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Study on the fire resistance performance of cementitious composites containing recycled glass cullets (RGCs)
Construction and Building Materials 242 (2020) 117992
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Study on the fire resistance performance of cementitious composites containing recycled glass cullets (RGCs) Binmeng Chen a,b, Honggang Zhu b,⇑, Bo Li c,⇑, Manlung Sham b, Zongjin Li d a
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China Nano and Advanced Materials Institute Limited, Hong Kong Science Park, Hong Kong, China c Department of Civil Engineering, The University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo, China d Institute of Applied Physics and Materials Engineering, University of Macau, Taipa, Macau, China b
h i g h l i g h t s RGCs (<100
lm) can increase the compressive strength via pozzalanic reaction.
Higher residual strength when incorporating smaller RGCs due to the binding effect. RGCs below 0.6 mm can apparently prevent spalling/cracks under high temperature. Lab-scale fire test is a promising method to simulate the full-scale fire test. RGCs under 0.6 mm are more promising materials in fire resistance concrete/mortar.
a r t i c l e
i n f o
Article history: Received 15 July 2019 Received in revised form 24 December 2019 Accepted 31 December 2019
Keywords: Recycled glass cullets Cementitious composites Fire resistance Integrity Thermal insulation
a b s t r a c t Glass manufacturing involves abundant amount of natural resources including thermal energy and it is important to explore various ways to ultilize the used glass instead of direct disposal to landfill. One way to recycle the glass products is to crush them into cullets and mix the cullets into cement as filler/aggregate. In this study, the effects of recycled glass cullets (RGCs) with different sizes on the fire resistance performance of cementitious composite were studied. Cementitious composite samples were prepared with RGCs at 4 different sizes, i.e. below 0.6 mm, 0.6–1.18 mm, 1.18–2.36 mm and 2.36– 4.75 mm. In order to evaluate the fire resistance of the composites with RGC, the composites samples were heated at different temperatures, and their residual strength, integrity and thermal conductivity were measured accordingly. Moreover, a specially designed furnace in accordance with modified BS 476-22 was employed to observe the thermal insulation and integrity of mortar panels under testing. The microstructure of RGCs mortar was observed under microscope and it was found that adding RGCs inside the composite improved the fire resistance of mortar. The beneficial effect of RGCs on fire resistance was found to reduce with increasing RGCs size, where RGCs finer than 0.6 mm showed the most significant beneficial effect. This is presumably attributed to the softening and state transformation of fine glass cullet at high temperature. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Tons of waste glass bottles were generated every day in the world, and recycling the disposed glasses is always a big challenge to achieve sustainable development. In particular, Hong Kong has over 300 tons of waste glass bottles being generated every day, and most of them were disposed to the landfill directly due to the lack of glass re-fabrication plants [1]. This not only increases ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Zhu), [email protected] (B. Li). https://doi.org/10.1016/j.conbuildmat.2019.117992 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
the burden of landfill and environment, but also leading to waste of natural resources and energy. The Hong Kong government has been encouraging the community to participate in glass recycling to promote resources recovery. Apart from melting the waste glass for reproduction of glass products, another approach is to recycle waste glass by crushing them into cullets and to integrate the cullets into cementitious composites as filler/aggregate. Cementitious materials, including concrete and mortar, are the most widely used materials in the world, not only serve as structural elements but also as various kinds functional materials [2–9]. Integrating glass cullets into cementitious composites is therefore a promising way to revitalizing the waste glass [10–15,17–19]. Bentchikou
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et al. [10] studied the performance of mortar containing both sand and recycled glass with different ratios; It was found that adding recycled glass reduced the density of mortar, and there was an optimized content of recycled glass based on the 28-day compressive strength results. Ling et al. [11] studied the feasibility of using 100% recycled glass aggregate to prepare architectural white cement mortar; It was found that recycled glass could improve the workability and drying shrinkage of mortar; However, the compressive and flexural strength of mortar decreased with increasing recycled glass content. So et al. [12] discussed the feasibility of using recycled heavyweight waste glass as fine aggregate in mortar and it was reported that utilizing heavyweight waste glass as fine aggregate in mortar was possible to reduce both dry shrinkage and alkali-silica reaction (ASR) expansion. BekirTopçu et al. [13] reported using 4–16 mm waste glass to replace 0–60% coarse aggregate in concrete had no obvious effect on workability but slightly reduced the strength. As a result, non-load bearing mortar and concrete products containing recycled glass have been developed and commercialized in various regions nowadays [14]. Apart from compressive strength, the fire resistance of different building materials determines the respective application scenarios. When being exposed to fire, evaporation of free water, dehydration of the hydrated products of cement as well as expansion of aggregates would all take place in mortar/concrete accompanied with significant strength degradation and loss of integrity and heat insulation [15]. The fire resistance of non-load bearing building elements is typically assessed in terms of thermal insulation and integrity according to relevant standards, for example BS EN 1364-1 [16]. The integrity is generally defined as the ability of the tested element to withstand collapse, free from holes, cracks and fissures and sustained flaming on the unexposed face, while the thermal insulation is generally defined as the ability of the tested element to restrict the temperature rise of the unexposed face of the element. As full scale standard fire test can only be conducted in standard fire testing labs which is rather time consuming and costly, researcher always prefer to evaluate the fire resistance performance of sample in laboratory. Investigations have been conducted to study the performance of mortar/concrete containing various recycled wastes at elevated temperature [15,17,20–25]. Guo et al. [17] and Ling et al. [15] reported that integrating recycled glass into mortar/concrete could enhance their residual mechanical performance after heating at 800 °C. Ali et al. [20] reported that integrating the glass powder into concrete could improve its residual strength after high temperature heating than silica fume. Leiva et al. [22] employed an inhouse designed furnace to evaluate the insulating capability of sample by monitoring the time for the unexposed face of sample reaching 180 °C increment while another side was heated. Arenas et al. [23] studied the fire resistance of concrete blocks containing coal combustion fly ashes and bottom ash. Nevertheless, most of the previous researches were conducted below 1150–1200 °C [17,20,22–25] as required in the standard fire test, and it is critical to understand the behavior of the materials under further elevated temperature. In the first part of this study, recycled concrete fine aggregate (RCFA) was selected as a comparable recycled material to demonstrate the benefits of using recycled glass cullets of small size since RCFA is commonly used as recycled fine aggregate in concrete. Mortar samples were prepared and heated at a series of temperatures in a furnace respectively (300 °C, 600 °C, 900 °C, and 1200 °C), and their residual strength, integrity and thermal conductivity were measured accordingly. Moreover, a specially designed in-house furnace in accordance with modified BS 47622 was employed (Fig. 1) to evaluate the thermal insulation and integrity of mortar panels. The effects of recycled glass cullets of different sizes on the fire resistance of mortar were then evaluated.
Fig. 1. Temperature rising curves of the furnace and BS 476-22.
