The Effect of Ethylene Diamine Tetra(Methylene Phosphonic Acid) Sodium on the Hydration of Tricalcium Silicate at 80 °C

Abstract

In the field of oil well cementing, controlling the hydration process of cement slurries is essential to ensure successful placement and long-term well integrity, especially under challenging high-temperature and high-pressure conditions. Organic phosphonates such as ethylene diamine tetra(methylenephosphonic acid) sodium (EDTMPS) have been identified as effective retarders that can delay the hydration of tricalcium silicate (C₃S), the primary phase responsible for early strength development in cement. This research explores the effect of EDTMPS on C₃S hydration, using a combination of analytical techniques including isothermal calorimetry, TGA, XRD, and SEM. The results demonstrate that 0.2% EDTMPS extended the induction period of C3S hydration by up to 4.27 h, reduced the heat flow during the acceleration period by 65%, and lowered the cumulative heat of hydration at 1 d by approximately 14%. As a result, EDTMPS significantly delayed the development of C-S-H gel and CH, thereby extending the setting time of the cement slurry and causing a reduction in the early compressive strength of the cement pastes. Therefore, EDTMPS had a slight effect on the compressive strength of cement pastes at 28 d. The results offer important insights into the retarding mechanism of organic phosphonates and their potential applications in enhancing the performance of oil well cement under high temperatures.

1. Introduction

In the oil and gas industry, well cementing is a vital process for maintaining wellbore integrity, ensuring zonal isolation, and protecting casing from the formation of corrosive fluids [1,2,3]. The success of this process depends heavily on the precise control of the cement slurry properties, particularly its rheology, thickening time, and early strength development. According to API (American Petroleum Institute) specification [4], cement slurries are designed to remain pumpable during placement and to achieve rapid strength development after placement, which requires a careful balance of hydration kinetics. One of the most critical challenges in well cementing is controlling the hydration process under the high-temperature and high-pressure conditions typical of downhole environments [4,5]. This is where retarders play a crucial role [6,7,8,9].

Retarders are chemical additives that delay the reaction of cement with water, extending the working time of the slurry [10,11,12]. This delay is essential for ensuring that the cement can be placed properly and reach the desired depth before setting begins. Among the various types of retarders used, organic phosphonates have emerged as highly effective agents [13,14,15]. Organic phosphonates, including compounds like ethylene diamine tetra(methylenephosphonic acid) sodium (EDTMPS) [16], are characterized by their strong chelating ability and their affinity for metal ions, particularly calcium. These properties make them particularly effective in interacting with cement components such as tricalcium silicate (C₃S) [17,18,19], which is the primary phase responsible for early strength development in Portland cement. It has super-retarding effects with even a small dosage. Furthermore, Kupwade et al. [20] reported that phosphonate retarders were not sensitive to variations in cement composition. Due to these advantages, EDTMPS retarders are commonly used in well cementing.

The primary hydration reaction in cement involves the reaction of cement with water, producing C-S-H gel and CH. The C-S-H gel is the major binding phase that imparts strength to the cement slurry [21,22], while CH is a crystalline byproduct. The rate at which these products form determines the setting time and early strength development [23]. Phosphonates such as ethylenediaminetetramethylene phosphonic acid (EDTMPS) have been increasingly studied for their role in modifying cement hydration processes. Recent studies have shown that phosphonates can act as effective retarders by interacting with calcium ions, delaying the precipitation of hydration products like calcium hydroxide and calcium silicate hydrate (C-S-H). For instance, research by Ma et al. [24] demonstrated that the adsorption of phosphonates onto clinker phases inhibited early hydration reactions, especially under high-temperature conditions. Moreover, a study by Pang et al. [15] highlighted the influence of phosphonates on the microstructure of C-S-H, suggesting that these additives could modify pore structure and enhance durability in long-term sealing. Previous studies have mostly focused on clarifying the work mechanism of EDTMPS, but the microstructure of cement pastes plays a more important role in the properties of cement pastes.

