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Article

Enhancing the Activation of Saudi Natural Pozzolan Using Thermal, Mechanical, Chemical, and Hybrid Treatment Approaches

by
Ardhymanto Am Tanjung
*,
Haitham M. Ahmed
and
Hussin A. M. Ahmed
Mining Engineering Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(24), 4535; https://doi.org/10.3390/buildings15244535
Submission received: 11 October 2025 / Revised: 7 November 2025 / Accepted: 24 November 2025 / Published: 16 December 2025
(This article belongs to the Special Issue Low-Carbon Concrete Structures Incorporating Sustainable Pozzolan and Recycled Materials)
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Abstract

Saudi Natural Pozzolan (SNP) can be processed and used in construction as a partial replacement for Ordinary Portland Cement (OPC). Its use as a supplementary cementitious material supports more sustainable and eco-friendly building practices. This study investigates various treatment methods for enhancing the reactivity of SNPs, including thermal, mechanical, thermo-mechanical, mechano-thermal, and chemical techniques. The activity of 18 different treated SNP mixtures was evaluated using the Strength Activity Index (SAI). Results identified the optimum conditions for each treatment: thermal treatment at 600 °C, mechanical treatment through 6 h of grinding, and chemical treatment with a 9% addition of hydrated lime. The SAI results demonstrated that a 6 h mechanical treatment was the most effective method for activating the raw pozzolan. X-ray diffraction (XRD) analysis revealed that phases such as quartz, anorthite, and aluminate are significant contributors to pozzolanic activity. The XRD analysis was further supported by scanning electron microscopy (SEM), which examined microstructural changes. This study highlights the potential of maximizing the utilization of extensive pozzolan resources in the Harrat region of the Kingdom of Saudi Arabia. Treated SNP can be applied in various industries, such as mining backfills, brick industry, and pozzolanic concrete, as a sustainable and environmentally friendly material.

1. Introduction

Ordinary Portland Cement (OPC) is an essential binder in most mega projects across civil, mining, and offshore engineering. However, its production poses significant environmental challenges, including high CO2 emissions [1] and the generation of cement kiln dust (CKD) [2,3]. The use of clinker, a primary component of OPC, can be reduced by incorporating supplementary cementitious materials (SCMs), such as natural, artificial, and by-product pozzolans [4,5]. The use of pozzolanic SCMs enhances the concrete performance by improving strength, durability, and resistance to acidic environments, which leads to cost reduction and environmental footprint minimization [4,6,7,8], particularly in underground metalliferous mining sites [9].
The supply of by-product pozzolan, such as fly ash, silica fumes, and slag, is insufficient to meet market demand [10]. Relying solely on artificial pozzolan is uncompetitive and unsustainable where resources are limited. In contrast, locally abundant natural pozzolan offers an economically viable and sustainable alternative. Saudi Arabia is advantaged by substantial deposits of natural pozzolanic material derived from scoria rock originating from the volcanic areas (Harrat) in the western region of Saudi Arabia (Figure 1), covering approximately 180,000 km2 [11]. However, these promising resources are not suitable for direct use as a cement replacement without activation.
Pozzolanic activity is typically evaluated using direct (Frattini and modified Chapelle tests) or indirect methods (Strength Activity Index (SAI) and lime reactivity test) [12]. Inactive materials can be activated through thermal treatment [13,14]; mechanical treatment (through prolonged grinding) [15]; chemical treatment, by adding chemical activators such as alkali (Sodium hydroxide, calcium hydroxide, and potassium hydroxide), sulfate (gypsum, sodium sulfate, and potassium sulfate), or other activators such as chlorides (calcium chloride dihydrate and sodium chloride) [16,17,18,19]; or a hybrid treatment that integrates two methods, such as thermo-mechanical or mechano-thermal, in the activation process [20,21].
These treatments stimulate a reaction between the pozzolan’s amorphous silica and calcium hydroxide, Ca(OH)2, which is also known as portlandite, forming secondary Calcium–Silicate–Hydrate (C-S-H) gels that fill the concrete pores, thereby increasing density and impermeability [22,23,24,25,26,27].
Previous research on the activation of Saudi Natural Pozzolan (SNP) includes a study by [28], which used mechanical treatment to achieve two particle sizes of 38 μm and 20 μm. A modified Chappelle test indicated that, despite increased fineness improving reactivity, these two sizes were less reactive than fly ash.
A subsequent thermal activation study [21], as a continuation of the mechanical treatment study [28], heated these same sizes (38 μm and 20 μm) at 550, 650, and 750 °C. Results showed that elevated temperatures reduced the pozzolan reactivity compared to untreated samples, whereas all heat-treated SNPs had a Chapelle value of approximately 809 mg (CaO/g), while the untreated ones obtained a value of more than 822 mg (CaO/g). Another study by [29] focused on the activation of Saudi Natural Pozzolan by thermal and mechanical treatments. In this study, SNP was pulverized to 25 μm and heated at temperatures of 600, 700, 800, and 900 °C. Heat treatment at 900 °C yielded the highest concrete strength. Internationally, studies on natural pozzolan in volcanic pumice from Turkey [13] and on German tuff [30] have shown that thermal activation either at 1000 °C for volcanic pumice mechanically treated before heating to about 20 μm, with a specific surface area of 2.14 m2/g, or at a low temperature of 500 °C for German tuff, with a specific surface area of 17.2 m2/g, can significantly enhance pozzolanic activity, as observed in the strength performance at 7 and 28 days. However, these studies, along with those on SNP, have been limited to a fineness range of approximately 20–38 μm.
Research on Syrian natural pozzolan [31] has explored finer size ranges, including not only micron-size (14 μm) but also nano-size (96 nm) particles, which contribute to a filling effect and accelerate the hydration reaction, mitigating the low early strength typically found in natural pozzolanic mixtures. However, SNPs research has not yet investigated these finer sizes. Furthermore, although the Syrian study [31] used finer sizes of natural pozzolan (NP), it tested pozzolan in concrete with varying dosages, which may lead to misinterpretation of the results, particularly the effect of activation treatment on the pozzolanic activity of NP. Therefore, standard pozzolanic activity evaluation techniques (e.g., SAI, Chapelle, or Frattini) are necessary to obtain a standardized activity for the tested material.
Thermal treatment is not a stand-alone process; preliminary grinding is required before thermal treatment, since raw pozzolan does not exhibit sufficient cementitious characteristics for direct use as a cement replacement [32]. While previous SNP studies have explored temperatures from 500 to 900 °C, the potential of temperatures below 550 °C and above 900 °C remains unexamined for SNP, despite showing promise for other natural pozzolans. Furthermore, the sequence of hybrid treatments (e.g., thermo-mechanical vs. mechano-thermal) and its impact on pozzolanic activity needs to be investigated.
Among chemical activators, hydrated lime (HL)—chemically identified as calcium hydroxide or slaked lime—is known to improve the early strength of the pozzolan [16,33,34]. The chemical activation of Saudi Natural Pozzolan (SNP) has received limited attention. Studies [17,35] added 7% HL in combination with 73% OPC and 20% SNP. The chemical activation effect was indirectly evaluated through concrete strength, rather than using a standard pozzolanic activity test (e.g., Strength Activity Index (SAI), or the Frattini or Chappelle test). While the results are positive, they do not clarify the optimum hydrated lime dosages that serve as an adequate substitute for cement.
A study on natural pozzolan from Morocco [36] used different hydrated lime dosages of 4%, 8%, and 12%. Although it showed strength improvement using 8% Ca(OH)2, the HL was used in combination with other aqueous chemical activators, such as sodium silicate and sodium hydroxide, to produce geopolymer concrete. Thus, the effect of hydrated lime, as an isolated effect, was not clearly identified.
Therefore, a comprehensive study that investigates the optimal treatment for SNP and compares different activation methods is lacking. This study aims to fill the gap by conducting an integrated experimental comparison of thermal, mechanical, hybrid (thermo-mechanical or mechano-thermal), and chemical treatments. It will examine the main affecting variables, including the activator percentage in chemical treatments, the duration of grinding in mechanical treatments, the temperature levels in thermal treatments, and the sequence of hybrid treatments. Pozzolanic activity will be evaluated using SAI and Frattini tests, with findings corroborated by XRD and SEM analysis to understand phase composition and microstructural changes.