2. Experimental 2.1. Materials The cement for mortar sample preparation was a typical market available 52.5 N Ordinary Portland Cement (OPC), with 3.15 g/cm3 specific gravity and surface area of 3310 cm2/g. Two kinds of recycled wastes, namely, recycled glass cullets and recycled concrete aggregate, were used as fine aggregate in the mortar. The recycled concrete aggregates were manufactured by crushing and sieving the concrete from those demolished buildings and tunnels locally in Hong Kong with maximum size 4.75 mm. It can be easily seen from the figure that there is a clear boundary in recycled concrete aggregate (Fig. 2(b)) due to it may contain binder and aggregate (Fig. 2(a)) when concrete was crushed, making recycled concrete aggregate higher water absorption of 7.4%. In a similar vein, the recycled glass cullets were manufactured by crushing and sieving from the disposed glass bottles. Fig. 3 shows the particle size distribution curves of the as-received recycled concrete fine aggregate (RCFA) and recycled glass cullets (RGCs). In order to study the effects of different sized glass cullets, the as-received glass cullets were also sieved to 4 kinds of different sizes, i.e. below 0.6, 0.6–1.18, 1.18–2.36 and 2.36– 4.75 mm, for sample preparation. The chemical composition of the employed OPC and RGCs were listed in Table 1. 2.2. Sample preparation The mix formulation of all mortar samples is shown in Table 2, and a water to cement ratio of 0.3 was used. As the densities of RCFA and RGC were similar, the volume ratios of RCFA and RGC were also similar. Among different mortar samples, the mix formulation is same but the fine aggregates employed are different. In order to ensure the different mortar materials have similar workability, suitable amount of water reducing agent was used if need. A planetary mixer was employed for preparing mortar material. The fine aggregates used were dried in a 105 °C oven for 24 h prior to mixing. To prepare mortar, required amount of cement and fine aggregates were mixed for 2 min, and then water and water reducing agent were added with another 2 more minutes mixing. All mortar specimens were demoulded after 24 h, and then cured in standard curing condition (27 ± 2 °C, RH 95%) for 27 days. In each mortar formulation, 40 mm 40 mm 40 mm cubes were prepared for compressive strength measurement, 40 mm 40 mm 160 mm prisms were prepared for observing spalling/cracks and measuring thermal conductivity, and 165 mm 315 mm 100 mm panels were prepared for fire resistance evaluation using the previous mentioned in-house furnace. All the specimens were cured in standard curing condition for 28 days prior to further processing or testing. 2.3. Test methods In order to study the fire resistance of mortar sample, 40 mm 40 mm 40 mm cube and 40 mm 40 mm 160 mm prism specimens of all different mortar formulation were heated at a specified temperature in furnace. Samples were heated gradually to the target temperature (300 °C, 600 °C, 900 °C, and 1200 °C) with a heating rate 3 °C/min, then kept at this temperature for another 2 h and cooled down naturally later. After cooling in furnace, the 40 mm 40 mm 40 mm cubes were subject to compression test for the residual strength measurement, while the 40 mm 40 mm 160 mm prisms were used for integrity and thermal conductivity evaluation. The heating temperatures employed include 300, 600, 900 and 1200 °C. For the purpose of comparison, the thermal
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(a)
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(b)
Fig. 2. Photo & optical image of recycled concrete fine aggregate.
2.3.2. Spalling/crack behavior Spalling or cracking is an important integrity indicator of the fire resistance of an element. In this study, the degrees of spalling and cracking of all specimens subject to heating were observed. As referred to previous publications [20], spalling/ cracking can be classified into three levels, i.e. no spalling/cracks, partially spalling/cracks and full spalling/cracks. The strength degradation of mortar samples subject to heating was also relevant to the degree of spalling/cracking. 2.3.3. Thermal conductivity A quick thermal conductivity meter KEM QTM 500 was used to measure the thermal conductivity of all mortar samples. The principle of measurement of the instrument is the hot wire method specified in relevant international standards, for example BS EN 993-15 and ASTM C-1113. The following equation is used to calculate the thermal conductivity,
k¼
Fig. 3. Particle size distribution of the as-received RCFA and RGCs.
Table 1 Chemical composition of the employed OPC & RGC. Chemical component
OPC (wt%)
RGC (wt%)
SiO2 Al2O3 CaO SO4 Fe2O3 MgO K2O TiO2 Na2O Cr2O3 MnO
19.85 3.68 65.14 5.40 2.90 1.78 0.91 0.27 – – –
65.12 11.46 5.61 – 0.25 1.52 0.31 0.03 15.66 0.03 0.01
Table 2 Mix formulation of mortar. OPC
RCFA/RGC
Water
1
0.5
0.3
q lnðt2=t1Þ 4pðT2 T1Þ
ð1Þ
where, k is the thermal conductivity of sample, q is the heat generated by heater per unit length and time, t1 & t2 are the measuring time, T1 & T2 are the temperature at t1 & t2. Prior to the test, calibration of instrument with standard reference sample was conducted, and all specimens were dried. For each specimen, the thermal conductivity was measured at three points, and the result was taken as the average of three measured values. And, the thermal conductivity of each sample is the average of results of three specimens. 2.3.4. Lab-scale fire test Apart from evaluating the fire resistance of mortar sample via observing their strength degradation, spalling and cracking as well as change of thermal conductivity after being heated in furnace, lab scale fire tests were also conducted for all mortar samples with the specially designed furnace as shown in Fig. 4. Prior to the labscale fire test, all panel specimens were dried at 80 °C for 72 h. And then, the panel specimens were mechanically fixed as the opening (315 mm 165 mm) of the furnace. One side of the panel specimen was therefore exposed to high temperature while the other face was in ambient atmosphere. Thermal insulation mat were also installed between the panel specimen and the furnace to avoid heat leakage through the gap between specimen and furnace. A type S thermocouple was installed inside the chamber of furnace to measure the temperature of furnace, and five type K thermocouples were installed on the unexposed face of panel specimen to measure the temperature rise of the unexposed face (Fig. 4). After the lab-scale fire test, the surfaces of panel specimens were observed under a Leica DM2700M microscope, while the internal microstructure of panel specimens was studied under JEOL JCM 6000PLUS SEM. Small debris from tested panels were employed for SEM observation, and was coated with gold prior to SEM observation.
3. Results and discussion conductivity and compressive strength of unheated specimens were also measured at ambient temperature for all mortar samples. Averaged value was calculated on the three tested specimens in these tests to ensure the accuracy. 2.3.1. Compressive strength measurement The compression tests, for both heated and unheated specimen, were conducted at a SUST CMT5202 material testing system, and the loading rate employed was 0.5 mm/min. The compressive strength of each sample is the average of results of three specimens.
3.1. Spalling and cracking Typically, spalling and cracking would occur in mortar samples under high temperature due to the accumulation of vapor pressure inside and varied deformation of binder materials and aggregate. Figs. 5 and 6 respectively show the appearance of various heated RCFA mortar (i.e. the mortar containing recycled concrete fine
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Fig. 4. Specially designed furnace & specimen installation for the lab-scale fire test.
