WANG Xinjie, XU Donghang, ZHU Pinghua*, CHEN Chunhong, QIAN Zhongqiu

(1. Department of Civil Engineering, Changzhou University, Changzhou 213164, China; 2. Architectural Research Institute Group Co., Ltd.,Changzhou 213164, China)

Abstract: To further strengthen the protective effect of aerogel cement paste (ACP) coating on selfcompacting concrete (SCC) in tunnel fire under the optimal mix proportion, the effect of curing temperature (from 5 to 80 ℃, based on site construction curing temperature and surrounding rock temperature) on fire insulation of ACP was investigated. The mechanical properties, thermal conductivity and porosity of ACP were tested.The microstructure of ACP was characterized by means of SEM, XRD and TG/DTG. The results reveal that 50℃ is the optimal curing temperature for ACP with good mechanical properties and fire insulation. Relatively high curing temperature can facilitate hydration and pozzolanic reactions, contributing to the generation of more stable substances (such as C-S-H gels, tobermorite and thenardite, etc). ACP under excessive low curing temperature brings inhomogeneous microstructure with coarse pores, leading to producing wider and longer microcracks when it is exposed to tunnel fire. The microcracks make the heat convection and thermal radiation more significant and thus accelerate the damage of ACP under fire. In case of the less than 7% difference of thermal conductivity, dense microstructure and stable substances are more conducive to strengthening fire insulation of ACP. In practical engineering applications, the thickness of protective layer of ACP can be further optimized when ACP is cured under about 50 ℃.

Key words: aerogel cement paste; curing temperature; tunnel fire insulation; microstructure; stable hydration products

1 Introduction

Tunnels play an important role in alleviating traffic congestion and promoting the transportation of goods[1]. The development and utilization of tunnels bring us many benefits, but the severe challenges to tunnel fire safety come with it[2]. Tunnel fire always displays the characteristics of high temperature (exceed 1 000 ℃)[3], dense smoke[4], rapid proliferation[5], and irreversible harm to tunnel structures, causing painful casualties and heavy economic losses[6,7]. Self-compacting concrete (SCC) widely used as tunnel linings is inclined to explosive spalling when tunnel fire occurs,which aggravates threats to properties and lives of human beings[3]. Therefore, the fire safety of tunnels has attracted extensive attention. The fire resistant coating technology for tunnel concrete is favored because of its advantageous effect and convenient construction.The fire resistant coating limits the heat transfer (heat convection and thermal radiation) between the concrete structure and flame to protect concrete from fire[8,9].Previous studies have showed that aerogel cement paste (ACP) as a fire resistant coating significantly improved heat insulation of concrete in tunnel fire[10]. Zhuet al[11,12]indicated that 40 mm of ACP layer effectively prevented the explosive spalling of SCC from tunnel fire. To date, the research on fire resistance of ACP is mainly focused on the content of aerogel or thickness of coating, but the effect of curing regime on ACP is still a research gap.

As is well-known, thermal insulation, hydration products and microstructure of ACP are critical factors for its fire resistance, while curing temperature is one of the main parameters affecting these performances of cement paste[13]. The hydration kinetics, porosity and hydration products change substantially in cement paste as curing temperature increases[14,15]. Zhaoet al[16]reported that the porosity of mortar subjected to 40℃ curing became lower than the mortar cured under 20 ℃, suggesting that higher curing temperature promoted the cement hydration. On the contrary, Wanget al[17]proposed that cement paste should be cured at low temperature (<20 ℃) which caused cement paste more compact and homogeneous. The stable and dense microstructure is beneficial to the fire resistance of cement paste. Penget al[18]showed that the high temperature resistance of ultra-high performance concrete with dense hydrates and microstructure were improved.Therefore, determining the optimal curing temperature for ACP is important for engineering application.