Understanding the interaction between organic phosphonates and cement hydration is crucial for optimizing their use in oil well cementing. Despite the significant advancements in understanding the effects of phosphonates on cement hydration under normal conditions, there remains a clear gap in the current research concerning the behavior of EDTMPS under elevated temperature conditions. High-temperature environments, often encountered in deep wells, can significantly alter the kinetics of cement hydration and the effectiveness of retarders. Quantifying the effect of EDTPMS on the degree of hydration of C₃S at these elevated temperatures is crucial for optimizing its use in well cementing applications where thermal stability and performance are critical.

This study focuses on the effect of organic phosphonates, particularly EDTMPS, on the hydration behavior of tricalcium silicate (C₃S), a critical component in oil well cement. By employing a combination of analytical techniques, including isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), this research seeks to provide a comprehensive understanding of how phosphonates interact with C₃S, delay hydration, and influence the overall properties of oil well cement. The induction period, heat release rate, cumulative heat of hydration, and calcium silicate hydrate (C-S-H) gel and calcium hydroxide (CH) content were investigated in this study. The dosage of EDTMPS was 0.2% and 0.4% by weight of cement.

2. Materials and Method

2.1. Materials

The primary material used in this work was tricalcium silicate (C₃S), which was synthesized in the laboratory to ensure high purity and control over its phase composition. The C₃S was synthesized by heating a stoichiometric mixture of CaCO₃ and SiO₂ at around 1450 °C for 10 h with a heating rate of 10 °C/min and then maintaining at 900 °C for 1 h. The resulting C₃S was then ground to a fine powder. The phase composition of the synthesized C₃S was confirmed by XRD analysis, and its particle size distribution was measured, with the results presented in Figure 1 and Figure 2. According to the quantitative XRD results, the C₃S contained 98.1 wt% monoclinic C₃S and 1.9 wt% CaO. The d0.1, d0.5, and d0.9 of the C₃S were 1.19, 9.88, and 32.73 μm, respectively.

The retarder used in this study, ethylene diamine tetra(methylenephosphonic acid) sodium (EDTMPS), was purchased from Macklin Reagent, a well-known supplier of high-quality chemical reagents. The EDTMPS was provided as an aqueous solution with a solid content of 10%.

Additionally, the deionized water used in all experiments was prepared using a laboratory-grade purification system to ensure consistency and eliminate any potential impurities that could have affected the hydration process.

2.2. C₃S Slurry and Sample Preparation

According to API specification [4], in order to simulate the slurry formula used in cementing, the C₃S slurry and cement pastes were prepared with a water-to-solid ratio of 0.44. The hydration products and morphology of C₃S slurry and C₃S slurry with added EDTMPS were tested at curing ages of 3 h, 8 h, 1 d, and 3 d. To terminate the hydration of C₃S, the sample was immersed in isopropanol for 24 h to stop the hydration process, followed by drying in a vacuum oven at 60 °C for 4 h.

2.3. Isothermal Calorimetry Test of C₃S Slurry

To study the variation in heat released during the C₃S hydration, the heat of the C₃S slurry was continuously measured for 72 h at 80 °C. The slurry was prepared by mixing C₃S with water in a water-to-C₃S ratio of 0.44. After thorough mixing, 8.4 g slurry was transferred into the sample chamber of the isothermal calorimeter, ensuring it was sealed to prevent evaporation and contamination. The calorimeter was set to a constant temperature of 80 °C for the test. The heat released by the hydration reactions was continuously measured and recorded over time. These data provided insights into the reactivity and setting behavior of the C₃S slurry, including the rate and total heat of hydration.

