Effect of Aging on Class G High Sulfate-Resistant Oil Well Cement Under High Relative Air Humidity

Abstract

Previous research on cement aging mainly focuses on construction cement, exploring the mechanisms through which aging conditions affect cement properties. However, the impact of aging on oil well cement remains understudied. Aging of cement under high-humidity conditions leads to significant alterations in its properties, indicating that the cement formulation needs to be adjusted to reduce the negative effects during cementing operations. The effect of aging on particle size, mineral composition, and early hydration behavior of oil well cement after 0, 7, 14, and 28 d at 90% relative humidity (±3%RH) and 25 °C (±2 °C) was investigated. The results showed that, during the aging process, the uptake of H₂O and CO₂ from the surrounding atmosphere by cement leads to slight hydration. This process was associated with a reduction in the specific surface area and surface energy. The contents of hydration products ettringite (AFt) and calcium hydroxide (CH) increased, whereas the amounts of C₃S and C₃A decreased. Consequently, the early hydration rate of cement decreased along with a reduction in the cumulative heat release. As the aging time increased, the compressive strength and thickening time of the cement pastes decreased, and the rheological properties deteriorated. Under the experimental temperature and humidity conditions, the permissible aging time without significant deterioration should not exceed 7 d, with a maximum permissible aging time of 14 d.

1. Introduction

Cement is an important aspect of oil and gas reservoir development that serves to protect and secure the casings of oil and gas wells. As the demand for oil and gas resources continues to increase, offshore oil fields have garnered increasing attention [1].

The uptakes of water vapor (H₂O) and carbon dioxide (CO₂) from the atmosphere by oil well cement resulting a slight hydration, which is referred as the aging of oil well cement. After leaving the cement factory, fresh cement ages to varying degrees with respect to the relative air humidity and aging time.

The aging of cement during transportation and storage has a significant effect on its properties. Owing to transportation challenges and negative weather conditions, bulk oil well cement is occasionally stored long-term, sometimes for tens of days, in high-humidity warehouses along the coast. In particular, in the summer and autumn, the air humidity in a warehouse exceeds 80% RH. Long-term storage in a high-humidity environment can cause changes in cement properties, which leads to the need to adjust the cement paste formulation. Cement aging has negative effects on cementing process. Consequently, it is important to study the aging mechanism of oil well cement.

According to previous studies [2,3], different mineral compositions in cement sorb water at varying relative humidities. The relative humidity thresholds for each mineral composition, ranked from low to high, are as follows: hemihydrate gypsum at 34% RH, orthogonal C₃A at 55% RH, C₃S at 63% RH, C₂S at 64% RH, C₄AF at 78% RH, and cubic C₃A at 80% RH [4]. When oil well cement was stored in high humidity areas such as coastal areas, the relative air humidity was usually greater than 80% RH [5]. This meant that almost all of the major clinker phases and gypsum in the cement began to sorb water, and the aging of the cement had a greater impact on its performance [6]. In view of the effect of aging on cement properties, some previous scholars have carried out studies on it.

