Palygorskite as an Extender Agent in Light Cement Pastes for Oil Wells: Performance Analysis

Abstract

Cementing operations are among the most critical steps in oil-well construction. When performed improperly, the integrity and useful life of the well can be significantly compromised. Light cement pastes are used to cement formations with a low fracture gradient to ensure zonal isolation and maintain the integrity of the casing. Extenders are additives used to reduce the density of cement pastes, ensuring that the paste has desirable properties before and after setting. This work aimed to evaluate the application of palygorskite clay as an additive in lightweight cement pastes for oil wells, highlighting how its fibrous morphology influences the microstructure and enhances the macroscopic properties of the hardened cement matrix. For this, the clay sample was initially characterized regarding its physicochemical properties using X-ray diffraction (XRD), X-ray fluorescence (XRF), thermogravimetry (TG), textural analysis (BET/N₂), and scanning electron microscopy (SEM). Lightweight pastes (1.56 g/cm³) were then formulated, varying the clay concentration by 1%, 3%, and 6% of the total mass. Cement pastes using bentonite were also formulated for comparison. Technological tests of atmospheric consistency, rheological behavior, free water, and stability were applied. It can be noted that the pastes formulated with palygorskite had lower viscosity, reflected in the reduced plastic viscosity and yield stress values, indicating easier flow behavior when compared with bentonite-based pastes. The pastes formulated with 6% palygorskite and 3% bentonite showed satisfactory stability and drawdown results. Therefore, applying palygorskite satisfies the minimum requirements for acting as an extending agent for lightweight cement pastes and is an option for application in oil-well cementing operations.

1. Introduction

Palygorskite, also known as attapulgite, is a fibrous clay mineral composed of a double layer of silicon tetrahedrons and a central layer of magnesium, aluminum, or iron octahedrons. This substance is found almost exclusively in soils of the world’s arid and semiarid regions, with Brazil’s primary deposits located in Piauí.

Palygorskite was not initially recognized as a distinct clay mineral when it was discovered due to its chemical composition and similar properties to montmorillonite [1]. However, it stands out as a representative of the group of fibrous clay minerals. Its fibrous morphology is characterized by long, thin fibers with diameters ranging from 20 to 30 nanometers and lengths that can reach several micrometers, whereas montmorillonite, which also has a double layer of silicon tetrahedra and a central layer of aluminum octahedra, exhibits a more lamellar structure [2].

Palygorskite belongs to the palygorskite and sepiolite groups, which are three-layer phyllosilicates with a T:O ratio of 2:1. Furthermore, these phyllosilicates have a fibrous morphology. The physicochemical properties of palygorskite are remarkable, making its applications diverse [3]. With a high specific surface area and a porous structure that varies from micropores to mesopores, palygorskite is an ideal material for applications that require high adsorption capacity such as catalysis, effluent treatment, and as an additive for the petroleum industry [4].

Oil-well cementing is a fundamental process in the construction of oil wells, aiming to ensure the structural stability of the well and the isolation of production zones. The procedure involves pumping a Portland cement slurry between the casing and the rock formation. Proper cementing is essential to ensure the integrity of the well throughout its life, preventing incidents such as blowouts and casing collapses. An effective paste must be adapted to each well scenario; for this purpose, chemical additives are used to achieve certain desired properties [5].

The admixtures used In cement slurries are crucial In modifying their rheological and performance properties. Among the various types of admixtures, extenders are of particular interest as they allow for cement savings and a reduction in the paste density, facilitating their application in easily fractured formations [6]. Extenders, such as clays, increase the volume of the paste without significantly compromising its strength after curing. They are instrumental in situations where it is necessary to minimize fluid invasion into the formation. Palygorskite is already known as a viscosifying additive in the oil industry for its application in drilling fluids [7,8]. This article aimed to investigate palygorskite clay as a potential extender additive for cement pastes used in oil wells. Bentonite clay, widely recognized and commonly used for this purpose, was used as a comparative reference [9].