In addition, porosity determines the thermal properties of a material. A higher porosity brings about a lower thermal conductivity and better thermal insulation[19,20]. Niubóet al[20]verified that the increased porosity of magnesium phosphate cement enhanced its thermal insulation performance. However, dense cement paste decreases its porosity[21,22]and results in a higher thermal conductivity, which adversely affects the thermal insulation performance of cement paste.Moreover, stable hydration products can enhance the ability to resist fire. Habibet al[23]found that pozzolan reacted with Ca(OH)2to form additional calcium silicate hydrate (C-S-H) improving fire resistance of cement paste (containing pozzolan), as the thermal stability of C-S-H is higher than that of Ca(OH)2. From these results, it can be deduced that curing temperature will determine the microstructure, hydration products and thermal conductivity of ACP. But the optimal curing temperature for ACP with good fire resistance is not clear. Hence, it is of great significance to investigate the effect of curing temperature on the fire insulation of ACP.

Previous studies[10-12]designed mix proportion of ACP based on different density (500-900 kg/m3). The results showed that ACP with a density of 700 kg/m3had the best fire resistance and the mix proportion at this density is the optimal mix proportion. In order to further enhance the fire insulation of ACP under the optimal mix proportion, the effect of curing temperature on it was studied. The determination of curing temperature was derived from the environment of four seasons,extreme weather in some regions, cold region tunnels and high-temperature geothermal tunnels. The SCC coated with a 40 mm thick ACP (ACP-SCC) was used for the simulated tunnel fire test (based on HC curve,1 100 ℃, 2.5 h). The interfacial temperature variation of ACP-SCC was recorded, which could directly reflect the difference of ACP’s fire insulation. Besides, the microstructure of ACP before and after tunnel fire simulation was characterized by scanning electron microscope(SEM), X-ray diffraction (XRD), thermal gravimetry(TG) and differential thermal gravimetry (DTG) analyses. Associated with porosity, thermal conductivity,mass loss, hydration products and microstructure, the fire resistance mechanism of ACP at different curing temperatures was clarified.

2 Experimental

2.1 Materials

The cement was ordinary portland cement (P·O 42.5). Silica fume was added to avoid alkaline silica reaction in aerogels[24]. Fly ash was used to improve the thermal insulation performance and water retention capacity of cement paste[25]. The chemical compositions of cement, silica fume and fly ash were shown in Table 1. The air entraining agent was supplied by Shandong Hongxiang Building Admixture Factory, introducing a great quantity tiny and stable bubbles into the samples as stirred and thus leading to a lower density[26,27]. J-1 polycarboxylate high performance water reducer was received from Jiangsu Subor Co. Ltd. Hydrophobic aerogel granules (thermal conductivity: 0.020 W/(m·K);particle size: 0-4 mm; particle density: 100 kg/m3) were provided by Guangdong Alison H-Tech Co. Ltd..

2.2 Preparation of ACP

ACP was prepared according to the premixed cement paste process[28]. Firstly, the cement, silica fume,fly ash, air entraining agent, water and water reducing agent were mixed and stirred for 60 s. Then the aerogels were added into the mixture and stirred for another 120 s. Previous studies[10-12]have determined that ACP with a density of 700 kg/m3is more suitable for tunnel engineering applications, and the specific mix proportion was designed as shown in Table 2.

2.3 Preparation and curing of SCC coated with ACP

The mix proportion of SCC is shown in Table 3.The SCC samples were coated with ACP as depictedin Fig.1. Firstly, the SCC surface was made a groove and a thermocouple was placed on it. Then the wellmixed ACP slurry was poured into the mold to make the coating thickness up to 40 mm. Lastly, the samples were kept in a standard curing chamber (20±1 ℃, 95%relative humidity) for 24 h to remove the molds.

Table 1 Chemical composition of cement, fly ash and silica fume/wt%

Table 2 Mix proportion of ACP/(kg/m3)

Fig.1 The preparation process of ACP composite SCC

Table 3 Mix proportion of C40 SCC/(kg/m3)

Table 4 Curing regimes of ACP

After that, all samples were cured under different conditions as presented in Table 4. Each curing temperature has been defined as follows: 5 ℃ represented winter temperature and ambient temperature of cold region tunnel[29]; 20 ℃ represented spring, autumn temperature and standard curing temperature; 35 ℃ represented summer temperature; 50 ℃ represented extreme high weather temperature in some areas; 65 ℃ and 80℃ were the curing temperature reached at the construction site and surrounding rock temperature of high-temperature geothermal tunnel[30]. Relative humidity was controlled as the standard moisture content (>95%RH).