Florian [7] reported that acicular ettringite is the main product of cement aging at 90% RH. The excessive growth of ettringite on the clinker surface hinders the entry of water into the unreacted portions of the clinker [8], thus reducing the early hydration heat. After 14 d aging, the compressive strength of the hardened cement pastes at 7 d of age was less than 3 MPa. Meier [9] reported that, during cement aging, the formation of ettringite and agglomeration of cement particles caused by water absorption are two antagonistic processes. They further suggested that the overall content of C₃A, particularly the content of orthogonal C₃A, along with the aging time, significantly affects the performance of the cement [10,11]. Both studies suggested that the formation of AFt during aging affected the performance of cement. Dubina’s [12] experiments revealed that the hydration heat release peak of cement delayed with increasing aging time. They proposed that C₃S exposed to air generated a thin layer of C-S-H on its surface, which acted as a barrier when mixed with water and hindered its hydration [13]. The results revealed that the hydrations of C₃A and C₃S, which are more sensitive to water, and their hydration products led to changes in the cement properties during the aging process. Stoian [14] also reported that the reduction in the early compressive strength of aged cement was linearly correlated with the decrease in the accumulated heat release from the early hydration of cement. They posited that the formation of hydration products during the aging process hindered the reaction between the cement and water. These authors suggested that adding fine lime powder to the cement, which serves as an additional nucleation site for hydration, could mitigate the effect of aging [15,16]. The previous studies suggested that aging hinders the hydration behavior of cement and reduces its compressive strength, but the influence of aging on other properties of oil well cement, such as the thickening time or rheological properties, was rarely studied. However, the thickening and rheological properties of cement are of equal concern during cementing. It is necessary to study the effects of aging on the properties and mechanism of oil well cement pastes.

In this paper, the effects of aging at 90% RH and 25 °C on the properties of oil well cement pastes were evaluated in terms of thickening time, rheological properties, and compressive strength. The underlying mechanism was also analyzed. The findings provide a theoretical basis for the time limits of transportation, storage, and use of oil well cement, filling the gap in the research field while offering important significance for the progress of oil well cement applications.

2. Materials and Methods

2.1. Materials

High sulfate-resistant (HSR) Class G oil well cement was obtained from China Gezhouba Group Cement Co., Ltd., Wuhan, China. The chemical composition of cement was determined by X-ray fluorescence spectrometry (ARL-9900 X-ray fluorescence spectrometer, Thermo Fisher Scientific, Waltham, MA, USA), using the fusion method to prepare cement powder into disc-shaped samples for measurement. The results are shown in

Table 1. Chemical composition of class G oil well cement (wt.%).

SiO2	Al2O3	Fe2O3	TiO2	CaO	MgO	SO3	K2O	Na2O	LOI
21.47	4.11	5.48	0.35	62.75	1.34	2.24	0.48	0.05	1.07

Note: LOI is the loss on ignition at 1000 °C.

. The mineral composition was analyzed by quantitative Rietveld refinement method (Rigaku-D max/RB X-ray diffractometer, Rigaku Corporation, Tokyo, Japan), with powder samples dried at 60 °C for 4 h before testing. The results are shown in

Table 2. Mineral composition of class G oil well cement (wt.%).

C3S	C2S	C3A	C4AF	CaSO4·2H2O	CaSO4·0.5H2O	CH
60.0	20.1	1.2	15.8	2.2	0.3	0.4

.

2.2. Aging of Cement

Class G oil well cement was placed on stainless steel trays with a thickness of 1–2 cm in a closed aging chamber maintained at 25 °C ± 2 °C, and the relative humidity was controlled by saturated KCl solutions to 90% RH ± 3% RH. The samples were aged for periods of 0 d (labeled A0), 7 d (A7), 14 d (A14), and 28 d (A28). The aged cement was collected upon completion of the predetermined aging times. The change of relative air humidity in the aging environment is shown in Figure 1. The uncertainty is computed based on the following formulae:

$$S_d=\sqrt{{\displaystyle \frac{1}{n-1}}{\sum }_{i=1}^n{\left(x_i-\overline{x}\right)}^2}$$

(1)

$${\mu }_A={\displaystyle \frac{S_d}{\sqrt{n}}}$$

(2)

$${\mu }_B={\displaystyle \frac{{\Delta }_{ins}}{divisor}}$$

(3)

$${\mu }_C=\sqrt{{\mu }_A^2+{\mu }_B^2}$$

(4)

In Equation (1), S_d–the standard deviation; n—the number of measurements; x_i—the i-th measured value; $\overline{x}$—the average of the measured values.

In Equation (2), μ_A—the Type A uncertainty resulting from repeated measurements; S_d—the standard deviation; n—the number of measurements.

