J. Cent. South Univ. (2020) 27: 3464-3476

DOI: https://doi.org/10.1007/s11771-020-4559-8

Hydration process of rice husk ash cement paste and its corrosion resistance of embedded steel bar

WANG Hui(汪晖)¹, ZHANG Ai-lian(张爱莲)², ZHANG Lin-chun(张林春)², WANG Qian(王倩)²,

HAN Yan(韩艳)², LIU Jun-zhe(柳俊哲)³, GAO Xiao-jian(高小建)⁴, SHI Fei-ting(石飞停)⁵,

LIN Xue-yan(林雪妍)¹, FENG Li-yu(冯立宇)¹

1. School of Civil and Environmental Engineering, Ningbo University, Ningbo 315000, China;

2. School of Civil Engineering, Sichuan College of Architectural Technology, Deyang 618000, China;

3. School of Construction Engineering, Qingdao Agricultural University, Qingdao 266000, China;

4. School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China;

5. Civil Engineering Department, Yancheng Institute of Technology, Yancheng 224051, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Thermogravimetric analysis and electrical resistivity were used to determine the hydration process of cement paste with rice husk ash (RHA) (0-15%) and water-cement ratio of 0.4 in this work. X-ray diffraction (XRD) method and scanning electron microscopy (SEM) were used to survey crystal composition and microstructures of specimens cured for 3 h, 1 d, 7 d and 28 d. Finally, electrical parameters (electrical resistance and AC impedance spectroscopy) of steel bars reinforced cement paste were investigated to study the effect of RHA on the corrosion resistance. Results showed that RHA could affect the cement hydration by hydration promotion and pozzolanic effect. The evaluation function for electrical resistivity and curing ages fitted well with linear increasing function. The addition of RHA higher than 5 % demonstrated a decreasing role in the electrical resistivity of cement paste at earlier curing ages (3-7 d). Meanwhile, when at later curing ages (7-28 d) the result was the opposite. Moreover, RHA demonstrated positive effects on corrosion resistance of steel bars in cement paste.

Key words: rice husk ash; thermogravimetric analysis; electrical parameters; X-ray diffraction; corrosion resistance

Cite this article as: WANG Hui, ZHANG Ai-lian, ZHANG Lin-chun, WANG Qian, HAN Yan, LIU Jun-zhe, GAO Xiao-jian, SHI Fei-ting, LIN Xue-yan, FENG Li-yu. Hydration process of rice husk ash cement paste and its corrosion resistance of embedded steel bar [J]. Journal of Central South University, 2020, 27(11): 3464-3476. DOI: https://doi.org/10.1007/s11771-020-4559-8.

1 Introduction

In China, billion tons rice husk is produced from agricultural production which brought a series of ecological damage and environmental pollution [1]. As reported in some papers, rice husk ash (RHA) is rich in amorphous silica, which can replace part of cement and improve the mechanical behavior and resistance of cementitious materials [2-4]. As reported from some researchers, RHA serves as a type of active mineral admixture whose chemical properties are similar to silica fume [5]. Moreover, some researches indicated that RHA was effective to reduce porosity, improve pore structures and promote hydration degree [6]. Meanwhile, RHA was confirmed to be effective to enhance the mechanical strength and durability of high strength/performance concrete [7, 8]. VAN [8] reported that the mechanical strength of high strength/ performance concrete was improved by RHA due to the reduced free water, thus decreasing the effective water cement ratios. Moreover, the compressive strength and impermeability can be enhanced by RHA induced by the decreased scales and amount of pores in concrete.

CHANDRASEKHAR et al [9] and SIDDIQUE et al [10] stated that concrete with RHA showed favourable compactness. WANG et al [11] pointed out that RHA can be used as an internal curing agent which was effective to prevent concrete from shrinkage. SHARMA et al [12] pointed out that RHA was effective to improve the hydration process of cement-based materials. Although, the influence of RHA in cement hydration process has been widely studied, few researches were involved in reflecting the hydration process of cement-based materials with RHA by combining electrical method and thermogravimetric analysis [13].

Cement hydration is a complex physical and chemical process [14]. The microstructure and macro performance of cement matrix varied during hydration process [15]. Lots of literatures concerning with hydration mechanism have been reported [16]. Scanning electronic microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) are usually applied to analyzing the hydration and hardening mechanisms [17]. However, the influence of RHA on the microstructure, hydration products and process of cement paste troubled some researchers.

