Study on macro performance and micro-analysis of high strength grouting material

Wind turbine duct put forward higher requirements for the performance of grouting materials. In order to obtain more economical high strength grouting materials (HSG), three kinds of low price main components, cement, quartz sand (QS) and fly ash microspheres (MS) were utilized and developed. Macro performance and micro-analysis of the HSG were conducted. The macro performance tests revealed that the mix proportion of HSG is determined as, cement: QS: MS: superplasticizer: swelling agent = 80:10:10:0.3:0.12, water binder ratio is determined as 0.26. Compressive and flexural strength of the HSG exceeding 120 MPa and 20 MPa respectively. The HSG also showed good fluidity and homogeneity. The micro-analysis showed that a complete and continuous micro-structure was formed and interface transition zone could not be observed. XRD and TG-DSC results meant that the amount of AFt increased. On the contrary, the amount of Ca(OH)2 decreased. pore diameter of the HSG was mostly 10 nm. The addition of swelling agent and MS did not changed the types of hydration products of HSG, but the pore structure was optimized. Due to the cheap components, HSG contributed higher profits with the lower price, and which perform good economic benefits and market competitiveness.


Introduction
Chinese government proposed at the united nations general assembly that China will achieve carbon neutrality by 2060 [1,2]. Therefore, vigorously developing clean and renewable energy to replace traditional fossil energy is one of the important ways to address global climate issues, build an environment-friendly economy, and strive to achieve carbon neutrality [3,4]. The global wind energy reserves are abundant and wind power generation is environmentally friendly, gradually becoming a highly respected renewable energy source. The reserves of marine wind energy resources are abundant, and offshore wind power generation is gradually becoming the focus of the new energy field [5,6].
Grouting connection is a common connection between the offshore fan foundation and the lower pile foundation. On the one hand, this connection can reduce the stress concentration and fatigue caused by welding, and on the other hand, it can also level the fan [7]. At the same time, it has excellent structural performance, low cost, convenient construction and other advantages [8]. According to DNV-OS-J101 Design of Offshore Wind Turbine Structures [9], grouting connection is a structural join composed of two concentric tubular parts, and grouting material is filled in the annular area between the external and internal pipes. A few foreign companies, such as Densit Company in Denmark, NAUTEC Company in Singapore, and BASF Company in Germany, have mastered offshore wind power grouting products and technologies, and have had a large number of successful engineering projects in Europe. The offshore wind power grouting materials have compressive strength of up to 100 MPa∼140 MPa [10].
The particularity of grouting of offshore wind power lies in that the offshore fan is under the action of reciprocating random load, and the grouting connection section is under the combined action of axial force, bending moment, torque and other loads, which makes the grouting material work in a complex stress state. Different from traditional grouting materials, wind power grouting materials are required to have higher fluidity and higher stability in the pumping stage, and also to have high mechanical properties [11,12]. Currently, researchers have conducted extensive research on the fluidity, volume stability, and mechanical properties of grouting materials.
Fei et al [13] showed that the incorporation of fly ash, steel slag and blast furnace slag could to some extent reduce viscosity and improve fluidity of grouts. Li et al [14] found that the fluidity and initial consistency were improved with the addition of fly ash. Further, bentonite caused the increased viscosity and the decreased fluidity of grouts. Cui et al [15] studied that the addition of waterborne epoxy resin can improve the fluidity and reduce the density of the grouts.
He et al [16] studied the synergistic effect of superabsorbent polymer (SAP) and MgO expansion agent on compensates shrinkage of high strength cement-based grouting materials (UHS-CGM). The result indicated that SAP improved hydration degree of UHS-CGM and expansion efficiency of MEA. Small size SAP showed better shrinkage compensation effect. Zhang et al [17] revealed that after the addition of 6% expansion agent composed of metakaolin and alunite, the grouting material showed good expansion. The addition of sulphoaluminate cement further improved the expansion effect. The expansion effect is attributed to the formation of more ettringite expansion components. Huang et al [18] found that when the addition of expansion agent mainly composed of aluminate sulfate and magnesium oxide increased from 4% to 8%, the expansion rate of grouting material increased from 0.08% to 0.16%.
Fu et al [19] directly filled high performance concrete (UHPC) into the HTRB600E steel sleeve joint. The addition of steel fibers significantly improved the bearing capacity of the steel sleeve joint, but reduced the fluidity of the grouting material, resulting in a decrease in grouting performance. Li et al [20] studied that the addition of liquid sodium silicate and sodium hydroxide significantly increased the compressive strength of slag based grout. Sodium hydroxide and liquid sodium silicate could increase the strength of grout by about 21% and 36% respectively. The formation of more silicoaluminate is the factor that promotes the strength of grout. Chen et al [21] found that wet-milling fine cement improved the mechanical properties of grout. The formation of dense hydration product network and the reduction of pore radius contributed the higher strength of grout.
However, at present, there is a lack of systematic research on the influence of components on the macro properties and microstructure of grout and the correlation between them. In addition, in order to achieve high strength performance in grouting materials, it is usually necessary to add essential materials, such as nano silica [22,23], liquid sodium silicate [24], and silica fume [25], which significantly increases the cost of grouting materials. Therefore, it is of practical significance to prepare an more economical HSG which composed of cheap components. In view of this, three main components, which include PII 525 cement, fly ash microsphere and quartz sand are utilized to develop HSG with compressive strength and flexural strength of 120 MPa and 20 MPa respectively. On this basis, the micro-analysis was conducted. The low cost make the HSG has practical engineering application significance.