Although palygorskite has been extensively studied for applications in drilling fluids, its use as an extender in oil-well cementing remains underexplored. Most extenders, such as bentonite, rely on lamellar structures with high swelling capacity but also impose drawbacks like excessive viscosity and reduction in mechanical strength. In contrast, palygorskite presents a fibrous morphology that promotes the formation of a physical network within the cement matrix. This network contributes not only to viscosity modulation, but also enhances the microstructural integrity of the hardened paste by improving the particle packing, reducing pore size, and mitigating sedimentation. This study sought to bridge this knowledge gap by correlating the microstructural characteristics induced by palygorskite with the macroscopic performance of cement pastes, offering an alternative extender with improved operational properties for oil-well cementing.

2. Materials and Methods

2.1. Materials

The materials used to develop this research were Portland cement Class G (density: 3.13 g/cm³) from Mizu Cimentos LTDA (Mossoro, Brasil), bentonite (density: 2.34 g/cm³) from Bentonisa S/A (João Pessoa, Brasil) and palygorskite (density: 2.42 g/cm³) from the state of Piaui, Brazil.

It is important to note that the performance of clay minerals such as palygorskite and bentonite can vary considerably depending on their geological origin and mineralogical composition. The findings of this study were based on a specific sample, and while they provide meaningful insights into the comparative behavior of these materials, the results should not be generalized to all types of palygorskite or bentonite without further validation.

2.2. Clay Characterization

The palygorskite and bentonite, previously crushed in an agate mortar, were subjected to the following physicochemical characterization techniques: X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM), thermogravimetric analysis (TGA/DTG), and the adsorption and desorption of nitrogen at 77 K. The XRD analysis was performed using Bruker (Billerica, MA, USA) D2Phaser equipment equipped with a Lynxeye detector and copper radiation (CuKα, λ = 1.54 Å) with a Ni filter, current of 10 mA, voltage of 30 kV, 2-theta range between 2° and 40°, divergent slit of 0.6 mm, central slit of 1 mm, step equal to 0.01°, and acquisition time of 0.2 s. The methods of powder (natural sample) and oriented blade (hydrated, glycolated, and calcined samples) were used.

The XRF analysis was performed using energy-dispersive X-ray fluorescence (EDX) equipment on a Bruker (Billerica, MA, USA) S2 PUMA—SERIES II device, using a Pd tube with a maximum power of 50 W, maximum voltage of 50 kV, maximum current of 1 mA and a HighSense Silicon Drift Detector.

The thermal stability of the clay was studied by thermogravimetry using a TG 209 F3 Tarsus—Netzsch thermal analyzer (Weimar, Germany). Samples of about 8 mg were placed in platinum crucibles and heated from 26 to 900 °C at a heating rate of 10 °C/min in an inert nitrogen atmosphere with a flow rate of 50 mL/min. Scanning electron microscopy (SEM) analysis was performed on a TESCAN (Brno, Czech Republic) VEGA 4 scanning electron microscope using a secondary electron (SE) detector with a beam energy of 10 KeV. The samples were coated with gold film and deposited on carbon strips for analysis in a high vacuum.

The N₂ adsorption and desorption analysis at 77 K was carried out using the NOVA 800 BET equipment from Anton Paar (Graz, Austria). The samples were previously degassed at 150 °C for 10 h. With the data collected, graphs of the adsorption and desorption isotherms were plotted. From the data obtained, the specific surface area was determined using the BET method, which followed the criteria suggested by IUPAC [10], and the total pore volume using the Gurvith method.

2.3. Technological Tests for Evaluating the Extender Agent for Cement Slurry

Initially, the formulations were calculated according to the API RP 10B-2 standards [11], which define a volume of 600 cm³ of cement slurry for carrying out technological tests. In addition, the density was set at 13.0 lb/gal (1.56 g/cm³). Seven formulations were defined, with the masses described in

Table 1. Cement slurry designs used in the tests.

