# The Effects of Magnesium Borate Whiskers on the Mechanical Performance of Oil Well Cement

Authors: Xianshu Gao; Ping Zhou; Xiaoliang Qin; Zhaijun Wen; Jing Wang; Hao Wang; Ming Li; Jia Zhuang; Chengxin Li
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Quasi-brittle behaviors of cement-based material can be reinforced through diverse methods due to its unique multiscale features. Among them, reinforcing agents are the most direct and effective means, such as slag, metakaolin, and fly ash. These materials can improve the performance of cement stone to a certain extent, but their chemical composition is still mainly silica, which does not in essence change the defects of cement. Therefore, in this experiment, magnesium borate whisker (Mg2B2O5) was used as reinforcer of 90°C class G oil well cement and magnesium borate whisker reinforced cement-based material (MBWRC) was prepared. On the one hand, the performance of mechanical strength was controlled, and the resistance to compression, traction, and bending was included. Furthermore, static stress-strain behaviors analysis and toughness behaviors analysis (dynamic multicycle loading test) were further conducted. Second, mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) were used to test the characteristics of pores and interfaces of cement-based materials. Through multiscale microstructure analysis, MBWRC was found to have excellent 90°C mechanical performances when compared to control sample’s, for which tensile strength increased to 235% of controlled sample’s, and flexural strength increased to 130%, plus a healthy developed compressive performance, and MBWRC showed much denser pore structure, in which harmless micropore increased from 13.3% to 14.40%, porosity decreased from 17.01% to 16.20%, and permeability decreased from 0.2533% to 0.0205%. Furthermore, MBWRC showed resistance capability to mechanical loading, which can be attributed to the formation of denser pore structure and more excellent mechanical performance.


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## Body

## 1. Introduction

Cement-based materials are brittle materials with low tensile strength and flexural strength, high brittleness, easy cracking, and serious durability problems. In the cementing process, the cement stone is easily affected by the downhole environment to produce cracks and gaps, thereby forming channeling channels, resulting in a weakening of the interlayer isolation effect of the cement sheath, which seriously affects the cementing quality and life [1–4]. Therefore, it is very necessary to use appropriate methods to improve the mechanical properties of cement stone. For example, the calcium-silicon ratio of cement is changed by adding silica fume, and reinforcing agent is added. Among them, the enhancer is the most direct and effective method, such as metallic/inorganic natural fibers, carbon nanotubes, nano-sized particles, synthetic and organic fibers, slag, metakaolin, fly ash, and a hybrid of these reinforcing materials [5–16]. However, the strengthening ability of these materials will be limited, such as poor compatibility, or because of its own hydration reaction, which has a negative impact on cement hydration, and the main component of these materials is still silica, which cannot solve the defect problem of cement stone from the root [17–19]. Therefore, it is extremely important to find a reinforcing agent with good dispersibility and can significantly improve the defects of cement stone.Whisker materials, a kind of single-crystal inorganic product, show a smaller size, bigger length/diameter ratio, and better compatibility to cement-based material [20, 21]. Also, whiskers show similar morphology formation as carbon fiber [22]. So, whisker materials may have broad application prospect in cement-based material for excellent interface compatibility. As whisker materials show no obvious defects in structure and strength properties, they can be used as additives for cement-based materials [4, 22], which exert extraordinary effectiveness in delaying microcracks formation and propagation [19, 23, 24]. It has been observed that whisker materials can improve not only compressive strength but also flexural strength of cement-based material and excellent load-deflecting and fracture control ability [22, 25]. Li et al. depicted that whiskers used in cement-based material can be beneficial to toughness performance of cement-based material when combined with carbon fibers [26–28].In this experiment, Magnesium borate (Mg2B2O5) whiskers are used as reinforcing agent for the phenomenon of strength decline of cement stone at 90°C, which is a type of synthesized whisker material, and show excellent physicochemical tolerance performance, especially high temperature, high salinity, and high alkalinity [29–32]. Also, it shows excellent mechanical performance compared to ordinary fiber materials, such as better compatibility, inertness, and lower thermal-expansion properties [33–35]. By testing the compressive strength, tensile strength, and flexural strength of the system, its mechanical properties were explored, and its mechanism of action was explored. This research will help to open up the application field of whisker materials, and at the same time, it has evident reference significance in the decline of cement stone strength.


## 2. Materials and Methods

### 2.1. Materials

In experiments, class G oil well cement (OWC) was used, as shown in Table1; Mg2B2O5 whisker, commercial product, was produced by Qinghai Citic Guoan. Ltd., Chengdu, China, as shown in Tables 2 and 3. Fluid loss additive was used for better free water control ability; dispersant was used for better rheological properties performance [36] (specification as Table 4), for industrial-grade products.Table 1
Chemical components of class G oil well cement.
ComponentsSiO2Al2O3Fe2O3CaOMgOK2Owt%22.73.394.8165.50.90.37ComponentsSO3TiOMnO2Na2OGeOP2O5wt%1.210.190.090.130.010.11Table 2
Chemical composition of Mg2B2O5 whiskers (wt %).
CompositionMg2B2O5CaOFe2O3Cl−WaterContent99.00.020.030.010.05Table 3
Mg2B2O5 whisker parameters.
MaterialLength (μm)Diameter (μm)Tensile strength (GPa)Bulk density (g·cm−3)Elastic modulus (GPa)Mg2B2O5 whiskers10∼700.5∼83.922.91264.6Table 4
Additives for cement-based material.
AdditivesSpecificationFluid loss additivePoly (acrylamide) (G33S), 2.0%DispersantSulfonic (acetone-formaldehyde) polycondensate (SXY-2), 2.0%.Note: % was weight percentage, by total weight of cement powders and whiskers used.


### 2.2. Sample Preparation of Cement-Based Material

#### 2.2.1. Preparation of Cement Slurry

Cement slurry was prepared and cured as standards GB/T 19139-2012 recommended. Formulas are depicted in Table5. Mg2B2O5 whiskers, G33S, and SXY-2 were all mixed with cement powders, and then, cement mixtures were poured into water to agitate uniformly with stirring machines, and mixing speed was 4000 r/min. Weight ratio of cement powders and whiskers to water was fixed to 0.44 [29, 30]. Then, mixing speed increased to 12000 r/min ± 250 r/min for another 35 s ± 1 s.Table 5
Formulas of cement slurry.
SamplesCement powders/gMg2B2O5 whiskers/gG33S/gSXY-2/gWater/gSample 0 (controlled Sample)80001616352Sample 179281616352Sample 2784161616352Sample 3776241616352Sample 4768321616352Note: Mg2B2O5 whiskers are contents by weight of cement powders and whiskers used, respectively, 0%, 1.0%, 2.0%, 3.0%, and 4.0%; controlled sample: 800 g cement powders, 16 g G33S, 16 g SXY-2, and 352 g water (Sample 0).


