Repair composition for self-heating cement composite material and repair method using same

Abstract

The present invention relates to: a repair composition for a self-heating cement composite material; and a repair method using same The present invention provides a repair composition for a self-heating composite material, comprising: calcium sulfoaluminate cement, calcium aluminate cement, a conductive filler, and a fluidizing agent

Description

Self-heating cement composite repair composition and repair method using the same

[0001] The present invention relates to a self-heating cement composite repair composition and a repair method using the same.

[0002] In the winter road and air transport sectors, surface icing caused by snow and ice acts as a major risk factor, leading to various problems such as accidents, flight delays, and safety threats. Current snow removal methods have disadvantages, including high manpower and resource consumption, negative environmental impact, and difficulty in real-time response.

[0003] To address these issues, cement-based self-heating composites have been developed. These composites contain cement and conductive fillers, generate heat using electricity, and enable effective snow removal. However, cement composites can crack under repeated loading and extreme environmental conditions, which degrades their heating performance and damages their physical properties. In particular, cracks disrupt the conductive network, causing a significant drop in heating performance even with small cracks.

[0004] Therefore, in order to maintain self-heating performance, research on materials capable of rapidly repairing cracks is necessary.

[0005] [Prior Art Literature]

[0006] [Patent Literature]

[0007] Republic of Korea Published Patent Application No. 10-2543840

[0008] The present invention aims to provide a repair composition for a self-heating composite material.

[0009] The present invention aims to provide a repair method for a self-heating composite material.

[0010] The present invention provides a repair composition for a self-heating composite material comprising calcium sulfoaluminate cement, calcium aluminate cement, a conductive filler, and a fluidizing agent.

[0011] The above repair composition comprises 50 to 60 weight percent calcium sulfoaluminate cement, 35 to 45 weight percent calcium aluminate cement, and 0.5 to 5 weight percent conductive filler, based on 100 weight percent of the above repair composition.

[0012] The above repair composition may additionally include gypsum.

[0013] Based on 100% by weight of the above repair composition, it may include 50-60% by weight of calcium sulfoaluminate cement, 35-45% by weight of calcium aluminate cement, 0.5-5% by weight of conductive filler, and 3-7% by weight of gypsum.

[0014] The above fluidizing agent may be one or more of a carboxylate-based fluidizing agent, a lignosulfonic acid-based fluidizing agent, a naphthalene-based fluidizing agent, a melamine-based fluidizing agent, a polyether-based fluidizing agent, and a gluconic acid-based fluidizing agent.

[0015] The conductive filler may be one or more of carbon nanotubes, graphene, PAN-based carbon fibers, and pitch-based carbon fibers.

[0016] The present invention also provides a method for repairing a self-heating composite material, comprising the steps of: preparing a self-heating composite material in which a crack has occurred; and injecting a repair composition into the crack.

[0017] The present invention can provide a repair composition for a self-heating composite material.

[0018] The present invention can provide a method for repairing a self-heating composite material.

[0019] Figure 1 shows (a) changes in electrical resistivity / conductivity over 28 days of a self-heating composite, (b) tunneling-induced electrical resistivity / conductivity, (c) increase in temperature according to various input voltages, and (d) the relationship between the increase in temperature and the input power.

[0020] Figure 2 shows (a) compressive strength and (b) electrical resistivity and thermal conductivity of repair compositions having various CNT contents.

[0021] FIG. 3 shows the hydration reaction kinetics and products: (a) isothermal calorimetry and (b) XRD results of a self-heating composite and a repair composition containing 2.5% CNT. Abbreviations are defined as follows: A: alite, B: belite, C: tricalcium aluminate, F: ferrite, G: gypsum, E: ettringite, P: portlandite, CA: calcium aluminate, Ge: gehlenite, Y: ye'elmite, M: monosulfate, S: stratlingite, AH: aluminum hydroxide.

[0022] FIG. 4 shows SEM-BSE images of samples of a self-heating composite and a repair composition: (a) self-heating composite on day 1, (b) repair composition on day 1, (c) self-heating composite on day 7, and (d) repair composition on day 7.

[0023] Figure 5 shows the grey value histograms of SEM-BSE images of samples on day 1 and day 7 of curing.

[0024] Figure 6 shows the self-heating performance of a repair composition with 2.5% CNT added: (a) relationship between time and temperature and (b) relationship between input power and temperature.

[0025] FIG. 7 illustrates the repair process of a cracked self-heating composite: (a) crack samples having various crack sizes, (b) preparation of a repair composition, (c) application of the repair composition into the crack, (d) monitoring of the electrical resistivity of the repaired self-heating composite for 7 days, and (e) a photograph of the repaired self-heating composite.