Slurry	Water (g)	Cement Class G (g)	Bentonite (g)	Palygorskite (g)
P0	440.76	493.87	-	-
B1	439.98	489.74	4.90	-
B3	438.46	481.71	14.45	-
B6	436.28	470.13	28.21	-
P1	439.79	489.93	-	4.90
P3	438.83	486.07	-	14.47
P6	436.99	469.46	-	28.17

: a standard formulation containing only water and cement and six slurries with additives of palygorskite and bentonite at 1%, 3%, and 6% by weight of cement. Due to its high consistency, the paste with 6% bentonite (B6) was not adequately mixed according to the requirements of API RP 10B-2 [11], making it not feasible to carry out the technological tests.

It is important to emphasize that the inability to prepare the slurry with 6% bentonite (B6) according to API RP 10B-2 [11] was not solely a laboratory limitation, but rather an indication of the excessive viscosifying effect of bentonite at higher concentrations. During the preparation procedure, the slurry reached a consistency level that exceeded the capacity of the mixing equipment and deviated from the operational standards established for slurry homogenization.

The slurries were prepared according to the procedure recommended in API RP 10B-2 [11]. All components were weighed on an electronic scale, ensuring an accuracy of 0.1% of the masses, and the mixture was mixed in a Chandler (Tulsa, OK, USA) brand paddle mixer, Model 3500, with speed and mixing time control. The bentonite and palygorskite were previously hydrated in the mixing water for 30 min at a rotation of 1000 rpm. The prepared slurries were subjected to atmospheric consistency, free water, stability, rheology, and compressive strength tests.

The atmospheric conditions for the tests were defined using the Schedule 2010 software, version 1.6. Considering a well with a depth of 1000 m and a geothermal gradient of 1.7 °F/100 ft, the following temperatures were established: bottomhole circulation temperature (BHCT) of 38 °C and bottomhole static temperature (BHST) of 57 °C.

Before measuring the atmospheric consistency and performing the free water test, the cement slurries were homogenized using a Chandler atmospheric consistometer, model 1200, operating at a rotation of 150 ± 15 rpm for 30 min at room temperature. After the homogenization period, the consistency of the slurries was assessed with a consistency measuring dial. Then, approximately 623 g of the paste was transferred to a 500 mL Erlenmeyer flask, which remained at rest for 2 h. The supernatant water was removed with a syringe and weighed. Finally, the percentage of free water present in the slurry was calculated.

In the stability test, the slurries were previously homogenized at 38 °C (simulating BHCT). After 30 min of homogenization, they were poured into decanting tubes and placed in a thermostatic bath, where they remained for 24 h. After being removed from the bath, the cylinders were opened at the top, and the existing fluids were eliminated. The remaining space up to the top of the cylinder was filled using a syringe, obtaining a volume in cubic centimeters (cm³). The “top reduction” was obtained by converting the volume to length, expressed in mm. Subsequently, the hardened slurry was removed from the cylinders and sectioned into four equal parts. Based on Archimedes’ principle, the difference in density between the top and bottom sections was verified.

The rheological test was performed using a Chandler viscometer, model 3500. The slurries, previously homogenized at a temperature of 38 °C, were subjected to variable rotation rates (ascending and descending) from 3 to 300 rpm, recording the respective torque values. Finally, the rheological parameters of the initial gel, final gel, yield point, and plastic viscosity were obtained using the Bingham mathematical model.

The compressive strength test was performed in triplicate. The slurries were placed in cubic metal molds with 50.00 mm edges and cured for 7 and 28 days in a thermostatic bath, under agitation, at a temperature of 57 °C. The rupture of the test specimens was performed using a Shimadzu (Kyoto, Japan) AG-I 300 kN universal mechanical testing machine controlled by Trapezium X software. The compression tests were performed at room temperature with a loading speed of 72.2 kN/min.

3. Results and Discussion

3.1. Characterization of Palygorskite Clay

3.1.1. X-Ray Diffraction (XRD)

X-ray diffraction analysis of the palygorskite sample was performed in four ways. Initially, an XRD analysis of the untreated material was performed. Next, a clay hydration procedure was performed, following the oriented blade methodology. The third analysis consisted of a glycolization process using ethylene glycol to monitor any significant changes in the diffractogram. Finally, the sample was calcined at a temperature of 600 °C, and then XRD analysis was performed. Figure 1 shows the diffractograms obtained for the four analyses performed with the palygorskite sample.