#### 2.2.2. Preparation of Samples and Operation Procedures for Mechanical Strength Tests

Prepared cement mud was prepared using a standard mold and all the mechanical tests were performed by electronic hydraulics. First is compression resistance test. Cured rectangular samples were fixed at 50.8 mm × 50.8 × 50.8 mm, and loading rate was 300 N/s. Second is tensile-strength test. Cured cylindrical samples 50.0 (diameter) × 25.0 (thickness) were used, and loading rate was 300 N/s. Third is flexural-strength test. Cured rectangular samples were as 40.0 mm × 40.0 mm × 160.0 mm. The test procedure was a 3-point bending method, with the use of motorized bending machines, in which the speed of the span and the cross member was set at 100 mm and 0.02 mm/min, respectively. Curing periods of testing specification as 1 in. (diameter) × 2 in. (height), were used for multicycle loading test and static stress-strain test, and operation procedure was as the Chinese standard GB/T 50266-99 recommended. For the multicycle load test, loading rate was at 1.6 KN/min and unloading rate was at 3.2 KN/min, test pressure was 20.7 MPa, and test temperature was 90°C. For static stress-strain test, the loading rate was at 300 N/s. Curing condition was atmospheric pressure/90°C (±2°C) water curing bath and kept for 28 days.


#### 2.2.3. Preparation of Samples for SEM Analysis and Mercury Intrusion Porosimetry (MIP) Test

Samples (3% Mg2B2O5 whiskers added and cured in condition of water bath, 90°C/28 days) used for interface microstructure SEM analysis were collected or prepared when mechanical test was conducted [37].Pore structure was tested according to mercury intrusion porosimetry (MIP) method, and cured samples (with 3% Mg2B2O5 whiskers) were used for MIP test used mold of 25.0 mm (diameter) × 40.0 mm (height) and then kept in water bath curing at 90°C, deviation as ± 2°C, and cured for 28 days (atmospheric pressure). The test was with AutoPore IV 9500 automatic mercury porosimeter for analysis of pore structure that diameter ranges from 3 nm to 1000 μm and carried out as mercury porosimetry test international standard, ISO 15901-1-2016 recommended, according to Washburn equation. The maximum outside pressure limit was 228 MPa and injected fixed mercury volume into pore structure. As pressure changed, volume of injected mercury changed, and this reflected pore size distribution and porosity. Also, for permeability parameters of cement-based material before and after adding magnesium borate whisker (MBWRC), the MIP method was also utilized according to Daray Rule and Poiseuille Rule and operated as standard ISO 15901-1-2016 recommended, with maximum test pressure.


## 2.1. Materials

In experiments, class G oil well cement (OWC) was used, as shown in Table1; Mg2B2O5 whisker, commercial product, was produced by Qinghai Citic Guoan. Ltd., Chengdu, China, as shown in Tables 2 and 3. Fluid loss additive was used for better free water control ability; dispersant was used for better rheological properties performance [36] (specification as Table 4), for industrial-grade products.Table 1
Chemical components of class G oil well cement.
ComponentsSiO2Al2O3Fe2O3CaOMgOK2Owt%22.73.394.8165.50.90.37ComponentsSO3TiOMnO2Na2OGeOP2O5wt%1.210.190.090.130.010.11Table 2
Chemical composition of Mg2B2O5 whiskers (wt %).
CompositionMg2B2O5CaOFe2O3Cl−WaterContent99.00.020.030.010.05Table 3
Mg2B2O5 whisker parameters.
MaterialLength (μm)Diameter (μm)Tensile strength (GPa)Bulk density (g·cm−3)Elastic modulus (GPa)Mg2B2O5 whiskers10∼700.5∼83.922.91264.6Table 4
Additives for cement-based material.
AdditivesSpecificationFluid loss additivePoly (acrylamide) (G33S), 2.0%DispersantSulfonic (acetone-formaldehyde) polycondensate (SXY-2), 2.0%.Note: % was weight percentage, by total weight of cement powders and whiskers used.


## 2.2. Sample Preparation of Cement-Based Material

### 2.2.1. Preparation of Cement Slurry

Cement slurry was prepared and cured as standards GB/T 19139-2012 recommended. Formulas are depicted in Table5. Mg2B2O5 whiskers, G33S, and SXY-2 were all mixed with cement powders, and then, cement mixtures were poured into water to agitate uniformly with stirring machines, and mixing speed was 4000 r/min. Weight ratio of cement powders and whiskers to water was fixed to 0.44 [29, 30]. Then, mixing speed increased to 12000 r/min ± 250 r/min for another 35 s ± 1 s.Table 5
Formulas of cement slurry.
SamplesCement powders/gMg2B2O5 whiskers/gG33S/gSXY-2/gWater/gSample 0 (controlled Sample)80001616352Sample 179281616352Sample 2784161616352Sample 3776241616352Sample 4768321616352Note: Mg2B2O5 whiskers are contents by weight of cement powders and whiskers used, respectively, 0%, 1.0%, 2.0%, 3.0%, and 4.0%; controlled sample: 800 g cement powders, 16 g G33S, 16 g SXY-2, and 352 g water (Sample 0).


### 2.2.2. Preparation of Samples and Operation Procedures for Mechanical Strength Tests

Prepared cement mud was prepared using a standard mold and all the mechanical tests were performed by electronic hydraulics. First is compression resistance test. Cured rectangular samples were fixed at 50.8 mm × 50.8 × 50.8 mm, and loading rate was 300 N/s. Second is tensile-strength test. Cured cylindrical samples 50.0 (diameter) × 25.0 (thickness) were used, and loading rate was 300 N/s. Third is flexural-strength test. Cured rectangular samples were as 40.0 mm × 40.0 mm × 160.0 mm. The test procedure was a 3-point bending method, with the use of motorized bending machines, in which the speed of the span and the cross member was set at 100 mm and 0.02 mm/min, respectively. Curing periods of testing specification as 1 in. (diameter) × 2 in. (height), were used for multicycle loading test and static stress-strain test, and operation procedure was as the Chinese standard GB/T 50266-99 recommended. For the multicycle load test, loading rate was at 1.6 KN/min and unloading rate was at 3.2 KN/min, test pressure was 20.7 MPa, and test temperature was 90°C. For static stress-strain test, the loading rate was at 300 N/s. Curing condition was atmospheric pressure/90°C (±2°C) water curing bath and kept for 28 days.