2. Materials and Methods

2.1. Raw Materials

The materials used in this research were Ordinary Portland Cement (OPC), standard graded sand (silica sand), Saudi Natural Pozzolan, and regular tap water (Figure 2). OPC was obtained locally from the western region of Saudi Arabia, and Saudi Natural Pozzolan was sourced from a quarry in the Madinah region. According to ASTM C618, Saudi Natural Pozzolan is classified as class N (natural pozzolan) as the combined content of SiO2, Al2O3, and Fe2O3 exceeds 70% (Table 1). Detailed characteristics of all raw materials are provided in Table 1, Table 2 and Table 3. Tap water was used for all cementitious mortar mixtures.

2.2. Test Methods and Pozzolan Mix Scenarios

2.2.1. Test Methods

This study employed the Strength Activity Index (SAI) method, as per ASTM Standard C311M-22 [37] to evaluate the pozzolanic activity. For each scenario, two mixtures were prepared: a control sample with 100% cement and a test mixture with 80% cement and 20% pozzolan. The control mixture consisted of 500 g OPC, 1375 g of standard graded sand, and 242 mL of water.
The test mixture contained 400 g of OPC, 100 g of SNP (the tested sample), 1375 g of standard graded sand, and 244.1 mL of water. The amount of water was determined by the standard, which allows for a variation of ±5 mL from the control mixture. This water-to-cementitious material ratio was maintained for all treatments.
The mixing procedure commences by mixing water, pozzolan, and cement for 30 s at a low speed (speed 3). Afterwards, gradually pour the standard graded sand over 30 s while mixing continues, and then stops. Subsequently, blend again at a medium-high speed (speed 5) for an additional 30 s. After that, 50 s of the 90 s final duration is allocated to ensure the mixed material remains unstuck from the bottom of the bowl. Then, blend the mortar for an additional 60 s to ensure the mixed material attains homogeneity.
After mixing, gradually pour the mixture into 50 mm cube molds, ensuring that each layer is sufficiently compacted. The samples are demolded after a 24 h period and immersed in tap water at 23 °C (±2 °C) for 7 and 28 days, as per the standard. The Unconfined Compressive Strength (UCS) was measured at 7 and 28 days of curing. For each mixture and curing period, three cubes were tested, resulting in a total of six specimens per mixture (Figure 3).
The SAI is calculated using Equation (1), in accordance with ASTM C311M-22 [37].
S A I   ( % ) = A B × 100
where
A is the average UCS value of the test mixture cubes (80% OPC, 20% Pozzolan), MPa
B is the average UCS value of the control mix cubes (100% OPC), MPa.
As per standard, a pozzolan is classified as active if its SAI value meets or exceeds the threshold of 75%; values below this are considered inactive [37].

