Influence of rice husk ash on mechanical properties and fire resistance of recycled aggregate high-calcium fly ash geopolymer concrete

Journal Pre-proof Influence of rice husk ash on mechanical properties and fire resistance of recycled aggregate high-calcium fly ash geopolymer concrete

Peem Nuaklong, Pitcha Jongvivatsakul, Thanyawat Pothisiri, Vanchai Sata, Prinya Chindaprasirt PII:

S0959-6526(19)34667-0

DOI:

https://doi.org/10.1016/j.jclepro.2019.119797

Reference:

JCLP 119797

To appear in:

Journal of Cleaner Production

Received Date:

01 August 2019

Accepted Date:

18 December 2019

Please cite this article as: Peem Nuaklong, Pitcha Jongvivatsakul, Thanyawat Pothisiri, Vanchai Sata, Prinya Chindaprasirt, Influence of rice husk ash on mechanical properties and fire resistance of recycled aggregate high-calcium fly ash geopolymer concrete, Journal of Cleaner Production (2019), https://doi.org/10.1016/j.jclepro.2019.119797

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Journal Pre-proof Influence of rice husk ash on mechanical properties and fire resistance of recycled aggregate high-calcium fly ash geopolymer concrete Peem Nuaklonga, Pitcha Jongvivatsakulb,*, Thanyawat Pothisiria, Vanchai Satac and Prinya Chindaprasirtc, d Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand b Innovative Construction Materials Research Unit, Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand c Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand d Academy of Science, The Royal Society of Thailand, Dusit, Bangkok, 10300, Thailand a

Abstract: The use of recycled aggregates in geopolymer concrete provides an eco-friendly alternative for the construction industry. Since geopolymer concretes made with recycled aggregates generally have weaker internal structure compared with those using natural aggregates, SiO2-rich materials have therefore been used to improve the strength properties, one of which is the well-known nano-SiO2 (nS). However, because nS is not environmental friendly, other green SiO2-rich materials, such as rice husk ash (RHA) from agricultural wastes, are needed. The objective of this study is to investigate the effectiveness of replacing nanoSiO2 with rich husk ash for improving the performance of recycled aggregate geopolymer concrete (RAGC) made from high-calcium fly ash that is able to set and harden without heat curing. The mechanical properties and fire-resistant performance of RHA-added and nS-added geopolymer concretes were tested and compared. The results show that the addition of RHA is effective in improving the strengths of RAGCs, particularly when the SiO2/Al2O3 ratio is increased to 4.17. The 28-day compressive strengths of RAGCs containing RHAs ranged from 36.0 to 38.1 MPa due to the improved microstructure and denser matrix and were comparable to those of RAGCs made with nS. However, the inclusion of SiO2-rich materials had an adverse effect on the post-fire residual strength of geopolymer concretes made from recycled aggregates due primarily to the reduced porosity. Keywords: Recycled aggregate; Geopolymer concrete; Nano silica, Rice husk ash; Fire resistance *Corresponding

author. Tel: +66-2218-6570, Fax: +66-2251-7304.

E-mail address: [email protected] (Pitcha Jongvivatsakul) No. of words in the whole file: 7,886 No. of tables: 8 No. of figures: 9