### 2.2.3. Preparation of Samples for SEM Analysis and Mercury Intrusion Porosimetry (MIP) Test

Samples (3% Mg2B2O5 whiskers added and cured in condition of water bath, 90°C/28 days) used for interface microstructure SEM analysis were collected or prepared when mechanical test was conducted [37].Pore structure was tested according to mercury intrusion porosimetry (MIP) method, and cured samples (with 3% Mg2B2O5 whiskers) were used for MIP test used mold of 25.0 mm (diameter) × 40.0 mm (height) and then kept in water bath curing at 90°C, deviation as ± 2°C, and cured for 28 days (atmospheric pressure). The test was with AutoPore IV 9500 automatic mercury porosimeter for analysis of pore structure that diameter ranges from 3 nm to 1000 μm and carried out as mercury porosimetry test international standard, ISO 15901-1-2016 recommended, according to Washburn equation. The maximum outside pressure limit was 228 MPa and injected fixed mercury volume into pore structure. As pressure changed, volume of injected mercury changed, and this reflected pore size distribution and porosity. Also, for permeability parameters of cement-based material before and after adding magnesium borate whisker (MBWRC), the MIP method was also utilized according to Daray Rule and Poiseuille Rule and operated as standard ISO 15901-1-2016 recommended, with maximum test pressure.


## 2.2.1. Preparation of Cement Slurry

Cement slurry was prepared and cured as standards GB/T 19139-2012 recommended. Formulas are depicted in Table5. Mg2B2O5 whiskers, G33S, and SXY-2 were all mixed with cement powders, and then, cement mixtures were poured into water to agitate uniformly with stirring machines, and mixing speed was 4000 r/min. Weight ratio of cement powders and whiskers to water was fixed to 0.44 [29, 30]. Then, mixing speed increased to 12000 r/min ± 250 r/min for another 35 s ± 1 s.Table 5
Formulas of cement slurry.
SamplesCement powders/gMg2B2O5 whiskers/gG33S/gSXY-2/gWater/gSample 0 (controlled Sample)80001616352Sample 179281616352Sample 2784161616352Sample 3776241616352Sample 4768321616352Note: Mg2B2O5 whiskers are contents by weight of cement powders and whiskers used, respectively, 0%, 1.0%, 2.0%, 3.0%, and 4.0%; controlled sample: 800 g cement powders, 16 g G33S, 16 g SXY-2, and 352 g water (Sample 0).


## 2.2.2. Preparation of Samples and Operation Procedures for Mechanical Strength Tests

Prepared cement mud was prepared using a standard mold and all the mechanical tests were performed by electronic hydraulics. First is compression resistance test. Cured rectangular samples were fixed at 50.8 mm × 50.8 × 50.8 mm, and loading rate was 300 N/s. Second is tensile-strength test. Cured cylindrical samples 50.0 (diameter) × 25.0 (thickness) were used, and loading rate was 300 N/s. Third is flexural-strength test. Cured rectangular samples were as 40.0 mm × 40.0 mm × 160.0 mm. The test procedure was a 3-point bending method, with the use of motorized bending machines, in which the speed of the span and the cross member was set at 100 mm and 0.02 mm/min, respectively. Curing periods of testing specification as 1 in. (diameter) × 2 in. (height), were used for multicycle loading test and static stress-strain test, and operation procedure was as the Chinese standard GB/T 50266-99 recommended. For the multicycle load test, loading rate was at 1.6 KN/min and unloading rate was at 3.2 KN/min, test pressure was 20.7 MPa, and test temperature was 90°C. For static stress-strain test, the loading rate was at 300 N/s. Curing condition was atmospheric pressure/90°C (±2°C) water curing bath and kept for 28 days.


## 2.2.3. Preparation of Samples for SEM Analysis and Mercury Intrusion Porosimetry (MIP) Test

Samples (3% Mg2B2O5 whiskers added and cured in condition of water bath, 90°C/28 days) used for interface microstructure SEM analysis were collected or prepared when mechanical test was conducted [37].Pore structure was tested according to mercury intrusion porosimetry (MIP) method, and cured samples (with 3% Mg2B2O5 whiskers) were used for MIP test used mold of 25.0 mm (diameter) × 40.0 mm (height) and then kept in water bath curing at 90°C, deviation as ± 2°C, and cured for 28 days (atmospheric pressure). The test was with AutoPore IV 9500 automatic mercury porosimeter for analysis of pore structure that diameter ranges from 3 nm to 1000 μm and carried out as mercury porosimetry test international standard, ISO 15901-1-2016 recommended, according to Washburn equation. The maximum outside pressure limit was 228 MPa and injected fixed mercury volume into pore structure. As pressure changed, volume of injected mercury changed, and this reflected pore size distribution and porosity. Also, for permeability parameters of cement-based material before and after adding magnesium borate whisker (MBWRC), the MIP method was also utilized according to Daray Rule and Poiseuille Rule and operated as standard ISO 15901-1-2016 recommended, with maximum test pressure.