2.4 Tunnel fire simulation

Tunnel fire simulation tests were carried out on a bogie-hearth resistance electric furnace[11]. The temperature was heat up to the target temperature of 1 100℃ in 40 minutes and the temperature was kept for 2.5 h (based on HC standard tunnel fire rising temperature curve[31]), as illustrated in Fig.2. In order to make sure only the coated surface was subjected to tunnel fire simulation, the SCC’s uncoated side surfaces were parceled in a 100 mm thick zirconium fiber blanket. The interfacial temperature of ACP-SCC was uninterruptedly recorded by thermocouple as shown in Fig.3.

Fig.2 HC and experimental curves of tunnel fire simulation

Fig.3 Schematic of tunnel fire simulation

2.5 Measurements

2.5.1 Physical properties of ACP

Three prismatic samples of ACP (40 mm×40 mm×160 mm) cured under various curing regimes were prepared for measuring the flexural strength. The compressive strength of ACP was tested by the flexural tests[32].

The ACP samples (50 mm× 50 mm× 50 mm)cured under different regimes for 28 days were prepared to measure the porosity[33]. The samples were dried at 100±5 ℃ until a constant weight was reached(Wd). Then, they were submerged in the distilled water for more than 3 days and weighed in water (Ww). After that, the samples were got out, and their surface were wiped with a damp cloth and recorded the weights in the air (Wa). Thus, the porosity was calculated based on the equation: (Wa-Wd)/(Wa-Ww)×100%.

The thermal conductivity of ACP samples (250 mm × 250 mm × 50 mm) were tested on a heat flow meter based on ISO 8301[10,34].

The mass of ACP samples (50 mm × 50 mm× 50 mm) before and after simulated tunnel fire (2.5 h)was measured to assess the fire resistance of ACP under various curing regimes.

2.5.2 Microstructure of ACP

A small number of samples were taken from the surface center of ACP for SEM, XRD, TG/DTG analyses before and after simulated tunnel fire. The obtained samples were immersed into acetone to stop hydration.These samples were then vacuum dried over 3 days.They were ground to a grain size below 0.063 mm before XRD and TG/DTG tests.

The SEM analysis were examined by Zeiss SUPRA55 scanning electron microscope. SEM images were performed under 20 kV accelerating voltage.

The XRD analyses were performed to assess the chemical composition of ACP before and after thermal treatment. XRD (D/MAX2500, Rigaku) worked at 40 kV and 30 mA.

TG/DTG patterns of ACP specimens were obtained by the thermogravimetric analyzer Labsys Evo/seteram in the temperature range from 30 to 1 100 ℃.The heating rate was 10 ℃/min under a nitrogen flow of 20 mL/min.

3 Results and discussion

3.1 Physical properties

The mechanical properties of ACP under different curing regimes are illustrated in Fig.4. The compressive and flexural strength of ACP cured from 5 CW to 50 HW increase with rising curing temperature. In comparison, the strength of ACP cured from 50 to 80 HW slightly drops as the curing temperature increases.The compressive (3.71 MPa) and flexural strength (0.88 MPa) are the highest when APC is cured under 50 HW,45% and 34% higher than that under 5 CW, respectively. To sum up, a proper increase in curing temperature is beneficial to improve the mechanical properties of ACP, but too high curing temperature will bring a little adverse effect[15].

Fig.5 illustrates that the thermal conductivity of ACP increases firstly and then decreases with rising curing temperature. The average thermal conductivity of ACP is 0.165 W/(m·K), indicating that ACP has good thermal insulation. The lowest thermal conductivity is 0.158 W/(m·K), when ACP is cured under 5 CW.The highest thermal conductivity of ACP under 35 HW is only 7% and 3% higher than that under 5 CW and 80 HW, respectively. This shows that curing temperature has a weak effect on the thermal conductivity of ACP.