[0026] Figure 8 shows an SEM image of a self-heating composite (a) and a part (b) repaired by a repair composition before cracking occurred.

[0027] Figure 9 shows thermal images over time after repair.

[0028] The present invention will be described in detail below through examples. The following examples are intended only to illustrate the present invention, and the present invention should not be interpreted as being limited by the following examples.

[0029] Examples

[0030] Manufacturing of self-heating composite materials

[0031] Type I general Portland cement was used as a binder material, and sand with a particle size of 0.17–0.7 mm was incorporated as aggregate. Silica fume (Elkem Inc., EMS-970) was added to improve both the mechanical strength and dispersion of nanoconductive fillers within the cement composite. The conductive fillers used in this study included carbon nanotubes (CNT) from Kumho Chemical Inc. and carbon fibers (CF) from Ace C&TECH Co., Ltd. The particle sizes of the CNTs and CFs used were 11–13 nm and 40–50 μm, respectively, and their lengths were 100–200 μm and 3 mm, respectively. The water-to-cement ratio was maintained at 0.5, and 1.6 wt% of a polycarboxylate-based high-performance superplasticizer (SP) (Dongnam Co., Ltd., FLOWMIX 3000U) was added to obtain desirable fluidity and improve the dispersion of CNTs within the cement composite. The mixing ratio used to manufacture the self-heating composite is shown in the table below. The figures shown in the table represent parts by weight per 100 parts by weight of cement.

[0032] Cement, sand, silica, fume, CNTCF, water, high-performance fluidizing agent 100150100.60.4501.6

[0033] The above self-heating composite was prepared using the following procedure. First, dry materials including cement, sand, and silica fume were mixed for 5 minutes. Then, CF was added to the mixture and mixed for an additional 3 minutes. Simultaneously, CNTs were added to water containing SP, and this solution was mixed for 5 minutes. Next, the solution was poured into the dry mixture, and the entire mixture was blended for another 5 minutes. The resulting mixture was poured into a mold measuring 40 x 40 x 160 mm. Copper electrodes coated with silver paste were inserted into the composite to minimize contact resistance. The electrode size was 20 x 60 mm, and the spacing between electrodes was 130 mm. The sample was cured at room temperature for 28 days. Preparation of a self-heating composite repair composition

[0034] A repair composition was prepared using two types of sustainable cement: calcium aluminate cement (CAC) and calcium sulfoaluminate (CSA) cement. Six different amounts of CNT (0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 wt%) were added to the repair composition to ensure high electrical conductivity. The water-to-binder ratio was fixed at 0.5. To achieve a molar ratio of calcium sulfate and calcium sulfoaluminate of 1.0 in the CSA cement (CSA clinker and gypsum), the proportions of CAC, CSA clinker, and gypsum were set to 50%, 43.9%, and 6.1%, respectively. The proportions of the repair composition are shown in the table below.

[0035] Classification CACCSA Gypsum CNTSP Example 1 (C0.5) 5043.96.1500.50 Example 2 (C1.0) 5043.96.1501.00 Example 3 (C1.5) 5043.96.1501.51.0 Example 4 (C2.0) 5043.96.1502.01.0 Example 5 (C2.5) 5043.96.1502.52.0 Example 6 (C3.0) 5043.96.1503.02.5

[0036] Experimental method

[0037] The electrical resistivity and conductivity of the self-heating composite were measured using a portable multimeter (DMM 34410A) with a two-probe method during a 28-day curing period. After 28 days of curing, a self-heating test was performed on the self-heating composite. In this test, a power supply (PL-3005S) applied five different DC voltages (10, 15, 20, 25, and 30 V) for one hour. Type K thermocouples were attached to both sides of the composite to monitor the surface temperature via a data logger (Agilent Technologies, 34972A). The relationship between the temperature rise of the composite and the applied input power was then determined. The self-heating performance of the self-heating composite repair composition was evaluated using the same method. The isothermal heat generation rate of a representative sample was measured for 120 hours using isothermal calorimetry (TAM Air, TA Instrument). XRD patterns were obtained using an Aeris-600w (Malvern Panalytical) with 1.54 Å Cu-Kα radiation over a 2θ angle range of 5°–65°. The generator voltage and tube current were set to 40 kV and 15 mA, respectively. Additionally, microstructure images were obtained by performing field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) analyses using a FE-SEM instrument (REGULUS8230, Hitachi) and an EDS instrument (ELECT SUPER, AMTEK Co., Ltd.). Furthermore, backscattered electron (BSE) analysis was performed to evaluate the hydration characteristics of the sample.

