Effects of epoxy resin on the mechanical performance

Materials and Design 109 (2016) 133–145

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Materials and Design journal homepage: www.elsevier.com/locate/matdes

Effects of epoxy resin on the mechanical performance and thickening properties of geopolymer cured at low temperature Jiapei Du a, Yuhuan Bu a,⁎, Zhonghou Shen a, Xianhai Hou a, Chengxing Huang b a b

College of petroleum engineering, China University of Petroleum, 266580 Qingdao, China College of pipeline and civil engineering, China University of Petroleum, 266580 Qingdao, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• A novel geopolymer-resin hybrid composite material is introduced as oil well cement. • Resin enhances the early-term strength of hardened geopolymer-based paste significantly. • Diluent acts as a compatibilizer which made the ER dispersing in geopolymer more easily. • High degree of miscibility leads to close interaction between geopolymer and ER.

a r t i c l e

i n f o

Article history: Received 9 May 2016 Received in revised form 29 June 2016 Accepted 3 July 2016 Available online 14 July 2016 Keywords: Epoxy resin Geopolymer Compressive strength Thickening time Low temperature Dilution effect

a b s t r a c t The research objectives were to investigate the influence of waterborne bisphenol-A epoxy resin on mechanical performance and thickening properties of metakaolin-slag based geopolymer at 10 °C. Test results showed that the epoxy resin has ability to decrease the thickening time, while the benzyl glycidyl ether diluent can prolong thickening time. Hydration heat test was performed to explain the different consistency increasing trend of pure geopolymer and hybrid composite samples. Epoxy equivalent analysis was carried out to explain the mechanism of dilution effect on thickening time. The 1-day compressive strength of geopolymer was reinforced by doping epoxy resin, but the 3-day strength was lower than pure geopolymer when the doping amount of epoxy resin was below 50%. The morphology and quantitative element results revealed that the homogenous and high degree of miscibility led to dense and uncracked morphology. In order to enhance the durability of cement sheath under deep water strata stress wave condition, we should increase the dosage of ER to achieve low Young's modulus. The benzyl glycidyl ether diluent is recommended to make thickening time adjustable and to reinforce the compressive strength. This novel material makes the geopolymer-based composites applicable for oil wells cementation at low temperatures. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction ⁎ Corresponding author at: College of Petroleum Engineering for Oil Well Cement Research and Testing, Economic and Technological Development Zone, Changjiang West Road 66, 266580 Qingdao, China. E-mail addresses: [email protected] (J. Du), [email protected] (Y. Bu).

http://dx.doi.org/10.1016/j.matdes.2016.07.003 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

Geopolymers are typically used as an alternative to ordinary Portland cement in construction industry [1]. These materials have many excellent characteristics, including environmental sustainability, the ability to provide high compressive strength at room temperature,

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J. Du et al. / Materials and Design 109 (2016) 133–145