2.4. XRD Testing of Hydration Products

The diffractometer was operated with Cu-Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA with a scanning rate of 5° 2θ/min. For XRD analysis, the fully cured and dried C₃S sample was first broken into small pieces, targeting a particle size of approximately 1–2 mm. The sample was then further ground into a fine powder until the particle size was reduced to less than 10 μm. Alumina was used as an internal standard mineral during the sample grinding process. The purpose of incorporating alumina was to ensure accurate calibration and to assist in identifying and quantifying the phases present in the C₃S sample. The amount of internal standard alumina was 20% by weight. The powder was sieved through a 200-mesh sieve to ensure uniformity. The dried powder was then uniformly pressed into a standard XRD sample holder. The prepared sample was placed in the XRD sample tray and scanned. The scanning rate was 5°/min and the 2θ range was 5° to 80°. The resulting diffraction data were analyzed using XRD software GSAS-II to determine the crystalline phases and structural characteristics of the sample.

2.5. TGA of Hydration Products

The dried sample was finely ground to ensure uniformity and to minimize particle size effects during analysis. In the TGA test, the ground sample was placed in a sample pan and heated from room temperature to 800 °C at a controlled rate of 10 °C/min under N₂ atmosphere. Sample mass was measured before and after testing, to monitor mass loss accurately. The weight loss provided insights into the thermal degradation and composition of the C₃S slurry, including the amount of bound water and the presence of other phases.

2.6. Morphology of Hydration Products

The morphology of C₃S hydration products was observed using scanning electron microscopy (SEM). The dried sample was mounted on an SEM sample holder using conductive adhesive to ensure proper conductivity. To enhance the surface conductivity and prevent charging effects during imaging, the sample was coated with a thin layer of gold. Finally, the prepared sample was placed into the SEM device and analyzed under high magnification to observe the detailed morphology of the hydration products, including their shape, size, and distribution.

2.7. Compressive Strength

The uniaxial compressive strengths of hardened cement pastes (HCPs) were evaluated with AEC-201 cement strength tester (Ruifeng, Shanghai, China) at a loading rate of 1.2 kN/s. Three samples were tested and the average was taken as the representative value.

3. Results and Discussion

3.1. C₃S Hydration Heat

The hydration heat of C₃S is the exothermic reaction heat released when C₃S reacts with water, crucial for the strength development and setting process of C₃S slurry. The hydration heat of C₃S slurry and slurry with added EDTMPS was continuously monitored at 80 °C for 72 h, as shown in Figure 3. As shown in Figure 3a, EDTMPS significantly influenced the hydration heat flow of C₃S. With an increase in the amount of EDTMPS, the peak of the hydration heat flow markedly decreased, and the induction period was noticeably extended. Therefore, EDTMPS had a significant impact on the accumulative heat of C₃S within three days. Especially with the addition of 0.4% EDTMPS, the accumulative heat of the C₃S slurry with EDTMPS was significantly lower than that of the neat slurry.

Table 1. Summary of the isothermal hydration kinetics results.

Notation	Induction Period		Acceleration Period		Accumulated Heat (J/g)
	Duration (h)	Minimum Heat Flow (mW/g)	Begin (h)	Maximum Heat Flow (mW/g)	24 h	48 h	72 h
Neat slurry	1.56	1.137	3.66	12.78	164.30	192.73	204.03
0.2% EDTMPS	4.27	0.409	9.17	4.49	141.52	180.37	197.00
0.4% EDTMPS	18.39	0.176	30.30	1.54	25.76	103.52	161.08

summarizes the characteristic parameters at each period of the hydration process of the C₃S slurry. As the amount of EDTMPS increased to 0.4%, the hydration induction period of the C₃S slurry extended from 1.56 h to 18.39 h, while the start time of the acceleration period was extended from 3.66 h to 30.30 h. In addition, EDTMPS significantly reduced the minimum hydration heat flow of the C₃S slurry during the induction period and the hydration heat flow peak during the acceleration period. With the addition of 0.4% EDTMPS, the minimum hydration heat flow of the C₃S slurry decreased to 0.176 mW/g, and the maximum hydration heat flow decreased to 1.54 mW/g. EDTMPS significantly influenced the 24 h accumulative heat of the C₃S slurry. Adding 0.4% EDTMPS reduced the accumulative heat of C₃S from 164.30 J/g in the neat slurry to 25.76 J/g, marking an 84.3% reduction rate. When the curing time was extended to three days, the reduction rate decreased to 21.1%, which still did not reach the accumulate heat value of the neat slurry. From the above analysis of the effect of EDTMPS on the hydration heat of C₃S, it can be seen that EDTMPS significantly retarded the hydration of C₃S at 80 °C.