2.2.2. Mixture Scenarios

This study investigated a total of 18 mixtures. This comprised 1 control mix and 17 mixtures from various pozzolan treatment conditions, which include the untreated raw pozzolan (1 mix), thermally treated pozzolan (4 mixes), mechanically treated pozzolan (6 mixes), hybrid-treated pozzolan (2 mixes), and chemically treated pozzolan (4 mixes). A total of 108 cube specimens were cast and tested. Details of the specific proportions and number of samples in each blend are listed in Table 4.
The composition of the control mixture differed from the test mixtures only in the binder percentage. The control mixture used 500 g of OPC, while each test mixture used 400 g of cement (OPC) and 100 g of treated or untreated pozzolan, following the 20% replacement ratio recommended by ASTM C311M-22 [37]. The quantities of standard graded sand (silica sand) and water were kept constant across all mixtures.

2.3. Natural Pozzolan Treatment Process

Saudi Natural Pozzolan was treated using four methods: thermal, mechanical, chemical, and hybrid treatments. The specific procedures for each are detailed in the following subsections.

2.3.1. Heat Treatment (HT)

For the thermal treatment, raw pozzolan was placed in a porcelain crucible and heated in a laboratory furnace (CHM-60G manufactured by JIN-BOMB Enterprise Co., Ltd., Kaohsiung, Taiwan) as illustrated in Figure 4. The furnace temperature was controlled automatically using a TAEI PY700 controller, manufactured by TAEI, New Taipei, Taiwan (Figure 4b). During operation, a manual heat rate was applied to ensure the temperature was maintained at the desired level (Figure 4a).
Previous studies on Saudi Natural Pozzolan have employed elevated temperatures ranging from 550 to 900 °C [21,29]. Internationally, thermal treatment at 1000 °C has been shown to improve the pozzolanic activity of Turkish volcanic pumice [13], while a lower temperature of 500 °C was effective for German tuff [30]. However, a comprehensive temperature profile for SNP, particularly at 500 °C and 1000 °C, is lacking. Although the previous research applied these temperatures, raw materials such as SNP may respond unevenly due to their heterogeneity. Therefore, this study comprehensively investigates a broader temperature range (500 °C, 600 °C, 800 °C, and 1000 °C) to determine the optimum temperature for SNP thermal treatment.
The heating procedures started by increasing the temperature from the ambient temperature to the target temperature using a manually controlled heating rate, gradually increasing from 0% to a maximum of 20%. Once the target temperature was achieved, samples were soaked for 30 min. At this stage, the heating rate was maintained at approximately 10–25%. After soaking, the furnace power was switched off, and the samples were allowed to cool naturally to ambient temperature over approximately 11 h. The heating curves are shown in Figure 5.

2.3.2. Mechanical Treatment (MT)

Mechanical treatment was performed using a laboratory-scale planetary mill (Retsch PM 400, manufactured by RETSCH GmBH, Haan, Germany). Approximately 150 g of raw pozzolan was placed into the mill, and the grinding time was controlled by the unit’s operation menu (Figure 6). Previous research on SNPs has been limited to a particle size range of 20–38 μm [28,29]. However, studies on non-SNPs at finer sizes, such as micron-size particles (14 μm) or nano-size particles (96 nm), have shown their potential to increase the early strength [31]. This size range has not been systematically investigated for SNP. However, achieving such small sizes from raw material is often impractical due to time and cost constraints. Therefore, to efficiently generate a range of finer sizes, grinding times of 2, 4, 6, 12, 18, and 24 h were employed. Subsequently, the particle size distribution of each ground sample was then analyzed using a Malvern Mastersizer 2000 particle size analyzer (equipped with Scirocco 2000 dry dispersion unit), manufactured by Malvern Panalytical, Worcestershire, UK
The raw SNP has an initial specific surface area of 0.365 m2/g. The grinding speed was kept at 150 revolutions per minute (rpm) for all mechanical treatments. During operation, the mill power consumption was maintained at around 67–70% of its maximum capacity. The particle size distribution of ground SNP at each grinding time, from 2 to 24 h, is presented in Figure 7. Analysis of these distributions yielded two key parameters for each scenario: the mean particle size (D50) and the specific surface area, which are summarized in Table 5. The results demonstrate that extended grinding successfully reduced the particle size to the micro-scale but not to the nanoscale, as defined by [31]. As anticipated, increasing the grinding time decreases the particle size of SNP, with a corresponding increase in its specific surface area.

2.3.3. Hybrid Treatment

Hybrid treatment refers to the treatment of natural pozzolan by integrating both thermal and mechanical treatments in a specific sequence. The treatment is designated thermo-mechanical (THM) when the pozzolan is heated first then followed by grinding. Conversely, the mechano-thermal (MCT) treatment applies grinding prior to thermal treatment.
A thermo-mechanical (THM) treatment was initially performed by heating the natural pozzolan in a laboratory furnace at 600 °C. Then, the heat-treated pozzolan material was subjected to grinding for 6 h (Figure 8(a)). Meanwhile, the mechano-thermal (MCT) treatment begins with grinding for 6 h, followed by heating at 600 °C (Figure 8(b)). Time and temperature parameters were standardized across both methods to maintain consistency as identified and optimized in previous experimental work.

