Fast Cured Mineral-Impregnated Carbon-Fiber (MCF) Reinforcements Made of Geopolymer as a Promising Alternative to Conventional Fiber Reinforced Polymer (FRP) Systems
Authors
:
Jitong Zhao, Marco Liebscher, Golrokh Airom, Viktor Mechtcherine
The chapter delves into the development of fast-cured mineral-impregnated carbon-fiber (MCF) reinforcements using geopolymer as a promising alternative to conventional fiber-reinforced polymer (FRP) systems. It highlights the advantages of MCF in terms of enhanced durability, fire resistance, and compatibility with concrete matrices. The study investigates the optimal curing conditions for MCF production, focusing on temperatures of 50°C and 75°C for durations ranging from 2 to 8 hours. The resulting MCF composites exhibit excellent mechanical properties, with flexural strengths reaching up to 590 MPa after 8 hours of curing at 50°C. Additionally, the chapter explores the bond-slip relationships between MCF and GP concrete substrates, demonstrating superior bond strength at elevated temperatures compared to traditional epoxy-coated carbon fibers. The findings suggest that MCF reinforcements hold significant potential for the construction industry, offering a robust and sustainable alternative to existing FRP systems.
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Abstract
This study introduces the design and realization of a fast-setting technology for an efficient industrial production of a novel mineral-impregnated carbon-fiber (MCF) reinforcements for the building sector. By employing mineral-based matrices for carbon fiber (CF) reinforcements, numerous advantages can be achieved, including high temperature resistance, cost-effectiveness, reliable bonding with concrete substrates, and enhanced flexibility in automated processing.
This study focuses on the impact of different thermal curing regimes for the forming process of the MCF composite. The fabrication process involves commercially available raw materials and the utilization of a continuous pultrusion line, followed by oven heating at temperatures of 50 ℃ and 75 ℃ for short durations. The purposefully designed impregnation suspension allowed a sufficient long-lasting processing window at the early age. Extensive experimental investigations have been conducted to examine the development of the resulting MCF performance and the implementation of the MCF as reinforcement in GP concrete at varying temperature levels.
1 Introduction
Amidst depleting energy reserves and increasing environmental pollution, carbon fiber reinforced polymer (CFRP) composites have gained prominence in modern society. By utilizing carbon fiber (CF) reinforcement, these composites enable the construction of lightweight structures and the strengthening of traditional building materials, resulting in reduced fuel consumption and harmful emissions [1]. For the available CF composites, commonly polymeric matrices are applied to secure their shape stability, inner stress-transfer, and reinforcing ability to the concrete matrix [2]. But, insufficient fire resistance and poor compatibility with the concrete matrices greatly restrict their broad application [3]. To tackle these challenges, a promising alternative to traditional steel or polymer-based reinforcements mineral-impregnated carbon fiber (MCF) reinforcements has emerged. This innovative impregnation technology involves currently utilizing minerals, specifically hydraulic micro-cements [1], silica fume [4] and alternative binders, i.e., aluminosilicate [5, 6] or calcium silicate cement [7]. MCF reinforcements, with their unique composition and profiles, enable comparable load-bearing capacity to FRP while offering improved durability, fire resistance, and compatibility with concrete. Additionally, their high geometrical flexibility during the fresh and forming stages unlocks vast potential for automation and digitalization [8]. Amongst abundant variants, geopolymer (GP) impregnating suspensions offer a promising solution in terms of the long-lasting processing window and reliable impregnation quality during the early stages for efficient industrial production [9]. The syntheses of GP via moderate thermal activation at temperatures below 100 ℃ facilitate rapid hardening and strength evolution, addressing the concept development of the fast-setting inorganic matrix composites as advanced construction reinforcement materials, akin to thermosetting resins.
The stimulus of the present study leans upon developing fast-setting forming process for GP-based MCF composite via brief, controlled thermal treatment. A highly automated inline MCF production was employed. To identify the optimal combination, the investigating parameters involve curing temperatures of 50 ℃ and 75 ℃ for varying durations, ranging from 2 h to 8 h. The resulting MCF composites were qualified regarding their physicochemical and mechanical behavior at the early age and 28 days and implemented in a GP concrete matrix. Load transfer capacity of MCF was validated at various temperature levels from 20 ℃ to 200 ℃.