## 3. Results and Discussion

### 3.1. Effect of Magnesium Borate Whiskers to Cement-Based Material

#### 3.1.1. Mechanical Performance of Magnesium Borate Whisker Reinforced Cement-Based Material (MBWRC)

As temperature changed, strength parameter of cement-based material showed differences. In experiment, compressive strength was utilized for confirming effect of temperature on cement-based material (controlled sample/sample 0, Table5). As shown in Figure 1, cement-based material showed stable 7-days compressive strength when temperature rose from 50°C to 90°C, respectively, 19.4 MPa, 18 MPa, and 17.5 MPa. However, as curing period extended to 28 days, cement-based material showed compressive strength decline, value decreased to 15.9 MPa when temperature was just 50°C. As temperature ascended to 70°C∼90°C, strength decline phenomenon disappeared, and compressive strength was kept at 21.5 MPa (70°C) or 20.5 MPa (90°C), respectively. So, cement-based material showed much weaker strength regression phenomenon when temperature improved to 70°C∼90°C, and these were coordinated with former research results [38]. However, experiments’ results also depicted that cement-based material showed weakening strength performance when temperature rose to 90°C [38, 39]. Thus, in this research, 90°C was chosen as temperature point for class G oil well cement, which used whiskers for mechanical reinforcement when working temperature reached to 90°C.Figure 1
Effect of temperature (50°C∼90°C) on compressive performance (controlled sample).The compressive strength, tensile strength, and flexural strength of the cement slurry system under different additional amounts of magnesium borate whiskers are shown in Figures2–4, respectively. Figure 2 shows that there is no definite correlation between the compressive strength of the cement slurry system and the amount of magnesium borate whiskers. When the amount of magnesium borate whiskers is 3%, the compressive strength under different hydration time is slightly enhanced. When the amount of whisker added is lower than or higher than 3%, the compressive strength of cement stone decreases. This is because the whisker content is too small, which is easy to pull out, which is conducive to the expansion of cracks, resulting in a decrease in compressive strength. If the content is too high, it is easy to agglomerate and reduce the compressive strength. Figure 3 shows that, under the same hydration time, the addition of magnesium borate whiskers helps to improve the tensile strength of cement stone and eliminates the phenomenon of strength decline at 28 d, and the enhancement effect is the most obvious when the addition amount is 3%. It shows that the magnesium borate whiskers increase the toughness of the cement stone. Figure 4 shows that compared with the control group, the flexural strength of the experimental group is improved, and it increases with the increase of the amount of magnesium borate whiskers (1%–3%); when the amount of whiskers is 4%, the flexural strength increases but decreases. It shows that magnesium borate whiskers help to enhance the ability of cement stone to resist bending damage. To sum up, magnesium borate whiskers help to improve the mechanical properties of cement stone and slow down and eliminate the phenomenon of strong decline of cement stone. This may be because the whiskers act as “bridge” in the cement system to help disperse the stress on the cement stone [5, 22, 40, 41]. However, when the amount of whiskers is too big, it is easy to cause defects due to agglomeration. Depending on the experimental results, the optimum amount is 3%.Figure 2
90°C compressive performance of MBWRC.Figure 3
90°C tensile performance of MBWRC.Figure 4
90°C flexural performance of MBWRC.


#### 3.1.2. Effect of Multicycle Loading to MBWRC

Dynamic multicycle loading test results are shown in Figures5 and 6, Tables 6 and 7. By comparison of Tables 6 and 7, with seven loading-unloading cycles, MBWRC showed bigger Poisson ratio than that of controlled sample; it shows that the lateral deformation of the cement stone is greater than the longitudinal deformation before plastic deformation occurs after being stressed, indicating that the toughness of the cement stone is enhanced. At the same time, the elastic modulus value of MBWRC decreases, indicating that the rigidity of the cement stone becomes weaker. The comprehensive results show that MBWRC helps to enhance the elastic properties of cement stone and has good deformation resistance when subjected to external multidirectional and different stresses.Figure 5
Multicycle loading curve of the controlled sample.Figure 6
Multicycle loading curve of MBWRC (3% whiskers added).Table 6
Multicycle loading test results of the controlled sample.
TestCycle no.Loading stepUnloading stepPoisson ratioElasticity modulus (MPa)Poisson ratioElasticity modulus (MPa)Controlled sample10.345601.60.484223020.5462283.30.493173330.5471783.20.547148740.581512.40.571129850.6121333.70.6091177.460.6461197.20.6121081.770.6551098.10.6414959Table 7
Multicycle loading test results of MBWRC (3% whiskers added).
TestCycle no.Loading stepUnloading stepPoisson ratioElasticity modulus (MPa)Poisson ratioElasticity modulus (MPa)MBWRC (3% whiskers added)10.34830300.3441076.220.441063.50.465874.930.524875.60.544786.440.590772.60.597693.750.643714.80.653671.960.686662.70.690629.770.732644.60.745608.9


### 3.2. Effect of Magnesium Borate Whiskers to Pore Structure of Cement-Based Material

#### 3.2.1. Pore Size, Pore Size Distribution, and Porosity of MBWRC

According to cumulative pore volumes method for testing porosity, pore size/pore size distribution and porosity were implied for microstructure analysis of cement-based material (0%∼3.0% magnesium borate whiskers (MBWs) added, Table5, the density is 1.90 g/cm3), as shown in Table 8 (the measured data are accessible pores). With the increase in mustache content, the total volume of pores was cut reduced and greatest reduction was displayed when 3.0% whiskers added. As a result, adding of whiskers could contribute to denser cement structure and smaller pore structure [42].Table 8
Pore size/pore size distribution and porosity of MBWRC (cumulative pore volumes).
Magnesium borate whiskers (%)Pore size distribution parameters (nm)/%Porosity (%)≦2020∼5050∼200≧200013.3044.0025.2817.4217.011.013.2231.5342.5212.7316.602.013.6341.0834.2511.0416.463.014.4036.9839.129.5016.20For pore size distribution analysis of MBWRC, the following pore ranges can be considered for evaluation of cement-based material [43–46]: smallest pores (≤20 nm), thought as no damage to structure of cement-based material; smaller pores (20∼50 nm), thought as nearly no damage; medium pores (50∼200 nm), thought with potential direct damage; and large pores (≥200 nm), thought of direct damage to cement-based material. In experiment, as shown in Table 8, with whiskers added, percentage of smallest pores has been increased from 13.30% to 14.40%; and percentage of large pores sharply dropped from 17.42% to 9.50%, and most have become medium pores and smaller pores. As a whole, adding of whiskers has optimized pore structure of cement-based material, indicating the filling effect owing to formation of more hydration products [43–46], most was calcium–silicate–hydrate (C–S–H) phase [47–49]. Following this, the pore structure of the cement-based material was improved. Because of the formation of more hydration products and the effect of mustache resistance on pore structure when 3.0% BMB was added [48, 49], cement-based material showed smaller porosity value of 16.20% when compared to 17.01% of controlled sample, and thus a denser pore structure was formed.


