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Effect of core–shell nanocomposites on the mechanical properties and rheological behaviors of cement pastes

SiO2 nanoparticles (50 nm in diameter) coated with poly(ethylene glycol) methyl ether methacrylate (PEGMA) were synthesized by radical polymerization. The SiO2/PEGMA nanocomposites were characterised using FITR, 1HNMR and TGA methods. The load of PEGMA in SiO2/PEGMA nanocomposites was 72.9 wt%. The hydration products, microstructure, pore structure, density, compressive strengths and rheological properties of cement were investigated. The SiO2/PEGMA nanocomposite could not only significantly improve the cement hydration and densify the microstructure by reducing the content of calcium hydroxide and promoting the production of calcium silicate hydrate, but also efficiently enhance the fluidity of the cement slurry. The compressive strength of cement with 2 wt% SiO2/PEGMA nanocomposites was increased by 40.1% curing for 28 days, which was much better than cement with the physical blending of SiO2 nanoparticles and superplasticizers. The SiO2/PEGMA nanocomposites with core–shell structure novelly combine the advantages of SiO2 nanoparticles and superplasticizers to significantly improve the performance of cement pastes. The results obtained provide a new understanding of the effect of the core–shell nanocomposites on cement pastes and demonstrate the potential of the nanocomposites for well cementing applications.


Introduction
During well cementing, oil well cement used to provide well integrity is susceptible to formation stress, periodic external pressure changes, and the difference in elasticity between the cement sheath and casing during the production of wells. 1-4 These stresses and pressures may produce unbalanced forces in the cement stone, causing brittle fracture or the formation of micro-annulus and micro-cracks. 5 The destruction of the downhole cement sheath seriously affects the productivity and production safety of wells. 6,7 With the deepening of oileld exploration and development, the requirements for the performance of oil well cement are getting higher and higher to meet the needs of cementing technology under complex geological conditions. Silica fume is a by-product of the ferroalloy industry, and its particle size is about 0.1-0.5 mm. 8,9 In addition, the main component of silica fume is amorphous silica, so the pozzolanic activity is higher than that of other mineral admixtures. 10 The effect of silica fume in cement is similar to that of SiO 2 nanoparticles. 11 Compared with silica fume, SiO 2 nanoparticles have more signicant advantages in pozzolanic effect, lling effect and nucleation ability in cement. 12 SiO 2 nanoparticles are higher pozzolanic activity than silica fume and can react with calcium hydroxide to generate extra calcium silicate hydrate (C-S-H). 13 In addition, the particle sizes of SiO 2 nanoparticles are smaller than that of silica fume, SiO 2 nanoparticles can ll smaller pores in nanoscale, thereby densifying the pore structure of cement. 14 Besides, SiO 2 nanoparticles can act as nucleation sites for C-S-H to greatly accelerate the hydration in the early stage. 15 Additionally, the presence of SiO 2 nanoparticles in cement pastes can effectively improve their rheological properties. 16,17 However, SiO 2 nanoparticles have a strong tendency to agglomerate, which can easily form weak zones in oil well cement, reducing the mechanical properties of cement. [18][19][20] As a result, the application of SiO 2 nanoparticles in cementing is restricted. The dispersion of SiO 2 nanoparticles can be improved by surface modication of nano-SiO 2 .
The dispersion stability of SiO 2 nanoparticles in oil well cement can be improved by adding superplasticizers, which was different types of polymers, such as phosphate polycarboxylate superplasticizers, polycarboxylate superplasticizers and polyether-based superplasticizers. 21,22 The superplasticizers can adsorb cement particles through the branched chains, which signicantly improves the rheological behaviors, accelerates the hydration process, and reinforces the mechanical properties of cement. [23][24][25] Rheological behaviors of the oil well cement are the main factors that decide the borehole sealing effectiveness. 26 However, the interaction between SiO 2 nanoparticles and superplasticizers is weak, which cannot essentially change the strong agglomeration trend of SiO 2 nanoparticles, thus the uniform dispersion of SiO 2 nanoparticles in oil well cement cannot be achieved on the nanoscale level. 27 With the maturity of nanotechnology in recent years, nanocomposite materials have been widely used in different research elds, and polymer/nanoparticles composite materials have become the focus of scientic research in this century. [28][29][30] The mechanical and thermodynamic properties of the composites were greatly improved by introducing nanoparticles into the polymer matrix. [31][32][33] Numerous studies have shown that, compared with the physical blending of polymer and nanoparticles, polymer/nanoparticle composites have been greatly improved in mechanical properties, thermal properties, electrical properties, rheological properties, ame-retardant, anticorrosive properties. 34-37 SiO 2 nanoparticles with graed polymers possessed excellent dispersion stability due to steric hindrance and electrostatic repulsion, [38][39][40][41] which probably resulted in better properties of oil well cement. And the graed polymers can endow the nanocomposite with multiple properties.
In the present study, SiO 2 nanoparticles coated with PEGMA (SiO 2 /PEGMA) were prepared by radical polymerization, and the effect of the SiO 2 /PEGMA nanocomposites on the hydration process, mechanical and rheological properties of cement was investigated. Hydration products composition, microstructure, density and pore structure of cement were characterised to investigate the reinforcement mechanism by means of TGA, SEM and MIP, respectively. Compressive strengths and rheological properties were also studied as important indicators for cement.

