes. This is attributed to the high pozzolanic activity of the natural pozzolana tuff powder in cement-based composites resulting from increased nucleation, filling, and pozzolanic reactions.
• ANOVA analysis revealed statistically significant relationships between NVT content and the compressive and flexural strengths in the concrete samples. The F-values and p-values indicate the strength of these relationships, highlighting the importance of NVT content in influencing the concrete’s mechanical properties.

Overall, the inclusion of natural volcanic tuff in cementitious composites represents a promising path forward in the construction industry as it improves the rheological and mechanical properties of concrete composites. Moreover, the use of natural materials is in line with sustainability goals by reducing the demand for traditional cement, thus reducing the construction industry’s carbon footprint.

Therefore, future research should focus on optimizing the percentage of volcanic tuff replacement across different cement and aggregate types to achieve the best balance between workability, mechanical strength, and durability. Investigating the potential use of volcanic tuff with other supplementary cementitious materials (SCMs) could also be an interesting area to explore synergistic effects. Taking into consideration that the physical and chemical properties of volcanic tuff can vary significantly depending on the source, these variances may affect its performance in concrete.