#### 3.2.2. Permeability of MBWRC

As shown in Figure7, as mercury penetrated into pore structure of cement-based material in MIP experiment, permeability can be calculated according to Daray Rule and Poiseuille Rule. Properties linked to permeability of cementitious materials, such as pore surface and structure, tested permeability value affected. When 3.0% whiskers were added, permeability of cement-based material decreased from 0.2533 m.D to 0.0205 m.D (m.D: milli-Darcy), and contributed to the hydration products of the cement matrix itself (most probably the formation of the C–S–H gel array) and the resistance of the whiskers to the pore structure [40, 41, 48, 49]. As permeability decreased, denser microstructure was formed. As a result, MBWRC can show superior multiscale microstructure and get better mechanical performance when reinforced with 3.0% whiskers.Figure 7
Permeability of MBWRC with different percentages of whiskers added.


### 3.3. Reinforcement Mechanism of Magnesium Borate Whiskers to Cement-Based Material

#### 3.3.1. Reinforcement Behaviors of Magnesium Borate Whiskers (SEM Analysis)

As shown in Figure8, Magnesium Borate Whiskers showed perfect adhesion properties with hydration products of cement matrix. So, bridging effect was demonstrated and cement matrix was bridged to inhibit the formation of microcracks. Also, microcracks’ deflection effect was created to avoid hefty amount of microcracks by adding whiskers. Also, pulling-out phenomenon of whiskers showed no side effect on the formation of microcracks. As the description of Figures 8(b) and 8(c), there was also no incompatibility between the cement matrix and whisker so that better adhesion interface between them was formed. In accordance with Figure 7 and Table 8, less amount of larger micropores can be observed on the inner interface of MBWRC. Figure 9(a) is the microstructure of the sample with added MBWRC, and Figure 9(b) is the microstructure of the control sample. In contrast, there are no obvious microcracks or micropores on the interface of the MBWRC sample, indicating that MBWRC contributes to improving the cement microstructure of stone. In summary, MBWRC contributes to improving the mechanical properties of cement-based materials, thus improving the microstructure of cement-based materials and reducing the generation of cracks.Figure 8
Reinforcement behaviors of magnesium borate whiskers. (a) MBWRC (scale at 10μm); (b) MBWRC (scale at 5 μm); (c) controlled sample (scale at 5 μm).
(a)(b)(c)Figure 9
Interface of cement-based materials. (a) MBWRC (scale at 50μm); (b) controlled sample (scale at 50 μm).
(a)(b)


#### 3.3.2. Reinforcement Ability of Magnesium Borate Whiskers to Cement-Based Material

As shown in Figure10 and Table 9, MBWRC showed bigger Poisson ratio and smaller elasticity modulus under certain stress loading, which result was similar to behaviors of first cycle in multicycle loading test. So, MBWRC showed better resistance to mechanical deformation and excellent elasticity.Figure 10
Static stress-strain behaviors of MBWRC (compared with the controlled sample).Table 9
Stress-strain test results of MBWRC (3% whiskers) and the controlled sample.
Test no.Elasticity modulus (MPa)Poisson ratioStress value (MPa)Area integralControlled sample10619.6 ± 0.20.132 ± 0.0624.1 ± 0.114 ± 0.02MBWRC7850.8 ± 0.10.220 ± 0.0529.6 ± 0.129 ± 0.01Specifically, MBWRC got bigger area integral than that of controlled sample (Table9). So, MBWRC showed better toughness behaviors after whiskers added [41, 50]. Due to mechanical function of MBWs, i.e., transition effect, deflection effect, and shrinking effect on cement-based materials [51–53], MBWRC has shown a better response to mechanical damage for increased mechanical elasticity.


## 3.1. Effect of Magnesium Borate Whiskers to Cement-Based Material

### 3.1.1. Mechanical Performance of Magnesium Borate Whisker Reinforced Cement-Based Material (MBWRC)

As temperature changed, strength parameter of cement-based material showed differences. In experiment, compressive strength was utilized for confirming effect of temperature on cement-based material (controlled sample/sample 0, Table5). As shown in Figure 1, cement-based material showed stable 7-days compressive strength when temperature rose from 50°C to 90°C, respectively, 19.4 MPa, 18 MPa, and 17.5 MPa. However, as curing period extended to 28 days, cement-based material showed compressive strength decline, value decreased to 15.9 MPa when temperature was just 50°C. As temperature ascended to 70°C∼90°C, strength decline phenomenon disappeared, and compressive strength was kept at 21.5 MPa (70°C) or 20.5 MPa (90°C), respectively. So, cement-based material showed much weaker strength regression phenomenon when temperature improved to 70°C∼90°C, and these were coordinated with former research results [38]. However, experiments’ results also depicted that cement-based material showed weakening strength performance when temperature rose to 90°C [38, 39]. Thus, in this research, 90°C was chosen as temperature point for class G oil well cement, which used whiskers for mechanical reinforcement when working temperature reached to 90°C.Figure 1
Effect of temperature (50°C∼90°C) on compressive performance (controlled sample).The compressive strength, tensile strength, and flexural strength of the cement slurry system under different additional amounts of magnesium borate whiskers are shown in Figures2–4, respectively. Figure 2 shows that there is no definite correlation between the compressive strength of the cement slurry system and the amount of magnesium borate whiskers. When the amount of magnesium borate whiskers is 3%, the compressive strength under different hydration time is slightly enhanced. When the amount of whisker added is lower than or higher than 3%, the compressive strength of cement stone decreases. This is because the whisker content is too small, which is easy to pull out, which is conducive to the expansion of cracks, resulting in a decrease in compressive strength. If the content is too high, it is easy to agglomerate and reduce the compressive strength. Figure 3 shows that, under the same hydration time, the addition of magnesium borate whiskers helps to improve the tensile strength of cement stone and eliminates the phenomenon of strength decline at 28 d, and the enhancement effect is the most obvious when the addition amount is 3%. It shows that the magnesium borate whiskers increase the toughness of the cement stone. Figure 4 shows that compared with the control group, the flexural strength of the experimental group is improved, and it increases with the increase of the amount of magnesium borate whiskers (1%–3%); when the amount of whiskers is 4%, the flexural strength increases but decreases. It shows that magnesium borate whiskers help to enhance the ability of cement stone to resist bending damage. To sum up, magnesium borate whiskers help to improve the mechanical properties of cement stone and slow down and eliminate the phenomenon of strong decline of cement stone. This may be because the whiskers act as “bridge” in the cement system to help disperse the stress on the cement stone [5, 22, 40, 41]. However, when the amount of whiskers is too big, it is easy to cause defects due to agglomeration. Depending on the experimental results, the optimum amount is 3%.Figure 2
90°C compressive performance of MBWRC.Figure 3
90°C tensile performance of MBWRC.Figure 4
90°C flexural performance of MBWRC.