In Equation (3), μ_B—the Type B uncertainty based on the instrument’s precision; Δ_ins—the instrument’s uncertainty, which is set to 3%RH in this paper; and the divisor is taken as √3.

In Equation (4), μ_C—the combined uncertainty; μ_A—the Type A uncertainty; and μ_B—the Type B uncertainty.

The calculated relative air humidity uncertainty based on the measured data is 1.7% RH.

2.3. Preparation and Properties of the Cement Paste

Cement paste preparation, rheological properties, thickening time, and compressive strength tests of the cement samples were conducted according to API RP 10B-2:2013 [17]. The fluidity of the cement paste was tested according to GB/T 8077–2012 [18] at room temperature.

Cement paste preparation: During cement paste preparation, an NHJJ-type mixer (Ningsai, Tianjin, China) was used with a rotational speed set at 4000 r/min ± 200 r/min for 15 s, followed by increasing the speed to 12,000 r/min ± 200 r/min for 35 s. The water-to-cement ratio of the cement paste was 0.44.

Rheological properties: A NCCQ-type atmospheric thickening instrument (Tianjin Ningsai) was employed to cure the cement paste at 80 °C for 20 min. Rheological property parameters were obtained using a Chandler 3500 six-speed rotational viscometer (Hisense, Qingdao, China) by measuring readings at 3 r/min, 6 r/min, 100 r/min, 200 r/min, 300 r/min, and 600 r/min.

Thickening time: Thickening curves of the cement paste at 80 °C were recorded using the NCCQ-type atmospheric thickening instrument (Tianjin Ningsai), with consistency measured in Burden (Bc). The thickening process was terminated when the consistency reached 70 Bc, and the curve was recorded.

Compressive strength: Compressive strength tests were performed using an AEC-201 cement strength testing machine (Wuxi Ailikang, Nanjing, China) with a loading rate of 2.4 kN/s. Cement paste was poured into pre-greased cylindrical molds (Φ2.54 cm × 2.54 cm), cured in an 80 °C constant-temperature water bath until 2 h after final setting, then demolded. The demolded cement stone was further cured in the 80 °C water bath until the specified age. Three samples were tested for each age, and the average value was recorded.

Fluidity: We wet the inside of the truncated cone circular mold (with an inner diameter of 36 mm at the top, 64 mm at the bottom, and a height of 60 mm) and the surface of the smooth glass plate in advance with water. Then, we placed the truncated cone circular mold at the center of the glass plate and quickly poured the prepared paste into the truncated cone circular mold while stirring. We immediately lifted the circular mold vertically, and measured and recorded the maximum diameter of the free-flowing cement paste within 30 s.

2.4. Particle Size and Specific Surface Area

Before testing, the powder density of the cement sample was measured using a Le Chatelier flask (Tianke, Tianjin, China). The specified amount of cement was poured into the Le Chatelier flask, and ethanol (AR) was used for dispersion. The powder density was obtained by recording the reading of the liquid-level rise in the flask.

The particle size distribution of the cement was measured with a Hydro 2000 M/MU laser particle size analyzer from Malvern, UK. Ethanol (AR) served as the dispersing medium, and the test was carried out under ultrasonic conditions [19]. The measurement range was from 0.02 μm to 2000 μm.

The specific surface area of the cement powder was tested using an FBT-9 Blaine air permeability apparatus (Shanghai Yichang, Shanghai, China). A specified mass of cement was put into the apparatus, and the specific surface area was determined by recording the time it took for the liquid level in the glass tube to pass between two marks [20].

2.5. Mineral Compositions

The mineral compositions of cement samples A0, A7, A14, and A28 were analyzed. X-ray diffraction (XRD) analysis was performed using a Rigaku D max/RB diffractometer (Rigaku, Japan) with Cu Kα radiation. The X-ray tube operated at 9 kV, with a step size of 0.01°, and the scans were conducted at a rate of 10°/min from 10° to 80° at 2θ. The mineral composition was determined using Rietveld refinement. Before the test, the cement powders with different aging times were dried in a vacuum drying oven (Lichen, Zhejiang, China) at 60 °C ± 2 °C for 4 h.