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2 Experimental Program
2.1 Materials and Manufacturing of Mineral-Impregnated Carbon-Fiber
To manufacture MCF, a commercial carbon tow, SIGRAFIL® C T50–4.4/255-E100, manufactured by SGL Group, Germany, was selected. This tow consisted of 50,000 individual filaments, each with a diameter of 6.9 µm and tensile strength of 4,400 MPa, treated with epoxy sizing. The impregnating suspension and corresponding mixing procedure were specifically tailored to attain a complete impregnation of the yarn, following a previous study [10]. The suspension comprised metakaolin (MK) (\({\text{Al}}_{{2}} {\text{O}}_{{3}} \cdot {\text{2SiO}}_{{2}}\)) from BASF, Germany, a superplasticizer (SP) Sapetin D27 from Woellner, Germany, and a commercial potassium silicate activator Geosil® 14517 from Woellner, Germany. The mixing was performed intensively using a T 50 digital ULTRA‐TURRAX® at a speed of 7000 rpm for 7 min. The designed GP impregnation matrix, made from metakaolin, revealed a delayed setting time up to approximately 14 h under room temperature condition. The manufacture of the unidirectional MCF element was achieved via an automated, continuous pultrusion process, as detailed in [5]. The carbon rovings were continuously pulled at a velocity of 6 m/min under constant tension. Pre-wetting was carried out using a kiss-coater, followed by impregnation in a bath equipped with five-roller-foulard. The finally shaped MCF was obtained using a conical nozzle with an inner opening diameter of 4.1 mm. After initial impregnation, the semi-finished composites were thermally cured under sealed conditions in oven. Curing parameters involved the temperature conditions of 50 ℃ and 75 ℃ for varying durations, ranging from 2 h to 8 h. Subsequently, the cured MCF specimens were stored at 20 ℃ and a relative humidity of 65% until testing. Figure 1 shows the used CF roving and the obtained MCF bar. The made reinforcement possessed a nearly circular cross-section after solidification with a fiber volume fraction of approximately 16 vol.-%.
Fig. 1.
Ready-made bar of GP-based MCF (below) and used CF roving (top).
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2.2 Characterization of Impregnating Matrix and MCF
To assess the impact of heating treatment on the flexural performance of MCF composites, three-point bending tests were conducted using a Zwick-Roell testing machine (model Z 1445) equipped with a load cell (capacity: 1 kN) on the young and 28 day age specimens. The tests were performed with a span of 100 mm, following the procedure outlined in [11]. The displacement rate during the tests was set at 5 mm/min. The bond-slip relationships between the yarn and GP concrete substrate was characterized by one-sided in-situ pullout tests with an Instron machine (model 8501), load cell and Instron climate chamber at a displacement rate of 1 mm/min at room temperature (~ 20 ℃), 100 ℃ and 200 ℃ following a previously established testing setup [12]. The considered GP concrete matrix comprised of MK, the same potassium silicate activator (Geosil® 14517), quartz sand and rough sand, associated with 28 d compressive strength of 62.8 MPa, as described in [13]. A commercial FRP, GRID Q85/85 – CCE – 21 from Solidian (Albstadt, Germany), impregnated with epoxy (EP) resin and with tensile strength of 3300 MPa was used as reference. The pullout data has been as well published previously in [13]. The fiber matrix distribution and fracture surface of the composites were observed using an environmental scanning electron microscopy (ESEM) with a Quanta 250 FEG instrument manufactured by FEI (The Netherlands).
3 Results and Discussion
3.1 Characterization of Impregnating Matrix
Figure 2 shows representative ESEM images of the impregnation matrices fractured after curing at 50 ℃ and 75 ℃ for 2 h and 8 h, respectively. The matrices treated at 50 ℃ for 2 h indicated an amorphous binder phase with more unreacted or partially reacted MK particles. When extended for 8 h, less unreacted particles and more built aluminosilicate gel were visible, declaring the superior mechanical performance for the MCFs with the extended curing regime. At 75 ℃ after 2 h of curing, the size and number of unreacted MK particles were apparently reduced, while both the GP gel and void contents stemming from dissolved MK particles increase slightly in the structure. With extending time to 8 h, a more porous and less ordered microstructure was particularly built.
Fig. 2.
ESEM images of the GP matrices cured at 50 ℃ and 75 ℃ (a, b) for 2 h and (c, d) for 8 h, respectively.
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The morphological characteristics of the homogeneous filament-matrix distribution provide compelling evidence in support of the high quality of the designed GP suspension; as determined using microscopic image of the cross-section of MCF; cf. Figure 3a. The specimen was cured at 50 ℃ for 8 h and these images accurately represent specimens produced with varying curing conditions. The black circles denote the locations of individual filaments. Instances of impregnation matrix accumulation without embedded filaments were observed infrequently. An even incorporation of carbon filaments within the matrix yields excellent interfacial stress transfer between components, thereby contributing to the exceptional mechanical performance of the composite. In Fig. 3b, a consistent and continuous embedding of the fiber without discernible gaps was evident, suggesting a favorable physical interaction between the components.