Fig. 5. Images of RCFA mortar after being heated at 300 °C, 600 °C, 900 °C and 1200 °C.
aggregate) samples and heated RGC mortar (i.e. the mortar containing recycled glass cullet) samples, while the degree of spalling/cracking of each sample were summarized in the Table 3. Under 300 °C, a few tiny cracks could be generally found on the surface of mortar samples, while all samples showed no spalling. Under this heating condition, the evaporation of free water could occur but the decomposition of cement hydration product and expansion of binder and aggregate were relatively insignificant. The thermal stress was not large enough to induce spalling, but fine cracks were still able to form during the cooling period due to the thermal stress induced by shrinkage when the temperature decreased. When the heating temperature increased from 300 °C to 600 °C and even 900 °C, more and more cement hydration products were expected to decompose along with strength degradation of the binder of mortar. Moreover, the expansion of binder materials and aggregate increased with increasing temperature, leading to larger thermal stress under heating as well as larger shrinkage stress under cooling. This is presumably the reason why the number and sizes of cracks formed in mortar samples, regardless of the type of fine aggregate, increased with the heating temperature. RGC mortar containing RGC larger than 2.36 mm even showed spalling after being heated at 900 °C. It can also be noticed that the number and size of cracks formed in RGC mortars increased with the sizes of RGC. Presumably the expansion and shrinkage difference between binder and aggregate increased with increasing
RGC size. It is also intersting to notice that the cracking/spalling degree of RCFA mortar sample was larger than that of RGC mortar containing RGC below 0.6 mm, but smaller than that of RGC mortar containing RGC larger than 0.6 mm. When the heating temperature was further increased to 1200 °C, the number and sizes of cracks formed in RCFA mortar further increased. However, the RGC mortars showed different behaviors. Although the RGC mortar containing RGC larger than 2.36 mm showed similar number and crack sizes to the condition of 900 °C, no spalling was noticed. In addition, the RGC mortars containing RGC smaller than 2.36 mm even showed reduced number and sizes of cracks, compared with the condition of 900 °C. This could be attributed to that glass cullet would start to soften or even melt gradually when the temperature reached 900 °C or above, as shown in Fig. 7, and the smaller RGC would be easier to soften and melt compared with larger RGC with similar amount of heat. When glass cullet was melt to liquid glass, the volume of glass was increased but the liquid glass would fill the voids around, leading to smaller volume change of mortar, which is different to the volume expansion of mortar being heated to lower temperature. Moreover, when the temperature decreases during cooling process, the melted glass will resolidify and act as binder to bind the decomposed binder materials together. This is presumably why the RGC mortars being heated at 1200 °C showed even less cracks/spalling when compared with the mortars being heated at 900 °C. 3.2. Compressive strength degradation Fig. 8 shows the measured compressive strength of RCFA mortar and RGC mortars after heating at various temperatures. Regarding to the ambient temperature condition, the compressive strength of RGC mortar decreased with increasing sizes of the RGC used, while the strengths of most RGC mortar are larger than that of the RCFA mortar. One possible reason is that pozzolanic reaction would occur in glass cullet especially when the sizes of glass cullet is below 100 lm [10], leading to the reduced porosity and increased binder/aggregate bonding strengths, and hence increased compressive strengths of mortar. In general, the reactivity of RGC increases with reduced size. While very few reaction would be expected to occur between RCFA and cement. When the mortars were subject to heating, as discussed in the above section, decomposition of cement hydration products and cracking would occur in the mortar sample. This led to strength reduction of mortar samples, as shown in Fig. 8. It is noted that the compressive strength of RCFA mortar were reduced after being subject to heating and the strength reduction increased with
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(a) 300oC
(b) 600oC
(c) 900oC
(d) 1200oC
Fig. 6. Images of RGC mortars after being heated at 300 °C, 600 °C, 900 °C and 1200 °C.
Table 3 Spalling/cracks level of RCFA and RGCs mortar at elevated temperature. Specimen ID
R-1 G-1 G-2 G-3 G-4
Spalling/cracks level 300 °C
600 °C
900 °C
1200 °C
NO NO NO NO NO
NO NO NO NO Partially
Partially NO Fully Fully Fully
Partially NO NO Partially Fully
Fig. 7. Images of RGCs after being heated at 300 °C, 600 °C, 900 °C and 1200 °C (sieved RGC at different temperatures).
increasing heating temperature, which is consistent to the degree of cracking of RCFA mortar against heating. Regarding the RGC mortars, besides the expected decomposition of cement hydration products and cracking, further pozzolanic reaction of glass cullet (<100 lm) under heating would also affect the strength of mortar. When the mortar samples were heated under 300 °C, the decomposition of cement hydration products
and cracking are insignificant, while the pozzolanic reaction of glass cullet (<100 lm) would contribute more, especially for mortar containing smaller RGC. This is why the compressive strength of mortar containing RGC below 0.6 mm was even increased and the compressive strength of mortar containing RGC of 0.6 mm1.18 mm showed almost no reduction after being heated at 300 °C. However, as the reactivity of RGC larger than 1.18 mm
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Fig. 9. Thermal conductivity of RCFA and RGC mortars before and after being heated at 300 °C, 600 °C, 900 °C and 1200 °C. Fig. 8. Compressive strength of RCFA and RGC mortars before and after being heated at 300 °C, 600 °C, 900 °C and 1200 °C.
was relatively low, the strength of mortars containing RGC above 1.18 mm was reduced after being heated at 300 °C. When the heating temperature increased to 600 °C and 900 °C, the strength of all RGC mortars were reduced, and the strength reduction increased with increasing heating temperature and RGC size. This is because the decomposition of cement hydration products and cracking of mortar become more serious at higher temperature. Under 900 °C, mortars containing RGCs larger than 0.6 mm showed serious cracking/spalling and could not even be tested under compression. When RGC mortars were heated at 1200 °C, as discussed in the above section, fine glass cullets were melted under heating and resolidify during cooling. This is probably the reason the compressive strength of mortar containing RGCs below 0.6 mm and 0.6–1.18 mm increased. It should be noted that the trends of strength reduction of mortar against heating temperature and aggregate size is generally consistent to the degrees of cracking/spalling of mortar against heating temperature and aggregate sizes. 3.3. Thermal conductivity Thermal conductivity is the property of a material to conduct heat. The thermal insulation performance of mortar specimens subject to fire highly depends on the thermal conductivity of mortar at different temperatures. The thermal conductivities of all mortar samples, before and after heating, were hence measured to evaluate the thermal insulation performance of mortar specimen subjected to fire. Fig. 9 shows the thermal conductivity of mortar samples measured under room temperature and measured after being heating at 300, 600, 900 and 1200 °C. Under ambient temperature, the thermal conductivities of RGC mortars, regardless of the size of RGC, are all smaller than that of RCFA mortar as a result of the lower thermal conductivity of RGC compared with RCFA. Regarding the RGC mortar samples, the thermal conductivity was found slightly increased with increasing RGC sizes, and the mortar containing RGC below 0.6 mm showed the lowest thermal conductivity and hence the best thermal insulation performance. As shown in Fig. 9, after being heated, all mortar samples showed reduced thermal conductivity, and the reduction in thermal conductivity increased with increasing heating temperature. This could be attributed to the increase of voids in mortar due to evaporation of moisture, decomposition of cement hydration products and cracking, as well as the phase change of binder and aggre-
gate under elevated temperatures. Generally speaking, the thermal conductivity of RGC mortars containing RGC of different sizes showed similar degree of reduction rate with increasing temperature. However, when the heating temperature reached 900 °C or above, the thermal conductivity of some RGC mortars cannot be measured due to serious cracking of specimens. Mortar containing RGC below 0.6 mm showed the lowest thermal conductivity amongst the five studied mortars, either under ambient temperature or after being heated, indicating that using fine RGC, instead of large size stone aggregate, could improve the thermal insulation of mortar under fire. 3.4. Lab-scale fire test Besides observing the cracking behavior, compressive strength degradation and thermal conductivity change of mortar subject to heating, the fire resistance of mortar sample was also evaluated using a high temperature furnace to simulate their behavior under actual fire test. Different mortar material as were fabricated as panels and was installed at the opening of a specially designed furnace in which the temperature rise roughly complies with the temperature rise requirements of BS 472 Part 22; During the test, one face (Heated face) of the panel was exposed to heat in furnace while the opposite face (Unheated face) was exposed to ambient atmosphere; Refer to the insulation and integrity criteria of standard fire test, the temperature rises at five points of each external side of the mortar panel were measured and the cracking of panel were recorded. All the panel specimens were subjected to heat in furnace for 5 h. The measured temperature rise curves of the RCFA mortar panel and the RGCs mortar panels containing RGCs below 0.6 mm and RGCs of 2.36–4.75 mm were shown in Fig. 10. The blue and red dotted lines in plots indicate the maximum temperature rise limit and the average temperature rise limit specified in BS 472 part 22, respectively. It can be seen that, the temperatures of the unheated faces of all mortar sample showed almost no increase within the first 30 min and then increased gradually. After 5 h testing, the maximum temperature rise and average temperature rise of the RCFA mortar were both below the limits, saying 180 °C and 140 °C respectively. Regarding the RGC mortars, the RGCs mortar containing RGCs below 0.6 mm showed similar temperature rise behavior to that of the RCFA mortar, however, the average and maximum temperature rises of the RGCs mortar containing RGCs of 2.36–4.75 mm exceeded the limits after about 220 min and 300 min respectively. This indicates that the RGCs mortar containing RGCs of 2.36–4.75 mm showed the worst fire resistance in
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(a) Mortar with recycled concrete fine aggregate
(b) Mortar with 2.36-4.75mm RGC
(c) Mortar with RGC below 0.6mm
Fig. 10. Temperature rising curves of (a) RCFA mortar and RGC mortars containing (b) RGC of 2.36–4.75 mm and (c) RGC below 0.6 mm in the lab scale fire test.