[0038] Cracks were induced in the self-heating composite using a general-purpose testing machine. A bending load was applied to the self-heating composite at a speed of 0.01 mm / s, and load data was recorded at 0.2-second intervals. Simultaneously, the self-sensing capability of the composite was evaluated by measuring the electrical resistance of the composite at 0.1-second intervals to monitor the load applied to the composite. The recorded electrical resistance was converted into a fractional change in resistance (FCR) (%), which represents the sensitivity of the composite.

[0039] SFNet, a leading segmentation network, was used for crack detection. This choice was based on the optimal balance between SFNet's parameter efficiency and detection accuracy. After detecting cracks at the pixel level, image coordinates were converted to real-world coordinates using an affine transformation based on the four vertex positions of the sample surface and their corresponding world coordinate data. The standardized sample size was assumed to be known. Detected cracks were measured for maximum diagonal length and width along each row, and the measurements were converted into real-world units and stored.

[0040] In this study, a total of four different crack widths (5, 10, 20, and 30 mm) were introduced, and self-heating composites were repaired using the repair compositions shown in Table 2. After the repair process, the electrical conductivity of the composites was monitored for 7 days to evaluate the hydration process. After 7 days, the self-heating performance of the composites was evaluated to compare the performance before the cracks occurred with that after the repair process.

[0041] result

[0042] Self-heating composite heat performance

[0043] The heating performance of the self-heating composite was evaluated by measuring the temperature rise at an ambient temperature of approximately 25°C. As shown in Figure 1, the self-heating composite exhibited higher heating performance as the input voltage increased.

[0044] Repair composition

[0045] The CNT content with the best effect was determined using the repair compositions in Table 2. For both the repair compositions and the self-heating composites, (1) high early strength, (2) high electrical conductivity, and (3) high thermal conductivity were considered. The compressive strength of the prepared repair compositions was tested on days 3 and 7 of curing, and electrical and thermal conductivity were measured on day 7. As shown in Figure 2, all samples exhibited an early strength higher than 18 MPa on day 3 of curing, regardless of the CNT content. These values ​​increased on day 7 of curing, and some composites containing 1.5–2.5% CNT reached approximately 28 MPa. These results can be attributed to the nucleation and bridging effects of CNTs in cement composites. When CNTs are added appropriately, they act as nucleation sites to promote the hydration reaction in cement composites. Additionally, CNT particles within the hydrate cause a bridging effect, thereby improving strength. However, when the CNT content increased to 3%, the strength decreased sharply. Excessive addition of CNTs can lead to the formation of poorly distributed CNT aggregates, which may result in reduced strength. Figure 2(b) shows the electrical resistivity and thermal conductivity of the composite. The penetration threshold at which electrical resistivity decreased sharply was identified within the CNT range of 1.5–2.5%. Similarly, thermal conductivity increased with CNT content until it began to decrease around the penetration threshold (>2%). As the CNT content increases, a large amount of pores are formed within the cementitious composite, which can lead to a decrease in thermal conductivity. Considering the three main factors of high initial strength, high electrical conductivity, and high thermal conductivity, the best effect was achieved when 2.5% CNT was mixed.

[0046] Isothermal calorimetry and XRD tests were performed to evaluate the hydration reaction rate and products of the repair composition compared to general self-heating composite materials. As shown in Fig. 3 (a), the repair composition of the present invention exhibited a faster reaction rate, a shorter induction period, and a higher heat release rate compared to general self-heating composite materials. In Fig. 3 (b), the XRD pattern of the repair composite during the initial curing stage showed rapid formation of hydration products. As the je-ellimite dissolved within one hour, the peak intensity of ettringite appeared weak after one hour of curing. Subsequently, the peak intensity of ettringite increased, while the peak intensity of je-ellimite decreased with prolonged curing. The peak associated with monosulfate was observed after 12 hours of curing, and its intensity increased over time. Based on the hydration reaction kinetics of the repair composite, this increase is thought to be due to the hydration of yellimite and calcium aluminate in the CSA-CAC system, where the calcium aluminate dissolved to form the AFm phase rather than the cartoite phase.