Concrete constructions are usually applied in complex environment when serving in coastal areas [18-21]. The complex service environment consists of freeze-thaw, long-term immersion and dry and wet alternation of salt solution by seawater, carbonization and their coupling effects, etc [22]. It should be noted that, electrons of iron element of steel bars in concrete are easy to lose when the concrete is applied in these environments, leading to the corrosion of steel bars and forming rust [23]. Moreover, conductive path of pore solution is blocked by rust and the conductivity is reduced. These working conditions created many factors which corrode steel bars in concrete [24]. Some researches pointed out that RHA was able to improve the compactness and promote the hydration process of cement matrix [25-30]. Therefore, RHA may have some positive effects on the corrosion resistance of steel bars in cementious materials. Meanwhile, little attention was paid to that.

This paper studied the hydration process of RHA cement paste. The proportion of rice husk ash varied from 0 to 15% by total mass of bind materials. Water cement ratio (w/c) in this research was 0.4. Thermogravimetric (TG) analysis, electrical resistivity, X-ray diffraction method and scanning electron microscopy (SEM) were determined to research the hydration process of cement paste. Additionally, the influence of RHA on corrosion resistance of steel bars in cement paste was investigated by analyzing the electrical parameters of steel reinforced cement paste. This research will further reveal the hydration mechanism and steel bars’ corrosion resistance of cement paste with RHA.

2 Experimental

2.1 Raw materials

42.5R ordinary Portland cement (China Anhui Conch Group Company Limited) was used as a cementitious material in this study. Polycarboxylate superplasticizer was applied in regulating the flowability of fresh paste. Rice husk, an agricultural waste material from Heilongjiang Province was burned in an airtight high temperature furnace to produce RHA. Firstly, origin rice husk was heated from 10 °C to 500 °C in 2 h, and then the residue was calcined and ground in a vibrating mill for 15 min as described in earlier research [26]. The particle size distribution and chemical compound were the same as that in Refs. [25, 31, 32]. Tables 1 and 2 show the particle passing percentage and chemical composition of cement and RHA respectively. In Table 2, “R₂O” means “Na₂O+K₂O”. Grade HPB300 plain round steel bars with diameter of 10 mm were used in experiments. The particle passing percentage was determined by the test sieving method for fineness of cement according to Chinese Standard (GB/T 1345—2005).

2.2 Mixing proportion

Mix proportion of the cement paste with RHA was shown in Table 3. In this experiment, specimens with water-cement ratio of 0.4 and four mass ratios of RHA (0%, 5%, 10% and 15% by mass of total cementitious materials) were manufactured. The mass of water reducing agent is 0.6% by the total mass of cementitious materials. Three samples of each group were used for every experiment while the average result was the final result of each test.

Table 1 Particle passing percentage of cementitious materials

Table 2 Chemical composition of cementitious materials in mass fraction

Table 3 Mixing mass proportion of fresh cement paste

2.3 Specimens preparation and measurement

NJ-160A cement paste mixer was provided to fabricate RHA cement paste. Firstly, cement, RHA and water were mixed homogeneously in mixer and stirred at a speed of 140 r/min for 2 min. Then a higher stirring speed (285 r/min) was supplied to mix the mixture for another 2 min. After that, fresh paste was poured into molds with dimensions of 40 mm×35 mm×55 mm. AC electrical resistance was measured by a TH2810D LCR digital electric bridge (Changzhou Tonghui Co., Ltd.). The testing frequency of this electric bridge ranged from 100 Hz to 10 kHz, and the testing AC voltage varied from 0.1 V to 1 V. In this study, the electrical resistance of each sample was determined at frequency of 10 kHz, and the testing voltage of each sample was 1 V. Two pieces of AISI 316 L stainless steel meshes (5.0 cm×3.0 cm) embedded samples served as testing electrodes. The distance between two electrodes was 4.0 cm. A German STA 449 F3 simultaneous thermogravimetric analyzer was selected for the thermal analysis experiment. Approximately 20 mg of sample was placed in alumina pan and heated from room temperature to 600 °C at a rate of 10 °C/min under the nitrogen atmosphere with a flow rate of 20 mL/min. All samples were held at 50 °C for 24 h and filtrated with a 74 μm sieve before thermogravimetric just as described in Ref [33]. JSM-6360LV scanning electron microscope (Japan electron optics laboratory, Tokyo, Japan) and D8 ADVANCE X-ray diffractometer (Bruker Corp., Tokyo, Japan) were applied for scanning electron microscopy (SEM) and X-ray diffractometry (XRD) experiments respectively. The treatment steps for SEM and XRD experiments are described as follows: Firstly, specimens with different curing ages were immersed in anhydrous alcohol for 5 d. After immersing, the specimens were dried in vacuum drying oven for 4 d. Dried samples with soybean size were metallized with gold and used for SEM observations. Meanwhile, else dried samples were pulverized and used for X-ray powder diffraction (XRD) tests.