2.3.4. Chemical Treatment (CT)

Chemical treatment involved blending raw pozzolan with hydrated lime (HL). Previous studies using SNP incorporated 7% HL showing enhanced pozzolan activation [17,35]. Another study on a different natural pozzolan used HL dosages of 4, 8, and 12%, with 8% proving effective in combination with other activators [36]. To comprehensively investigate the effect of HL dosages on SNP, this study extended the dosages range to include 3%, 6%, 9%, and 12% HL by mass. The predefined amounts of pozzolan and hydrated lime were thoroughly dry-blended to ensure a uniform mix of HL and raw pozzolan was achieved, as shown in Figure 9.

3. Results and Discussion

3.1. Colors of Treated Materials

The application of various treatments to the original pozzolan—thermal (HT), mechanical (MT), hybrid (THM, MCT), and chemical (CT)—resulted in a distinct color change in the pozzolan. A portable whiteness meter (PCE-WSB1) manufactured by PCE Instruments UK Ltd. from Manchester, UK was used to quantify these changes by measuring the material’s blue whiteness index (Figure 10). This instrument complies with the international standards of ISO 2410 and ISO 3688 [38].
The whiteness meter scale ranges from 0 to 100, where 0 represents absolute black and 100 represents white. The measured whiteness values for all materials are presented in Table 6. Color changes and alterations have predominantly occurred in the heat treatment (HT) and hybrid scenarios, compared to slight changes from the mechanical (MT) and chemical (CT) treatments (Table 6).
The color changes began at 500 °C, with the material turning a slightly greyish yellow, then to a more distinct yellowish hue at 600 °C. The initial color change is likely due to the formation of hematite (Fe2O3) crystals, which alter the light reflection and absorption of the material. As the temperature increased to 800 °C, the color shifted to reddish orange and finally to red at 1000 °C as a result of agglomeration and the sintering of small hematite particles. The resulting larger and more perfect crystals reflect less light and exhibit increased absorption, leading to observed reddish coloration [39].
The color of the MT samples is in contrast with that of HT, becoming brighter with increased grinding time. This phenomenon is linked to particle size reduction. From raw material up to 6 h of grinding, the color remained a dark greyish. However, from 12 to 24 h, a lighter grey was observed. As particles become smaller, internal light scattering and total reflectance are enhanced, causing powders to appear brighter or greyer because dark pigments are more effectively absorbed [40].
The mechanical grinding process can decrease the particle size, particle morphology, and increase the specific surface area [41]. Furthermore, prolonged grinding induces dehydration and lattice distortion. This process reduces the crystallinity of the material and can convert crystalline phases into an amorphous state [42,43]. The resulting amorphous particles generate a weak reflective surface, decreasing luminance and depicting a dark color of the material [44,45].
The color of chemically treated materials was not significantly distinct from that of raw pozzolan. This is expected, as the white hydrated lime (CaOH2) does not dissolve but merely coats the pozzolan particles. Consequently, as the HL dosage increases, the mixture’s color becomes progressively brighter.

3.2. Performance of Treatments

3.2.1. Heat Treatment (HT)

The compressive strength of all thermally treated samples increased from 7 to 28 curing days due to ongoing hydration. At an early age (7 days), samples treated at 600 °C, 800 °C, and 1000 °C demonstrated significantly higher strength (20.6, 20.8, and 19.7 MPa, respectively), compared to the control mix, untreated pozzolan, and the 500 °C sample (18.2, 17.8, and 15.6 MPa, respectively). However, while the strengths of the HT 600–1000 °C samples remained above the control, the untreated pozzolan mixture achieved the highest strength (27.7 MPa) among all pozzolan-containing mixtures at 28 days (Figure 11). The Strength Activity Index (SAI) was calculated for each mixture at 7 and 28 curing days based on the average values of compressive strengths (Figure 12). Each value in Figure 12 represents the final SAI for a given mixture, as per the standard [37].
All thermally treated (HT) pozzolans were classified as active, as their SAI values exceeded the 75% threshold at both 7 and 28 days (Figure 12). Samples treated at 600 °C and 800 °C showed the highest activation at 7 days. However, the pozzolanic activity of all HT samples decreased by 28 days. Among them, the 500 °C sample exhibited the most significant deterioration, although it remained just above the activity threshold. The decline in performance over time may be attributed to the conversion of the natural pozzolan from the natural amorphous phase of the pozzolan to a less reactive crystalline phase [21].
When compared to raw material, heat treatment alone was not the most effective method. However, considering only heat treatment scenarios, 600 °C is established as the optimal condition. It provided consistent strength performance and lower energy consumption compared to similar performing treatment at 800 °C. This finding aligns with a previous study that identified 650 °C as the most promising temperature, noticing a significant calcium hydroxide consumption, indicating high pozzolanic reactivity [46].
The optimal result of the HT 600 °C is supported by the XRD analysis. The quartz (SiO2) phase intensity in the HT 600 °C was higher than that of the control sample, as depicted in the two-theta at 36.7° and 39.6°. Another contributing factor is a high anorthite (CaAl2Si2O8) intensity observed at 50.3°, which can compact the cement matrix by acting as an inert filler [47]. Furthermore, the presence of aluminate (C3A), a primary cement clinker phase, at 42.5° and 50.3°, contributes to early strength development by rapidly interacting with water and gypsum to form ettringite and monosulfate [48]. The C-S-H formation as a binding phase in the cement was also confirmed by the SEM analysis (Figure 13).