terms of insulation, and reducing the size of RGC cullets could significantly improve the fire resistance of mortar in terms of insulation. The temperature rise results were consistent with the results obtained in above sections. The following reasons can explain the improved thermal insulation property of ultra-fine recycled glass cullet (size < 0.6 mm):
fire resistance performance. Especially, glass cullets with smaller size are more likely to become soften and turn into liquid form when the same amount heating energy is applied since it has larger surface area in comparison with larger size. 3.5. Optical & Scanning Electron Microscopy
a) Compared to larger size of recycled glass cullet, ultra-fine recycled glass cullet (<100 lm) in the group of size smaller than 0.6 mm is more reactive than larger size glass, which accords with the result of compressive strength. Pozzolanic reaction can take place between ultra-fine recycled glass (<100 lm) and cement hydrated products (CH) and generate C-S-H [10], which can reduce the amount of CH since CH is unstable at elevated temperature owing to decomposition of CH. b) Recycled glass cullets were softened during temperature increment, and then turn into liquid with low viscosity under high temperature (e.g., 1200 °C), the glass liquid would flow into the porous structure inside the concrete/ mortar and generate more smaller voids as indicated in SEM photographs (Fig. 13(c)), which is the possible mechanism of the reduction of thermal conductivity and improved
Fig. 12 shows the high-powered microscope images before and after lab-scale fire tests. Glass was surrounded by cement hydrated products under room temperature while many holes appeared after fire test. Decomposition of cement hydrated products and melting of RGCs account for the formation of these holes. RGCs are in liquid form when heated to 1200 °C, so the RGCs liquid can fill the voids inside the mortar and the gaps (interface zones) as showed in SEM photos (a) and (b) in Fig. 13. After heating, large holes emerge where RGCs locates, which implied RGCs has been melt and flow into pores and gaps, Fig. 13(c) is the reflection of heating. It is worth to mentioning that even RGCs is melt under 1200 °C, smaller size of glass can also be formed when the specimen is cooled down naturally as indicated in Fig. 13. Although majority of melt glass play a role binder, it is easy to find that small particle
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(a) Before heating
(b) After heating
Fig. 11. Optical images of RGC mortar (<0.6 mm) (a) before and (b) after being heated at 1200 °C.
Fig. 12. SEM images of RGC mortar (<0.6 mm) (a) (b) before and (c) after the lab scale fire test.
Fig. 13. SEM images of heated RGC Mortar (<0.6 mm).
appearing in heated specimen (Fig. 11) when the specimen is naturally cooled down.
4. Conclusions In this paper work, RCFA and RGCs with different sizes were used to prepare fire-resistance mortar. The respective performance in terms of compressive strength, residual strength at elevated temperature, spalling/cracks behavior, thermal conductivity and insulation performance under lab-scale fire test were carried out.
Optical and SEM were used to observe the appearance and microstructure of RGCs mortar. Conclusions can be drawn according to the previous results and discussions that the glass cullets with size below 0.6 mm could increase the compressive strength via pozzalanic reaction between ultra-fine recycled glass cullets (<100 lm) and cement hydrated products (CH). Compressive strength was reduced in general with temperature rising whereas the residual strength was higher when incorporating smaller size glass cullets. Unsieved glass cullet would have interior performance when compared to RCFA in lab-scale fire test. However, glass cullets demonstrate increasing better fire-resistance when size stays under 0.6 mm in both integrity and thermal insulation, especially under high temperature due to the change of solid to liquid phase. High-powered microscope and SEM images reveal another possible mechanism why glass can have better thermal insulation (lower thermal conductivity). To conclude, recycled glass cullets under 0.6 mm would be favorable to improve the fire resistance of concrete/mortar compared to RCFA. CRediT authorship contribution statement Binmeng Chen: Data curation, Writing - original draft. Honggang Zhu: Formal analysis. Bo Li: Methodology. Manlung Sham: Writing - review & editing. Zongjin Li: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This study was sponsored by the Innovation and Technology Commission, Hong Kong SAR (Grant numbers: ITP/072/14NI). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117992.