Specimens with dimensions of 50 mm×50 mm×50 mm were selected to determine the electrical properties of steel bars reinforced cement paste. All specimens were covered by plastic sheets for 2 d storage at room environment (20 °C and 40% relative humidity) and then demolded. The demolded specimens were cured at a standard fog room with temperature of 20 °C and relative humidity of above 95% for another 26 d. After curing, all specimens were immersed in 3% NaCl solution for a year. Each specimen was embedded with a steel bar with length of 6 cm and diameter of 10 mm. A 316L stainless steel mesh and steel bar were served as two electrodes of each specimen, space between steel mesh and axis of steel bar was 2.0 cm. Electrochemical workstation (Shanghai Princeton Instrument Co., LTD) was used to test the electrochemical impedance spectroscopy. The measurement of electrical parameters is shown in Figure 1.

3 Results and discussions

3.1 Interpretation of TG Curves

Figure 2 shows the TG and DTG curves of RHA cement paste at different curing ages. Detailed inspection of TG and DTG curves showed that bound water decomposed in three successive steps (Figures 2(a)-(h)). To quantify the hydration degree of RHA cement paste at different curing ages, TG/DTG tests were carried out at a temperature ranging from 20 to 600 °C. As shown in Figure 2, the TG results are presented in the left column, while the corresponding DTG results are shown in the right column. For all the samples, the total mass loss increased with the increase of curing ages, indicating that the hydration degree continuously increased during curing. Based on the DTG curves, the mass loss of all samples can be mainly divided into three steps, corresponding to the desorption of water and the decomposition of hydration products. The first step occurring between 50 °C and 150 °C with mass loss ranging from 2.1% to 5.2% was caused by the evaporation of free water in cement paste. Then, the continuous mass loss below 400 °C can be mainly attributed to the desorption of water in the C—S—H pores and the decomposition of C—S—H gel. As temperature varied from 400 °C to 460 °C, a steep mass loss ranged from 3.1% to 17%, which was attributed to the decomposition of Ca(OH)₂(CH) crystals. It was noteworthy that, when the curing age was 3 h, this mass loss was not obvious, corresponding to a negligible content of CH at early-age hydration. With the hydration proceeding, the fraction of CH grew rapidly. In addition, the TG curves of 7 d were nearly comparable with those of 28 d, indicating a gradual completion of hydration from 7 d to 28 d [34-39].

3.2 Electrical resistivity

Figure 3 shows the resistivity of RHA cement paste tested by a TH2810D LCR digital electric bridge (Changzhou Tonghui Co., Ltd.) with a frequency ranging from 100 Hz to 10 kHz. All specimens with curing ages of 3 h, 1, 7 and 28 d were taken from the water and wiped off water, and then their electrical resistivity was tested. Resistivity of cement paste increased lineally as the curing ages ranged from 3 h to 28 d. This was attributed to the fact that the increased curing age was able to improve the hydration degree of cement thus increasing the electrical resistance. When the RHA content ranged from 0 to 5%, the increasing speed of resistivity of samples was decreased by the addition of RHA due to the delayed hydration process by RHA [26]. However, when the RHA content ranged from 5% to 15%, the increasing speed of resistivity of samples was increased by the addition of RHA due to the fact that RHA at high dosage (more than 5%) effectively helped accelerate the hydration rate of cement paste. As shown in Figure 3, the relationship between electrical resistivity and cured ages can be expressed by linear function relation. Fitting degree of each specimen was 0.99; therefore, the experimental results fit well with fitting results.

Figure 1 Measurement of electrical parameters

Figure 2 Thermogravimetric analysis curves of specimens with RHA:

Figure 3 Electrical resistivity of specimens with RHA

3.3 XRD analysis

Figure 4 shows XRD images of specimens after standard curing for 28 d. From Figure 4, it can be observed that the diffraction peaks of 3CaO·SiO₂ (C₃S), 2CaO·SiO₂ (C₂S), cristobalite (SiO₂) of all groups were very strong and the crystallinity is good [33-35]. Diffraction peak of calcium hydroxide (CH) first increased and then decreased with the increasing curing ages as shown in Figure 4. This might be attributed to the fact that at earlier curing ages (3 h-7 d), main mineral phases in cement clinker like 3CaO·SiO₂ (C₃S) or 2CaO·SiO₂ (C₂S) reacted with H₂O, and then calcium silicate hydrates (C—S—H) and CH formed. For the curing age of 1 d, it can be observed that the diffraction peak of calcium hydroxide (CH) increased with the increase of RHA content, indicating that the early-age hydration can be accelerated with the incorporation of RHA. As shown in Figure 4(d), when the RHA content was the highest (RHA-4), the diffraction peak of CH first increased and then decreased with the increase of curing ages, which might be attributed to the pozzolanic reaction of SiO₂ with CH. Combined with the TGA results, the formation of additional C-S-H can be verified. Meanwhile, from all the XRD patterns, it can be observed that the main peak of C₃S located at about 29° (2θ) decreased with the curing process, indicating the gradually depletion of C₃S during the hydration especially with the highest amount of RHA (RHA-4) [40-47].