As can be seen, the diffractograms of the four analyses were similar, presenting only a slight change in the intensity of the peaks. In agreement with the literature [12,13], one can observe four prominent peaks in the diffractogram above. The first refers to the palygorskite itself, in the order of 2θ = 8.4°, followed by a peak in the order of 2θ = 20°, also from palygorskite. The two peaks at 2θ = 21° and 2θ = 27° refer to quartz, which was also present in this clay sample, as is well-referenced in other works [14].

The X-ray diffraction (XRD) patterns of sodium bentonite subjected to distinct physicochemical treatments—powder, hydrated, glycolated, and calcined—revealed a structural behavior typical of smectite-group minerals, particularly montmorillonite, as shown in Figure 2.

In its powder form, the (001) basal reflection was observed at approximately 2θ ≈ 6.8°, corresponding to a basal spacing of about 13 Å, which is characteristic of natural sodium montmorillonite with ambient moisture. Upon hydration, this reflection shifts slightly toward lower angles, indicating an expansion of the interlayer spacing due to water intercalation. When glycolated, the (001) peak moved significantly to lower 2θ values (≈5.6–6.0°), reaching a basal spacing of around 17 Å—a diagnostic indicator of expandable smectites such as montmorillonite. Conversely, the calcined sample exhibited a substantial reduction in the (001) peak intensity and the collapse of the layered structure, consistent with dehydroxylation and partial amorphization resulting from high-temperature treatment. The appearance of minor peaks suggests the persistence of thermally stable impurities such as quartz.

This sequential shift of the (001) peak under glycolation and collapse under calcination confirmed the dominant presence of sodium montmorillonite and its characteristic responsiveness to hydration, swelling, and thermal degradation, in agreement with established clay mineralogical behavior [15].

3.1.2. X-Ray Fluorescence (XRF)

XRF analysis was performed using energy-dispersive X-ray fluorescence (EDX) equipment.

Table 2. Chemical characterization of the palygorskite and bentonite samples.

Oxide	% Weight
	Palygorskite	Bentonite
MgO	5.35	3.81
Al2O3	18.36	23.85
SiO2	59.78	56.62
K2O	3.69	0.55
CaO	0.58	1.26
TIO2	1.08	0.63
Fe2O3	11.16	13.28

shows the results obtained from the equipment.

The results are in line with the chemical analysis of palygorskite and bentonite, showing high values in the quantity of silicon, aluminum, and iron. Another metal that stood out in these samples was the presence of magnesium. These data are well-referenced in the literature [16,17,18].

3.1.3. Scanning Electron Microscopy (SEM)

Below are two SEM images, Figure 3, of the palygorskite sample captured at 10,000× and 20,000× magnification and 10 KeV electron beam energy. The detector used was the secondary electron (SE).

The micrograph above is characteristic of palygorskite; it is possible to see a fibrous structure and a coiling of these fibers. These images align with what can be observed in the literature [16]. Bentonite was analyzed using the same equipment and with the same image acquisition parameters, as can be seen in Figure 4.

The scanning electron microscopy (SEM) image of the bentonite sample revealed a characteristic lamellar aggregated structure, commonly associated with smectite group minerals, particularly montmorillonite. The particles displayed a plate-like or flaky morphology, forming stacked and irregularly arranged aggregates. This structural organization is consistent with the typical expandable layered nature of montmorillonite [19].

3.1.4. Thermogravimetry (TG)

Thermogravimetric analysis was performed using NETZSCH TG 209F3 equipment. A heating ramp up to 900 °C was programmed with a heating rate of 10 °C/min. Figure 5 shows the TG curves of the palygorskite and bentonite samples analyzed.