acid resistance, high temperature and fire resistance, low shrinkage, as well as low manufacturing energy consumption [2–4]. Geopolymers are usually obtained from inexpensive materials, such as metakaolin, clay, coal fly ash and metallurgical slag [5]. For these reasons, the application of geopolymer-based materials covers many fields like applied as fire resistant materials, as foam tiles with low energy consumption, as corrosion-resistant coatings on steel, etc. Nevertheless, in the field of oil well cementation, geopolymer-based materials are seldom used due to two main reasons: First, the high reactivity of geopolymer leads to nonadjustable thickening time at room temperature, let alone high temperatures [6–8]; Second, at low temperatures in deep water conditions, the early-term strength development of these materials are slow, so it is hard to satisfy the sealing demand of deep water wells [9]. These drawbacks lead to the thickening property and early-term strength of geopolymer-based materials at temperatures below 20 °C barely referred by other researches. The synthesis of geopolymers is performed according to mixing high reactive aluminosilicate under strong alkaline environment. At room temperature or elevated temperatures, the reactive aluminosilicate materials dissolved and formed a three-dimensional geopolymeric network structure rapidly [10]. But at temperatures below 20 °C, the cross linking reaction of silico-aluminate materials is tardy. So the bottleneck problem which confined the using of geopolymer in deep water oil wells could overcome by accelerating the setting rate and the strength development of geopolymers at low temperatures. Recently, the enhancement of geopolymers has been concentrated on the optimization of mechanical properties [11–13]. Several kinds of fillers, such as carbon fibers, glass fibers or organic resins, are blended in geopolymer to produce geopolymer-based composite [14,15]. Epoxy resin, whose curing reaction with curing agent under alkali-activated condition is not much sensitive to temperatures compared to geopolymers [16]. Meanwhile, geopolymer is mixed with a small amount of epoxy resin can improve geopolymeric mechanical performances. In this field, lots of efforts have been done on the incorporation of different organic resins [17,18]. Zhang at al. [19] synthesized a series of alkali-activated metakaolin/granulated blast furnace slag based geopolymer composites by mixing with different amount of resin. The mechanical properties were enhanced remarkably by doping resin. Roviello at al. [20] produced a novel material displaying high compatibility between the organic fire resistant melamine and geopolymer phases. The hybrid composite showed higher durability and good thermal stability. Ferone at al. [21]invented organic-inorganic materials by an approach based on co-reticulation of metakaolin based geopolymer and epoxy based resin. These materials present a good and homogeneous dispersion and significantly enhanced compressive strength. In this paper, an innovative method for the geopolymer-based composite utilizing in deep water oil well cementation is described. The new recommended approach is based on the mixing of a diluted epoxy resin to a geopolymer suspension, when both polymerization reactions are just started. By using this new approach, novel geopolymer-resin composites have been prepared and characterized by thickening time, hydration heat, compressive strength, Young's modulus, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis. These new materials showed interesting thickening properties and mechanical properties. For the sake of prolong thickening time, the diluent whose key component is benzyl glycidyl ether is used. After the addition of diluent, the thickening time and early-term strength of hybrid materials could satisfy the demand of deep water oil wells sealing. Moreover, these materials can also be useful as adhesive materials at low temperatures.

solutions. Metakaolin and GBFS were provided by Jiaozuo Yukun Mining Corporation, China. The chemical composition and physical properties of metakaolin and GBFS are presented in Table 1. Sodium hydroxide and sodium silicate were obtained from Sinopharm Chemical Reagent Co., Ltd. Waterborne bisphenol-A epoxy resin (ER) and waterborne polyamine epoxy curing agent (CA) were obtained from Shanghai Hanzhong Coating Corporation, China. Diluent was provided by Suzhou Composite Company, China. The key component of diluent is benzyl glycidyl ether (BGE). 2.2. Methods 2.2.1. Synthesis of geopolymer composites The starting materials for synthesis of metakaolin-slag based geopolymer composites were mixed in the mass proportion of metakaolin: GBFS: activator (sodium silicate which modulus is 1.0): water = 1:0.2:0.3:0.6. The alkaline activator is a powder that must dissolve in water. A mechanical blender was used to mix the solution homogeneously. Sodium silicate was added into the blender after which the blender was started on a rotation rate of 1000 rpm for 5 min. Then, the solution was prepared 12 h prior to use in order to make sure that the activator component was blended uniformly and allowed to cool to 10 °C. Finally, while the blender was running, the metakaolin and GBFS were gradually added to the activator solution and mixing for 5 min to achieve complete reactions between activator solution and powder materials. 2.2.2. Manufacturing process The mix proportion of ER and CA is 2:3. The dosage of diluent is 5%– 20%wt of ER. The diluent was dissolved in ER and then after stirred for 5 min with muddler, the CA was added into the mixture. Subsequently, an aqueous solution of geopolymer was put into the stirrer for 15 s with a stirring rate of 4000 rpm and mixed for 35 s at 12,000 rpm to be homogeneously mixed. Defoamer was used to remove air bubbles. Mix design parameters and their designation were provided in Table 2. The calculation of resin dosage should take into account both ER and CA. Therefore, the respective dosage of resin will be: 12.5% (S1 and D1), 25% (S3, D2 and D3) and 50% (S4). After being prepared, the slurry was cast into 5 cm cubes for compressive strength test and pressurized consistometer for consistency measurement. 2.2.3. Thickening time test The thickening time tests were aimed to determine the length of time which is related to pumpability time [22]. Considering the low temperature in deep water wells, the consistency was tested at 10 °C. The consistency of the geopolymer-resin composite slurry was measured in Bearden units (Bc), and thickening time test ended when the slurry reaches a consistency of 100 Bc. The slurry container which driven by the motor is equipped with a stationary paddle assembly. 2.2.4. Heat of hydration In order to study on effects of resin on the heat of hydration, the hydration heat test equipment was designed (Fig. 1). The water bath was used to maintain constant temperature and the vacuum cup was prepared to prevent heat loss. The plastic cup was filled with geopolymer slurry. Immediately after placing, copper pipe which used to prevent the temperature sensor from being polluted was placed through the Table 1 Chemical composition and physical properties of metakaolin and GBFS (wt.%).