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Journal Pre-proof 1. Introduction Geopolymer is a type of alkali-activated cementitious systems (Provis, 2014) which is acknowledged as a green binder that can be used to substitute Portland cement in the construction industry (Zhuang et al., 2016). It is produced by mixing solid aluminosilicate, such as metakaolin and fly ash with an alkali activator (Hajimohammadi and van Deventer, 2017; Provis, 2018). Unlike Portland cement composites, in which calcium silicate hydrate (CSH) gel is the principal binding compound, this material contains geopolymeric gel as the main cementing compound (Provis et al., 2012). Under moderate temperature curing of 40-90 °C, the geopolymeric material using low-calcium fly ash or metakaolin shows superior cementing properties, compared to when the mixture is cured at ambient temperature (Nath and Sarker, 2015). Without heat, the rate of chemical reaction in the system is slow, and thus the geopolymer composite takes more time to harden and exhibits low early strengths (Nath and Sarker, 2015). In contrast, geopolymers made from high-calcium fly ash (HCF) are able to set and harden without heat curing. With moderate contents of calcium in the fly ash, higher geopolymer strength can be obtained when cured at room temperature (Provis, 2018). This helps expand the application of fly ash geopolymers beyond precast elements to the building construction where concrete members are cast on-site. In addition, further economic saving can be obtained when the heat curing process is not required. The SiO2/Al2O3 ratio of the mix is one of the factors controlling the properties of geopolymer composite, which varies with the chemical composition of the starting materials. The increase in this ratio tends to increase silicate species and thus improves the strength of composite product (Chindaprasirt et al., 2012). Moreover, the SiO2-modified composites are more durable against corrosion from aggressive solutions such as sodium chloride and acid solutions. (Adak et al., 2014; Deb et al., 2016; Nuaklong et al., 2019). Early efforts to increase the SiO2/Al2O3 ratio of geopolymer-based cementitious materials were concentrated on the inclusion of SiO2-rich products, one of which is the well-known “nano-SiO2 (nS)”. However, the manufacture of nS involves melting quartz sand soda ash (sodium carbonate) at high temperature of about 1,300 °C (Affandi et al., 2009; Liou and Yang, 2011). Because of the large amount of energy required to produce the material, the harmful consequences on the environment are unavoidable (Wang et al., 2016). The use of alternative SiO2-rich materials is needed to make the composites more environmental friendly. Abundant agricultural wastes containing high content of SiO2, such as rice husk ash (RHA), are viewed as cleaner substitutes for improving the performance of geopolymeric composites. RHA is a by-product obtained by burning rice husk for several heating purposes (Mor et al., 2017; Prasara-A and Gheewala, 2017). It is comprised mostly of reactive amorphous SiO2 phase depending on the burning process (Khan et al., 2012). It is available and suitable for silicate polymeric condensation (Pimraksa et al., 2011). Using RHAs in the concrete industry will not only provide an opportunity for reducing the nS consumption but also help decrease the pollution problems created by disposal of RHAs in landfill, especially in rice-producing countries (e.g., Thailand, India, and other countries in Asia). Recycled aggregate concrete (RAC) is an eco-friendly material made from crushing of old concrete from demolished buildings and other concrete structures (Bassani et al., 2019). The economic and environmental benefits of concrete recycling are achieved through reducing the natural aggregate consumption, decreasing the need for landfill space, and reducing the cost of waste disposal (Bostanci et al., 2018; Muduli and Mukharjee, 2019). However, it has been shown that the performance of concrete is reduced due to the inclusion of recycled concrete aggregates (RCAs). Similarly, this is also observed in geopolymer concrete (Nuaklong et al., 2016; Nuaklong et al., 2018a; Nuaklong et al., 2018b; Shi et al., 2012). This is because of the porous structure of old mortar on the surface of recycled aggregates (Shi et al., 2016). In addition, RCA itself is comprised of an original interfacial transition zone (ITZ)—the boundary 2

Journal Pre-proof between adhered old mortar and original aggregate. This means that the recycled aggregate concrete contains more interfacial transition zones, the weakest zone of concrete, than the natural aggregate (Shi et al., 2016). In this study, an attempt has been made to use RHA to improve the performance of high-calcium fly ash geopolymer concrete made from coarse recycled concrete aggregates. The mechanical properties and fire endurance are investigated in order to provide a holistic view of the performance of concrete during its life cycle. The RAGC made with RHAs is expected to show a comparable performance to that of the nS-added RAGC. Furthermore, the mechanical properties of SiO2-modified RAGC should be comparable to those of the natural aggregate geopolymer concrete (NAGC). The RHA-added RAGC is expected to be an eco-friendly alternative to conventional concrete as a green construction material that is Portland cementfree with a considerably reduced natural aggregate consumption.

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Journal Pre-proof 2. Materials and experimental details 2.1 Alkali activators, fly ash and additive materials 10-M sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solutions were used as alkali activators. The sodium silicate solution consisted of 30.3% SiO2, 12.3% N2O and 57.3% H2O. The primary binder for geopolymer preparation was high-calcium fly ash (HCF) collected from Mae Moh power plant, Thailand. Commercially available nS with an average size of 12 nm was used as an additive material, while the RHA was used as the alternative source of SiO2. The RHAs were ground by the ceramic ball mill for 8 hours to achieve a median particle size (d50) of 13.9 µm. The physical properties and chemical compositions of fly ash, nS and RHA used in the current study are listed in Table 1. Note that besides the SiO2 content, another significant difference between nS and RHA is the surface area. The BET specific surface areas of nS and RHA were 200 and 16.3 m2/g, respectively. The X-ray diffraction (XRD) patterns of the source materials are shown in Fig. 1. The high-calcium fly ash used in this study was mainly in amorphous phase. Traces of crystalline products, including quartz (SiO2), anhydrite (CaSO4), calcium oxide (CaO), and magnesioferrite (MgFe2O4), were also detected as can be seen by the sharp peak feature in the XRD pattern. The nS is highly reactive due to the presence of highly amorphous silica in the material, which is expected to have positive effects on the structure of geopolymer gel. Unlike nS, the XRD result indicates that the RHA consists of amorphous silica with cristobalite and crystal of quartz. Similar findings were reported by He et al. (2013). The presence of these crystalline products in the RHA can decrease its reactivity (Sturm et al., 2016). However, the effectiveness of RHA is improved by fine grinding (Chindaprasirt et al., 2007). 2.2 Coarse and fine aggregates The physical properties of the aggregates used to prepare geopolymer concrete in this study are listed in Table 2. The natural coarse aggregate for making the natural aggregate geopolymer concrete was limestone with a specific gravity, water absorption and unit weight of 2.65, 0.61% and 1,511 kg/m3, respectively. The recycled concrete aggregates used were obtained by crushing old concrete with compressive strengths ranging from 30 to 40 MPa. Both recycled concrete and natural aggregates were immersed in water to obtain the saturated surface dry (SSD) condition. The fine aggregates used for all mixtures were natural river sand with a fineness modulus of 2.63, which were also prepared in the SSD condition. 2.3 Experimental details The mix proportions for geopolymer concretes are shown in Table 3. For all geopolymer mixtures, the alkali activators-to-fly ash and Na2SiO3-to-NaOH weight ratios were kept at 0.6 and 1, respectively. For comparative purpose, the SiO2-modified samples with the same SiO2/Al2O3 ratio were used. The SiO2/Al2O3 mole ratios of the silica added mixtures varied in the range of 4.17 to 4.33. Since the RHA contains a lower SiO2 content compared with the nS, a larger amount of RHAs is required to get the same SiO2/Al2O3 ratio as for the nS-added samples. For example, 1nS mixture was designated for specimens with 1% nS content, whereas 1.2RHA mixture was for specimens with 1.2% RHA content; both having the same SiO2/Al2O3 ratio of 4.17. To prepare geopolymer specimens, a mechanical mixer was used. The procedures for mixing geopolymer concrete are as follows. First, a dry mixture of fly ash and nS (or RHA) was mixed with NaOH. After 5 min of mixing, coarse and fine aggregates were added and 4