3.2. Hydration Products of C₃S Slurry

During the hydration of C₃S, CH and C-S-H gel are formed, which can be represented by the following chemical equation. The amount of these hydration products can be used to evaluate the effects of retarders on C₃S hydration:

$$2{\mathrm{C}\mathrm{a}}_3\mathrm{S}\mathrm{i}{\mathrm{O}}_5+6{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}·2\mathrm{S}\mathrm{i}{\mathrm{O}}_2·3{\mathrm{H}}_2\mathrm{O}+3\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

Previous study has shown that EDTMPS adsorbs onto the surface of cement particles and the first hydration products, thereby delaying the hydration of the cement particles. XRD quantitative analysis was conducted to determine the hydration degree of C₃S at different hydration ages [25]. The XRD patterns of the C₃S slurry cured for 3 h, 8 h, 1 d, and 3 d are shown in Figure 4. CH was the only crystalline phase product. The results showed a clear CH diffraction peak at 18.12° for neat C₃S slurry at 8 h. This diffraction peak increased with the increase of curing age. EDTMPS delayed the appearance of the CH peak, as the CH diffraction peak appeared at 1 d for C₃S slurry with 0.2% EDTMPS. The addition of more EDTMPS further delayed the appearance of the CH peak. With 0.4% EDTMPS, a clear diffraction peak of CH was observed at 3 d.

The composition and changes of C₃S hydration products were quantitatively analyzed using XRD refinement techniques. Alumina (α-Al₂O₃) was used as an internal standard mineral. The amorphous phase product in the C₃S slurry was classified as C-S-H gel. The results are shown in

Table 2. The quantitative XRD results of the hydration products of C₃S (wt.%).

Notation	Time	C3S	CH	C-S-H
Neat slurry	3 h	88.48	3.05	8.47
	8 h	57.68	13.31	29.01
	1 d	51.29	15.63	33.08
	3 d	40.08	16.90	43.02
0.2% EDTMPS	3 h	95.37	1.24	3.39
	8 h	87.46	2.58	9.96
	1 d	55.64	14.06	30.30
	3 d	47.93	16.04	36.03
0.4% EDTMPS	3 h	95.56	1.60	2.84
	8 h	94.71	1.90	3.39
	1 d	87.44	3.79	8.77
	3 d	50.82	15.09	34.09

. For C₃S neat slurry, the C₃S content gradually decreased with the curing age and dropped to 40.08% at 3 d, and the contents of CH and C-S-H gel gradually increased, reaching 16.09% and 43.02% at 3 d, respectively. For C₃S slurry with 0.2% EDTMPS, the hydration of C₃S was significantly inhibited at 8 h. The C3S content was 77.46%, which was much higher than that of the C₃S neat slurry. At the same time, the amounts of CH and C-S-H gel generated were also small. When more EDTMPS was added to the C₃S slurry, the initial hydration degree of the C₃S was lower. At 1 d, the C₃S content was 79.44% for C₃S slurry with 0.4% EDTMPS, and the amounts of CH and C-S-H gel generated were only 3.79% and 16.77%. At 3 d, the contents of hydration products, especially C-S-H gel, were still significantly lower than in C₃S neat slurry.