### 3.1.2. Effect of Multicycle Loading to MBWRC

Dynamic multicycle loading test results are shown in Figures5 and 6, Tables 6 and 7. By comparison of Tables 6 and 7, with seven loading-unloading cycles, MBWRC showed bigger Poisson ratio than that of controlled sample; it shows that the lateral deformation of the cement stone is greater than the longitudinal deformation before plastic deformation occurs after being stressed, indicating that the toughness of the cement stone is enhanced. At the same time, the elastic modulus value of MBWRC decreases, indicating that the rigidity of the cement stone becomes weaker. The comprehensive results show that MBWRC helps to enhance the elastic properties of cement stone and has good deformation resistance when subjected to external multidirectional and different stresses.Figure 5
Multicycle loading curve of the controlled sample.Figure 6
Multicycle loading curve of MBWRC (3% whiskers added).Table 6
Multicycle loading test results of the controlled sample.
TestCycle no.Loading stepUnloading stepPoisson ratioElasticity modulus (MPa)Poisson ratioElasticity modulus (MPa)Controlled sample10.345601.60.484223020.5462283.30.493173330.5471783.20.547148740.581512.40.571129850.6121333.70.6091177.460.6461197.20.6121081.770.6551098.10.6414959Table 7
Multicycle loading test results of MBWRC (3% whiskers added).
TestCycle no.Loading stepUnloading stepPoisson ratioElasticity modulus (MPa)Poisson ratioElasticity modulus (MPa)MBWRC (3% whiskers added)10.34830300.3441076.220.441063.50.465874.930.524875.60.544786.440.590772.60.597693.750.643714.80.653671.960.686662.70.690629.770.732644.60.745608.9


## 3.1.1. Mechanical Performance of Magnesium Borate Whisker Reinforced Cement-Based Material (MBWRC)

As temperature changed, strength parameter of cement-based material showed differences. In experiment, compressive strength was utilized for confirming effect of temperature on cement-based material (controlled sample/sample 0, Table5). As shown in Figure 1, cement-based material showed stable 7-days compressive strength when temperature rose from 50°C to 90°C, respectively, 19.4 MPa, 18 MPa, and 17.5 MPa. However, as curing period extended to 28 days, cement-based material showed compressive strength decline, value decreased to 15.9 MPa when temperature was just 50°C. As temperature ascended to 70°C∼90°C, strength decline phenomenon disappeared, and compressive strength was kept at 21.5 MPa (70°C) or 20.5 MPa (90°C), respectively. So, cement-based material showed much weaker strength regression phenomenon when temperature improved to 70°C∼90°C, and these were coordinated with former research results [38]. However, experiments’ results also depicted that cement-based material showed weakening strength performance when temperature rose to 90°C [38, 39]. Thus, in this research, 90°C was chosen as temperature point for class G oil well cement, which used whiskers for mechanical reinforcement when working temperature reached to 90°C.Figure 1
Effect of temperature (50°C∼90°C) on compressive performance (controlled sample).The compressive strength, tensile strength, and flexural strength of the cement slurry system under different additional amounts of magnesium borate whiskers are shown in Figures2–4, respectively. Figure 2 shows that there is no definite correlation between the compressive strength of the cement slurry system and the amount of magnesium borate whiskers. When the amount of magnesium borate whiskers is 3%, the compressive strength under different hydration time is slightly enhanced. When the amount of whisker added is lower than or higher than 3%, the compressive strength of cement stone decreases. This is because the whisker content is too small, which is easy to pull out, which is conducive to the expansion of cracks, resulting in a decrease in compressive strength. If the content is too high, it is easy to agglomerate and reduce the compressive strength. Figure 3 shows that, under the same hydration time, the addition of magnesium borate whiskers helps to improve the tensile strength of cement stone and eliminates the phenomenon of strength decline at 28 d, and the enhancement effect is the most obvious when the addition amount is 3%. It shows that the magnesium borate whiskers increase the toughness of the cement stone. Figure 4 shows that compared with the control group, the flexural strength of the experimental group is improved, and it increases with the increase of the amount of magnesium borate whiskers (1%–3%); when the amount of whiskers is 4%, the flexural strength increases but decreases. It shows that magnesium borate whiskers help to enhance the ability of cement stone to resist bending damage. To sum up, magnesium borate whiskers help to improve the mechanical properties of cement stone and slow down and eliminate the phenomenon of strong decline of cement stone. This may be because the whiskers act as “bridge” in the cement system to help disperse the stress on the cement stone [5, 22, 40, 41]. However, when the amount of whiskers is too big, it is easy to cause defects due to agglomeration. Depending on the experimental results, the optimum amount is 3%.Figure 2
90°C compressive performance of MBWRC.Figure 3
90°C tensile performance of MBWRC.Figure 4
90°C flexural performance of MBWRC.


## 3.1.2. Effect of Multicycle Loading to MBWRC

Dynamic multicycle loading test results are shown in Figures5 and 6, Tables 6 and 7. By comparison of Tables 6 and 7, with seven loading-unloading cycles, MBWRC showed bigger Poisson ratio than that of controlled sample; it shows that the lateral deformation of the cement stone is greater than the longitudinal deformation before plastic deformation occurs after being stressed, indicating that the toughness of the cement stone is enhanced. At the same time, the elastic modulus value of MBWRC decreases, indicating that the rigidity of the cement stone becomes weaker. The comprehensive results show that MBWRC helps to enhance the elastic properties of cement stone and has good deformation resistance when subjected to external multidirectional and different stresses.Figure 5
Multicycle loading curve of the controlled sample.Figure 6
Multicycle loading curve of MBWRC (3% whiskers added).Table 6
Multicycle loading test results of the controlled sample.
TestCycle no.Loading stepUnloading stepPoisson ratioElasticity modulus (MPa)Poisson ratioElasticity modulus (MPa)Controlled sample10.345601.60.484223020.5462283.30.493173330.5471783.20.547148740.581512.40.571129850.6121333.70.6091177.460.6461197.20.6121081.770.6551098.10.6414959Table 7
Multicycle loading test results of MBWRC (3% whiskers added).
TestCycle no.Loading stepUnloading stepPoisson ratioElasticity modulus (MPa)Poisson ratioElasticity modulus (MPa)MBWRC (3% whiskers added)10.34830300.3441076.220.441063.50.465874.930.524875.60.544786.440.590772.60.597693.750.643714.80.653671.960.686662.70.690629.770.732644.60.745608.9