Preparation of SiO 2 /PEGMA
2 wt% SiO 2 nanoparticles in 10 wt% ethanol solution were sonicated for 15 min to achieve the homogeneous suspension. 3 wt% BSA was added to the above dispersion and stirred for 6 h at 50 C under nitrogen. Aer cooling to atmospheric temperature, the dispersion was centrifuged and washed by alcohol multiple times to remove the unreacted BSA. Subsequently, the modied SiO 2 nanoparticles (SiO 2 -BSA) were dried at 60 C for 15 h.
2 wt% SiO 2 -BSA in anhydrous ethanol was sonicated for 30 min to uniform dispersion. 10 wt% PEGMA and 0.025 wt% AIBN were added to the dispersion and mixed for 4 h under nitrogen at 60 C. Thereaer, the obtained product was centrifuged and rinsed several times with alcohol and labelled as SiO 2 / PEGMA nanocomposites. The preparation process for this nanocomposite is illustrated in Fig. 1. The chemical structure of synthesized SiO 2 /PEGMA nanocomposites was then characterised by FTIR spectrometry and 1 HNMR spectroscopy techniques.

Preparation of cement samples
Four kinds of cement samples were prepared to study the inuence of SiO 2 /PEGMA nanocomposites on the performance of oil well cement, including blank oil well cement, oil well cement with 2 wt% SiO 2 nanoparticles, oil well cement with 2 wt% SiO 2 nanoparticles and 0.3 wt% PEGMA/AA superplasticizer, oil well cement with 2 wt% SiO 2 /PEGMA nanocomposites. These samples were labelled as blank, SiO 2 , SiO 2 + P and SiO 2 /PEGMA, respectively.

Compressive strength measurement
The compressive strength of set cement was evaluated according to the GB/T 19139-2012 National Standard of China. The prepared cement pastes having a water-cement ratio of 0.44 was poured into a cubic copper mold with a side length of 50.8 mm and cured at 75 C and 21 MPa for a period of time. The curing time was set to 3, 7 and 28 days, respectively. The compressive strength of hardened cement pastes was tested under a loading rate of 72 kN min À1 by the triaxial rock mechanics testing system (TAW-1000, China). The compressive strength of cement samples at each curing time was tested 5 times.

Density of cement samples
The dry mass of cement samples with a curing time of 3 days was measured aer vacuum drying at 105 C for 12 h. The bulk density of cement could be written as follows: where M and V are the dry mass (g) and specimen volume (cm 3 ) of the cement samples, respectively. All results were the arithmetical average of 3 parallel tests.

Rheological behavior of cement paste
Rotary rheometer (MCR 302 with parallel plates, Austria) was utilised to analyse the rheological properties of cement paste based on API Recommended Practice 10B-2, where the plates gap and plate diameter were selected as 2 mm and 25 mm, respectively. The cement pastes having a water-cement ratio of 0.44 was stirred for 2 min before performing the rheological test. The test was kept 25 C through a water circulation system around the sample container.

Characterization
Thermogravimetric analysis (TGA, TA 5200, USA) was applied to determine the weight loss of the SiO 2 /PEGMA nanocomposites and cement with a curing time of 3 days. Scanning electron microscope (SEM, IT100, Japan) was utilised to observe the microstructure of oil well cement aer curing for 3 days. Mercury intrusion porosimeter (MIP, AutoPore III, USA) was applied to characterise the pore structure of hardened cement pastes with a curing time of 3 days.