2.6. Thermal Analysis

A TGA STA449 F3 Jupiter analyzer (NETZSCH, Selb, Germany) was used for the analysis of cement samples A0 and A28. Powder samples (10 mg) were taken for testing, the working atmosphere was nitrogen with a gas flow rate of 30 mL/min. The measurement range was from 25 °C to 1000 °C at a heating rate of 20 °C/min. The cement weight was recorded every 1 °C until 1000 °C.

2.7. Hydration Behavior

The heat release of the cement during early hydration was recorded using a TAM Air isothermal calorimeter (Valencia, TA, USA). Before the test, the isothermal calorimeter was calibrated in advance, and the temperature was set at 20 °C and maintained for 24 h to ensure the stability of the test baseline. Cement samples A0, A7, A14, and A28 were mixed with deionized water and then placed in an isothermal calorimeter. The cement-to-water weight ratio is 0.44. The temperature of the calorimetric channel was set at 20 °C, the indicator was recorded every 30 s, and the cement paste was continuously collected from this location to the end of 72 h.

3. Results and Discussion

3.1. Influence of Aging on the Engineering Performances of Oil Well Cement

3.1.1. Compressive Strength

The compressive strength of hardened oil well cement pastes is a critical factor in ensuring proper cementing during construction. The compressive strengths of samples A0, A7, A14, and A28 at 80 °C are shown in

Table 3. The compressive strength of hardened cement pastes at 80 °C after different aging time (MPa).

Cement Sample	Curing Time (d)
	1	2	3	7	28
A0	26.8 ± 1.2	32.5 ± 1.3	37.2 ± 1.7	40.1 ± 1.4	35.8 ± 1.6
A7	21.8 ± 1.5	24.7 ±1.4	25.2 ± 1.6	32.1 ± 1.3	30.5 ± 1.8
A14	19.6 ± 1.2	23.0 ± 1.3	26.7 ± 1.5	29.8 ± 1.9	29.8 ± 0.8
A28	18.9 ± 0.9	24.1 ± 0.7	24.6 ± 1.5	26.8 ± 0.4	25.9 ± 0.8

.

The compressive strength of the hardened cement pastes at 1 d of age generally exceeds 14 MPa [21]. Specifically, at 1 d of aging, the compressive strength of A7 was 5 MPa lower than that of A0. The strength of A14 approached that of A28, with A28 exhibiting a compressive strength 8 MPa lower than that of A0. The results revealed that the compressive strength of cement samples decreased with increasing aging time. However, the compressive strength of the cement samples with different aging durations remained above 14 MPa at 1 d of curing.

The 28 d compressive strength of the A0 and A7 samples was lower than the 7 d compressive strength. The 28 d compressive strengths of the A14 and A28 samples were close to the 7 d compressive strength. With increasing aging time, the strengths of the A14 and A28 samples almost did not decrease at 28 d, but their 28 d compressive strengths were lower than those of the A0 and A7 samples.

3.1.2. Thickening Time

There are different requires for the thickening time of oil well cement, which are generally set according to the actual operation situation [22]. The transition time of cement, defined as the time required for cement consistency to shift from 30 to 70 Bc, typically does not exceed 40 min [23]. Prolonged transition times may lead to gas channeling in oil wells. The thickening times of samples A0, A7, A14, and A28 at 80 °C are shown in Figure 2.

The thickening time of the A0 sample was 103 min, and that of A28 was reduced to 53 min, which is approximately 50% less than that of the A0 sample. Moreover, the transition time of the A0 sample was 8 min, and that of the A28 sample increased to 19 min. As the aging time increased, the thickening time of the cement paste decreased, whereas the transition time slightly increased.