2. Materials and methods

Component (wt.%)

2.1. Materials Geopolymer utilized in this research was synthesized by activating metakaolin and granulated blast furnace slag (GBFS) with alkali

Metakaolin GBFS

CaO

SiO2

Fe2O3

Al2O3

SO3

MgO

Na2O

K2O

Loss on ignition

0.17 36.57

55.06 28.3

0.76 0.83

42.12 13.16

0.15 1.65

0.06 7.58

0.06 0.49

0.55 0.5

1.2 9.65

J. Du et al. / Materials and Design 109 (2016) 133–145 Table 2 Mix design parameters and their designation. Samples

Geopolymer (wt%)

ER (wt%)

CA (wt%)

Diluent (wt%)

W/S

S1 S2 S3 S4 D1 D2 D3

100 87.5 75 50 87.5 75 75

0 5 10 20 5 10 10

0 7.5 15 30 7.5 15 15

0 0 0 0 0.5 1.5 2

0.6 0.6 0.6 0.6 0.6 0.6 0.6

center of plastic cup and into the geopolymer suspension. This temperature sensor recorded the temperature of the slurry via a data logger. The initial rise and following drop of temperature was recorded every 10 min for the first 6 h and then every 1 h until the temperature stabilizes.

2.2.5. Compressive strength Geopolymer-resin composite slurries were cured for different time periods at 10 °C in a low temperature kettle. The low temperature kettle was filled with distilled water and sealed to keep the CO2 in the air out of the kettle. Three samples of each kind of hardened paste were prepared to compressive strength test and the average value was recorded. This test was carried out on wetted specimens with a 0.5 MPa/s loading rate by WEW-300B forcing press machine which obtained from Liaocheng Building Material Equipment Factory.

2.2.6. Young's modulus The Young's modulus mentioned here is dynamic Young's modulus. For homogeneously elastic media, Eqs. (1) and (2) is tenable [23].