3.2.2. Mechanical Treatment (MT)

Mechanical treatment was conducted at different grinding times (e.g., 2, 4, 6, 12, 18, and 24 h), at a constant speed of 150 rpm, utilizing a total of 88 balls. The compressive strength (UCS) values for all mechanically treated samples improved from 7 to 28 curing days. However, the sample ground for only 2 h at 7 and 28 days had a lower strength than that of the untreated material, indicating that this short grinding time is ineffective. The strength began to improve at 4 h and increased to reach a peak at 6 h. However, material strength decreases at 12 h and then remains constant at 18 and 24 h (Figure 14).
Regarding the activity, all samples are classified as active pozzolans (SAI > 75%), with 6 h of grinding being the most effective. This treatment improved the SAI of the raw pozzolan by approximately 30% at 7 days and 10% at 28 days (Figure 15). This enhanced activity is linked to a significant increase in the specific surface area, from 0.37 m2/g (Raw Pozzolan) to 2.07 m2/g after grinding for 6 h (MT6h) (Table 5)
The XRD analysis of the MT6h supports this finding, showing a reduction in the portlandite (CH) peak intensity at a 2-theta angle of 18.38°, which indicates high pozzolanic reactivity due to CH consumption [46]. This reaction produces secondary C-S-H gel, contributing to long-term strength (Figure 16). The consistent presence of quartz in MT6H, also likely contributed to strength maintenance by acting as an inert filler and providing nucleation sites that accelerate the hydration process, thereby enhancing early strength [49].

3.2.3. Hybrid Treatment

Hybrid treatments were investigated to determine the impact of the treatment sequence of thermo-mechanical (TMC) vs. mechano-thermal (MCT) on strength and pozzolanic activity at 7 and 28 days. Compressive strength (UCS) results showed that both hybrid methods improved strength over raw pozzolan at 7 and 28 days. There was no significant difference between the treatments, with MCT being about 1% higher than THM on both curing days (Figure 17).
Both hybrid treatments generated high pozzolanic activity. Particularly, the MCT treatment increased the SAI by about 35% in 7 days, compared to a 34% increase for THM. The difference becomes narrower at 28 days, where MCT maintained a higher activity level (8% increase vs. 6% for THM) over the raw material (Figure 18). Notably, the SAI for both hybrid treatments (THM and MCT) decreased from 7 to 28 days by approximately 8%, yet remained higher than that of the raw untreated pozzolan.
XRD analysis revealed higher intensities of reactive phases (quartz, aluminate, alite, and anorthite) in the MCT samples compared to both the control and TMC. SEM also confirmed the distinction of MCT, where the C-S-H gel was the major constituent (Figure 19). In contrast, the THM exhibited an excess of portlandite Ca(OH)2, visible as hexagonal plates or tabular structures [50]. This excess indicates lower pozzolanic activity, as the available lime was not fully consumed in the pozzolanic reaction [51] (Figure 19). The presence of such crystalline portlandite is known to reduce compressive strength and increase porosity by accumulating in the interfacial transition zone (ITZ) [52], explaining the inferior performance of the THM sequence.

3.2.4. Chemical Treatment (CT)

Chemical treatment involved blending raw pozzolan with hydrated lime (CaOH2) at proportions of 3%, 6%, 9%, and 12%. The compressive strength (UCS) results demonstrated that all HL additions effectively enhanced early-age strength. While all CT scenarios are higher than the control sample at 7 days, only the CT 9% HL mixture maintained this superiority at 28 days, also exceeding the strength of the untreated raw pozzolan (Figure 20). Specifically, the CT 9% mixture improved strength by about 31% at 7 days and 2% at 28 days, compared to the raw material.
All chemically treated materials are classified as active pozzolans at both 7 and 28 curing days, as they exceed the threshold (SAI > 75%) (Figure 21). Early curing days demonstrate that activation treatments are effective in all scenarios, as they are more active than the original material. However, a decreasing trend in SAI was observed at 28 days for most mixtures. The CT9% is the most effective, as it was the only one that could maintain a high level of pozzolanic activity throughout the curing periods.
The high activity of CT 9% HL is confirmed by the XRD analysis (Figure 22), where the high intensity of quartz, aluminate, and anorthite phases contributes significantly to the strength by acting as inert filler [47]. The high intensity of quartz in CT9% maintains its strength by the filling effect that densifies the mixture. A similar inert filler effect was contributed by anorthite [47]. The low intensity of CH in the XRD diffractogram indicates high pozzolanic activity, confirming that the calcium hydroxide was effectively consumed to form additional C-S-H gel, thereby enhancing strength development [53].