[0047] Figure 4 shows the results of SEM and EDS analysis of the self-heating composite and repair composition after day 1 and day 7 of hardening. The gray value of each sample was calculated from the images obtained as shown in Figure 5. The gray value distribution graph represents the distribution of hydrates, and a lower gray value indicates a higher amount of hydrate. However, a gray value of less than 50 is interpreted as pores within the cementitious composite. The degree of hydration can be inferred from the cumulative area under the graph. As shown in Figure 5, unreacted clinker was observed after day 1 of hardening, but it became indistinguishable after day 7 of hardening. Notably, the clinker area in the self-heating and repair composites was measured at 38,214 and 23,300, respectively, after day 1 of hardening. These values ​​decreased to 11,737 and 8,799, respectively, on day 7 of hardening. The repair composition had a significantly smaller clinker area than the self-heating composite, with changes of 30.83% and 37.73% between the 1st and 7th days of curing, respectively. These results suggest that rapid hydration occurred in the repair composition, which is in good agreement with the isothermal and XRD results shown in Fig. 3(b).

[0048] Figure 6 shows the self-heating performance of a repair composition prepared with 2.5% CNT. The repair composition can be heated at a lower input voltage than that used in Figure 1(c). This can be explained by a smaller sample volume (125 cm³) compared to the volume (256 cm³) shown in Figure 1(c). Notably, the relationship between temperature increase and input power shown in Figure 6(b) exhibits a slope of 2.4081, which is similar to that of the self-heating composite (2.4374). This demonstrates similar heating performance between the self-heating and repair composites, implying that they can be used to repair cracks in the self-heating composite. Furthermore, the R-squared value is 0.9827, indicating a high correlation within this input power range.

[0049] Self-heating performance after repair

[0050] Figure 7 illustrates the repair process of a cracked self-heating composite using a repair composition containing 2.5% CNT. The repair process is as follows. First, molds with lengths of 165 mm, 170 mm, 180 mm, and 190 mm were prepared, and crack samples with crack sizes of 5 mm, 10 mm, 20 mm, and 30 mm, respectively, were placed into the molds (see Figure 7 (a)). Next, the repair composition was prepared as shown in Figure 7 (b). Then, the repair composition was poured into the molds to fill the cracks (see Figure 7 (c)). The electrical properties of the repaired self-heating composite were monitored for 7 days to investigate changes caused by the hydration process. After 7 days, the repaired self-heating composite underwent a self-heating test to evaluate the recovery of its heating performance. It was observed that the electrical properties of the composite remained stable for 7 days regardless of the hydration process (see Figure 7 (d)).

[0051] As shown in Fig. 1, the electrical resistivity of general self-heating cementitious composites tends to increase as the curing time progresses during the hydration process. Therefore, stable electrical properties must be achieved before a specific material can be used as a self-heating composite. The composite repaired by the repair composition of the present invention exhibited stable electrical properties from the first day of curing. This indicates that such a composite can be used as a self-heating material without a long curing process, and means that existing self-heating composites can be quickly repaired in various infrastructures such as highways, tunnels, or airports.

[0052] As shown in Fig. 8, it can be seen that a conductive network is well formed in both the self-heating composite before cracking and the self-heating composite repaired by the repair composition of the present invention.

[0053] Figure 9 shows a thermal image analysis of a sample, which means that the composite material can recover its original self-heating performance after repair using the repair composition of the present invention.

Claims

1. A repair composition for a self-heating composite material comprising calcium sulfoaluminate cement, calcium aluminate cement, a conductive filler, and a fluidizing agent.

2. In Paragraph 1, A repair composition comprising, based on 100% by weight of the above repair composition, 50-60% by weight of calcium sulfoaluminate cement, 35-45% by weight of calcium aluminate cement, and 0.5-5% by weight of a conductive filler.

3. In Paragraph 1, A repair composition further comprising gypsum.

4. In Paragraph 3, A repair composition comprising, based on 100% by weight of the above repair composition, 50-60% by weight of calcium sulfoaluminate cement, 35-45% by weight of calcium aluminate cement, 0.5-5% by weight of conductive filler, and 3-7% by weight of gypsum.

5. In Paragraph 1, A maintenance composition wherein the above-mentioned fluidizing agent is one or more of a carboxylate-based fluidizing agent, a lignosulfonic acid-based fluidizing agent, a naphthalene-based fluidizing agent, a melamine-based fluidizing agent, a polyether-based fluidizing agent, and a gluconic acid-based fluidizing agent.

6. In Paragraph 1, The above conductive filler is a repair composition comprising one or more of carbon nanotubes, graphene, PAN-based carbon fibers, and pitch-based carbon fibers.

7. Step of preparing a self-heating composite material with cracks; A method for repairing a self-heating composite material, comprising the step of injecting the repair composition of claim 1 into the above crack.

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