## 3.2. Effect of Magnesium Borate Whiskers to Pore Structure of Cement-Based Material

### 3.2.1. Pore Size, Pore Size Distribution, and Porosity of MBWRC

According to cumulative pore volumes method for testing porosity, pore size/pore size distribution and porosity were implied for microstructure analysis of cement-based material (0%∼3.0% magnesium borate whiskers (MBWs) added, Table5, the density is 1.90 g/cm3), as shown in Table 8 (the measured data are accessible pores). With the increase in mustache content, the total volume of pores was cut reduced and greatest reduction was displayed when 3.0% whiskers added. As a result, adding of whiskers could contribute to denser cement structure and smaller pore structure [42].Table 8
Pore size/pore size distribution and porosity of MBWRC (cumulative pore volumes).
Magnesium borate whiskers (%)Pore size distribution parameters (nm)/%Porosity (%)≦2020∼5050∼200≧200013.3044.0025.2817.4217.011.013.2231.5342.5212.7316.602.013.6341.0834.2511.0416.463.014.4036.9839.129.5016.20For pore size distribution analysis of MBWRC, the following pore ranges can be considered for evaluation of cement-based material [43–46]: smallest pores (≤20 nm), thought as no damage to structure of cement-based material; smaller pores (20∼50 nm), thought as nearly no damage; medium pores (50∼200 nm), thought with potential direct damage; and large pores (≥200 nm), thought of direct damage to cement-based material. In experiment, as shown in Table 8, with whiskers added, percentage of smallest pores has been increased from 13.30% to 14.40%; and percentage of large pores sharply dropped from 17.42% to 9.50%, and most have become medium pores and smaller pores. As a whole, adding of whiskers has optimized pore structure of cement-based material, indicating the filling effect owing to formation of more hydration products [43–46], most was calcium–silicate–hydrate (C–S–H) phase [47–49]. Following this, the pore structure of the cement-based material was improved. Because of the formation of more hydration products and the effect of mustache resistance on pore structure when 3.0% BMB was added [48, 49], cement-based material showed smaller porosity value of 16.20% when compared to 17.01% of controlled sample, and thus a denser pore structure was formed.


### 3.2.2. Permeability of MBWRC

As shown in Figure7, as mercury penetrated into pore structure of cement-based material in MIP experiment, permeability can be calculated according to Daray Rule and Poiseuille Rule. Properties linked to permeability of cementitious materials, such as pore surface and structure, tested permeability value affected. When 3.0% whiskers were added, permeability of cement-based material decreased from 0.2533 m.D to 0.0205 m.D (m.D: milli-Darcy), and contributed to the hydration products of the cement matrix itself (most probably the formation of the C–S–H gel array) and the resistance of the whiskers to the pore structure [40, 41, 48, 49]. As permeability decreased, denser microstructure was formed. As a result, MBWRC can show superior multiscale microstructure and get better mechanical performance when reinforced with 3.0% whiskers.Figure 7
Permeability of MBWRC with different percentages of whiskers added.


## 3.2.1. Pore Size, Pore Size Distribution, and Porosity of MBWRC

According to cumulative pore volumes method for testing porosity, pore size/pore size distribution and porosity were implied for microstructure analysis of cement-based material (0%∼3.0% magnesium borate whiskers (MBWs) added, Table5, the density is 1.90 g/cm3), as shown in Table 8 (the measured data are accessible pores). With the increase in mustache content, the total volume of pores was cut reduced and greatest reduction was displayed when 3.0% whiskers added. As a result, adding of whiskers could contribute to denser cement structure and smaller pore structure [42].Table 8
Pore size/pore size distribution and porosity of MBWRC (cumulative pore volumes).
Magnesium borate whiskers (%)Pore size distribution parameters (nm)/%Porosity (%)≦2020∼5050∼200≧200013.3044.0025.2817.4217.011.013.2231.5342.5212.7316.602.013.6341.0834.2511.0416.463.014.4036.9839.129.5016.20For pore size distribution analysis of MBWRC, the following pore ranges can be considered for evaluation of cement-based material [43–46]: smallest pores (≤20 nm), thought as no damage to structure of cement-based material; smaller pores (20∼50 nm), thought as nearly no damage; medium pores (50∼200 nm), thought with potential direct damage; and large pores (≥200 nm), thought of direct damage to cement-based material. In experiment, as shown in Table 8, with whiskers added, percentage of smallest pores has been increased from 13.30% to 14.40%; and percentage of large pores sharply dropped from 17.42% to 9.50%, and most have become medium pores and smaller pores. As a whole, adding of whiskers has optimized pore structure of cement-based material, indicating the filling effect owing to formation of more hydration products [43–46], most was calcium–silicate–hydrate (C–S–H) phase [47–49]. Following this, the pore structure of the cement-based material was improved. Because of the formation of more hydration products and the effect of mustache resistance on pore structure when 3.0% BMB was added [48, 49], cement-based material showed smaller porosity value of 16.20% when compared to 17.01% of controlled sample, and thus a denser pore structure was formed.


## 3.2.2. Permeability of MBWRC

As shown in Figure7, as mercury penetrated into pore structure of cement-based material in MIP experiment, permeability can be calculated according to Daray Rule and Poiseuille Rule. Properties linked to permeability of cementitious materials, such as pore surface and structure, tested permeability value affected. When 3.0% whiskers were added, permeability of cement-based material decreased from 0.2533 m.D to 0.0205 m.D (m.D: milli-Darcy), and contributed to the hydration products of the cement matrix itself (most probably the formation of the C–S–H gel array) and the resistance of the whiskers to the pore structure [40, 41, 48, 49]. As permeability decreased, denser microstructure was formed. As a result, MBWRC can show superior multiscale microstructure and get better mechanical performance when reinforced with 3.0% whiskers.Figure 7
Permeability of MBWRC with different percentages of whiskers added.