3.3. Optimum Result and Confirmation from Other Analyses

Among the five treatment scenarios, the optimum conditions for each have a great impact on early-age strength and pozzolanic activity. However, the thermal treatment at 600 °C was not a viable method as it resulted in lower activity than the untreated pozzolan. In contrast, other scenarios, such as MT, hybrid, and CT, positively enhanced the pozzolanic activity of raw materials. Mechanical treatment is the most effective treatment. Grinding for 6 h (MT6h) yielded the best combination of strength performance and pozzolanic activity, achieving a specific surface area of 2.07 m2/g and a mean particle size (d50) of 5.8 μm (Figure 23). Unlike other treatment scenarios, the MT6h exhibited remarkable consistency, maintaining its high activity, with only a 1% difference between the 7- and 28-day SAI values.
The pozzolanic reactivity was further validated using the Frattini test, a direct method that measures a material’s ability to reduce portlandite concentration in solution [54,55]. A sample is classified as active if its data point falls below the CaO solubility curve [56]. The Frattini test confirmed that all treated materials, including the original, were active, similar to the findings of the SAI test (Figure 24). The low CaO content in the filtrate for most samples indicated high pozzolanic activity [51].
Mechanical treatment requires a single energy input (electricity), rendering it a cost-effective operation in contrast to the mechano-thermal approach, which utilizes dual energy outputs: mechanical and thermal. Therefore, based on its performance, consistency, and economic perspective, the 6 h mechanical treatment (MT6h) is identified as the optimal pozzolan activation approach.

4. Conclusions

This study comprehensively evaluated the efficacy of thermal (HT), mechanical (MT), hybrid, and chemical treatments (CT) in enhancing the reactivity of Saudi Natural Pozzolan (SNP). The performance was assessed against a control mixture and untreated SNP, using the Strength Activity Index (SAI) across 18 distinct mixtures, with key findings validated by XRD and SEM analysis.
In summary, the key findings are outlined as follows:
  • All applied treatment methods effectively activated the SNP, with its SAI exceeding the 75% threshold for pozzolanic activity.
  • For the heat treatment (HT) scenarios, the highest UCS value was obtained at 600 °C.
  • For the mechanical treatment (MT) scenarios, the highest UCS value was obtained by grinding (MT) for 6 h.
  • For the chemical treatment (CT) scenarios, the highest UCS value was obtained with the addition of 9% hydrated lime (HL).
  • For hybrid treatment scenarios, mechano-thermal (MCT) demonstrated superior performance compared to thermo-mechanical (TMC).
  • The mechanical treatment at 6 h of grinding (MT6h) is identified as the most promising treatment. It produces the highest and most consistent pozzolanic activity across 7 and 28 curing days from the SAI perspective, which is in line with the Frattini pozzolanic activity test.
  • Microstructural analyses confirmed the mechanism behind the enhanced performance. XRD analysis supported these findings by detecting key strength-contributing phases, such as quartz, aluminate, and anorthite. The decreasing intensity of HL peaks indicates the successful secondary C-S-H formation, which was directly observed and confirmed by SEM.
This study is limited to the evaluation of mechanical performance (UCS) associated with SAI. However, future studies could include quantitative phases and conduct a detailed analysis of energy consumption and associated cost for each treatment condition. Industrial applications of the optimum conditions in real-world applications, e.g., backfill in underground mining or concrete applications, are recommended to validate these findings at an industrial scale.

Author Contributions

Conceptualization, A.A.T. and H.A.M.A.; methodology, A.A.T. and H.M.A.; software, A.A.T.; validation, A.A.T., H.M.A., and H.A.M.A.; formal analysis, A.A.T.; investigation, A.A.T.; resources, A.A.T. and H.M.A.; data curation, A.A.T., H.M.A., and H.A.M.A.; writing—original draft preparation, A.A.T.; writing—review and editing, A.A.T., H.M.A., and H.A.M.A.; visualization, A.A.T.; supervision, H.M.A. and H.A.M.A.; project administration, H.M.A.; funding acquisition, H.M.A. All authors have read and agreed to the published version of the manuscript.

