A new implementation of electrochemical impedance spectroscopy (EIS) and other methods to monitor the progress of hydration of strontium monoaluminate (SrAl2O4) cement
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Electrochemical impedance spectroscopy has been employed to monitor hydration of strontium monoaluminate (SrAl2O4) cement. Other supported techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, thermal analysis (DSC–TG–EGA) and microcalorimetry were also used. In the impedance spectrum at 102 day on a 0.5 water/cement ratio paste, a large double depressed low-frequency arc, a single depressed arc at middle-frequency region and a small part of a large depressed arc at high-frequency region were discriminated. It was due to the specific phase composition and crystal phase content in the fully hardened cement paste. Hence, the new electrochemical equivalent model R1(C1(R2W1))(C2(R3W2))(C3(R4W3))(C4(R5W4)) was implemented and fitted to the experimental results of the fully hydrated and hardened SrAH cement paste. Various hydration products including crystalline Sr3AH6, and amorphous phases SrAH7 and AH3-gel were formed at an early age of hydration. At final hydration process, the main reaction products detected are the ones most thermodynamically stable, i.e. crystalline Sr3AH6 and Al(OH)3. The heat evolution of SrAl2O4 cement under different temperatures (20 °C and 40 °C) was examined by isothermal calorimetry. The curing temperature was found to have a visible effect on cement hydration kinetics.
KeywordsStrontium monoaluminate (SrAl2O4) Electrochemical impedance spectroscopy (EIS) Equivalent circuit models Hydration Microstructure
With the recent dynamic quest for developing sustainable ceramics, refractory materials and building materials, it has been found that there is a need for more advanced material characterization techniques that can provide valuable insight into the nature and fundamental behaviour of the new classes of cementitious materials as fast as they are becoming available. These methods can be implemented for understanding and predicting, for example, cement hydration kinetics, microstructure development and long-term performance of various cementitious systems. Examples of these novel techniques that have been recently used for cementitious material characterization include X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance spectroscopy (NMR), X-ray microtomography and atomic force and lateral force microscopy (AFM and LFM) [1, 2, 3] apart from the most commonly used in cement chemistry, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS). Recently, AC electrochemical impedance spectroscopy (EIS) has been demonstrated to be a promising technique for tracing hydration feature of different cementitious systems [4, 5, 6, 7, 8, 9, 10]. It is well known that, immediately after mixing cement, a gel layer forms on the surfaces of the cement grains. Substantially, morphological changes in microstructure during the cement hydration process occur, mainly due to its transformation from the viscous liquid to the solid state. The cement minerals are replaced by new solid hydration products, whereas the pore solution acts as a necessary transition zone between the two solid states. Hence, the unique EIS response of hydrating cement paste should be expected at each stage of reaction.
The early stage of the cement paste hydration, where a liquid phase of the saturated cement paste dominates and a significant movement of ions occurs in the pore solution, can be considered as a simple electrochemical system. A typical electrical equivalent circuit of R1(C(R2W)) type can be used to simulate the experimental results , where R1 denotes the solution resistance, C is related to the double-layer capacitance of the electrodes/electrolyte interface, R2 stands for charge transfer resistance of the electrodes/electrolyte interface and W considers the Warburg character associated with some diffusional processes at the electrode/paste interface [4, 8, 11]. In a recent study, Madej and Kruk  develop a novel electrochemical equivalent model R1(CPE(R2W1))W2 that can better explain phenomena of the early stage hydration process of cementitious compounds. At the mid of the cementing materials hydration process, cement paste hardens, its ion content decreases gradually and the high-frequency arc in the impedance spectrum appears. Hence, the impedance spectrum shows both electrode and bulk features, and the following electrochemical model R1(C1(R2W1))(C2(R3W2)) successfully describes the impedance response of the hardened cement paste [4, 8, 12].