Figure 4 X-ray diffraction patterns of specimens with RHA:

3.4 SEM analysis

The RHA cement paste microstructures can be clearly observed by SEM images shown in Figure 5. All samples were determined at curing ages of 3 h, 1, 7 and 28 d, respectively. As observed in these figures, the amount of CH first increased and then decreased with the increasing curing ages. When the curing age was 7 d, the amount of CH in RHA cement paste was the highest. The increased curing ages demonstrated a positive effect on the compactness of cement paste. Moreover, the increased amount of RHA led to ascending the amount of flocculent hydration products at earlier curing age (less than 28 d) and improving the compactness of specimens.

3.5 Corrosion resistance of steel bars

As described in some researches, electrons loss of iron element on the interface between steel bar and air or water is the essential of steel bars’ corrosion. After corrosion, the conductive path of pore solution is blocked by the rust leading eventually to reducing the conductivity of steel bars reinforced concrete [48, 49]. Therefore, specimens with higher electrical resistivity possess better corrosion resistance. Figure 6 shows the AC electrical resistance of steel bars reinforced RHA cement paste with standard curing ages ranging from 1 month to 12 months. As shown in Figure 6, the increase of curing ages led to ascending the electrical resistance of samples. The addition of RHA demonstrated an increasing effect on the electrical resistance of steel bars reinforced RHA cement paste (except that 5% RHA presented a dropping tendency in electrical resistance compared with specimens without RHA) when cured for 1 month. Moreover, the AC electrical resistance results in Figure 6 show less values for samples with 10% and 15% of RHA in comparison to reference (0%) from 4 month of curing on. This might be attributed to the fact that the pozzolanic effect by RHA was effective to decrease the electrical polarization resistance of samples [26, 32]. Moreover, the pozzolanic effect could improve the corrosion resistance of steel bars in specimens [50, 51]. Therefore, specimens with 10% and 15% of RHA corroded less seriously than samples with 0% RHA. Therefore, samples with higher content of rust behaved lower conductivity. Additionally, the addition of RHA could decrease the electrical polarization, leading to decreasing the electrical resistance.

Hence, the AC electrical resistances of samples with 10% and 15% RHA were lower than that of samples with 0% RHA. Figure 7 depicts the variation of electrical resistance with increasing curing ages. As shown in Figure 7, the increasing speed of specimens decreased in the order of RHA- 5%>RHA-0%>RHA-10%>RHA-15%, confirming that specimens with RHA higher than 5% corroded less seriously, thus possessing more excellent corrosion resistance of steel bars.

The electrochemical impedance spectroscopies (EIS) for the specimens with different dosage of RHA are shown in Figure 8. In Figure 8, real component (Z_r) represents electrical resistance while the imaginary component (-Z_i) means electrical reactance. Figure 9 shows the corresponding equivalent circuits selected to analyze the mechanism of conductive properties. It can be observed from Figure 8 that the imaginary components (-Z_i) of all groups ascended linearly with real components (Z_r). In EIS curves, -Z_i represents electrical reactance, while Z_r means electrical resistance. As shown in Figure 8 that, EIS values of all groups move from left to right, confirming that curing ages demonstrated an ascending trend in electrical resistance. Moreover, as illustrated in Figure 8 that curing ages had little influence on electrical reactance of specimens.

Figure 5 SEM micrographs of specimens with RHA:

Figure 6 AC electrical resistance with curing ages

Figure 9 shows the following corresponding equivalent circuits stemming from EIS curves. In Figure 9, steel bar buried in the axis position of specimen can serve as an electrode. The circular section can be represented by the parallel of resistance and capacitance elements. The initial resistance (R_s) represented the contact resistance between specimen and electrodes. Meanwhile,R₁, R₂ and R₃ are electrical resistances of pore solution, cement matrix and rust, respectively. Additionally, C₁, C₂ and C₃ represent the corresponding electrical capacitance. R_i and C_i (i=1-3) are paralleled. While, each paralleled combined electrical component is in series. Conductive path way was formed by pore solution in cement matrix. The addition of RHA higher than 5% was effective to reduce the electrical conduction, thus preventing electron migration in samples. The electrical impedance (Z) can be obtained from equation (1) (where X_ci is the capacitive reactance of each electrical component). The Chi-squared of each specimen was less than or equal to the value of 8.225×10^-4, which indicated that this equivalent circuit model was rational and appropriate to the conductive mechanism.