Cement
Components involved in the grouts in this study is shown in figure 1. PII 52.5 Portland cement was used in this work, which was produced by Guangdong Yingde Conch Cement Co., Ltd Initial and final setting time are

Fly ash microsphere (MS)
The microspheres were provided from Lingshou Jiagong mineral products Co., Ltd Physical properties of microsphere is shown in table 1.

Quartz sand (QS)
Quartz sand with 40-70 mesh, which purchased from Guangdong heyuan quartz sand Co., Ltd was used in this study. The particle size distribution is shown in table 2.

Additive
Water reduction rate of white powder superplasticizer is 25%. The specific area and average particle size of yellow powder swelling agent are 485m 2 /kg and 2.5±0.6 μm respectively. Swelling agent is a kind of powdery composite expansion component composed of calcium oxide and magnesium oxide.

Preparation
The W/B of 0.28 was added into the mixer pot, then the required content of additives was added with a low speed mixing for 30 s. Cement, MS and QS were gradually added, and then the slurry was mixed for 5 min with the 1000 rpm.

Fluidity test
The test methods were conducted in accordance with the requirements of 'Standard Test Method for Flow of Hydraulic Cement Mortar' (ASTM C1437-15).

Mechanical performance test
The mechanical tests were performed according to 'Cement mortar strength test method (ISO method)' (GB/ T17671-1999).

Free expansion rate test
The test was conducted according to 'Technical Specification for Construction of Highway Bridge and Culvert' (JTG/T 3650-2020, China).

Scanning electron microscope (SEM) test
SEM test was conducted on JSM-7500F machine. The grouting material was cut into square samples with the size of 1 mm × 5 mm × 5 mm, and then the sample was placed in an oven to dry for 24 h at 100°C. The dried thin sample was then sprayed with gold coating for test.

X-ray diffraction (XRD) test
The powdered sample was dried at 60°C for 48 h before the operation. The test was operated with diffraction angle ranged from 5°to 90°. The scanning frequency is controlled as 10°/min. Computer software (MDI Jade) was used to determine the identity of the mineralogical compositions of the specimen by comparison to a database.

Mercury intrusion porosimetry (MIP) test
MIP test was operated on the machine AutoPore Iv 9510. The dried sample with cube of 10 mm * 10 mm * 10 mm was prepared. The testable aperture diameter is ranged from 5 nm∼800um.
The main properties requirements of grouting materials according to the 'Grouting materials for offshore wind power duct supports'(Q31/0120000414C001-2016-01, China) is shown in table 3.
The optimized mix ratio based on the single factor test result was designed, and which is shown in table 4.