Major progress has also been made to date in the development of novel cement-based systems such as C–A–Z–H, C–Sr–A–Z–H and Sr–A–H (C = CaO, A = Al2O3, Sr = SrO, Z = ZrO2) materials containing mainly calcium/strontium aluminate hydrates [13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Nowadays, cements containing strontium aluminate phases are currently being evaluated for technological applications owing to their unique both physical and chemical properties over other high alumina cements such as calcium aluminate cements (CACs). The applications, such as dense or thermal insulating refractory materials, and the binding materials used for preparing radiation shielding concrete to protect against X-ray and gamma radiation are supposed for strontium aluminate cements . Strontium monoaluminate SrAl2O4 (SrA; S = SrO, A = Al2O3) is an analogue of the well-known calcium monoaluminate CaAl2O4. According to the calculated binary phase equilibrium diagram of SrO–Al2O3 given by Ye et al. , the congruent melting point of SrAl2O4 is determined to be 1960 °C.
This scientific paper aims at providing a new implementation of electrochemical impedance spectroscopy (EIS) method to monitor the hydration of strontium monoaluminate (SrAl2O4) cement. In this aspect, this study was undertaken to examine more closely the impedance behaviour of special cement paste throughout the hydration process, giving special attention to a novel electrochemical equivalent model.
Materials and methods
The reagent grade chemicals, strontium carbonate (99.00% SrCO3, Merck) and alumina α-Al2O3 (99.8% Al2O3, Acros Organics), were used as the starting materials. Synthesis of strontium monoaluminate SrAl2O4 was accomplished in a two-step firing process. An intermediate grinding/mixing stage of calcined mixture was necessary in order to improve its homogeneity. In a first step, raw materials were mixed with the 1:1 molar ratio of SrO and Al2O3 oxides, and then the mixture was homogenized for 2 h in a zirconium ball mill and pressed into cylinders having a diameter of 2 cm. All green pellets were calcined at 1300 °C for 10 h. In a second step, solid-state sintering of the pellets made from the calcined powder at 1550 °C for 15 h resulted in a phase SrAl2O4. The heating rate for both the calcination and sintering was 2 °C min−1.
Cement paste preparation, tests and procedures
The specimen for the impedance measurement was prepared by hand mixing in an ambient atmosphere with water-to-cement ratio (w c−1) of 0.5. The water-to-cement ratio of 0.5 was applied to achieve a viscous suspension without any undesirable sedimentation of neat SrAH cement paste. In a typical experiment, after casting the fresh cement paste, two pieces of stainless steel with smooth surfaces were separated from each other by a spacing between the electrodes being 1.5 cm and immediately immersed in the SrAH cement paste. Each electrode comprised a 2.5-mm-diameter stainless steel rod (approximately 3 cm in length) with the comparable working electrode active area. The sample was cured at 95% humidity conditions. Two-point electrical impedance measurements were obtained on each sample using a Potentiostat/Galvanostat AutoLab PGSTAT302 N frequency response analyser (FRA). The signal amplitude used in the experimental programme was 0.05 V with the impedance measured over the frequency ranges 1 MHz–100 Hz (4-day-hydrated SrAH cement paste), 1 MHz–0.1 Hz (18-day-hydrated SrAH cement paste) and 1 MHz–170 Hz (102-day-hydrated SrAH cement paste) using a logarithmic sweep with 100 frequency points per decade. In order to obtain the equivalent circuits to explain the different possible behaviours of the cement paste after different moist curing periods, EIS experimental data were correlated with data from the simulation EIS Spectrum Analyser software. Kramers–Kronig relationships (K–K test) were adapted in order to analyse the validity of the experimental impedance results.
Simultaneous thermal analysis [i.e. differential scanning calorimetry (DSC), thermogravimetry (TG) and evolved gas analysis (EGA)], X-ray diffraction (XRD), infrared spectroscopy (IR) and scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS) were used to trace phase changes in SrAl2O4–H2O system as a function of time. SrAH cement paste was made from SrAl2O4 micro-powder and water (w c−1 = 0.5). Paste was hand-mixed for 1 min, then poured into sealed polyethylene bag and cured in a climatic chamber with the relative humidity maintained at 95% and temperature of 20 °C. At time intervals of 4, 18 and 102 days, the samples were immersed in acetone to terminate the hydration and air-dried. The NETZSCH STA 449 F5 Jupiter coupled to QMS 403 D Aëolos apparatus was designed for simultaneous thermal analysis (heating rate of 10 °C min−1, a flow of Ar (50 mL min−1) and α-Al2O3 as a standard substance). The Fourier transform infrared spectrometer from Bruker (Vertex 70) was designed to record IR spectra in the 400–4000 cm−1 range. KBr pellets method was applied. Nova NanoSEM 200 from FEI Europe Company with EDS analyser from EDAX was used to observe microstructure and to find the chemical composition in micro-areas of cement paste, respectively.