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 1 1 νs ¼ ¼ ρ 2ð1 þ σ Þ Δts

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ð2Þ

Eq. (3) can be deduced by Eqs. (1) and (2). E¼

2 2 ρ 3Δt s −4Δt p Δts Δt 2s −Δt 2p

ð3Þ

Where νp is the longitudinal wave velocity, νs is the shear wave velocity, E is the dynamic Young's modulus, σ is the Poisson ratio, ρ is the solid density, Δtp is the longitudinal wave travel time and Δts is the shear wave travel time. Ultrasonic transducer was utilized to measure Δtp and Δts, and then E was calculated by Eq. (3). 2.2.7. Epoxy titration The epoxy equivalent weight (EEW) of ER is a value which can reflect the activity of ER. It refers to the equivalent number of epoxy groups in per 100 g of ER [24]. The ER used in this research was titrated as per ASTM D1652 to measure their EEW value. The EEW of ER before and after diluted was measured to explain the dilution mechanism of thickening time. The dosage of diluent is the weight proportion of ER. 2.2.8. Microstructural characterization To determine the microstructure change of the geopolymer composite samples, SEM imaging was performed on selected samples in a scanning electron microscope ETD. The EDX spectra were obtained using an OXFORD INCA X-act equipment installed in the same SEM instrument. 3. Results and discussion 3.1. Influence of ER on thickening property

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E ð1–σ Þ 1 ¼ νp ¼ ρ ð1 þ σ Þð1–2σ Þ Δtp

ð1Þ

3.1.1. The undiluted ER The thickening time of geopolymer with undiluted ER was tested, and the results are shown in Fig. 2. When the consistencies were

Fig. 1. Hydration heat test equipment.

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Fig. 2. The thickening time of geopolymer with undiluted ER.

measured at 10 °C, undiluted resin of which respective percentages are 12.5% w/w (S2), 25% w/w (S3) and 50% w/w (S4) could obviously shorten the thickening time. The thickening time of pure geopolymer slurry

(S1) is 281 min which indicated the reactivity of geopolymer is low at 10 °C. Meanwhile, geopolymer slurry with 5% and 10% undiluted ER is respectively shortened by 123 min and 196 min. However, when the

Fig. 3. Hydration heat of samples S1–S4.


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Fig. 4. The thickening time of geopolymer with diluted ER.

dosage of undiluted resin is 50%, the thickening time is only 24 min. The setting time of pure ER-CA system which used in this research is 120– 140 min at 10 °C. Due to the high alkalinity of geopolymer slurry, the curing reaction of ER-CA has been accelerated [25]. Therefore, undiluted ER could act as an early strength agent in geopolymer. 3.1.2. Hydration heat Hydration heat was performed to explain the different consistencies increasing trend of pure geopolymer and hybrid composite samples. The results shown in Fig. 3 demonstrate that the time of heat release is in the order of S1 N S2 N S3 N S4. In other words, the temperature rises with the dosage of resin increasing. The numbers on the curves are the time of special heat release points. Heat releasing of pure geopolymer slurry sample (S1) was sluggish, which released much heat after 190 min. The heat releasing time reached earlier after the addition of resin, which decreased by 50 min, 170 min and 176 min of S2, S3 and S4, respectively. Meanwhile, transition time of hybrid composite slurries reached highest hydration heat reduced from 650 min to 250 min, 120 min and 100 min, respectively. The heat release time was in accordance with the time of consistencies started to increase. The reduction of thickening time due to the two reactions in geopolymer-resin composite system: (a) geopolymer polymerization; (b) ER-CA curing reaction. The condensation reaction of geopolymer occurred under alkaline circumstance, the [SiO(OH)3]− which derived from Si(OH)4 reacts with NaOH. And then, 4 mol [SiO(OH)3]− react

Fig. 5. The thickening time of geopolymer with diluted ER.

with 5 mol [Al(OH)4]− to yield to a hydration product which has a chain-like structure of [AlO4]5− and [SiO4]4 − tetrahedral interlinked by sharing oxygen atoms. During the cross-linking process of the epoxy resin, the formation of epoxy groups makes the resin phase compatible with the geopolymer aqueous phase due to epoxy opening ring reaction in the setting stage. The high alkalinity attributed to the

138


Fig. 8. The compressive strength of geopolymer with undiluted ER.

Fig. 6. Molecular formula of ER and diluent: (a) Bisphenol-A epoxy resin (b) Benzyl glycidyl ether.