In the red curve (palygorskite), the three significant losses were related to water. The 7% mass loss, which occurred up to 105 °C, was attributed to water physisorbed on the palygorskite surface and micropore water [20]. The second peak of 2% mass loss, between 120 °C and 240 °C, was attributed to the loss of the remaining micropore water and the loss of the first coordination waters. Finally, the last peak of 5% mass loss, which occurred up to 569 °C, referred to the loss of residual coordination waters and the loss of structural waters [21].

The thermogravimetric behavior of bentonite, represented by the blue curve, revealed a significant initial mass loss of 11.9%, primarily attributed to the release of physically adsorbed and interlayer water. This phenomenon is characteristic of the smectite group, particularly montmorillonite, which constitutes the dominant mineral phase in bentonite. This dehydration process typically occurs at temperatures up to approximately 150 °C and reflects the high water retention capacity of bentonite due to its expansive lamellar structure [22]. A second mass loss of 4.6% was observed between 450 °C and 670 °C, corresponding to the dehydroxylation of structural hydroxyl groups within the clay layers. This thermal behavior is consistent with previous studies that emphasized the thermal response of montmorillonite-based clays under progressive heating [23].

When comparing bentonite and palygorskite, it was evident that bentonite underwent a more pronounced initial mass loss, indicative of its higher adsorption capacity, which is a direct consequence of its expandable smectitic structure. In contrast, palygorskite exhibited a more gradual and distributed thermal response due to its fibrous morphology and internal channels that retain water in various forms (adsorbed, channel, and structural).

3.1.5. Nitrogen Adsorption at 77 K

The palygorskite and sodium bentonite samples were analyzed for the adsorption and desorption of N₂ at 77 K. Important information on the textural properties of both materials was obtained, and Figure 6 shows the graph of the results.

The plotted graph showed a curve similar to a type II isotherm with slight hysteresis, type H3, for the palygorschite sample, while for sodium bentonite, the isotherm was type II with hysteresis type H4. It is worth remembering that the difference in the types of hysteresis of the materials is due to the geometry of the pores of the materials. The presence of hysteresis is indicative of the presence of mesopores [24]. The area data were calculated from the BET equation following the criteria suggested by IUPAC.

After making the equation of the line and calculating the angular and linear coefficients, it was possible to calculate the constant C and the capacity of the monolayer (N) to then apply the final formula for the specific area and obtain the value of 142.59 m²/g [11]. The literature offers various specific area values for palygorskite, but all of the values are as high as the one calculated experimentally [25]. The properties of sodium bentonite and palygorskite are shown in the

Table 3. The N₂ adsorption and desorption characteristics of palygorskite and bentonite.

Material	SBET (m2/g)	Constante C	VTP (cm3/g)	Vmeso(cm3/g)
Palygorskite	142.59	218.28	32.75	29.25
Bentonite	47.05	344.12	0.08	0.073

.

3.2. Technological Tests for Evaluating Extenders for Cement Slurries

3.2.1. Atmospheric Consistency

After mixing the slurries added with palygorskite or bentonite as an extending agent and the standard slurry, their consistency was verified at room temperature and pressure. Figure 7 shows the results after 30 min of testing.

It can be observed that the standard slurry (only cement and water) presented a low consistency, which characterized it as a light paste. An increase in consistency was observed with the rising concentration of palygorskite. The cement paste containing 3% palygorskite, compared with the standard paste (0%), exhibited a consistency increase from 1 b.c. to 4 b.c. A similar behavior was observed for bentonite, where the consistency reached 7 b.c. in the paste with the 3% addition.

Bentonite clay has a 2:1 structure, that is, an octahedral layer between two tetrahedral layers (as can be seen in Figure 8), and in the interlayers, there are exchangeable cations, which give them a high swelling capacity, and therefore a high swelling aptitude. This behavior offered a higher viscosity when compared with the other pastes [26]. The inability to prepare the B6 slurry (6% bentonite) due to excessive consistency demonstrates a key operational limitation of bentonite at high concentrations. Its strong swelling behavior causes an exponential viscosity increase, making the slurry unworkable. In contrast, palygorskite maintained a workable consistency even at 6%.