Hydration heat evolution and kinetics of the SrAH cement paste with w c−1 = 0.5 were analysed with a TAM air microcalorimeter (TA Instruments) at 20 °C and 40 °C by integrating the continuous heat flow signal during the 72-h hydration process. An admix ampoule which enables cement paste homogenization inside calorimeter was used for this purpose. The apparatus and the measurement procedure were described in detail in Ref. .
In situ electrochemical impedance spectroscopy (EIS) experiment
Kinetics of strontium monoaluminate hydration and detection of its hydration product
The intensity of the initial peak (Fig. 9a initial reaction period) corresponding to the wetting of the cement and the beginning of SrAl2O4 dissolution increases with an increase in the temperature measurement from 20 to 40 °C (Fig. 9b). The two exothermic stages were more clearly isolated from one another than in the case of the SrAH cement paste measured at 20 °C. The first exothermic stage corresponds to the formation of saturated solution, and the second exothermic part (acceleration and deceleration periods) to the formation of solid hydration products. The induction period (slow reaction period), i.e. a time of minimal hydration activity between the initial hydration reactions upon wetting and the later primary SrAl2O4 reaction with water to form hydrates, occurs between ca. 5 and 20 h of hydration (Fig. 9a).
Figure 10 shows an integration of the heat evolved over time of hydration, i.e. the total evolved heat. During the course of hydration, the cumulative heat evolved due to hydration of SrAl2O4 cement at 40 °C was higher than from hydrated at 20 °C after 72 h. Here, it is interesting that the hydration process was retarded between ca. 5 and 20 h since a dense Sr–A–H envelope forms around the strontium monoaluminate cement grains within the first few hours after mixing with water.
The IR spectra (Fig. 12b, c) confirmed the mineralogical composition of the investigated hydrating sample as determined by means of XRD. As a result of the interaction between cement particles and water, the infrared band of the SrAl2O4 decreased in intensity and the other new bands belonging to hydration products started to appear and increased in intensity upon the hydration time. The IR spectrum of the 102-day paste (Fig. 12c) is practically identical with absorptions assignable to cubic hydrate and aluminium hydroxide. The characteristic infrared bands of Sr3AH6 are 3665 cm−1 (OH stretching band), 785 cm−1 and 517 cm−1. According to Tarte , the assignment of the strong band observed at 517 cm−1 to the “pure” AlO6 vibration is evident. This spectrum exhibits a series of well-defined bands observed at 3622, 3530, 3466, 3395, 1026 and 972 cm−1, confirming the attribution of this band to gibbsite Al(OH)3 . In particular, the characteristic strong bands of gibbsite Al(OH)3 are 3530 and 3466 cm−1 due to the O–H–O stretching vibration and 1026 cm−1 due to the –OH bending vibration. The mid-IR spectrum of the fully hydrated SrAH cement paste (Fig. 12c) clearly shows strong ν2 CO32− and ν4 CO32− modes around 858 cm−1 and 663 cm−1, respectively. The band at 1460 cm−1 was generated by the antisymmetric stretching vibration (ν3) of carbonate CO32− anion.
The IR spectrum of the partly hydrated cement paste (Fig. 12b) contains absorption bands of both un-hydrated SrAl2O4 and strontium aluminate hydrates. The sharp band at 3665 cm−1 can be taken as characteristic of OH group vibrations in hydrated strontium aluminate of cubic type. The formation of Sr3AH6 hydrate in this sample was also confirmed by XRD. Moreover, a very broadband in the region between 3100 and 3600 cm−1 may indicate the presence of hydrated strontium aluminate other than the cubic one and the alumina gel (AH3 gel). Similarly, a weak band at 1645 cm−1 corresponding to H2O deformation mode supported the evidence for the presence of hydrated strontium aluminate other than the cubic type. Additionally, the absorption band at 1408 cm−1 is characteristic of AH3 gel .