Characterisation of SiO 2 /PEGMA nanocomposites
The FTIR spectra of the SiO 2 nanoparticles, SiO 2 -BSA, and SiO 2 / PEGMA nanocomposites were shown in Fig. 2(a). For the SiO 2 nanoparticles, 1100 and 1675 cm À1 were corresponded to the Si-O-Si and bending vibration of H 2 O, respectively. The peak centred at 3450 cm À1 was ascribed to the presence of Si-OH. The spectra of SiO 2 -BSA exhibited a new absorption band at 2950 cm À1 , which was attributed to the stretching vibration of the C-H bond due to the presence of BSA. Compared with the SiO 2 -BSA, the absorption band around 1575 cm À1 in SiO 2 / PEGMA nanocomposites can correspond to C]O, and the intensity of absorption peaks at 3450 cm À1 is signicantly enhanced, which conrms that PEGMA has been graed to SiO 2 nanoparticles.
To further verify the successful preparation of the SiO 2 / PEGMA nanocomposites, 1 HNMR was carried out and illustrated in Fig. 2(b). The peaks centred at 1.08 and 1.75 ppm were related to the -CH 3 and -CH 2 , respectively. Besides, the peak values at 3.26 and 3.59 ppm were ascribed to the O-CH 3 and -CH 2 -CH 2 -O. The above characteristic peaks of PEGMA were found in the 1 HNMR spectrum of SiO 2 /PEGMA nanocomposites, further conrming the successful preparation of nal nanocomposites.
The TGA curves of SiO 2 , SiO 2 -BSA and SiO 2 /PEGMA were illustrated in Fig. 3. For the SiO 2 -BSA, the mass loss embraced two parts, specically one slightly dwindled in the temperature range between 30 and 200 C while the other diminished in the region 200-510 C due to the decomposition of chemically bonded BSA from the particle surface. The SiO 2 /PEGMA has a signicant weight loss within the temperature range of 220-420 C, mainly due to the decomposition of PEGMA. The weight percentage of PEGMA on the SiO 2 nanoparticles can be written as eqn (2): DW PEGMA ¼ W 220,SiO 2 /PEGMA À W 420,SiO 2 /PEGMA À (W 220,SiO 2 ÀBSA À W 420,SiO 2 ÀBSA )  where DW PEGMA is the amount of PEGMA on the SiO 2 nanoparticles. W 220,SiO 2 -BSA and W 420,SiO 2 -BSA are the respective SiO 2 -BSA residual weight at 220 and 420 C. W 220,SiO 2 /PEGMA and W 420,SiO 2 /PEGMA are the respective SiO 2 /PEGMA residual weight at 220 and 420 C. The load of PEGMA in SiO 2 /PEGMA calculated from the above equation was 72.9 wt%. Aer anchoring BSA to the surface of SiO 2 nanoparticles, PEGMA formed a shell structure on the nanoparticle surface.

Thermal gravimetric analysis
The TGA curves for blank, SiO 2 , SiO 2 + P and SiO 2 /PEGMA cement samples aer curing for 3 days was shown in Fig. 4. The SiO 2 / PEGMA nanocomposite was more effective in promoting the hydration process of cement and reducing the content of CH in the hydration process. According to the TGA curves, the mass loss of the samples at 400-500 C was caused by the decomposition of CH, and the mass loss of CH (M CH ) can be written as follows: where W CH is the mass loss of the samples at 400-500 C. 42 The hydration degree of oil well cement could be represented by the non-evaporable water content, which was the mass loss between 105 C and 1000 C, and M non-evaporable is the content of non-evaporable water. According to Table 2, the mass of nonevaporable water of SiO 2 /PEGMA is more than that in the rest of the cement, while the content of CH was lowest. SiO 2 nanoparticles can decrease the crystal size and content of CH by reacting with CH, reducing the orientation of CH crystals aggregated at the interface and effectively improving the interface. 43 The pozzolanic reaction from SiO 2 nanoparticles and CH is benecial to improve the mechanical properties of cement. 20 SiO 2 nanoparticles aggregates due to their small size, affecting the pozzolanic activity. SiO 2 nanoparticles graed with PEGMA had a higher pozzolanic activity, which could signicantly promote the hydration process of cement and consume CH to generate C-S-H.