Funding

The project was funded by the KAU Endowment (WAQF) at King Abdulaziz University, Jeddah, Saudi Arabia. The authors, therefore, acknowledge with gratitude the WAQF and the Deanship of Scientific Research (DSR) for their technical and financial support.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors gratefully acknowledge the support received from the Modern Technology Laboratory (MTL) company (Jeddah), which provided essential materials used in this study. The authors also gratefully acknowledge the support received from Saudi Lime Company (Jeddah) for providing the hydrated lime utilized in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Map of the Harrats region in Saudi Arabia, modified from [11].
Figure 1. Map of the Harrats region in Saudi Arabia, modified from [11].
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Figure 2. Materials used (OPC, pozzolan, standard sand, and tap water).
Figure 2. Materials used (OPC, pozzolan, standard sand, and tap water).
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Figure 3. Strength Activity Index (SAI) determination procedure (a) mixing all materials, (b) casting mortar to mold, (c) submerging samples in the water, (d) UCS test, (e) final UCS test product at 7 days.
Figure 3. Strength Activity Index (SAI) determination procedure (a) mixing all materials, (b) casting mortar to mold, (c) submerging samples in the water, (d) UCS test, (e) final UCS test product at 7 days.
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Figure 4. Treating stages of heat treatment using a laboratory furnace (CHM-60H-JIN-BOMB, Kaohsiung, Taiwan): (a) Heat rate manual control, (b) automatic target temperature control (TAEI PY700, New Taipei, Taiwan).
Figure 4. Treating stages of heat treatment using a laboratory furnace (CHM-60H-JIN-BOMB, Kaohsiung, Taiwan): (a) Heat rate manual control, (b) automatic target temperature control (TAEI PY700, New Taipei, Taiwan).
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Figure 5. Heating curves (cycles of heat treatment) recorded for (a) 500 °C, (b) 600 °C, (c) 800 °C, and (d) 1000 °C for thermal treatment of SNP; The two red lines indicate a 30-min soaking time.
Figure 5. Heating curves (cycles of heat treatment) recorded for (a) 500 °C, (b) 600 °C, (c) 800 °C, and (d) 1000 °C for thermal treatment of SNP; The two red lines indicate a 30-min soaking time.
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Figure 6. Mechanical treatment stages: (a) Original sample, (b) pouring sample inside the bowl, (c) clamping bowl in the planetary mill, (d) close condition and the manual operation menu, (e) ground product of treated MT, (f) size measurement using particle size analyzer.
Figure 6. Mechanical treatment stages: (a) Original sample, (b) pouring sample inside the bowl, (c) clamping bowl in the planetary mill, (d) close condition and the manual operation menu, (e) ground product of treated MT, (f) size measurement using particle size analyzer.
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Figure 7. Laser particle analysis result from Mastersizer 2000.
Figure 7. Laser particle analysis result from Mastersizer 2000.
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Figure 8. Hybrid treatment scenarios: (a) Thermo-Mechanical; (b) Mechano-Thermal.
Figure 8. Hybrid treatment scenarios: (a) Thermo-Mechanical; (b) Mechano-Thermal.
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Figure 9. Chemical treatment process.
Figure 9. Chemical treatment process.
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Figure 10. Portable whiteness meter (PCE-WSB-1): (a) Standard white, (b) standard black, (c) target sample, (d) whiteness meter PCE, (e,f) testing process.
Figure 10. Portable whiteness meter (PCE-WSB-1): (a) Standard white, (b) standard black, (c) target sample, (d) whiteness meter PCE, (e,f) testing process.
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Figure 11. UCS of heat-treated samples at 7 and 28 curing days (3 samples for each curing day).
Figure 11. UCS of heat-treated samples at 7 and 28 curing days (3 samples for each curing day).
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Figure 12. SAI of heat treatment (HT) scenario at 7 and 28 curing days (3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
Figure 12. SAI of heat treatment (HT) scenario at 7 and 28 curing days (3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
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Figure 13. XRD diffractogram and SEM of the HT Scenario for 600 °C condition.
Figure 13. XRD diffractogram and SEM of the HT Scenario for 600 °C condition.
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Figure 14. UCS of mechanical treatment (MT) scenario at 7 and 28 curing days (3 samples for each curing days).
Figure 14. UCS of mechanical treatment (MT) scenario at 7 and 28 curing days (3 samples for each curing days).
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Figure 15. SAI of mechanical treatment (MT) scenario at 7 and 28 curing days (3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
Figure 15. SAI of mechanical treatment (MT) scenario at 7 and 28 curing days (3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
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Figure 16. XRD and SEM of the optimum mechanical treatment (MT) scenario for the 6 h condition.
Figure 16. XRD and SEM of the optimum mechanical treatment (MT) scenario for the 6 h condition.
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Figure 17. UCS of hybrid treatment scenarios at 7 and 28 curing days—(3 samples for each curing days).
Figure 17. UCS of hybrid treatment scenarios at 7 and 28 curing days—(3 samples for each curing days).
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Figure 18. SAI of hybrid treatments at 7 and 28 curing days—(3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
Figure 18. SAI of hybrid treatments at 7 and 28 curing days—(3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