The quantification of CH in the C₃S slurry was also be carried out using TGA. By analyzing the mass loss in the temperature range (400–500 °C) corresponding to the decomposition of CH, the amount of CH present in the hydrated C₃S slurry was accurately determined. This method provided a reliable measure of the CH content. The TG curves are shown in Figure 5. As shown in Figure 5a, due to the relatively short hydration period (3 h), the weight loss in the C₃S neat slurry and the C₃S slurry with the EDTMPS added was relatively small. However, the weight loss in the neat slurry was slightly higher compared with the slurry with the EDTMPS. The three slurries cured for 8 h showed significant differences, as shown in Figure 5b. The neat slurry exhibited significantly higher weight loss compared with the two slurries with EDTMPS added, indicating that the EDTMPS delayed the hydration of C₃S. As the curing period extended to 1 d, as shown in Figure 5c, the weight loss of the neat slurry and the slurry with 0.2% EDTMPS showed little difference. However, the loss of weight from the slurry with 0.4% EDTMPS remained relatively low, indicating that increasing the amount of EDTMPS resulted in a stronger retarding effect.

Based on the weight loss between 400 and 500 °C, the CH content in each sample was calculated. The results are shown in Figure 6. Comparing the CH content of each sample at different curing periods, as shown in Table 2 and Figure 6, the results obtained from the two methods were quite similar. This consistency indicates that both methods were accurate for the quantitative analysis of CH content. The trend of CH variation observed from the TGA aligned with the trend observed in the accumulative heat of hydration.

3.3. Morphology of C₃S Hydration Products

The morphology of the hydration products in each sample at different curing periods was observed using scanning electron microscopy (SEM), with the results shown in Figure 7. From Figure 7, it can be seen that distinct hydration products were observed on the surface of C₃S particles in the neat slurry as early as 3 h. As the curing period increased, the amount of calcium C-S-H gel gradually increased, with particles bonding together. In contrast, in the C₃S slurry with added EDTMPS, almost no hydration products were observed within 8 h of curing. The slurry with 0.2% EDTMPS showed significant C-S-H gel formation at 1 d, while the slurry with 0.4% EDTMPS did not show C-S-H gel formation until 3 d. The SEM observations were consistent with the results from the hydration heat tests and the quantitative analysis of the hydration products, confirming that EDTMPS effectively delayed the hydration of C₃S.

3.4. Compressive Strength of Hardened Cement Pastes

The compressive strengths of HCPs with different dosages of EDTMPS cured at 80 °C for different lengths of time are shown in Figure 8. As the dosage of EDTMPS increased, the compressive strength of HCPs decreased significantly. Especially at 1 d, compared with neat cement pastes, the compressive strength of HCPs with 0.4% EDTMPS decreased by 78.9%. At 28 d, the compressive strengths of all samples were comparable. The delayed formation of hydration products, particularly C-S-H and CH, due to the presence of the retarder, directly impacted the development of compressive strength in the early stages. As hydration products are responsible for the binding and densification of the cement matrix, a reduced amount of these products resulted in lower compressive strength. This was especially true during the early hydration period, where the strength gain was closely tied to the formation of C-S-H gel. Consequently, the delay in hydration caused by the retarder led to slower strength development in the initial stages, which could have impacted the mechanical performance of the material in early-age applications. However, this effect diminished over time as hydration eventually progressed, resulting in comparable long-term strength.

4. Conclusions

This study explored the effectiveness of organic phosphonates, particularly EDTMPS, as retarders in oil well cementing, by examining their influence on the hydration of C₃S. The results from isothermal calorimetry, XRD, TGA, and SEM consistently indicated that EDTMPS delayed the hydration process of C₃S, leading to a prolonged induction period and delayed development of hydration products such as C-S-H gel and CH. Such a delay in hydration is beneficial for extending the working time of cement slurries in high-temperature and high-pressure environments, ensuring that they can be effectively placed and achieve the desired performance. By using EDTMPS as a retarder, the hydration process of the cement slurry can be better controlled, allowing extended working times and improved placement of the cement in the wellbore. This helps ensure the long-term integrity of the well, preventing issues like premature setting or incomplete cementation. The consistent findings across multiple analytical techniques confirm that EDTMPS and similar organic phosphonates are promising candidates for optimizing the performance of oil well cements. These insights not only contribute to the understanding of hydration kinetics in the presence of retarders but also offer practical implications for the formulation and application of advanced cement systems in challenging downhole environments.