Fig. 3.
ESEM images showing in a) the cross-section and in b) fracture surface of MCF cured at 50 ℃ for 8 h.
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Differences obtained in the morphological evolution correlated well with considerable changes in the flexural performance of MCF composites; see Fig. 4. Since the thermal curing effectively accelerated the geopolymerization reaction and therewith a rapid development in matrix strength in the early stage of the forming process, outstanding flexural performance was attained within few hours. With rising temperature, here from 50 and 75 ℃, a faster strength gain both under flexural load can be seen merely before 4 h, relating to the advanced geopolymerization [14], whereas further constantly heating yielded in inferior flexural strength. For the thermally cured specimens for 8 h at 50 ℃, the early-age flexural strength reached the highest value of 590 MPa, 30% greater than 75 ℃. This decreasing trend was attributed to the increased matrices’ porosity, seen in Figs. 2 and 3. At both temperatures, the extension of curing, here from 2 h to 8 h, resulted in a gradual increase in flexural strength, triggered by the increased degree of geopolymerization [5]. After storing for additional 28 days at 20 ℃, the flexural strength exhibited similar values to the young age except for 2 h-cured composite counterparts and generated also an increasing trend with extended curing. Consequently, the above findings suggested an extended curing duration at a moderate temperature, e.g., 50 ℃ for 16 h, for the post-treatment of MCF, which was further used for the resulting MCF in the yarn pullout tests with GP concrete.
Fig. 4.
(a) Flexural strength and (b) tensile strength of MCFs tested immediately after curing at 50 ℃ and 75 ℃ as well as at the age of 28 days.
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Finally, pullout results highlighted comparable average shear bond strength in GP concrete to the EP yarn for the developed MCF reinforcement at room temperature; see Fig. 5a. Whereas, distinct pullout curve profiles were seen between them; see Fig. 5b. The MCF bars revealed a fast ascending branch in the initial loading stage until reaching the peak debonding force at a small displacement and with a higher shear modulus followed by a sudden drop to a moderate pullout force level. The particular curve profile emphasized good chemical compatibility of MCF towards GP concrete matrices. The reference EP yarn was characterized by a slowly and nonlinearly ascending branch and afterwards a gradual decay in frictional force. The bond of EP yarn was dominated by the mechanical interlock from the surface deformation, rather than the chemical adhesion between components. With rising environmental temperature, a significant downward trend was accompanied by reductions of 21% and 95% at 100 ℃ and 200 ℃, respectively. The significantly diminished bond observed in the EP yarns was triggered by the substantial softening of the viscoelastic epoxy and the degradation of the shear modulus upon surpassing its glass transition temperature [13]. However, considerable enhancement in bonding behavior at elevated temperatures levels were depicted with GP impregnation, maintaining approx. 60% bond strength at 200 ℃.
Fig. 5.
(a) Shear bond strengths and (b) representative pullout force-displacement curves at different temperature levels for the epoxy yarns and the MCFs, adapted from [13].
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4 Summary and Conclusions
This paper highlights the potential of automated produced and fast-setting MCF as promising fiber reinforcement system for the construction industry. Efficient rapid MCF setting was achieved via a purposefully designed GP impregnation matrix based on MK. A sufficiently long-lasting processing window allows an easy usage of the mineral suspension as well as a high flexibility for industrial applications.
By applying targeted heating at 50 ℃ and 75 ℃, a rapid setting and early strength gain of the derived MCF prototype was realized at different rates and almost completed merely within the initial several hours, similar to the forming process of FRP with thermosets. The produced MCF achieved a maximal flexural strength of 590 MPa after 8 h of curing at 50 ℃, being comparable to conventional FRP. The temperature treatment with prolonged duration indicated the advancement of geopolymerisation of the impregnation matrix and enhanced mechanical performance of resulting MCFs. However, negative impacts of the heat curing promoted the formation of a more porous and less ordered microstructure. For this reason, longer curing duration at relatively low temperatures, e.g., 50 ℃, is expected to be beneficial for durable MCF.
Finally, the novel type of reinforcement delivered equivalent bond strength to commercial CF yarns with epoxy coating at room temperature and evident improvement in bond strength at elevated temperatures, as proven by yarn pullout tests.
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Fast Cured Mineral-Impregnated Carbon-Fiber (MCF) Reinforcements Made of Geopolymer as a Promising Alternative to Conventional Fiber Reinforced Polymer (FRP) Systems
Authors
Jitong Zhao Marco Liebscher Golrokh Airom Viktor Mechtcherine