3.1.3. Rheological Properties

An effective rheology is essential to ensure the proper flow of cement paste [24]. When the fluidity is low or the rheological properties of the cement paste are poor, it is difficult to pump the cement paste to a predetermined position, which negatively affects the cementing operation. The rheological curves of the A0, A7, A14, and A28 samples at 80 °C are shown in Figure 3, and the specific rheological curve fitting parameters are listed in

Table 4. The rheological curve fitting results of cement pastes after different aging time at 80 °C.

Cement Sample	Rheological Model	Fitting Function	R2	µp (Pa·s)	τ0 (Pa)	Fluidity (cm, 25 °C)
A0	Bingham fluid	τ = 0.068γ + 0.920	0.995	0.068	0.920	19.0
A7	Bingham fluid	τ = 0.077γ + 1.333	0.999	0.077	1.333	19.0
A14	Bingham fluid	τ = 0.091γ + 2.467	0.998	0.091	2.467	18.0
A28	Bingham fluid	τ = 0.176γ + 12.864	0.947	0.176	12.864	16.0

.

Samples with different aging times all exhibited Bingham fluid behavior. The fluidity of A0, A7, and A14 samples was similar, ranging between 18 and 19 cm, with the rheological curves closely arranged. As aging time increased, both the plastic viscosity (µ_p) and yield stress (τ₀) of the cement pastes gradually increased, indicating a gradual deterioration in rheological properties. Additionally, the fluidity of the A28 sample significantly decreased to 16 cm, and the measured results exceeded the working range of the six-speed rotational viscometer at 600 r/min, indicating severe deterioration of the rheological properties.

3.2. Influence of Aging on the Particle Size of Oil Well Cement

When the estimated aging time was reached, the aged cement was found to be caked, with the extent of caking increasing with increasing aging time, notably, A28 cement was bonded to the stainless steel plate. The particle size distributions of samples A0 and A28 were characterized using a Hydro 2000 M/MU laser particle size analyzer from Malvern, UK, as shown in Figure 4 and

Table 5. The specific particle size distributions of cement samples A0 and A28.

Cement Sample	Partical Size (μm)			Specific Surface Area (m2·kg−1)
	d10	d50	d90
A0	2.54	15.15	36.69	375
A28	3.66	16.56	37.55	310

. The specific surface areas of samples A0 and A28 were measured with an FBT-9 Blaine air permeability apparatus (Shanghai Yichang), as listed in Table 5.

According to laser particle size analysis, compared with those of the A0 sample, the d₁₀, d₅₀, and d₉₀ values of the A28 sample increased by 1 μm, but this difference was not significant. In terms of the specific surface area, the A28 cement showed a reduction of 17% when measured by the Blaine method. Notably, the content of fines with particle size below 1 μm in A0 and A28 samples showed little change, indicating that aging had less significant impact on the particle size distribution of cement but more significant influence on its specific surface area.

The decrease in the specific surface area indicated that with increasing aging time, the surface energy of the cement powder decreased, which was usually accompanied by a decrease in the cement hydration reaction rate and compressive strength.

3.3. Influence of Aging on the Mineral Composition of Oil Well Cement

Powder X-ray diffraction (XRD) was used to characterize the mineral composition changes throughout the cement aging process. The XRD patterns of samples A0, A7, A14, and A28 are shown in Figure 5, with the specific compositional changes shown in

Table 6. The mineral compositions of cement samples after different aging time (wt.%).

Cement Sample	C3S	C2S	C3A	C4AF	CaSO4·0.5H2O	CaSO4·2H2O	AFt	CH	CaCO3
A0	60.0	20.1	1.2	15.8	0.3	2.2	0.0	0.4	0.0
A7	59.6	20.4	1.1	15.6	0.0	2.0	0.5	0.8	0.0
A14	59.0	20.7	0.9	15.5	0.0	1.6	1.2	1.1	0.1
A28	58.9	20.5	0.7	15.5	0.0	1.6	1.5	1.2	0.1

.