Fig.4 Compressive and flexural strength of ACP under various curing regimes

Fig.5 Thermal conductivity of ACP under various curing regimes

Unlike the thermal conductivity, the porosity of ACP decreases firstly and then increases as the curing temperature increases (Fig.6). The porosity of ACP under 35 HW (the lowest) is 10% lower than that under 5 CW (the highest). On the contrary, the porosity of ACP cured from 35 to 80 HW increases by about 5%.

Combined with Fig.4, it is found that ACP with high porosity weakens its mechanical properties.Similarly, high porosity reduces thermal conductivity(Fig.5). The higher the porosity, the lower the thermal conductivity. Therefore, the porosity is an important factor affecting mechanical and thermal properties of ACP[20,35].

3.2 Interfacial temperature of ACP-SCC

Based on Chinese standard GB 28375-2012[36],the interfacial temperature of SCC should not exceed 380 ℃ to protect SCC from explosive spalling. To confirm the protective effect of ACP under different curing regimes, the interfacial temperatures of ACP-SCC are tested and recorded, as shown in Fig.7. All the changes of interfacial temperature are divided into four stages(20 SC as an example: I→IV): stage I (0-73 min) is a slow rise; stage II (73-80 min) is a steep rise lasting about 7 minutes; stage III (80-133 min) is a slower rise,but slightly faster than stage I; stage IV (133–150 min)is a relatively fast rise (except 50 HW), implying that ACP begins to damage severely in tunnel fire.

Fig.6 Porosity of ACP under various curing regimes

Fig.7 Interfacial temperature of ACP-SCC under various curing regimes

All ACP-SCC specimens cured show good fire insulation to effectively resist tunnel fire for 2.5 h. The maximum surface temperature of ACP-SCC is 213 ℃(less than 380 ℃), suggesting that ACP cured under many natural states used as fire resistant coating of tunnels is feasible. But curing temperature affects the interfacial temperature of ACP-SCC, namely, the fire insulation. The interfacial temperature decreases firstly(from 5 CW to 50 HW), and then increases (from 50 HW to 80 HW), as curing temperature increases. The interfacial temperature of ACP-SCC cured under 50 HW is the lowest (103 ℃), 52% and 36% lower than that under 5 CW and 80 HW, respectively. In addition,the rising tendency of the interfacial temperature of ACP-SCC under 50 HW is the gentlest, especially at stage IV. Consequently, it is concluded that ACP under 50 HW has better fire insulation.

As is well-known, the lower the thermal conductivity, the better the thermal insulation performance[19,37]. When ACP is cured under 5 HW, its porosity is the highest and the thermal conductivity is the lowest, but the fire insulation is indeed the worst. This is contrary to the expectation because except for thermal conductivity, hydration products and microstructure of ACP are also the key factors determining its fire insulation.

3.3 Microstructure

3.3.1 XRD analysis

XRD patterns of ACP under different curing regimes before simulated tunnel fire are given in Fig.8(a).It is found that the intensity of peak at 18.1° belonging to Ca(OH)2declines as the curing temperature increases, reflecting a decrease in Ca(OH)2content.More Ca(OH)2react and combine with pozzolans due to rising curing temperature, generating more products(C-S-H gels) with agglutination characteristics[38]. More C-S-H products lead to lower cement paste pores as well as more outstanding adhesion between the cement paste and aerogel, bringing about better mechanical properties[39]. This result is in accordance with the mechanical properties test outcome in Fig.4. Besides,a type of crystalline hydrate (tobermorite) is formed,because part of C-S-H products are converted from gels into crystals[40]. The presence of pozzolans raise the Na2O content, leading to the formation of thenardite[41]. These stable hydration products (tobermorite and thenardite) are conducive to the elevated temperature resistance of ACP. It is noted that the peak of ettringite disappear when the ACP samples cured under 80 HW,because ettringite decomposed above 70 ℃[42].