Scanning electron micrograph analysis
The hardened cement pastes were also characterised by SEM, and the images were displayed in Fig. 5. It can be seen from Fig. 5(a) that the microstructure of blank cement was loose with a lot of holes and cracks, and the compactness was poor. As shown in Fig. 5(b), there were also many obvious pores in cement with SiO 2 nanoparticles. According to Fig. 5(c), although the microstructure of SiO 2 + P cement stone became denser, a few holes can be still observed. According to Fig. 5(d), it had the densest microstructure without any obvious pores. The mechanical properties of cement depend on its microstructure. 44 SiO 2 nanoparticles not only promote the hydration reaction and produces C-S-H to ll the pores of the cement paste, but also ll the voids between the grains in the cement matrix. 20 SiO 2 nanoparticles were easy to agglomerate, resulting in a signicant decrease in pozzolanic activity and nucleation effect. 45 The physical blending of SiO 2 nanoparticles and PEGMA/AA, and graing PEGMA on the surface of SiO 2 nanoparticles improved the microstructure of cement. As for SiO 2 + P, the superplasticizers could improve the dispersion stability of SiO 2 nanoparticles and accelerate the hydration process, which made the microstructure more compact. However, the interaction between SiO 2 nanoparticles and superplasticizers is weak, resulting in the agglomeration of SiO 2 nanoparticles in the cement slurry. 27 As for SiO 2 /PEGMA, the SiO 2 /PEGMA nanocomposites effectively optimized the microstructure of oil well cement. This may be mainly due to the core-shell structure of   SiO 2 /PEGMA nanocomposites which could signicantly improve the pozzolanic activity and nucleation effect of SiO 2 nanoparticles.

Pore structure analysis
The hydration process of cement is transformed cement particles and water into a solid and low permeability material, which is a chemically driven process. 46 The microstructure of cement was heterogeneous and multiphase with different types of pores inside, including gel pores (3-10 nm), capillary pores (10-1000 nm) and large pores (>1000 nm). 47 As shown in Fig. 6 and Table 3, the pore size distribution, pore structure and density of cement samples were most affected by the addition of the SiO 2 / PEGMA nanocomposites. The density of cement with SiO 2 / PEGMA was the highest, reaching 2.32 g cm À3 . According to the effect of pore size on cement properties, the pores are classied into harmless pores (<20 nm), less harmful pores (20-50 nm), harmful pores (50-200 nm) and more harmful pores (>200 nm). 48 The main peak of the pore size distribution of SiO 2 / PEGMA cement was obviously shied to the le, and the value of the peak was the lowest. In addition, the total porosity and average pore diameter of SiO 2 /PEGMA decreased from 35.6% and 25.8 nm to 17.5% and 12.6 nm, respectively, which indicates that the SiO 2 /PEGMA nanocomposites had a dramatical rening effect on the pore structure of oil well cement. The main reason for improving the pore structure of cement was the lling effect and the pozzolanic reaction. SiO 2 /PEGMA nanocomposites with a small particle size and high pozzolanic activity could ll the pores in the nanoscale and produce C-S-H with CH to optimize the distribution of the pore structure. SiO 2 / PEGMA nanocomposites could not only act as an activator to promote the hydration reaction, but also play a lling role to reduce the pore volume and make the cement more compact.

Mechanical properties analysis
The compressive strengths of these four cement samples aer curing for 3, 5 and 28 days were presented in Fig. 7. The compressive strengths of all samples increased continuously with the prolonging of the curing time. Moreover, the compressive strengths of the cement with SiO 2 nanoparticles, the physical blending of SiO 2 nanoparticles and PEGMA/AA superplasticizer, and the SiO 2 /PEGMA nanocomposites aer curing for 28 days increased by 9.1%, 15.4% and 40.1%, respectively. The main reason for improving the compressive strength of hardened cement pastes is the pozzolanic reaction, lling effect and nucleation ability of SiO 2 /PEGMA nanocomposites. The SiO 2 nanoparticles with graing PEGMA could react with hydration products to generate additional C-S-H, ll up smaller holes to optimize the distribution of the pore structure, and act as nucleation sites for C-S-H in the early hydration stage to greatly accelerate the hydration of cement.
These results further proved that the SiO 2 nanoparticles with core-shell structure could possess excellent pozzolanic activity, lling effect and nucleation ability to accelerate the hydration process and reinforce the mechanical properties of cement.