3.1.3. The diluted ER From Figs. 4 and 5 it can be seen that the diluent can extend the thickening time at 10 °C, and with the dosage of diluent increasing, the thickening time become longer. When the dosage of diluent is 0.5%, the thickening time of S2 is increased by 30 min. The phenomenon in which the consistency showed the stair-like trend vanished. The thickening time of S3 with 1.5% diluent is extended by 24 min and with 2% diluent is increased by 34 min. Therefore, as an epoxy active diluent, BGE can slow down the reaction proceeding of ER.

acceleration of curing reaction, which made the temperature of geopolymer-resin system increase. Due to temperature rises, the reactivity of geopolymers enhanced [26]. Therefore, the polymerization of geopolymers became faster and led to the reduction of thickening time. In addition, note that there is a phenomenon in which the consistency and the hydration heat of sample S3 both displayed the stair-like increasing trend (Figs. 2 S3 and 3 S3), it may be caused by the geopolymer polymerization and ER-CA curing reaction were not conducted at the same time.

3.1.4. Epoxy equivalent analysis Epoxy equivalent analysis was carried out to explain the mechanism of dilution effect on thickening time. The increase in thickening time was due to two main reasons: Firstly, it is obvious that, the BisphenolA epoxy resin molecule (Fig. 6a) has two epoxy group which benzyl glycidyl ether has only one (Fig. 6b). As we can see in Fig. 7, with the dosage of BGE increasing, the epoxy equivalent becomes lower. As the dosage (by weight of ER) of BGE continuous increasing, the linear trend of epoxy equivalent presents continuous declination. Secondly, ER has a tendency to crystallization at low temperatures. However, the diluent will destroy the ordered structure of ER. As a result, the

Fig. 7. Results of epoxy equivalent test.

Fig. 9. The compressive strength of geopolymer with diluted ER.


Fig. 10. SEM images of sample S3 after curing for 3 days.

dispersion property of resin enhanced and the mass polymerization in early hydration age was avoided.

3.2. Influence of ER on mechanical performance 3.2.1. Compressive strength The compressive strength of geopolymer (S1) and geopolymer-resin hybrid composite specimens containing 12.5% w/w (S2), 25% w/w (S3), 50% w/w (S4) of resin and specimen S2 containing 0.5% w/w (D1), S3 containing 1.5% w/w (D2), 2% w/w (D3), of diluent are shown in Figs. 8–9. In this research, the samples used for compressive strength test have been prepared and cured in the same condition. In Fig. 8, we can observe that geopolymer displayed high 3-day compressive strength but low 1-day strength. Undiluted ER enhances the 1day compressive strength of hardened geopolymer paste. But the 3-day compressive strength of geopolymer containing undiluted ER decreased when the dosage of ER is lower than 50%. This can be explained by low

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dosage of ER can accelerate the geopolymer polymerization reaction (see Sections 3.1.1 and 3.1.2) and lead to high 1-day strength. But at 10 °C, due to the ER is difficult to disperse in geopolymer at low temperatures, low dosage of ER phases cannot form effective connection with inorganic phases. Besides, the organic and inorganic phases tend to perform their polymerization reaction solely instead of react together. After three days curing, the barriers of organic and inorganic phases became cracks, so the connection of these two phases is weak, as evidenced by SEM observation (Fig. 10). These factors lead to low 3-day compressive strength. In particular, for both series of curing time, the compressive strength tends to increase with the ER dosage. The results are accordance with the previous research which mixed geopolymer with organic resin [20]. When the dosage of ER in hybrid composites system increasing, the organic phases becoming adequate to bond the inorganic phases. Effective connections were formed between ER and geopolymer phases. This is the reason why the compressive strength tends to increase with the ER dosage. In the case of diluted ER specimens, for both series of curing time, the strength was higher than undiluted ER specimens. The diluent dosage of 0.5% by weight of ER showed an increasing of 1-day compressive strength 36% in respect to the undiluted specimen. This might be due to the dilution effect of diluent which made geopolymer and ER homogeneous mixed. The best compressive strength has been acquired for the diluent dosage of 1.5%. Finally, the D3 sample has still very good properties with a compressive strength of 7.6 MPa for one day and 20.4 MPa for three days. 3.2.2. Young's modulus Fig. 11 shows the dynamic Young's modulus of pure geopolymer and geopolymer-resin hybrid composite cured at 10 °C. The dosage of diluent is 5%wt of ER. As we can see in Fig. 11, the cured geopolymer-resin composite shows a drop of the Young's modulus with the increasing of ER dosage. The diluted specimens showed higher Young's modulus than undiluted ones. The presence of diluent increased the brittleness of geopolymer-resin composites, but the Young's modulus data is still much less than the data of pure geopolymer. Pure geopolymer shows high stiffness but it is brittle, thanks to the presence of ER, the hybrid system becomes significantly tenacious. Taking the propagations of cracks with increasing strain into consideration, the ER might play double effects: on the one hand, it could act a toughening effect according to a typical crack skewing mechanism; on the other hand, ER could counteract part of the load by plastic deformation [21]. These factors made the geopolymer-resin hybrid displays high strength and less brittle. 3.3. Microstructural characterization