Cement-based materials are prone to failure after suffering from high temperature of more than 600℃[43,44]. Therefore, the structure of ACP is destroyed after tunnel fire simulation for 2.5 h, even though it protects SCC from spalling. Fig.8(b) shows the XRD patterns of ACP after tunnel fire simulation. There is no doubt that the peaks assigned to CaCO3and Ca(OH)2basically vanish, which are decomposed from 500 to 800 ℃[45]. Besides, the peaks belonging to wollastonite and Ca2Al2SiO7appear, produced by cement paste when it is heated to 800 ℃[46]. The intensity of peak at 29.97° (wollastonite from C-S-H decomposition[47])increases firstly and then decreases with increasing curing temperature, suggesting that different content of C-S-H form in ACP due to different curing temperature. Consequently, it can be concluded that higher curing temperature (50 HW) facilitates C-S-H formation,thus improving the mechanical properties of ACP.

Fig.8 XRD patterns of ACP samples under various curing regimes: (a) before simulated tunnel fire; (b) after simulated tunnel fire

3.3.2 Thermogravimetric analysis

The TG/DTG curves of ACP specimens subjected to different curing regimes are indicated in Fig. 9. The absorbed and capillaries water in ACP is released from 30 to 250 ℃[47]. The dehydration of C-S-H gels occurs in a wide temperature range of 40-550 ℃, because C-S-H gels have variable Ca/Si ratios with different decomposition temperatures[48]. The dehydroxylation of Ca(OH)2happens from 400 to 550 ℃. The decomposition of CaCO3occurs from 550 to 800 ℃, mainly around 690 ℃. The decline in mass over 800 ℃ is related to the decomposition of C-S-H crystals, like the decomposition of tobermorite into wollastonite[18].

The mass loss of endothermic peaks at 500 and 690 ℃ decrease with rising curing temperature, showing that the contents of Ca(OH)2and CaCO3decrease as the curing temperature increases. This suggests that more Ca(OH)2is reacted to form C-S-H in ACP cured under higher temperature. In addition, the mass loss of peak at over 800 ℃ is gradually increasing, because ACP with higher curing temperature produces more C-S-H crystals like tobermorite, which is dehydroxylated above 700 ℃[49]. The total mass loss of ACP samples is between 20% and 28%, and samples with higher curing temperature suffer smaller total mass loss. The higher the curing temperature, the lower intensities of dehydration, dehydroxylation and decarbonation of hydration products produced in ACP, which is beneficial to the fire resistance of ACP.

The mass loss of ACP blocks (50 mm×50 mm×50 mm, after simulated tunnel fire) and ACP powder(10 mg, after the TG experiment), and their ratio are showed in Fig.10. The mass loss of ACP powder decreases with rising curing temperature. In contrast, the mass loss of ACP block decreases firstly, and then increases. The difference is mainly attributed to the mass loss of ACP block determined by the combination of hydration products and microstructure. The mass loss ratio is used to better show the difference in microstructure. The mass loss ratio decreases firstly and then increases. The lowest mass loss ratio of ACP under 50 HW is 3% and 5% lower than that under 5 CW and 80 HW, respectively. Excessive curing temperature (over 65 ℃) leads to the formation of larger pores and uneven microstructure, which have a negative impact on the final fire resistance of ACP.

To sum up, the lower the mass loss of ACP blocks,the better performance of the combination of hydration products and microstructure. The minimum mass loss occurs in the ACP blocks cured under 50 HW. Hence,ACP under 50 HW has better fire insulation. The mass loss of the ACP blocks results are in accordance with the interfacial temperature ones in Fig.7.