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aggregates incorporated by addition and substitution, Energy Procedia 139 (December 2017) 499–504. Tung-Chai Ling, Chi-Sun Poon, Shi-Cong Kou, Feasibility of using recycled glass in architectural cement mortars, Cem. Concr. Compos. 33 (2011) 848–854. So YeongChoi, Yoon SukChoi, Eun IkYang, Characteristics of volume change and heavy metal leaching in mortar specimens recycled heavyweight waste glass as fine aggregate, Constr. Build. Mater. 165 (20) (March 2018) 424–433. _ Ilker BekirTopçu, Mehmet Canbaz, Properties of concrete containing waste glass, Cem. Concr. Res. 34 (2) (2004) 267–274. Tung-Chai Ling, Chi-Sun Poon, Hau-Wing Wong, Management and recycling of waste glass in concrete products: current situations in Hong Kong, Resour. Conserv. Recycl. 70 (January 2013) 25–31. Tung-Chai Ling, Chi-Sun Poon, Stress-strain behaviour of fire exposed selfcompacting glass concrete, Fire Mater. 37 (4) (2012) 297–310. BS EN 1364-1:2015. Fire resistance tests for non-loadbearing elements: Walls. Ming-Zhi Guo, Zhen Chen, Tung-Chai Ling, Chi-Sun Poon, Effects of recycled glass on properties of architectural mortar before and after exposure to elevated temperatures, J. Cleaner Prod. 101 (August 2015) 158–164. Tomas Ucol Ganiron Jr., Use of recycled glass bottles as fine aggregates in concrete mixture, Int. J. Adv. Sci. Technol. 61 (2013) 17–28. Tomas Ucol Ganiron Jr., Recycled window glass for non-load bearing walls, Int. J. Innovation Manage. Technol. 3 (6) (2012) 725–730. Msheer Hasan Ali, Youkhanna Zayia Dinkha, James H. Haido, Mechanical properties and spalling at elevated temperature of high performance concrete made with reactive and waste inert powders, Eng. Sci. Technol. 20 (2) (2017) 536–541. Venkatesh Kodur, Properties of concrete at elevated temperatures, ISRN Civ. Eng. 2014 (2014) 1–15, https://doi.org/10.1155/2014/468510. Carlos Leiva, Luis F. Vilches, Xavier Querol, Jose Vale, Constantino Fernández Pereira, Use of zeolitised fly ashes in fire resistant plates. 2007 World of Coal Ash (WOCA), May 7–10, Covington, Kentucky, USA, 2007. Celia García Arenas, Madelyn Marrero, Carlos Leiva, Jaime Solís-Guzmán, Luis F. Vilches Arenas, High fire resistance in blocks containing coal combustion fly ashes and bottom ash, Waste Manage. 31 (8) (2011) 1783–1789. Zhao Xiao, Tung-Chai Ling, Chi-Sun Poon, Shi-Cong Kou, Qingyuan Wang, Runqiu Huang, Properties of partition wall blocks prepared with high percentages of recycled clay brick after exposure to elevated temperatures, Constr. Build. Mater. 49 (December 2013) 56–61. Okan Karahan, Transport properties of high volume fly ash or slag concrete exposed to high temperature, Constr. Build. Mater. 152 (2017) 898–906, https://doi.org/10.1016/j.conbuildmat.2017.07.051.
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Study on the fire resistance performance of cementitious composites containing recycled glass cullets (RGCs) Binmeng Chen a,b, Honggang Zhu b,⇑, Bo Li c,⇑, Manlung Sham b, Zongjin Li d a
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China Nano and Advanced Materials Institute Limited, Hong Kong Science Park, Hong Kong, China c Department of Civil Engineering, The University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo, China d Institute of Applied Physics and Materials Engineering, University of Macau, Taipa, Macau, China b
h i g h l i g h t s RGCs (<100
lm) can increase the compressive strength via pozzalanic reaction.
Higher residual strength when incorporating smaller RGCs due to the binding effect. RGCs below 0.6 mm can apparently prevent spalling/cracks under high temperature. Lab-scale fire test is a promising method to simulate the full-scale fire test. RGCs under 0.6 mm are more promising materials in fire resistance concrete/mortar.
a r t i c l e
i n f o
Article history: Received 15 July 2019 Received in revised form 24 December 2019 Accepted 31 December 2019
Keywords: Recycled glass cullets Cementitious composites Fire resistance Integrity Thermal insulation
a b s t r a c t Glass manufacturing involves abundant amount of natural resources including thermal energy and it is important to explore various ways to ultilize the used glass instead of direct disposal to landfill. One way to recycle the glass products is to crush them into cullets and mix the cullets into cement as filler/aggregate. In this study, the effects of recycled glass cullets (RGCs) with different sizes on the fire resistance performance of cementitious composite were studied. Cementitious composite samples were prepared with RGCs at 4 different sizes, i.e. below 0.6 mm, 0.6–1.18 mm, 1.18–2.36 mm and 2.36– 4.75 mm. In order to evaluate the fire resistance of the composites with RGC, the composites samples were heated at different temperatures, and their residual strength, integrity and thermal conductivity were measured accordingly. Moreover, a specially designed furnace in accordance with modified BS 476-22 was employed to observe the thermal insulation and integrity of mortar panels under testing. The microstructure of RGCs mortar was observed under microscope and it was found that adding RGCs inside the composite improved the fire resistance of mortar. The beneficial effect of RGCs on fire resistance was found to reduce with increasing RGCs size, where RGCs finer than 0.6 mm showed the most significant beneficial effect. This is presumably attributed to the softening and state transformation of fine glass cullet at high temperature. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Tons of waste glass bottles were generated every day in the world, and recycling the disposed glasses is always a big challenge to achieve sustainable development. In particular, Hong Kong has over 300 tons of waste glass bottles being generated every day, and most of them were disposed to the landfill directly due to the lack of glass re-fabrication plants [1]. This not only increases ⇑ Corresponding authors. E-mail addresses: [email protected] (H. Zhu), [email protected] (B. Li). https://doi.org/10.1016/j.conbuildmat.2019.117992 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
the burden of landfill and environment, but also leading to waste of natural resources and energy. The Hong Kong government has been encouraging the community to participate in glass recycling to promote resources recovery. Apart from melting the waste glass for reproduction of glass products, another approach is to recycle waste glass by crushing them into cullets and to integrate the cullets into cementitious composites as filler/aggregate. Cementitious materials, including concrete and mortar, are the most widely used materials in the world, not only serve as structural elements but also as various kinds functional materials [2–9]. Integrating glass cullets into cementitious composites is therefore a promising way to revitalizing the waste glass [10–15,17–19]. Bentchikou
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et al. [10] studied the performance of mortar containing both sand and recycled glass with different ratios; It was found that adding recycled glass reduced the density of mortar, and there was an optimized content of recycled glass based on the 28-day compressive strength results. Ling et al. [11] studied the feasibility of using 100% recycled glass aggregate to prepare architectural white cement mortar; It was found that recycled glass could improve the workability and drying shrinkage of mortar; However, the compressive and flexural strength of mortar decreased with increasing recycled glass content. So et al. [12] discussed the feasibility of using recycled heavyweight waste glass as fine aggregate in mortar and it was reported that utilizing heavyweight waste glass as fine aggregate in mortar was possible to reduce both dry shrinkage and alkali-silica reaction (ASR) expansion. BekirTopçu et al. [13] reported using 4–16 mm waste glass to replace 0–60% coarse aggregate in concrete had no obvious effect on workability but slightly reduced the strength. As a result, non-load bearing mortar and concrete products containing recycled glass have been developed and commercialized in various regions nowadays [14]. Apart from compressive strength, the fire resistance of different building materials determines the respective application scenarios. When being exposed to fire, evaporation of free water, dehydration of the hydrated products of cement as well as expansion of aggregates would all take place in mortar/concrete accompanied with significant strength degradation and loss of integrity and heat insulation [15]. The fire resistance of non-load bearing building elements is typically assessed in terms of thermal insulation and integrity according to relevant standards, for example BS EN 1364-1 [16]. The integrity is generally defined as the ability of the tested element to withstand collapse, free from holes, cracks and fissures and sustained flaming on the unexposed face, while the thermal insulation is generally defined as the ability of the tested element to restrict the temperature rise of the unexposed face of the element. As full scale standard fire test can only be conducted in standard fire testing labs which is rather time consuming and costly, researcher always prefer to evaluate the fire resistance performance of sample in laboratory. Investigations have been conducted to study the performance of mortar/concrete containing various recycled wastes at elevated temperature [15,17,20–25]. Guo et al. [17] and Ling et al. [15] reported that integrating recycled glass into mortar/concrete could enhance their residual mechanical performance after heating at 800 °C. Ali et al. [20] reported that integrating the glass powder into concrete could improve its residual strength after high temperature heating than silica fume. Leiva et al. [22] employed an inhouse designed furnace to evaluate the insulating capability of sample by monitoring the time for the unexposed face of sample reaching 180 °C increment while another side was heated. Arenas et al. [23] studied the fire resistance of concrete blocks containing coal combustion fly ashes and bottom ash. Nevertheless, most of the previous researches were conducted below 1150–1200 °C [17,20,22–25] as required in the standard fire test, and it is critical to understand the behavior of the materials under further elevated temperature. In the first part of this study, recycled concrete fine aggregate (RCFA) was selected as a comparable recycled material to demonstrate the benefits of using recycled glass cullets of small size since RCFA is commonly used as recycled fine aggregate in concrete. Mortar samples were prepared and heated at a series of temperatures in a furnace respectively (300 °C, 600 °C, 900 °C, and 1200 °C), and their residual strength, integrity and thermal conductivity were measured accordingly. Moreover, a specially designed in-house furnace in accordance with modified BS 47622 was employed (Fig. 1) to evaluate the thermal insulation and integrity of mortar panels. The effects of recycled glass cullets of different sizes on the fire resistance of mortar were then evaluated.