3.2.2. Free Water

The free water content refers to the amount of supernatant water, that is, water that does not participate in the cement hydration reactions. The amount of free water is a determining factor for the integrity and durability of the cementing job. A low amount of free water is crucial to ensure the strength and stability of the cement paste, avoiding problems such as cracks and loss of adhesion. According to the API standard [11], free water is an essential parameter for slurries intended for cementing oil wells. It cannot exceed 5.9% of the volume of the paste prepared with Class G Portland cement.

Table 4. Free water results of the formulated pastes.

Standard	P1	P3	P6	B1	B3
18.62%	16.69%	12.61%	4.25%	5.08%	2.10%

presents the free water results for the standard paste and the slurries with palygorskite and bentonite.

The standard slurry presented a high percentage of free water, much higher than the 5.9% recommended by the API [11]. On the other hand, it was found that the addition of both clays promoted a reduction in this percentage. Concentrations of 1% and 3% of bentonite were sufficient to reduce the free water in the paste to levels below the maximum recommended by the standard. Concentrations of 1% and 3% of palygorskite resulted in higher percentages than 5.9% of free water. However, with the addition of 6% palygorskite, it was possible to reduce the free water content to 4.25%.

These results indicate that the addition of palygorskite, especially at appropriate concentrations, can be an effective strategy to control the free water content, similar to bentonite. This contributes to optimizing the cement slurry properties, ensuring a more efficient and safer cementing job.

3.2.3. Rheology

After being conditioned in an atmospheric consistometer for 30 min at a temperature of 38 °C, the rheological behavior of the slurries was verified using a Chandler viscometer, model 3500. The instrument is equipped with coaxial cylinder geometry, featuring a bob with a diameter of 34.49 mm and a slit size of 1170 mm. The ascending and descending readings were taken from 3 to 300 rpm, respecting 10 s after each speed. Figure 9 shows the flow curves of the formulated pastes.

All of the analyzed pastes presented non-Newtonian behavior, with a decrease in viscosity and an increase in the shear rate. The Birgham model presented a good fit for the curves, with coefficients of determination greater than 0.99. The standard paste, P0, and the paste, P1, presented the lowest shear stress values with the increase in the shear rate, with a gradual increase with the increase in the addition of palygorskite clay to the mixture. The addition of bentonite clay provided a more significant increase in viscosity to the pastes, consistent with what was observed in the atmospheric consistency behavior [27].

Figure 10 presents the results of the rheological parameters of the formulated pastes’ yield point (a), plastic viscosity (b), and gel strength (c), where Gi represents the initial gel and Gf the final gel.

The yield strength of a cement paste is a critical property in the area of oil- and gas-well cementing. It refers to the stress at which the slurry begins to deform and flow, indicating its ability to start flowing under stress, which is essential for pumping operations. A higher yield strength helps to suspend solid particles uniformly in the paste, preventing sedimentation and ensuring uniform distribution. An adequate yield strength helps to achieve good zonal isolation, ensuring that the paste fills the annular space and maintains its integrity until it hardens [28]. By analyzing Figure 10a, it was possible to notice that bentonite clay performed better when compared with the other formulations. This advantage is directly related to its hydration and swelling capacity due to the exchangeable cations in its interlayers.

The same observation can be said for both the parameters of plastic viscosity (Figure 10b) and gel strength (Figure 10c). However, although presenting even lower values than bentonite, palygorskite clay with an addition higher than 3% presented a better rheological performance when compared with the standard paste.

The lower plastic viscosity and yield point observed in slurries containing palygorskite, compared with those with bentonite, are directly related to the distinct morphology of the additive. While bentonite exhibits a lamellar structure with high swelling capacity due to interlayer water adsorption, palygorskite is composed of rigid, rod-like fibrous particles that do not swell but form a physical entangled network within the slurry. This fibrous arrangement Increases the slurry’s structural integrity without significantly increasing the viscosity. From an operational perspective, this characteristic translates into improved pumpability and lower frictional pressure losses during displacement. The reduced viscosity facilitates the more efficient removal of drilling fluids from the annulus, potentially enhancing the zonal isolation quality.