## 3.3. Reinforcement Mechanism of Magnesium Borate Whiskers to Cement-Based Material

### 3.3.1. Reinforcement Behaviors of Magnesium Borate Whiskers (SEM Analysis)

As shown in Figure8, Magnesium Borate Whiskers showed perfect adhesion properties with hydration products of cement matrix. So, bridging effect was demonstrated and cement matrix was bridged to inhibit the formation of microcracks. Also, microcracks’ deflection effect was created to avoid hefty amount of microcracks by adding whiskers. Also, pulling-out phenomenon of whiskers showed no side effect on the formation of microcracks. As the description of Figures 8(b) and 8(c), there was also no incompatibility between the cement matrix and whisker so that better adhesion interface between them was formed. In accordance with Figure 7 and Table 8, less amount of larger micropores can be observed on the inner interface of MBWRC. Figure 9(a) is the microstructure of the sample with added MBWRC, and Figure 9(b) is the microstructure of the control sample. In contrast, there are no obvious microcracks or micropores on the interface of the MBWRC sample, indicating that MBWRC contributes to improving the cement microstructure of stone. In summary, MBWRC contributes to improving the mechanical properties of cement-based materials, thus improving the microstructure of cement-based materials and reducing the generation of cracks.Figure 8
Reinforcement behaviors of magnesium borate whiskers. (a) MBWRC (scale at 10μm); (b) MBWRC (scale at 5 μm); (c) controlled sample (scale at 5 μm).
(a)(b)(c)Figure 9
Interface of cement-based materials. (a) MBWRC (scale at 50μm); (b) controlled sample (scale at 50 μm).
(a)(b)


### 3.3.2. Reinforcement Ability of Magnesium Borate Whiskers to Cement-Based Material

As shown in Figure10 and Table 9, MBWRC showed bigger Poisson ratio and smaller elasticity modulus under certain stress loading, which result was similar to behaviors of first cycle in multicycle loading test. So, MBWRC showed better resistance to mechanical deformation and excellent elasticity.Figure 10
Static stress-strain behaviors of MBWRC (compared with the controlled sample).Table 9
Stress-strain test results of MBWRC (3% whiskers) and the controlled sample.
Test no.Elasticity modulus (MPa)Poisson ratioStress value (MPa)Area integralControlled sample10619.6 ± 0.20.132 ± 0.0624.1 ± 0.114 ± 0.02MBWRC7850.8 ± 0.10.220 ± 0.0529.6 ± 0.129 ± 0.01Specifically, MBWRC got bigger area integral than that of controlled sample (Table9). So, MBWRC showed better toughness behaviors after whiskers added [41, 50]. Due to mechanical function of MBWs, i.e., transition effect, deflection effect, and shrinking effect on cement-based materials [51–53], MBWRC has shown a better response to mechanical damage for increased mechanical elasticity.


## 3.3.1. Reinforcement Behaviors of Magnesium Borate Whiskers (SEM Analysis)

As shown in Figure8, Magnesium Borate Whiskers showed perfect adhesion properties with hydration products of cement matrix. So, bridging effect was demonstrated and cement matrix was bridged to inhibit the formation of microcracks. Also, microcracks’ deflection effect was created to avoid hefty amount of microcracks by adding whiskers. Also, pulling-out phenomenon of whiskers showed no side effect on the formation of microcracks. As the description of Figures 8(b) and 8(c), there was also no incompatibility between the cement matrix and whisker so that better adhesion interface between them was formed. In accordance with Figure 7 and Table 8, less amount of larger micropores can be observed on the inner interface of MBWRC. Figure 9(a) is the microstructure of the sample with added MBWRC, and Figure 9(b) is the microstructure of the control sample. In contrast, there are no obvious microcracks or micropores on the interface of the MBWRC sample, indicating that MBWRC contributes to improving the cement microstructure of stone. In summary, MBWRC contributes to improving the mechanical properties of cement-based materials, thus improving the microstructure of cement-based materials and reducing the generation of cracks.Figure 8
Reinforcement behaviors of magnesium borate whiskers. (a) MBWRC (scale at 10μm); (b) MBWRC (scale at 5 μm); (c) controlled sample (scale at 5 μm).
(a)(b)(c)Figure 9
Interface of cement-based materials. (a) MBWRC (scale at 50μm); (b) controlled sample (scale at 50 μm).
(a)(b)


## 3.3.2. Reinforcement Ability of Magnesium Borate Whiskers to Cement-Based Material

As shown in Figure10 and Table 9, MBWRC showed bigger Poisson ratio and smaller elasticity modulus under certain stress loading, which result was similar to behaviors of first cycle in multicycle loading test. So, MBWRC showed better resistance to mechanical deformation and excellent elasticity.Figure 10
Static stress-strain behaviors of MBWRC (compared with the controlled sample).Table 9
Stress-strain test results of MBWRC (3% whiskers) and the controlled sample.
Test no.Elasticity modulus (MPa)Poisson ratioStress value (MPa)Area integralControlled sample10619.6 ± 0.20.132 ± 0.0624.1 ± 0.114 ± 0.02MBWRC7850.8 ± 0.10.220 ± 0.0529.6 ± 0.129 ± 0.01Specifically, MBWRC got bigger area integral than that of controlled sample (Table9). So, MBWRC showed better toughness behaviors after whiskers added [41, 50]. Due to mechanical function of MBWs, i.e., transition effect, deflection effect, and shrinking effect on cement-based materials [51–53], MBWRC has shown a better response to mechanical damage for increased mechanical elasticity.


## 4. Conclusions

(1)
There is a phenomenon of strong decline of pure cement stone at 90°C. In order to improve the mechanical properties of cement stone, Mg2B2O5 whiskers are used as reinforcing agent in this paper, and its application performance and mechanism of action are studied.(2)
Mechanical properties of MBWRC were investigated by compressive strength, tensile strength, flexural strength, and alternating load tests. The results show that when the MBWRC content is 3%, the compressive strength is stable; the tensile strength is raised to 235%; the flexural strength is increased to 130%; the elastic modulus is increased. The comprehensive results show that MBWRC can significantly improve the mechanical properties of cement stone.(3)
The effect of MBWRC on the pore structure of cement was explored through pore size test, and the results showed that MBWRC could help to optimize the pore distribution of the cement system, thereby improving the mechanical properties of cement stone. The action mechanism of MBWRC was investigated by micromorphology and stress-strain test, and the results showed that the enhancement mechanism of MBWRC was mainly attributed to the bridging effect, microcrack deflection effect, and pull-out effect.



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*Source: 8442648-2022-09-24.xml*