Fig. 1. Temperature rising curves of the furnace and BS 476-22.
2. Experimental 2.1. Materials The cement for mortar sample preparation was a typical market available 52.5 N Ordinary Portland Cement (OPC), with 3.15 g/cm3 specific gravity and surface area of 3310 cm2/g. Two kinds of recycled wastes, namely, recycled glass cullets and recycled concrete aggregate, were used as fine aggregate in the mortar. The recycled concrete aggregates were manufactured by crushing and sieving the concrete from those demolished buildings and tunnels locally in Hong Kong with maximum size 4.75 mm. It can be easily seen from the figure that there is a clear boundary in recycled concrete aggregate (Fig. 2(b)) due to it may contain binder and aggregate (Fig. 2(a)) when concrete was crushed, making recycled concrete aggregate higher water absorption of 7.4%. In a similar vein, the recycled glass cullets were manufactured by crushing and sieving from the disposed glass bottles. Fig. 3 shows the particle size distribution curves of the as-received recycled concrete fine aggregate (RCFA) and recycled glass cullets (RGCs). In order to study the effects of different sized glass cullets, the as-received glass cullets were also sieved to 4 kinds of different sizes, i.e. below 0.6, 0.6–1.18, 1.18–2.36 and 2.36– 4.75 mm, for sample preparation. The chemical composition of the employed OPC and RGCs were listed in Table 1. 2.2. Sample preparation The mix formulation of all mortar samples is shown in Table 2, and a water to cement ratio of 0.3 was used. As the densities of RCFA and RGC were similar, the volume ratios of RCFA and RGC were also similar. Among different mortar samples, the mix formulation is same but the fine aggregates employed are different. In order to ensure the different mortar materials have similar workability, suitable amount of water reducing agent was used if need. A planetary mixer was employed for preparing mortar material. The fine aggregates used were dried in a 105 °C oven for 24 h prior to mixing. To prepare mortar, required amount of cement and fine aggregates were mixed for 2 min, and then water and water reducing agent were added with another 2 more minutes mixing. All mortar specimens were demoulded after 24 h, and then cured in standard curing condition (27 ± 2 °C, RH 95%) for 27 days. In each mortar formulation, 40 mm 40 mm 40 mm cubes were prepared for compressive strength measurement, 40 mm 40 mm 160 mm prisms were prepared for observing spalling/cracks and measuring thermal conductivity, and 165 mm 315 mm 100 mm panels were prepared for fire resistance evaluation using the previous mentioned in-house furnace. All the specimens were cured in standard curing condition for 28 days prior to further processing or testing. 2.3. Test methods In order to study the fire resistance of mortar sample, 40 mm 40 mm 40 mm cube and 40 mm 40 mm 160 mm prism specimens of all different mortar formulation were heated at a specified temperature in furnace. Samples were heated gradually to the target temperature (300 °C, 600 °C, 900 °C, and 1200 °C) with a heating rate 3 °C/min, then kept at this temperature for another 2 h and cooled down naturally later. After cooling in furnace, the 40 mm 40 mm 40 mm cubes were subject to compression test for the residual strength measurement, while the 40 mm 40 mm 160 mm prisms were used for integrity and thermal conductivity evaluation. The heating temperatures employed include 300, 600, 900 and 1200 °C. For the purpose of comparison, the thermal
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(a)
3
(b)
Fig. 2. Photo & optical image of recycled concrete fine aggregate.
2.3.2. Spalling/crack behavior Spalling or cracking is an important integrity indicator of the fire resistance of an element. In this study, the degrees of spalling and cracking of all specimens subject to heating were observed. As referred to previous publications [20], spalling/ cracking can be classified into three levels, i.e. no spalling/cracks, partially spalling/cracks and full spalling/cracks. The strength degradation of mortar samples subject to heating was also relevant to the degree of spalling/cracking. 2.3.3. Thermal conductivity A quick thermal conductivity meter KEM QTM 500 was used to measure the thermal conductivity of all mortar samples. The principle of measurement of the instrument is the hot wire method specified in relevant international standards, for example BS EN 993-15 and ASTM C-1113. The following equation is used to calculate the thermal conductivity,
k¼
Fig. 3. Particle size distribution of the as-received RCFA and RGCs.
Table 1 Chemical composition of the employed OPC & RGC. Chemical component
OPC (wt%)
RGC (wt%)
SiO2 Al2O3 CaO SO4 Fe2O3 MgO K2O TiO2 Na2O Cr2O3 MnO
19.85 3.68 65.14 5.40 2.90 1.78 0.91 0.27 – – –
65.12 11.46 5.61 – 0.25 1.52 0.31 0.03 15.66 0.03 0.01
Table 2 Mix formulation of mortar. OPC
RCFA/RGC
Water
1
0.5
0.3
q lnðt2=t1Þ 4pðT2 T1Þ
ð1Þ
where, k is the thermal conductivity of sample, q is the heat generated by heater per unit length and time, t1 & t2 are the measuring time, T1 & T2 are the temperature at t1 & t2. Prior to the test, calibration of instrument with standard reference sample was conducted, and all specimens were dried. For each specimen, the thermal conductivity was measured at three points, and the result was taken as the average of three measured values. And, the thermal conductivity of each sample is the average of results of three specimens. 2.3.4. Lab-scale fire test Apart from evaluating the fire resistance of mortar sample via observing their strength degradation, spalling and cracking as well as change of thermal conductivity after being heated in furnace, lab scale fire tests were also conducted for all mortar samples with the specially designed furnace as shown in Fig. 4. Prior to the labscale fire test, all panel specimens were dried at 80 °C for 72 h. And then, the panel specimens were mechanically fixed as the opening (315 mm 165 mm) of the furnace. One side of the panel specimen was therefore exposed to high temperature while the other face was in ambient atmosphere. Thermal insulation mat were also installed between the panel specimen and the furnace to avoid heat leakage through the gap between specimen and furnace. A type S thermocouple was installed inside the chamber of furnace to measure the temperature of furnace, and five type K thermocouples were installed on the unexposed face of panel specimen to measure the temperature rise of the unexposed face (Fig. 4). After the lab-scale fire test, the surfaces of panel specimens were observed under a Leica DM2700M microscope, while the internal microstructure of panel specimens was studied under JEOL JCM 6000PLUS SEM. Small debris from tested panels were employed for SEM observation, and was coated with gold prior to SEM observation.