However, while lower viscosity improves the flow properties, it may pose challenges regarding the suspension of heavier particles during static periods. Nevertheless, the rheological parameters observed—particularly the yield stress and gel strength for formulations with 3% and 6% palygorskite—indicate that the slurry maintains an adequate suspension capability, balancing flowability with stability.

3.2.4. Stability Test

Figure 11 and Figure 12 show the density variation data and the drawdown of the formulated slurries after one day of curing. According to API RP10 [11], if the bottom and top density differences are more significant than 0.5 lb/gal (0.06 g/cm³), or the top’s drawdown is superior to 5 mm, the paste is considered unstable and must be reformulated.

Only slurries P6 and B3 presented a satisfactory difference in density between the top and bottom within the limits established by the standard. This is because the concentration of dispersed solids was small, which resulted in less interaction between particles, causing the solids to settle. Silva et al. [29] studied cement pastes with bentonite added as an extender, varying between 12 lb/gal (1.44 g/cm³) and 13.5 lb/gal (1.62 g/cm³), and observed that the lighter pastes presented less stability when compared with those with higher density, having a more significant drawdown. Bentonite was more effective in pastes with higher density, 13.5 lb/gal (1.62 g/cm³), and clay concentrations greater than 3%.

The stability results observed for slurries containing 6% palygorskite and 3% bentonite were closely related to their ability to control free water. In the case of bentonite, stability was mainly achieved through water absorption into its interlayer spaces, characteristic of smectite clays with high swelling capacity. In contrast, palygorskite stabilized the slurry via a distinct mechanism: its fibrous morphology promoted the formation of an interwoven physical network within the cementitious matrix. This fiber-based network mechanically trapped water within the capillary spaces between fibers and enhanced particle retention, effectively reducing sedimentation and limiting free water migration toward the surface.

This structural reinforcement explains why the slurry containing 6% palygorskite exhibited a significant reduction in free water (4.25%) and maintained a uniform density profile within the stability thresholds defined by the API standard. The physical entrapment of water, coupled with the scaffolding effect provided by the fibrous network, contributed to enhanced homogeneity and structural integrity of the slurry during both the fluid phase and subsequent setting.

3.2.5. Compressive Strength

The evaluation of the mechanical strength of the cement slurries through a compression test is essential to ensure the full functioning of the paste in an oil well. The paste must have sufficient resistance to ensure the physical stability of the well and prevent cracks that could compromise its safety. The slurries were cured for 7 and 28 days at 57 °C for the uniaxial compression strength test. The pastes selected for this test were the standard paste, B1 (bentonite 1%), B3 (bentonite 3%), P1 (palygorskite 1%), and P6 (palygorskite 6%). Figure 13 shows the maximum stress results supported by each sample in the compression test.

One of the main factors in the search for a replacement for bentonite is its negative influence on the mechanical strength of the slurry. When comparing the standard slurry with those containing 1% and 3% bentonite, a decrease of approximately 35.58% and 38.42% in strength at 28 days was observed. This reduction may be critical for the inadequacy of these formulations in cementing projects that require high mechanical strength values [30]. Among the slurries studied, P1, containing 1% by mass of palygorskite, presented the best performance, having an increase of 4.07% in strength for the 7-day period. By analyzing the 28-day period, there was a decrease of only 11.19% in mechanical strength, a value three times lower compared with the same concentration of bentonite at 28 days. Adding an extender to the cementitious matrix resulted in a decrease in cementitious particles, and consequently a drop in mechanical strength [31]. This phenomenon was observed when adding more palygorskite to the paste and increasing the bentonite concentration.

The compressive strength behavior of the slurries containing palygorskite indicates that its influence extends beyond the simple dilution of the cementitious phase. The increased strength observed in the P1 slurry (1% palygorskite) can be attributed to a microstructural reinforcement mechanism, where the fibrous particles act as physical bridges within the cement matrix. This effect likely enhances particle interlocking, improves matrix cohesion, and may refine the pore structure, reducing the initiation and propagation of microcracks.