As the aging time increased, the C₃S and C₃A contents in the cement decreased, whereas the hydration product AFt emerged and continued to increase, along with an increase in the CH content. The total content of CaSO₄ decreased because of its involvement in the hydration of C₃A [25].

Thermogravimetric analysis (TGA) has provided a more comprehensive investigation of mineral composition changes during the aging process [3,12,14]. The TG-DSC curves of samples A0 and A28 are shown in Figure 6, the specific mass changes and the decomposition of compounds corresponding to different temperature ranges are shown in

Table 7. The specific mass loss of cement samples A0 and A28.

Temperature (°C)	Mass Loss (%)		Added Mass (%)	Origins of Mass Loss
	A0	A28
25~140	0.50	1.00	0.50	Physically adsorbed water
140~400	0.10	0.60	0.50	Decomposition of calcium sulfate hemihydrate
400~550	0.20	0.30	0.10	Decomposition of calcium hydroxide and calcium aluminosulfate hydrate
550~1000	0.60	0.85	0.25	Decomposition of calcium carbonate and C-S-H
25~1000	1.40	2.75	1.35	-

.

Compared with sample A0, the mass loss of A28 increased by 1.35% between 50 °C and 1000 °C. In the temperature range of 110 °C to 400 °C, the mass loss of the A28 cement increased by 1% compared with that of the A0 cement, which accounted for 74% of the total increase. This mass increase was attributed primarily to the moisture absorption of the cement and the transformation of CaSO₄·0.5H₂O into CaSO₄·2H₂O during aging. In the range of 400 °C to 550 °C, the decomposition of hydration products, such as CH and calcium sulfoaluminate hydrate, led to an additional mass loss increase of 0.1%. During the aging process, C₃S partially hydrated to form a C-S-H gel, and CH reacted with H₂O and CO₂ to produce CaCO₃; both the C-S-H gel and CaCO₃ decomposed between 550 °C and 1000 °C, leading to a further increase of 0.25% in mass loss. This was consistent with the results of the XRD analysis.

The CaCO₃ content of A28 sample analyzed by XRD increased by only 0.1% compared with that of A0 sample, which was consistent with the thermogravimetric results. This meant that, during the aging process, the cement samples sorbed more H₂O than CO₂ from the air. Florian [7] suggested that the H₂O sorbed by cement samples from the air during aging was less than the CO₂ sorbed. The reason for the opposite was related to the aging condition of the closed curing box. In the box, the water from the saturated KCl solutions diffused into the air was constantly sorbed by the cement, whereas CO₂ was not replenished from the surrounding atmosphere.

3.4. Influence of Aging on the Hydration Behavior of Oil Well Cement

The hydration reaction is the most important physical and chemical reaction of cement and is directly related to the strength of hardened cement pastes. The change in the mineral composition of cement during the aging process inevitably affects its hydration. The early hydration behaviors of the A0, A7, A14, and A28 samples at 20 °C were analyzed using the heat flow thermal method, as shown in Figure 7.

With the aging time increased, the intensity of the main heat release peak decreased. This could be attributed to the prehydration of C₃S and C₃A during the aging process. The decrease of C₃S and C₃A led to the decrease in hydration heat release. A0 sample had only one heat release peak, but A7 and A14 samples showed a second heat release peak. This was caused by the decrease in gypsum content, which could not fully react with C₃A. After the gypsum was depleted, the remaining C₃A continued the hydration reaction, and a second hydration heat release peak occurred [10,26].

According to the analysis of mineral composition and specific surface area, the specific surface area and the clinker phases of aged cement decreased [27]. This meant that the hydration nucleation sites of the cement decrease, which caused a decrease on the hydration rate of cement. Moreover, the reduction in the specific surface area meant that the surface energy of the cement powder and the energy that cement can release during the hydration reaction reduced. These factors led to a reduction in the cumulative heat evolution during cement hydration, which reduced the compressive strength of the hardened cement pastes.