Fig. 11. Young's modulus of geopolymer with resin after curing for 3 days.

We try to study the reason why compressive strength of geopolymer is reinforced or weaken by doping ER. Fig. 12 shows images of sample S1 after cured for 3 days. The pure geopolymer shows a homogeneous morphology with bulk crystals on the surface. EDX analysis of sample S1 can explain the elementary composition of bulk crystals (Fig. 13). It is used to offer a qualitative evidence for the formation of ER or geopolymer in hybrid composites, rather than quantitative indication. The spectra and the inserted tables show the average chemical composition of elements contained in the area which the arrow pointed out. The Si/Al atomic ratio of this crystal is approximately equal to 5:3 and the Na/Ca is roughly 1:2.5. According to other studies, the nubby crystals might be garronite which arises from two-step reaction. The first reaction is condensation reaction carries by the loss of water molecule between 3 mol aluminum hydroxide and 5 mol orthosilicic acids to produce chain-like hydration products. The second reaction is 1 mol Na+ ions and 2.5 mol Ca2 + ions to form garronite with alkaliactivated [19]. It is convinced that garronite is characterized by a smooth dehydration and subsequent re-crystallization into stable anhydrous alkaline alumino-silicates without destruction of the framework.

140


Fig. 12. SEM images of sample S1 after the curing for 3 days, (a) 500 magnification; (b) 5000 magnification.

This may be the reason why sample S1 displayed good three days strength. As for SEM images of hybrid sample S2 (Fig. 14), a significantly different structure can be observed, with the presence of obvious defined resin phases unevenly dispersed in the geopolymer matrix. Meanwhile, agglomerations phenomena were observed. The dispersed resin phases display a heterogeneous and unsmooth surface. Plenty of cracks are generated due to the strict adhesion between resin and geopolymer phases are poor (Fig. 14b). A different morphology which indicated that the dispersed resin particles show a homogeneous and smooth sphere surface in geopolymer phases has been reported for hybrid composites mixed with different ER [27,28]. This disparity is because the curing agent used in this research is waterborne, the organic aggregations are hydrophilic. In the geopolymer solution, the resin particles can't perform reversed-phase polymerization to form sphere but to form randomly arranged nubby aggregations. In addition, the ER has a tendency to crystallization at low temperatures which lead to the organic phases polymerization in early age. So the sphere resin particles were not observed. Meanwhile, ER at low concentration is not adequate on the binding of geopolymer phases. These reasons lead to the sample S2 shows lower compressive strength than sample S1. EDX analysis of sample S2 can help us to define the mixing degree of geopolymer and ER. Na and Si element existed on behalf of geopolymer while high C element content represented the ER. Fig. 15 shows the elemental composition of different microstructures in sample S2 matrix. Noticeably shown in spectrum 1, the Na and Si element was detected,