Preparation of HSG
The single factor effect of admixtures on mechanical properties of grouts is shown in figure 2. It can be seen that the mechanical performance of grouts slightly increased with the addition of 5%-20% MS. After the addition of 20% MS, the compressive strength and flexural strength of grouts was increased to 72 MPa and 14.5 MPa, respectively. It's worth noting that when the content of QS is less than 15%, the mechanical properties of grouts gradually increased. when the content of QS is larger than 15%, the mechanical properties of grouts gradually  decreased. The reason for the existence of an optimal dosage is that QS can play the role of skeleton in grouts at an appropriate dosage. Figure 3 shows the optimized mix ratio test result. When the content of QS was kept at 5%, the mechanical property of grouts slightly increased with the decrease of cement, and increase of MS. In this work, the main particle size of quartz sand and cement is 160 μm and 40 μm respectively, and the main particle size of MS is 1-3 μm. So MS showed double positively effect of pore filling and secondary hydration. Although the hydration products decreased with the decrease of cement, this positive effects of MS overcome the negative effect of the reduction of cement hydration products. When the content of cement is reduced 75%, and the content of MS is increased to 20%, the properties of cement are sharply reduced due to insufficient cement hydration products.
When the content of cement was kept at 75% or 80%, it can be seen that with the addition of QS from 5% to 15%, the mechanical properties of grouts are improved firstly and then decreased. After the addition of 5% QS, a close-packed structure was formed. The QS acts as the skeleton structure, the cement could gel QS and MS to form a whole structure, and MS could fill the structural pores, and further improve the structural compactness through secondary hydration. The whole structure showed the most stability and compactness, which contributed the best mechanical properties of the grouts. Therefore, The optimal amount of cement, QS and MS is thus determined as 80%, 10% and 10% respectively.

Effect of water binder ratio
The effect of water binder ratio on mechanical property of grouts is shown in figure 4. It can be seen that the mechanical property of the grouts increased with the decrease of water binder ratio. When W/B was decreased to 0.22, the compressive and flexural strength of grouts were increased to 128.4 MPa and 28.1 MPa respectively.  The effect of water binder ratio on liquidity loss of grouts slurry is shown in figure 5. It can be seen that the reduction of water binder ratio caused the increase of fluidity loss. When the water binder ratio decreased to 0.22, the fluidity loss after stewing 30 min and 60 min increased to 7.6 s and 11.6 s respectively. With the decrease of water binder ratio, fluidity loss degree tended to increase. In the actual construction process, the grouts slurry should maintain good fluidity for a long time in order to be better pumped in the narrow pipeline. Under the condition of small water binder ratio, in order to achieve the required liquidity, the raw material cost of grouts will be substantially increased because of the increase of superplasticizer. Considering the mechanical properties (generally larger than 120 MPa), cost and fluidity loss, the optimum water binder ratio of the HSG is determined as 0.26.

Effect of superplasticizer and swelling agent
The effect of superplasticizer on fluidity of grouts is shown in figure 6. Initial and 30 min fluidity gradually increased with the addition of superplasticizer. After the addition of 0.35% superplasticizer, the initial and 30 min fluidity increased to 357 mm and 297 mm respectively. In addition, the increase degree of initial and 30 min fluidity gradually decreased with the superplasticizer. Considering the requirements for initial and 30 min fluidity in the specification and materials cost, the optimum dosage of superplasticizer is determined as 0.3%.  The effect of expansive agent on free expansion rate of grouts is shown in figure 7. With the addition of expansive agent, the free expansion rate gradually increased. After the addition of 0.18% expansive agent, the 3 h and 24 h free expansion rate increased to 1.85% and 0.62% respectively. Considering the requirements of free expansion rate and materials cost, the optimum dosage of expansive agent is determined as 0.13%.
The final mix proportion of HSG is shown as table 5. After the grouts slurry was sited for an hour, the slurry state is shown in figure 8. It can be seen that the HSG had good settlement stability, and there is no segregation generated. Good homogeneity and excellent working performance ensure that the HSG shows good construction convenience.