Rheological properties
Mixing well oil cement slurry exhibiting yield stress is one of the most challenging tasks during well cementing, and effectively reducing the yield pressure of cement can improve the cement operation. 49 To better clarify the effect of SiO 2 /PEGMA nanocomposite on the uidity of oil well cement, the changes in shear stress of cement slurries as a function of shear rate were determined. It can be seen from Fig. 8, the shear stresses of the cement were approaching a similar value with the growth of the shear rate due to the solidication of the cement slurry. The modied Bingham model can better reect the effect of very low shear stresses induced in the low shear rate region of ow  curves. 50 The modied Bingham model can be written as follows: where s, m p , _ g and c are considered as the shear stress, plastic viscosity, shear rate and a regression constant, respectively. The modied Bingham model was used to estimate the rheological behavior of cement pastes. Shear stress of the blank cement suspension was about 3 Pa and steadied with the growth of the shear rate. The yield stress of cement suspension with SiO 2 nanoparticles was highest among all samples, which was about 10 Pa. And with the increase of the shear rate, its shear stress decreased gradually to the value, which was slightly lesser than that of blank cement. As for the SiO 2 + P cement suspension, its yield stress is as low as 0.01 Pa. While for the cement suspension with SiO 2 /PEGMA nanocomposites, its yield stress is even lower than that of the SiO 2 + P cement suspension. The SiO 2 cement samples present higher yield stress than the blank cement due to the silica-silica interactions. 51 The adding PEGMA/AA improved the rheological behavior of the cement slurries due to the adsorption of active branched polymer sites onto the surface of cement grains. The graed PEGMA of SiO 2 / PEGMA nanocomposites could adsorb on the cement grains through its polyethylene glycol branch and play a substantial role in the uidity of cement pastes to effectively reduce the yield stress of the cement suspension.

Conclusion
In this investigation, the SiO 2 /PEGMA nanocomposites with coreshell structure were prepared by radical polymerization, and the load of PEGMA in SiO 2 /PEGMA calculated from the above equation was 72.9 wt%. The hydration products, microstructures, mechanical and rheological properties of blank, SiO 2 , SiO 2 + P and SiO 2 /PEGMA cement samples were studied by contrast analysis. The hydration degree of SiO 2 /PEGMA was more than that in the rest of the cement, while the content of CH was lowest. The SiO 2 /PEGMA nanocomposites were able to efficiently accelerate the hydration process by promoting the production of C-S-H and decreasing Ca(OH) 2 content. The microstructure of SiO 2 /PEGMA cement was the densest without any obvious pores, which was due to the higher pozzolan activity and lling effect of SiO 2 /PEGMA. The density of cement with SiO 2 /PEGMA was the highest, reaching 2.32 g cm À3 . In addition, the total porosity and average pore diameter of SiO 2 /PEGMA decreased from 35.6% and 25.8 nm to 17.5% and 12.6 nm, respectively, which indicates that the SiO 2 /PEGMA nanocomposites had a dramatical rening effect on the pore structure. The pozzolanic activity, nucleation effect and lling effect of SiO 2 /PEGMA nanocomposites made the microstructure of the cement more compact and thus improved the mechanical properties of cement. The cement with SiO 2 / PEGMA nanocomposites increased the compressive strength by 40.1% curing for 28 days, which was much better than other cement samples. The SiO 2 nanoparticles with core-shell structure could promote the generation of more C-S-H by reacting with CH, ll the gaps between the cement grains to improve the microstructure of cement, and provide nucleation sites for hydration as well as the growth of hydrated products to greatly accelerate the hydration process. Besides, the modied Bingham model was used to estimate the rheological behavior of cement pastes. Compared with the physical blending of polymers and SiO 2 nanoparticles, SiO 2 /PEGMA nanocomposite can better improve the uidity properties of cement slurries, and its yield stress is even lower than 0.01 Pa, which provided very suitable workability conditions. The SiO 2 /PEGMA nanocomposites could not only signicantly improve the cement hydration and densify the microstructure but also efficiently enhance the uidity of the cement slurries. The core-shell structure endowed the nanocomposite with multiple functions show broad potential applications in the eld of oil well cementing.

Author contributions
All authors made critical contributions to the collection and interpretation of data. Gang Wang conceived and designed the experiments; Hua Tan and Chunjing Lu performed the experiments; Gang Wang and Ao Sun analysed the data; Gang Wang wrote the paper.

Conflicts of interest
There is no conict of interest to be reported.