which represent the geopolymer particle in resin phases. For spectrum 2, the C element was only detected, which indicate that this area is pure resin. Additionally, although Na and Si element was identified in spectrum 3, the high ratio of C element and the SEM images can still be qualitative indicators illustrating that this area is mainly composed of resin. All of these phenomena indicate that the mixing of geopolymer and resin was heterogeneous. Fig. 16 shows the SEM images of sample S4, whose resin content is 50%. A very different morphology can be observed, instead in the case of the sample S2. As apparent from the images, the resin phases are dispersed in alveolate-like structure. Considering the ratio of resin and geopolymer is 1:1, the matrix and the particles cannot distinguish clearly. The ER particles which present as isolated nubby aggregations cannot observe in any images of sample S4. This is because the compatibility of waterborne resin and geopolymer solution is good, a small quantity of geopolymer was blended uniformly in resin matrix, see from spectrum 2 in Fig. 16. So the typical morphology of resin particles and its interface with the matrix cannot observe. But according to the EDX spectra, we found that geopolymer phases are linked by the resin phases. The joint of geopolymer particles is composed of resin and geopolymer, which shows fully dense structure. The resin dosage of 50% by weight may be adequate to bond the inorganic phases. Accordingly, sample S4 display improved mechanical properties if compared to samples with different resin dosage, as confirmed by compressive strength and young's modulus data (see Sections 3.2.1 and 3.2.2). It is worth pointing out that, if we compare mechanical properties of the pure geopolymer

Fig. 13. EDX spectra of sample S1 after the curing of 3 days.


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Fig. 14. SEM images of sample S2 after the curing of 3 days, (a) 500 magnification; (b) 5000 magnification.

with the hybrid composite sample S4, it can be easily observed that the compressive strength is the same, the young's modulus is strongly reduced by the addition of 50% resin. This is because the alveolate-like structure (Fig. 16a) shows better elasticity than granular pile structure (Fig. 12a). Fig. 17 presents the SEM images of sample D2, a different morphology is observed instead in the case of the S2 sample (Fig. 14a and b). Good homogeneity and uniformity of the microdispersion of the resin phase in geopolymer are evident. Few resin agglomerations can observe in hybrid composite matrix. This different microstructure attributed to

the presence of diluent that acts as a compatibilizing agent improving the interactions between the resin and geopolymer phases. The EDX spectrum of the sample D2 is shown in Fig. 18. See from the element weight percentage of diluted hybrid composite matrix, the C/Na ratio is about 1. Note that the C/Na ratio of inhomogeneous mixing area in sample S2 is about 7 or even 64 which indicate that these areas have high resin content. So the C/Na ratio of 1:1 illustrated the dispersion of resin and geopolymer phases are uniform. It is obvious that, in good agreement with compressive strength results, the diluent improved the strength of sample S2 by means of compatibilization effect.

Fig. 15. EDX spectra of sample S2 after the curing of 3 days.

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Fig. 16. SEM images and EDX spectra of sample S4 after the curing of 3 days, (a) 500 magnification; (b) 5000 magnification.

3.4. Reaction mechanism During the cross-linking process of the resin, the formation of epoxy groups makes the resin phase compatible with the geopolymer aqueous phase due to epoxy opening ring reaction in the setting stage. By this

approach, the ER phases are able to chemically interact with the geopolymer suspension to form a new chain-like framework which composed of [AlO4]5−, [SiO4]4− tetrahedral and polyamine. This new material shows an excellent dispersion up to micrometric level between the organic resin and the inorganic geopolymer. Thus a remarkably

Fig. 17. SEM images of sample D2 after the curing of 3 days, (a) 500 magnification; (b) 20,000 magnification.


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Fig. 18. EDX spectrum of sample D2 after the curing of 3 days.

reduced brittleness and improved early term compressive strength were present by the hybrid composites if compared to pure geopolymer [21,28].