3. Results and discussion conductivity and compressive strength of unheated specimens were also measured at ambient temperature for all mortar samples. Averaged value was calculated on the three tested specimens in these tests to ensure the accuracy. 2.3.1. Compressive strength measurement The compression tests, for both heated and unheated specimen, were conducted at a SUST CMT5202 material testing system, and the loading rate employed was 0.5 mm/min. The compressive strength of each sample is the average of results of three specimens.
3.1. Spalling and cracking Typically, spalling and cracking would occur in mortar samples under high temperature due to the accumulation of vapor pressure inside and varied deformation of binder materials and aggregate. Figs. 5 and 6 respectively show the appearance of various heated RCFA mortar (i.e. the mortar containing recycled concrete fine
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Fig. 4. Specially designed furnace & specimen installation for the lab-scale fire test.
Fig. 5. Images of RCFA mortar after being heated at 300 °C, 600 °C, 900 °C and 1200 °C.
aggregate) samples and heated RGC mortar (i.e. the mortar containing recycled glass cullet) samples, while the degree of spalling/cracking of each sample were summarized in the Table 3. Under 300 °C, a few tiny cracks could be generally found on the surface of mortar samples, while all samples showed no spalling. Under this heating condition, the evaporation of free water could occur but the decomposition of cement hydration product and expansion of binder and aggregate were relatively insignificant. The thermal stress was not large enough to induce spalling, but fine cracks were still able to form during the cooling period due to the thermal stress induced by shrinkage when the temperature decreased. When the heating temperature increased from 300 °C to 600 °C and even 900 °C, more and more cement hydration products were expected to decompose along with strength degradation of the binder of mortar. Moreover, the expansion of binder materials and aggregate increased with increasing temperature, leading to larger thermal stress under heating as well as larger shrinkage stress under cooling. This is presumably the reason why the number and sizes of cracks formed in mortar samples, regardless of the type of fine aggregate, increased with the heating temperature. RGC mortar containing RGC larger than 2.36 mm even showed spalling after being heated at 900 °C. It can also be noticed that the number and size of cracks formed in RGC mortars increased with the sizes of RGC. Presumably the expansion and shrinkage difference between binder and aggregate increased with increasing
RGC size. It is also intersting to notice that the cracking/spalling degree of RCFA mortar sample was larger than that of RGC mortar containing RGC below 0.6 mm, but smaller than that of RGC mortar containing RGC larger than 0.6 mm. When the heating temperature was further increased to 1200 °C, the number and sizes of cracks formed in RCFA mortar further increased. However, the RGC mortars showed different behaviors. Although the RGC mortar containing RGC larger than 2.36 mm showed similar number and crack sizes to the condition of 900 °C, no spalling was noticed. In addition, the RGC mortars containing RGC smaller than 2.36 mm even showed reduced number and sizes of cracks, compared with the condition of 900 °C. This could be attributed to that glass cullet would start to soften or even melt gradually when the temperature reached 900 °C or above, as shown in Fig. 7, and the smaller RGC would be easier to soften and melt compared with larger RGC with similar amount of heat. When glass cullet was melt to liquid glass, the volume of glass was increased but the liquid glass would fill the voids around, leading to smaller volume change of mortar, which is different to the volume expansion of mortar being heated to lower temperature. Moreover, when the temperature decreases during cooling process, the melted glass will resolidify and act as binder to bind the decomposed binder materials together. This is presumably why the RGC mortars being heated at 1200 °C showed even less cracks/spalling when compared with the mortars being heated at 900 °C. 3.2. Compressive strength degradation Fig. 8 shows the measured compressive strength of RCFA mortar and RGC mortars after heating at various temperatures. Regarding to the ambient temperature condition, the compressive strength of RGC mortar decreased with increasing sizes of the RGC used, while the strengths of most RGC mortar are larger than that of the RCFA mortar. One possible reason is that pozzolanic reaction would occur in glass cullet especially when the sizes of glass cullet is below 100 lm [10], leading to the reduced porosity and increased binder/aggregate bonding strengths, and hence increased compressive strengths of mortar. In general, the reactivity of RGC increases with reduced size. While very few reaction would be expected to occur between RCFA and cement. When the mortars were subject to heating, as discussed in the above section, decomposition of cement hydration products and cracking would occur in the mortar sample. This led to strength reduction of mortar samples, as shown in Fig. 8. It is noted that the compressive strength of RCFA mortar were reduced after being subject to heating and the strength reduction increased with
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(a) 300oC
(b) 600oC
(c) 900oC
(d) 1200oC
Fig. 6. Images of RGC mortars after being heated at 300 °C, 600 °C, 900 °C and 1200 °C.
Table 3 Spalling/cracks level of RCFA and RGCs mortar at elevated temperature. Specimen ID
R-1 G-1 G-2 G-3 G-4
Spalling/cracks level 300 °C
600 °C
900 °C
1200 °C
NO NO NO NO NO
NO NO NO NO Partially
Partially NO Fully Fully Fully
Partially NO NO Partially Fully
Fig. 7. Images of RGCs after being heated at 300 °C, 600 °C, 900 °C and 1200 °C (sieved RGC at different temperatures).
increasing heating temperature, which is consistent to the degree of cracking of RCFA mortar against heating. Regarding the RGC mortars, besides the expected decomposition of cement hydration products and cracking, further pozzolanic reaction of glass cullet (<100 lm) under heating would also affect the strength of mortar. When the mortar samples were heated under 300 °C, the decomposition of cement hydration products
and cracking are insignificant, while the pozzolanic reaction of glass cullet (<100 lm) would contribute more, especially for mortar containing smaller RGC. This is why the compressive strength of mortar containing RGC below 0.6 mm was even increased and the compressive strength of mortar containing RGC of 0.6 mm1.18 mm showed almost no reduction after being heated at 300 °C. However, as the reactivity of RGC larger than 1.18 mm
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Fig. 9. Thermal conductivity of RCFA and RGC mortars before and after being heated at 300 °C, 600 °C, 900 °C and 1200 °C. Fig. 8. Compressive strength of RCFA and RGC mortars before and after being heated at 300 °C, 600 °C, 900 °C and 1200 °C.