Micro-analysis
The micro morphology of HSG is shown in figure 9. It can be seen from figure 9(a) that the HSG showed a dense micro-structure. Pores and cracks were rarely observed. In figure 9(b), the MS showed rough surface, which means that secondary hydration has been occurred. Meanwhile, MS and QS were closely connected and   surrounded by cement hydration products. No weak interface transition zone could be observed. So a complete and continuous structure was formed at the micro level, which contributed high strength properties of the HSG. XRD tests on HSG with different components were conducted, and the results are shown in figure 10 (Grouts: the grouting materials without swelling agent and MS; Grouts-S: the grouting materials with swelling agent; Grouts-S-M: the grouting materials with swelling agent and MS). The results indicate that the main mineral phases of grouts, grouts-S and grouts-S-M are the same, which are C 3 S, C 2 S, Ca(OH) 2 and AFt. The types and amounts were not changed by adding the swelling agent. After the addition of MS, the intensity of characteristic peaks AFt increased, while the intensity of characteristic peaks C 3 S, C 2 S and Ca(OH) 2 decreased. It means that the addition of MS furtherly enhanced the hydration degree of the system and more hydration products were generated.
TG-DSC test results of grouts with different components in shown in figure 11. Hydration products C-S-H and AFt have a mass loss between 60°C and 120°C [26,27], and calcium hydroxide has a mass loss between 420°C and 500°C [28,29]. It can be found that the total amount of hydrated calcium silicate, AFt and calcium hydroxide has not been significantly changed after the addition of swelling agent. The addition of MS significantly reduced the content of calcium hydroxide in system, and also increased the total amount of hydrated calcium silicate and AFt. The TG and XRD results showed that the addition of swelling agent did not change the type and amount of components in system, and the addition of MS occurred secondary hydration, resulting in a higher hydration degree of the system. Differential porosity of grouts with different components is shown in figure 12. It can be found that the addition of swelling agent slightly increased the pore size in grouts, as well as increased the number of pore diameter below 10 nm. This is also related to the action mechanism of swelling agent. The swelling agent reduced the shrinkage of  grouts by producing gases such as nitrogen, and did not significantly affect the pore structure of the system. After the addition of MS, the main pore diameter about 20 nm decreased, while the number of pores with diameter below 10 nm significantly increased. It's pointed out that the pore diameter below 20 nm is belongs to harmless pores [30,31]. Therefore, the addition of MS modified the pore structure and pore diameter distribution of the grouts system, which shows an improvement in mechanical properties from a macro point of view.

Economic benefit
The cost and profit of high-strength grouting material are shown in table 6. It can be found that cost of the HSG is 470.4 RMB/t. Selling price can be as low as 625 RMB/t if a 20% profit is retained, which is far lower than the   current market price of similar products. Due to the cheap components in the HSG, which can generate higher profits at the lower price, and also perform good economic benefits and market competitiveness.

Conclusion
In this paper, the effects of multiple admixtures (cement, quartz sand, silica fume and microsphere) on mechanical properties of grouts were studied through single and multiple factor tests. The effect of superplasticizeron working properties, and swelling agent on hydration products was studied. The HSG was then developed through a simple combination of the two kinds of low price components quartz sand and fly ash microspheres, and the high strength formed mechanism was studied. The follow conclusion can be drawn: (1) The mechanical properties of grouts can be increased with the addition of 5%-15% quartz sand. When the content of microsphere is less than 20%, the mechanical properties of grouts can be slightly improved. The addition of silica fume weakens the mechanical properties of grouts. The reason is that the large specific surface area of silica fume leads to agglomeration during the grouts slurry stirring process. The agglomerated silica fume becoming a weak point, and which reduced the mechanical properties of grouts.
(2) When the water binder ratio decreased to 0.22, the fluidity loss after stewing 30 min and 60 min increased to 4.6 s and 9.6 s respectively. With the decrease of water binder ratio, fluidity loss degree tended to increase. Initial and 30 min fluidity gradually decreased with the addition of superplasticizer. With the addition of expansive agent, the free expansion rate gradually increased. The mix proportion of HSG is determined as, cement : quartz sand : MS : superplasticizer: swelling agent = 80:10:10:0.3:0.12.
(3) The HSG showed good homogeneity and excellent working performance. MS and quartz sand were closely connected and surrounded by cement hydration products. A complete and continuous structure was formed. The addition of MS and swelling agent did not changed the types of hydration products. The addition of MS increased the total amount of hydrated calcium silicate and Ettringite in the system, which means that MS inproved the hydration degree of the system. In addition, MS also optimized the pore structure. High strength-formed mechanism of wind turbine duct grouting is the synergism of 'squeezed structure' and 'cement strengthen'.