As far as the dilution effect, the diluent makes the mixing degree of the organic phases and inorganic phases increase, while the epoxy equivalent of hybrid composite has no significant change. Before

Fig. 19. Dilution mechanism of BGE.

144


Fig. 20. The Si\ \Na\ \C triangle of hybrid composite matrix.

dilution, the resin phases and geopolymer phases tend to polymerization disparately. Therefore, it can be observed in Fig. 14, plenty of aggregations distribute in the hybrid composite matrix. After the addition of diluent, it can act as a compatibilizer which made the ER phase dispersing in geopolymer suspension more easily. The higher degree of miscibility leads to a closer interaction between the phases, thus the diluted hybrid composite is more inclined to produce the resin-geopolymer coalition, which as shown in Fig. 19. So the compressive strength of hybrid composite with diluent is reinforced compared to the undiluted samples. 3.5. Suggestion of geopolymer-resin composites using as oil well cementing material The results of our experiment indicate that undiluted ER has negative effects on the thickening property of geopolymer, which will decrease the thickening time. We hope to use ER to enhance the 1-day compressive strength at low temperatures, but the negative effect will limit its application. The thickening time decreasing phenomena is mainly due to the high reactivity of ER which release lots of heat in early-term curing reaction. Therefore, we should use diluent to reduce the reactivity of ER. Meanwhile, the diluent can also improve both 1day and 3-day compressive strength. The geopolymer-resin composites showed high early-term strength and less brittleness at 10 °C. In order to enhance the durability of cement sheath under deep water strata stress wave condition, we should increase the dosage of ER to achieve good elasticity. If we aim to achieve a hybrid composite with homogenous, dense and uncracked morphology (Figs. 16–17), we should control the ratio of Si, Na and C atomic content in the red circle labeled area (Fig. 20), which C content is in the range of 40%–50%, Na at about 20% and Si at 30%–40%. In a word, producing geopolymer-resin using MK, GBFS and ER are feasible. The use of geopolymer-resin composite as a deep water oil well cementing material not only reduces carbon emission but also enhances the early-term strength of low temperature well cementing materials. 4. Conclusions In this study, effects of epoxy resin on thickening properties and mechanical performance of geopolymer were tested at 10 °C. Based on the test results, the following conclusions can be drawn.

(1) The undiluted ER has the ability to accelerate the polymerization reaction and reduces thickening time, but because the high reactivity of hybrid composite, the thickening time becomes shorter at high ER dosage. After the addition of diluent, the thickening time became longer due to the decrease of epoxy equivalent. Meanwhile, the diluent can also avoid the mass polymerization of ER in early hydration age. (2) Pure geopolymer displayed high 3-day compressive strength but low 1-day strength. Undiluted ER enhances the 1-day compressive strength of hardened geopolymer paste. But the 3-day compressive strength of geopolymer containing undiluted ER reduced when the dosage of ER is lower than 50%. This is because the strict adhesion between ER and geopolymer phases is poor and ER at low concentration is not adequate on the binding of geopolymer phases. In the case of diluted ER specimens, for both series of curing time, the strength was higher than undiluted ER specimens attributed to the diluent that acts as a compatibilizing agent improved the interactions between the ER and geopolymer phases. (3) Before dilution, the ER phases and geopolymer phases tend to polymerization disparately. After the addition of diluent, it can act as a compatibilizer which made the ER phase dispersing in geopolymer suspension more easily. The higher degree of miscibility leads to a closer interaction between the geopolymer and ER phases, thus the diluted hybrid composite is more inclined to produce the ER-geopolymer coalition. (4) The hybrid composites should control the C content in the range 40%–50%, Na at about 20% and Si at 30%–40% to achieve a homogeneous, dense and uncracked microstructure. In order to enhance the durability of cement sheath under deep water strata stress wave condition, we should increase the dosage of ER to achieve good elasticity. In addition, the BGE diluent is recommended to make the thickening time adjustable and to reinforce the compressive strength.

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