was relatively low, the strength of mortars containing RGC above 1.18 mm was reduced after being heated at 300 °C. When the heating temperature increased to 600 °C and 900 °C, the strength of all RGC mortars were reduced, and the strength reduction increased with increasing heating temperature and RGC size. This is because the decomposition of cement hydration products and cracking of mortar become more serious at higher temperature. Under 900 °C, mortars containing RGCs larger than 0.6 mm showed serious cracking/spalling and could not even be tested under compression. When RGC mortars were heated at 1200 °C, as discussed in the above section, fine glass cullets were melted under heating and resolidify during cooling. This is probably the reason the compressive strength of mortar containing RGCs below 0.6 mm and 0.6–1.18 mm increased. It should be noted that the trends of strength reduction of mortar against heating temperature and aggregate size is generally consistent to the degrees of cracking/spalling of mortar against heating temperature and aggregate sizes. 3.3. Thermal conductivity Thermal conductivity is the property of a material to conduct heat. The thermal insulation performance of mortar specimens subject to fire highly depends on the thermal conductivity of mortar at different temperatures. The thermal conductivities of all mortar samples, before and after heating, were hence measured to evaluate the thermal insulation performance of mortar specimen subjected to fire. Fig. 9 shows the thermal conductivity of mortar samples measured under room temperature and measured after being heating at 300, 600, 900 and 1200 °C. Under ambient temperature, the thermal conductivities of RGC mortars, regardless of the size of RGC, are all smaller than that of RCFA mortar as a result of the lower thermal conductivity of RGC compared with RCFA. Regarding the RGC mortar samples, the thermal conductivity was found slightly increased with increasing RGC sizes, and the mortar containing RGC below 0.6 mm showed the lowest thermal conductivity and hence the best thermal insulation performance. As shown in Fig. 9, after being heated, all mortar samples showed reduced thermal conductivity, and the reduction in thermal conductivity increased with increasing heating temperature. This could be attributed to the increase of voids in mortar due to evaporation of moisture, decomposition of cement hydration products and cracking, as well as the phase change of binder and aggre-
gate under elevated temperatures. Generally speaking, the thermal conductivity of RGC mortars containing RGC of different sizes showed similar degree of reduction rate with increasing temperature. However, when the heating temperature reached 900 °C or above, the thermal conductivity of some RGC mortars cannot be measured due to serious cracking of specimens. Mortar containing RGC below 0.6 mm showed the lowest thermal conductivity amongst the five studied mortars, either under ambient temperature or after being heated, indicating that using fine RGC, instead of large size stone aggregate, could improve the thermal insulation of mortar under fire. 3.4. Lab-scale fire test Besides observing the cracking behavior, compressive strength degradation and thermal conductivity change of mortar subject to heating, the fire resistance of mortar sample was also evaluated using a high temperature furnace to simulate their behavior under actual fire test. Different mortar material as were fabricated as panels and was installed at the opening of a specially designed furnace in which the temperature rise roughly complies with the temperature rise requirements of BS 472 Part 22; During the test, one face (Heated face) of the panel was exposed to heat in furnace while the opposite face (Unheated face) was exposed to ambient atmosphere; Refer to the insulation and integrity criteria of standard fire test, the temperature rises at five points of each external side of the mortar panel were measured and the cracking of panel were recorded. All the panel specimens were subjected to heat in furnace for 5 h. The measured temperature rise curves of the RCFA mortar panel and the RGCs mortar panels containing RGCs below 0.6 mm and RGCs of 2.36–4.75 mm were shown in Fig. 10. The blue and red dotted lines in plots indicate the maximum temperature rise limit and the average temperature rise limit specified in BS 472 part 22, respectively. It can be seen that, the temperatures of the unheated faces of all mortar sample showed almost no increase within the first 30 min and then increased gradually. After 5 h testing, the maximum temperature rise and average temperature rise of the RCFA mortar were both below the limits, saying 180 °C and 140 °C respectively. Regarding the RGC mortars, the RGCs mortar containing RGCs below 0.6 mm showed similar temperature rise behavior to that of the RCFA mortar, however, the average and maximum temperature rises of the RGCs mortar containing RGCs of 2.36–4.75 mm exceeded the limits after about 220 min and 300 min respectively. This indicates that the RGCs mortar containing RGCs of 2.36–4.75 mm showed the worst fire resistance in
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(a) Mortar with recycled concrete fine aggregate
(b) Mortar with 2.36-4.75mm RGC
(c) Mortar with RGC below 0.6mm
Fig. 10. Temperature rising curves of (a) RCFA mortar and RGC mortars containing (b) RGC of 2.36–4.75 mm and (c) RGC below 0.6 mm in the lab scale fire test.
terms of insulation, and reducing the size of RGC cullets could significantly improve the fire resistance of mortar in terms of insulation. The temperature rise results were consistent with the results obtained in above sections. The following reasons can explain the improved thermal insulation property of ultra-fine recycled glass cullet (size < 0.6 mm):
fire resistance performance. Especially, glass cullets with smaller size are more likely to become soften and turn into liquid form when the same amount heating energy is applied since it has larger surface area in comparison with larger size. 3.5. Optical & Scanning Electron Microscopy
a) Compared to larger size of recycled glass cullet, ultra-fine recycled glass cullet (<100 lm) in the group of size smaller than 0.6 mm is more reactive than larger size glass, which accords with the result of compressive strength. Pozzolanic reaction can take place between ultra-fine recycled glass (<100 lm) and cement hydrated products (CH) and generate C-S-H [10], which can reduce the amount of CH since CH is unstable at elevated temperature owing to decomposition of CH. b) Recycled glass cullets were softened during temperature increment, and then turn into liquid with low viscosity under high temperature (e.g., 1200 °C), the glass liquid would flow into the porous structure inside the concrete/ mortar and generate more smaller voids as indicated in SEM photographs (Fig. 13(c)), which is the possible mechanism of the reduction of thermal conductivity and improved
Fig. 12 shows the high-powered microscope images before and after lab-scale fire tests. Glass was surrounded by cement hydrated products under room temperature while many holes appeared after fire test. Decomposition of cement hydrated products and melting of RGCs account for the formation of these holes. RGCs are in liquid form when heated to 1200 °C, so the RGCs liquid can fill the voids inside the mortar and the gaps (interface zones) as showed in SEM photos (a) and (b) in Fig. 13. After heating, large holes emerge where RGCs locates, which implied RGCs has been melt and flow into pores and gaps, Fig. 13(c) is the reflection of heating. It is worth to mentioning that even RGCs is melt under 1200 °C, smaller size of glass can also be formed when the specimen is cooled down naturally as indicated in Fig. 13. Although majority of melt glass play a role binder, it is easy to find that small particle
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(a) Before heating
(b) After heating
Fig. 11. Optical images of RGC mortar (<0.6 mm) (a) before and (b) after being heated at 1200 °C.
Fig. 12. SEM images of RGC mortar (<0.6 mm) (a) (b) before and (c) after the lab scale fire test.
Fig. 13. SEM images of heated RGC Mortar (<0.6 mm).
appearing in heated specimen (Fig. 11) when the specimen is naturally cooled down.
4. Conclusions In this paper work, RCFA and RGCs with different sizes were used to prepare fire-resistance mortar. The respective performance in terms of compressive strength, residual strength at elevated temperature, spalling/cracks behavior, thermal conductivity and insulation performance under lab-scale fire test were carried out.
Optical and SEM were used to observe the appearance and microstructure of RGCs mortar. Conclusions can be drawn according to the previous results and discussions that the glass cullets with size below 0.6 mm could increase the compressive strength via pozzalanic reaction between ultra-fine recycled glass cullets (<100 lm) and cement hydrated products (CH). Compressive strength was reduced in general with temperature rising whereas the residual strength was higher when incorporating smaller size glass cullets. Unsieved glass cullet would have interior performance when compared to RCFA in lab-scale fire test. However, glass cullets demonstrate increasing better fire-resistance when size stays under 0.6 mm in both integrity and thermal insulation, especially under high temperature due to the change of solid to liquid phase. High-powered microscope and SEM images reveal another possible mechanism why glass can have better thermal insulation (lower thermal conductivity). To conclude, recycled glass cullets under 0.6 mm would be favorable to improve the fire resistance of concrete/mortar compared to RCFA. CRediT authorship contribution statement Binmeng Chen: Data curation, Writing - original draft. Honggang Zhu: Formal analysis. Bo Li: Methodology. Manlung Sham: Writing - review & editing. Zongjin Li: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This study was sponsored by the Innovation and Technology Commission, Hong Kong SAR (Grant numbers: ITP/072/14NI). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117992.
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