The H2O–EGA curve of the 4-day-cured SrAH sample (Fig. 13a) records two stages of volatile product evolution, with the peaks at ca. 100 °C and 270 °C, by hydration products, and these are also indicated by two endothermic effects. It can be noted that the first endothermic peak at ca. 100 °C represents two overlapping dehydration of both amorphous SrAH7 and AH3 gel. The second endothermic effect at 272 °C corresponds to decomposition of the cubic tri-strontium aluminate hexahydrate (Sr3AH6).
It should be noted that a small quantity of minor phases can be undetectable by XRD analysis, especially in this case of diffraction patterns with wide reflections of un-hydrated phase and hydration products. However, SrCO3 in relatively minor amount appears slightly above background at ca. 2θ = 25.232º (JCPDS Card No. 01-074-1491) in the X-ray diffraction pattern of 4-day-hydrated sample (Fig. 11b). It is worth mentioning that the presence of SrCO3 is dependent on the sample preparation conditions. As presented in the “Cement paste preparation, tests and procedures” section, cement paste sample was soaked or ground with an excess of acetone and inevitably exposed to CO2 from air.
The work has opened up new area for the direct application of impedance spectroscopy techniques, namely as a method for control of SrAl2O4 hydration features.
Equivalent circuit models and impedance formulas R1(C1(R2W1))W2 and R1(C1(R2W1))(C2(R3W2))W3 for the SrAH neat cement paste and SrAH rigid gel were proposed, respectively. The results of this modelling are adequate for the early hydration characteristics of SrAl2O4.
Impedance spectra of the fully hydrated strontium monoaluminate cement paste showed a large double depressed low-frequency arc, a single depressed arc at middle-frequency region and a small part of a large depressed arc at high-frequency region. It was due to the specific phase composition and crystal phase content (Sr3AH6 and Al(OH)3) of the fully hardened cement paste. Thus, a new electrochemical equivalent circuit model R1(CPE1(R2W1))(C2(R3W2))(C3(R4W3))(C4(R5W4)) was established and used for the long-term age of SrAl2O4 cement.
From the XRD, FT-IR, DSC–TG–EGA and SEM–EDS measurements, it was found that the crystalline and thermodynamically stable Sr3AH6 and Al(OH)3 hydration products were formed in a fully hydrated SrAl2O4 cement paste, while amorphous SrAH7 and AH3 gel together with crystalline Sr3AH6 were formed at an early and middle ages of hydration.
From the isothermal calorimetry tests at 20 °C and 40 °C, it was found that the curing temperature was found to have a visible effect on SrAl2O4 cement hydration kinetics.
This project was financed by the National Science Centre, Poland, project number 2017/26/D/ST8/00012.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 21.Pöllmann H, Stöber S, Mohr P, Kaden R. In: Fentiman ChH, Mangabhai R, Scrivener KL, editors. Synthesis and crystal chemistry of strontium aluminates, calcium aluminates: proceedings of the international conference, IHS BRE Press, Garston; 2014. pp. 89–98.Google Scholar
- 22.Pöllmann H, Kaden R. In: Fentiman ChH, Mangabhai R, Scrivener KL, editors. Mono- (strontium-, calcium-) aluminate based cements, calcium aluminates: proceedings of the international conference, IHS BRE Press, Garston; 2014. pp. 99–108.Google Scholar
- 24.Ptáček P. In: Ptacek P, editor. Applications of strontium aluminate cements, strontium aluminate—cement fundamentals, manufacturing, hydration, setting behaviour and applications. London: IntechOpen; 2014. p. 141–85.Google Scholar
- 30.Fernández-Carrasco L, Torrens-Martín D, Morales LM, Martínez-Ramírez S. Infrared spectroscopy in the analysis of building and construction materials. In: Prof. Theophanides T, editor. Infrared spectroscopy—materials science, engineering and technology, ISBN: 978-953-51-0537-4, InTech; 2012. https://doi.org/10.5772/36186. http://www.intechopen.com/books/infrared-spectroscopy-materials-science-engineering-and-technology/infrared-spectroscopy-of-cementitious-materials.
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