However, at higher concentrations (6%), the excessive amount of fibers appeared to disrupt the continuity of the cementitious matrix. This oversaturation can lead to weak interfacial zones, limit the full development of hydration products, and introduce additional porosity due to the presence of excessive non-reactive material.

3.3. Microstructural Analysis of Hardened Cement Paste Containing Palygorskite and Bentonite

Scanning electron microscopy (SEM) analysis was performed on the hardened cement pastes containing palygorskite and bentonite to evaluate the influence of their distinct morphologies on the cement matrix.

The SEM images of the paste containing palygorskite (Figure 14) clearly revealed the presence of a fibrous network embedded within the cementitious matrix. The palygorskite fibers were distributed throughout the structure, forming an interlaced network that appeared to contribute to the physical cohesion of the matrix. This fibrous arrangement likely acts as a reinforcing framework, bridging hydration products, filling capillary spaces, and potentially mitigating the development of microcracks. The fibers also seemed to contribute to a denser and more homogeneous microstructure, which correlated with the enhanced stability and improved rheological behavior observed in the macroscopic tests.

In contrast, the SEM image of the paste containing bentonite (Figure 15) displayed a markedly different microstructure. The morphology was dominated by aggregated clusters of lamellar particles characteristic of smectite-type clays. These plate-like structures were less integrated into the cementitious matrix compared with palygorskite. The bentonite clusters tended to form localized porous zones, where the layered arrangement may have trapped water and promoted microvoid formation. This microstructural feature likely contributed to the higher free water content and lower overall matrix cohesion observed in the bentonite-based slurry formulations.

These microstructural observations strongly support the rheological and mechanical performance trends discussed earlier. The fibrous morphology of palygorskite provides not only viscosity control and stability benefits in the fresh slurry, but also contributes to the physical integrity of the hardened cement matrix.

4. Conclusions

The main objective of this study was to evaluate the effectiveness of palygorskite clay as an extending agent in lightweight cement pastes for application in oil-well cementing operations, comparing it with the agent most commonly used by the industry, bentonite. Some conclusions can be drawn based on the results of palygorskite characterization and technological application.

• Palygorskite has proven to be a very versatile clay. As it is a 2:1 phyllosilicate, its specific area is large, providing essential industry applications. The nitrogen adsorption analysis allowed for the calculation of the specific area, which resulted in 142 m²/g, which is a high specific area value, and is one of its significant advantages.
• As it does not have exchangeable cations in its interlayers, after treatments, no changes were observed in the diffractogram patterns obtained for the palygorskite, with only slight increases in intensity in some peaks.
• The addition of palygorskite in light cement pastes influenced the rheological behavior, presenting lower viscosities when compared with pastes added with bentonite;
• The results obtained from the rheological parameters of the bentonite slurries were superior, given their more significant swelling capacity after hydration.
• The pastes with 6% palygorskite and 3% bentonite were those that presented superior stability, showing no sedimentation and providing better applicability in cementing operations.
• Additionally, the rheological behavior of the palygorskite-based slurries demonstrated a relevant operational advantage. The lower viscosity resulting from its fibrous morphology improved the pumpability and displacement efficiency, reducing the risk of high frictional pressures during cementing operations. At the same time, the physical entanglement of fibers ensured a sufficient yield strength and gel strength to maintain particle suspension, achieving a balance between flowability and stability.
• The paste with 1% palygorskite presented a superior result to the pastes with the addition of bentonite when their compressive strengths were evaluated.
• The microstructural evaluation confirmed that palygorskite, due to its fibrous morphology, promoted the formation of a more interconnected and homogeneous cement matrix. This characteristic contributed significantly to enhancing the mechanical integrity of the hardened cement paste. These findings position palygorskite as a technically viable alternative to traditional extenders, with additional microstructural benefits that extend beyond a simple density reduction.