With increasing aging time, the cement sorbed H₂O and CO₂ in the air, and the agglomeration of the cement particles led to a decrease in the specific surface area and surface energy of the powder. After water was sorbed, the clinker phase and gypsum in the cement continued to undergo a slight hydration reaction and produced hydration products, reducing the reaction phase with the mixing water. Therefore, the rate of nucleation of the cement hydration products decreased, and the heat release of hydrated cement decreased accordingly. Slight hydration during the aging process had an effect on the hydration of cement.

Measured using an FBT-9 Blaine air permeability apparatus (Shanghai Yichang), the specific surface area of cement decreases with aging time. Meanwhile, the thickening time of cement paste also decreases. This phenomenon can be attributed to the decreased gypsum content in aged cement, which inadequately limits the rapid hydration of C₃A [28,29]. The longer the aging time is, the lower the gypsum content. Consequently, the inhibition of gypsum to the rapid hydration of C₃A was weakened, the longer the aging time, the shorter the cement thickening time.

As the specific surface area decreased, the fluidity and rheological properties of the unaged oil well cement improved [30]. Conversely, the aging process negatively impacted the aged cement. This phenomenon can be attributed to a reduction in the surface energy of the cement powder during aging, coupled with the formation of hydration products that increase particle friction. The increased friction leads to elevated µ_p and τ₀, thereby impairing the rheological properties of the cement paste [31].

4. Conclusions and Observations

4.1. Conclusions

In existing studies on the effects of aging on cement, most scholars have focused on construction cement. The impact of aging on oil well cement has not been thoroughly investigated, yet this issue becomes prominent in practical applications. Therefore, to determine the allowable time to avoid significant performance changes due to aging, this paper studies the changes in compressive strength, thickening time, and rheological properties of cement with aging time, and conducts a mechanism analysis. The specific research conclusions are as follows:

• In an aging environment with 90% RH relative air humidity, the cement sorbed H₂O and CO₂ from the air, led to a reduction in the specific surface area and surface energy of the powder. The contents of the clinker phases C₃S, C₃A, and gypsum decreased, whereas the quantities of the hydration products AFt and CH increased. This process hindered the reaction between the cement and the water, leading to a decrease in both the hydration rate and heat release.
• The compressive strength, thickening time, and rheological properties of aged cement were affected. As the hydration of the cement slowed, the surface energy of the cement powder decreased, which negatively impacted the compressive strength. The reduction in the gypsum content was insufficient to adequately limit the hydration of C₃A, leading to a shortened thickening time. Additionally, the increase in hydration products increased the friction between the particles, further degrading the rheological properties.
• For Class G oil well cement aged in an environment with 90% relative humidity and at a temperature of 25 °C, the permissible time to avoid significant changes in cement properties should be less than 7 d, with a maximum aging period not exceeding 14 d. Furthermore, at a usage temperature of 80 °C, the performance of the cement paste aged for 28 d was found to be inadequate to meet the necessary operation requirements.

4.2. Observation

In this study, the aging of oil well cement was investigated by placing the cement flat in a pan and exposing it to humid air. This method allowed for a detailed exploration of the aging mechanism under extreme environmental conditions, which amplified the observed effects on cement. However, oil well cement is typically stored in well-sealed packaging, such as bags or bulk cement tanks, which limits its exposure to similar conditions in real-world applications.

As a result, the influence of humidity and temperature on oil well cement under these more controlled conditions is likely to be less pronounced than what was observed in the experiments. To better understand the actual effects of aging during use and identify an optimal aging time that minimizes these effects, it is necessary to simulate both the aging conditions and packaging scenarios found at construction sites. This approach provides more realistic insights into how aging impacts oil well cement in practice, thereby guiding the development of better storage and handling protocols.