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Figure 19. XRD and SEM of the hybrid treatment scenarios.
Figure 19. XRD and SEM of the hybrid treatment scenarios.
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Figure 20. UCS of chemical treatment (CT) scenario at 7 and 28 curing days—(3 samples for each curing day).
Figure 20. UCS of chemical treatment (CT) scenario at 7 and 28 curing days—(3 samples for each curing day).
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Figure 21. SAI of chemical treatments (CT) at 7 and 28 curing days—(3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
Figure 21. SAI of chemical treatments (CT) at 7 and 28 curing days—(3 samples for each curing day); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
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Figure 22. XRD of the chemical treatment at CT 9% HL.
Figure 22. XRD of the chemical treatment at CT 9% HL.
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Figure 23. The optimum activity of Saudi Natural Pozzolan through the SAI test); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
Figure 23. The optimum activity of Saudi Natural Pozzolan through the SAI test); The gray line at 75% represents the pozzolanic activity threshold for SAI test.
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Figure 24. Frattini pozzolanic activity test of the optimum treatment scenarios.
Figure 24. Frattini pozzolanic activity test of the optimum treatment scenarios.
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Table 1. Chemical composition of OPC and Natural Pozzolan (%).
Table 1. Chemical composition of OPC and Natural Pozzolan (%).
MaterialCaOSiO2Al2O3Fe2O3SO3MgOK2OK2ONa2OTiO2LOI *
OPC34.6814.911.89N/A2.921.24N/AN/A1.51N/AN/A
Scoria5.8854.3814.2118.430.12.573.913.914.883.527.8
Note: For pozzolan ∑(SiO2 + Al2O3 + Fe2O3) = 87.02%; * LOI = Loss on ignition.
Table 2. Physical properties of Natural Pozzolan.
Table 2. Physical properties of Natural Pozzolan.
MaterialDensity (g/cm3)Specific Surface Area/Fineness (m2/g)Retained on a 45-μm Sieve (%)
Scoria2.520.36514.21
Table 3. Chemical and physical analysis of tap water.
Table 3. Chemical and physical analysis of tap water.
No.Elements, SymbolDetection LimitValue
μg/Lμg/L
1SilverAg0.1<0.1
2AluminiumAl0.145.65
3ArsenicAs0.1<0.1
4BoronB0.11033.16
5BariumBa0.14.68
6BerylliumBe0.5<0.5
7BismuthBi0.1<0.1
8BromineBr0.1304.54
9CadmiumCd0.1<0.1
10ChromiumCr0.10.31
11CesiumCs0.1<0.1
12CupperCu0.16.63
13MercuryHg0.1<0.1
14IodineI0.10.26
15LithiumLi0.14.86
16ManganeseMn0.11.2
17NickelNi0.14.84
18LeadPb0.11.03
19RubidiumRb0.10.39
20AntimonySb0.5<0.5
21SeleniumSe0.12.51
22TinSn0.10.29
23StrontiumSr0.112.17
24TantalumTa0.1<0.1
25ThalliumTl0.1<0.1
26UraniumU0.1<0.1
27ZincZn0.189.1
ParameterUnitDetection LimitValue
Camg/L0.0120.67
Mgmg/L0.011.16
Namg/L0.0154.09
Kmg/L0.012.67
Femg/L0.010.02
Clmg/L181
HCO3mg/L168
NO3mg/L0.4<0.40
SO4mg/L0.012
Fmg/L0.010.5
NO2mg/L0.04<0.04
T.D.Smg/L195195
Conductivityµs/cm302302
pH 17.1
TurbidityNTU0.020.27
Table 4. Proportions of material and total samples for all mixtures (each mixture has 3 samples for 7 days and 3 samples for 28 days).
Table 4. Proportions of material and total samples for all mixtures (each mixture has 3 samples for 7 days and 3 samples for 28 days).
MaterialTreatmentOPC (g)Pozzolan (g)Standard Graded Sand (g)Water (g)Samples
(7 and 28 d)
Control MixtureControl500-1375242.06
Raw PozzolanUntreated4001001375244.16
PZ-HT500 °CThermal (HT)4001001375244.16
PZ-HT600 °C4001001375244.16
PZ-HT800 °C4001001375244.16
PZ-HT1000 °C4001001375244.16
PZ-MT-2 hMechanical (MT)4001001375244.16
PZ-MT-4 h4001001375244.16
PZ-MT-6 h4001001375244.16
PZ-MT-12 h4001001375244.16
PZ-MT-18 h4001001375244.16
PZ-MT-24 h4001001375244.16
PZ-thermo-mechanicalHybrid4001001375244.16
PZ-mechano-thermal4001001375244.16
PZ-CT-3% HLChemical
(CT)		4001001375244.16
PZ-CT-6% HL4001001375244.16
PZ-CT-9% HL4001001375244.16
PZ-CT-12% HL4001001375244.16
Total7300170024,7504391.7108
Table 5. Grinding characteristics and the size of each sample.
Table 5. Grinding characteristics and the size of each sample.
Grinding Time (hours)Grinding Speed (rpm)Recorded Power (%) by the CrusherAmount of Big and Medium BallsNumber of Small BallsSpecific Surface Area
(m2/g)		Particle Size
d50
(μm)
0Raw Material (Original Sample)0.37NA
2150677881.6811.12
4150677882.077.69
6150677882.525.80
12150677883.663.71
18150677883.812.96
24150707884.043.13
NA: Not Available.
Table 6. Natural pozzolan whiteness test result for all mixtures.
Table 6. Natural pozzolan whiteness test result for all mixtures.
CodeMaterialWb *Color of Pozzolans
aPZ-Original11.14Buildings 15 04535 i001
bHT-50010.24
cHT-6009.02
dHT-8006.4
eHT-10005.24
fMT-2H16.2
gMT-4H20.5
hMT-6H18.66
iMT-12H22.66
jMT-18H21.9
kMT-24H23.92
lTHMC15.36
mMCTH17.36
nCT-3% HL12.16
oCT-6% HL13.36
pCT-9% HL14.36
qCT-12 HL15.7
* Wb: Whiteness index blue, or blue whiteness.
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Tanjung, A.A.; Ahmed, H.M.; Ahmed, H.A.M. Enhancing the Activation of Saudi Natural Pozzolan Using Thermal, Mechanical, Chemical, and Hybrid Treatment Approaches. Buildings 2025, 15, 4535. https://doi.org/10.3390/buildings15244535

AMA Style

Tanjung AA, Ahmed HM, Ahmed HAM. Enhancing the Activation of Saudi Natural Pozzolan Using Thermal, Mechanical, Chemical, and Hybrid Treatment Approaches. Buildings. 2025; 15(24):4535. https://doi.org/10.3390/buildings15244535

Chicago/Turabian Style

Tanjung, Ardhymanto Am, Haitham M. Ahmed, and Hussin A. M. Ahmed. 2025. "Enhancing the Activation of Saudi Natural Pozzolan Using Thermal, Mechanical, Chemical, and Hybrid Treatment Approaches" Buildings 15, no. 24: 4535. https://doi.org/10.3390/buildings15244535

APA Style

Tanjung, A. A., Ahmed, H. M., & Ahmed, H. A. M. (2025). Enhancing the Activation of Saudi Natural Pozzolan Using Thermal, Mechanical, Chemical, and Hybrid Treatment Approaches. Buildings, 15(24), 4535. https://doi.org/10.3390/buildings15244535

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