Fig.9 TG/DTG curves of ACP specimens subjected to different curing regimes: (a) 5 CW; (b) 20 SC; (c) 35 HW; (d) 50 HW; (e) 65 HW; (f)80 HW

Fig.10 Mass loss of ACP blocks and ACP powder and their ratio

3.3.3 SEM analysis

The SEM images of ACP under different curing regimes are pictured in Fig.11. The microstructure of ACP cured from 5 CW to 50 HW become denser with rising curing temperature, which is attributed to the production of hydration products[50]. ACP cured under 50 HW possesses more uniform paste and denser microstructure (Fig.11(d)). In comparison, ACP samples cured under 65 and 80 HW have a larger number of rough pores and an inhomogeneous microstructure as the curing temperature increases. This shows that 50 ℃is the optimal curing temperature for APC, and too fast/slow hydration rate caused by excessive/too low curing temperature will bring an adverse effect on the microstructure and performances of ACP[17,51].

Fig.12 shows that aerogel in ACP atrophies and crystallizes after undergoing simulated tunnel fire for 2.5 h, resulting in huge gaps at the interface between cement paste and aerogel[52]. At this point, the aerogel lost its protective effect on the ACP. In addition, it can be observed that the microcracks along the cement paste become shorter and thinner firstly and then become coarser and longer with rising curing temperature. The microcracks make the heat convection and thermal radiation more significant[53], reducing the thermal insulation of ACP. In combination with Fig.10, it can be concluded that coarse and long microcracks tend to accelerate ACP damage, thus increasing the mass loss of ACP blocks.

Fig.11 SEM images of ACP under different curing regimes: (a) 5 CW; (b) 20 SC; (c) 35 HW; (d) 50 HW; (e) 65 HW; (f) 80 HW

Fig.12 SEM images of ACP under different curing regimes after simulated tunnel fire: (a) 5 CW; (b) 20 SC; (c) 35 HW; (d)50 HW; (e) 65 HW; (f) 80 HW

The decomposition of hydration products give rise to the shrinkage of concrete and create microcracks[54]. From the view of Fig.9, ACP cured under 5 CW possessed the maximum content of decomposed hydration products. Consequently, ACP cured under 5 CW is easier to produce microcracks. In addition, the more numerous coarse pores, the more microcracks (in Fig. 11). Fig.13 shows that the propagation model of microcracks in ACP with tiny and coarse pores[55]. The coarser the pores, the wider and longer of the microcracks.

Fig.13 Schemes of the microcracks propagation in cement paste under different microstructure when ACP exposed to tunnel fire: (a) the dense microstructure with uniform and tiny pores; (b) the inhomogeneous microstructure with coarse pores

ACP with high porosity decreases its thermal conductivity, and accelerates the formation and propagation of microcracks when it is exposed to elevated temperature, thus reducing its fire resistance. In the case of less than 7% differences in thermal conductivity, hydration products and microstructure are critical factors for fire insulation of ACP. The optimal fire resistance and heat insulation of ACP is cured under 50 HW.

4 Conclusions

Curing temperature is the key factor affecting the performances of ACP. High curing temperature accelerates hydration reaction rate of cement-based materials and generates stable hydration products (such as C-S-H gels, tobermorite and thenardite,etc). However, excessive temperature causes adverse effects on the performances of ACP. The microstructure of ACP becomes dense and its porosity reduces continuously with rising curing temperature when APC is cured from 5 to 50 ℃.On the contrary, the microstructure of ACP cured from 65 to 80 ℃ becomes uneven with larger pores as the curing temperature increases. ACP with high porosity decreases its thermal conductivity, but also produces more microcracks when tunnel fire occurs, thus reducing its fire resistance. With little change of thermal conductivity, dense microstructure and stable hydration products are more conducive to improving the fire insulation performance of ACP.

Based on the analysis of thermal conductivity,hydration products, microstructure of ACP and the interfacial temperature of ACP-SCC, 50 ℃ is the optimal curing temperature for ACP to protect SCC tunnel linings from spalling after tunnel fire.

In practical engineering, when ACP is cured in cold regions, heat storage and heat preservation method or greenhouse method can be used to improve the curing temperature. When ACP is cured under high-temperature geothermal regions, the ACP thickness should be increased appropriately. When the curing temperature is about 50 ℃, the thickness of protective layer of ACP can be further optimized.