Figure 7 Resistance-time curves of reinforcement cement paste

Figure 8 EIS curves of steel bars reinforce specimens:

Figure 9 Corresponding equivalent circuits of steel bar reinforced specimens with RHA

                 (1)

As shown in Figure 8 and Table 4 that, the relationship between -Z_i and Z_r fits well with linear function relation since the fitting degree of each group is higher than 0.94. The fitting equation is expressed as follows:

-Z_i=a×Z_r+b                             (2)

Table 4 Fitting results between imaginary components and real components

4 Conclusions

1) It could be obtained from the results of electrical resistivity of RHA cement paste that the relationship between electrical resistivity and curing time could be described through linear function. The addition of RHA less than 5% led to decreasing the electrical resistivity of cement paste, while the addition of RHA 5%-15% demonstrated increasing roles on the electrical resistivity.

2) Through microstructure analysis, the influence of RHA on cement hydration could be summarized as hydration promotion and pozzolanic effect. Based on the TG analysis, it could be observed that the total mass loss increased with the increase of RHA content, indicating that the overall degree of hydration could be accelerated by the incorporation of RHA. However, when temperature ranged from 400 to 460 °C, based on the mass loss, the content of CH especially at 28 d decreased with the increase of RHA content. Combined with the following XRD and SEM results, the existence of RHA mainly accelerated the consumption of unhydrated cement (i.e., C₃S). At the later period of hydration (28 d), the reduction of CH was caused by the reaction of CH with pozzolanic RHA.

3) RHA was effective to improve the corrosion resistance of steel bars in cement paste. Additionally, EIS was sensitive to reflect the corrosion resistance and corrosion degree of steel bars in RHA cement paste. The conduction of steel bars reinforced RHA cement paste could be simplified as the equivalent circuit which consisted of the contact resistance of electrodes, shunt-wound electrical resistance and capacitance of pore solution, cement matrix and rust, respectively. Based on this model, the changing law of conductive performance of steel bars reinforced concrete could be obtained through EIS analysis.

Contributors

WANG Hui conducted conceptualization, methodology, data curation, writing-original draft, writing-review and editing. ZHANG Ai-lian finished methodology, investigation, data curation. ZHANG Lin-chun conducted software and validation. WANG Qian conducted conceptualization and methodology. HAN Yan conducted data curation, software, validation. LIU Jun-zhe provided writing-review and editing, resources, project administration and funding acquisition. GAO Xiao-jian finished some supervision and paper writing. SHI Fei-ting corrected some written errors in the manuscript. LIN Xue-yan conducted some experiments. FENG Li-yu conducted some experiments.

Conflict of interest

All authors declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

稻壳灰水泥浆的水化进程及其内置钢筋的耐腐蚀性能

摘要：本文采用热重分析和电阻率来测试稻壳灰水泥净浆的水化进程。水泥净浆中稻壳灰的掺量为0~15%，净浆的水灰比为0.4。用X射线衍射方法和扫描电镜技术研究试件养护3 h、1 d、7 d和28 d后的晶体组成和微观结构。最后研究了RHA对水泥净浆埋入钢筋后的电学性能参数(电阻和交流阻抗谱)的影响。研究结果表明，RHA能够通过对水化的促进作用以及火山灰效应影响水泥的水化。RHA水泥净浆的电阻率与其养护龄期符合良好的线性增长关系。当养护龄期在早期(3 h~7 d)，水泥净浆中RHA掺量超过5%时，RHA引起净浆电阻的降低；然而当养护龄期为晚期(7 d~28 d)时，结果恰好相反。此外, RHA能够对水泥净浆中钢筋的耐腐蚀性能起到促进作用。

关键词：稻壳灰；热重分析； 电学性能参数；X射线衍射； 耐腐蚀

Foundation item: Projects(51808300, 51778302) supported by the National Natural Science Foundation of China; Project supported by the K.C. Wong Magna Fund in Ningbo University, China

Received date: 2020-03-01; Accepted date: 2020-09-22

Corresponding author: LIU Jun-zhe, PhD, Professor; Tel: +86-15825570195; E-mail: junzheliu2@126.com; ORCID: https://orcid.org/ 0000-0001-7246-1621