Journal Pre-proof mixed for further 5 min. Finally, Na2SiO3 was poured into the mixture and the materials were mixed for another 5 min. Except for the aggregates, the same proportions of materials were used for manufacturing the pastes as those used for concretes. The geopolymer pastes were made by following the procedures recommended by Hanjitsuwan et al. (2014) and Tho-In et al. (2018). After demolding, both the paste and the concrete samples were wrapped with plastic sheets and then cured at room temperature until the testing day. 2.4 Testing methods In this study, the influence of RHA addition on the mechanical performance of RAGC, as compared with nS, has been investigated through the testing of fresh and hardened concrete properties as well as the microstructure analysis. The XRD patterns of geopolymer pastes cured for 7, 28 and 90 days were investigated. Understanding the solid phase development of the geopolymer matrix based on the XRD analysis is helpful in interpreting the test results of concrete, especially the change in compressive strength with time. The slump flow of fresh geopolymer concrete was determined in accordance with ASTM C1611 (2018). The details of testing for hardened concrete specimens are summarized in Table 4. To evaluate the fire-resistant performance of RAGC, the 28-day cured concrete samples were heated in a gas furnace according to the standard fire curve given by ISO 834 (1999). The fire exposure durations of 30, 60 and 90 min were chosen. After heating, the samples were cooled down naturally to ambient temperature inside the furnace. The fire endurance of geopolymer concrete, including spalling and residual compressive strength, was evaluated.

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Journal Pre-proof 3. Results and Discussions 3.1 Microstructure development by XRD The XRD patterns of geopolymer pastes are shown in Fig. 2. The broad amorphous hump around 20° to 40° (2θ), which typically appears on the XRD patterns of high-calcium fly ash based geopolymer composites (Guo et al., 2010; Phoo-ngernkham et al., 2014), indicates the formation of the disordered glassy structure in the systems (Chindaprasirt and Rattanasak, 2010; Chindaprasirt et al., 2013a; Chindaprasirt et al., 2013b). Since the fly ash used in this investigation had high calcium contents, the crystalline peaks associated with calcium-rich gel were detected in addition to the geopolymeric gel. While the broad hump, centered around 30° (2θ), showed the formation of sodium aluminosilicate hydrate (NASH) (Chindaprasirt et al., 2012), two sharp peaks of calcium silicate hydrate (CSH) phase were found at the 2θ values of 29.5° and 32.0°. In addition, a peak of crystalline quartz (Q) was also detected at the 2θ value of 26.7°. This peak overlapped with the calcium aluminosilicate hydrate (CASH) phase according to Bernal et al. (2011). After 7 days of curing, it can be observed that the glassy phase in geopolymer was affected by adjusting the SiO2/Al2O3 mole ratio of the starting material, similar to that reported by Pimraksa et al. (2011). The increase in the hump peaks around 25° to 30° (2θ) was obtained when the SiO2-rich material was added. In addition, the diffraction peak at the 2θ value of 26.7° was intensified by the inclusion of RHA, but no difference can be noticed between the samples made with pure fly ash (control mixture) and the nS-added samples (1nS mixture). It is possible that the pores in the geopolymer matrix were filled with the crystalline quartz derived from the RHA. An increase in curing time increases the intensity of the diffraction peaks of crystalline products. A similar trend was obtained by Bernal et al. (2011). Moreover, a few sharp peaks started to develop in the XRD results, especially after 90 days of curing; in other words, the microstructure of the samples was observed to become more ordered. 3.2 Workability The test results on slump flow of geopolymer concretes are presented in Fig. 3. The normal concrete samples made from RCAs (CON-R) showed a much higher slump flow than those made from natural limestones (CON-N). The flow value of CON-R concrete was 789 mm, which was about 14% higher than CON-N concrete. The results are in line with previous studies (Nuaklong et al., 2016; Nuaklong et al., 2018b; Poon et al., 2004). This is attributed to the fact that RCAs normally require more water to reach the SSD condition compared to natural limestones. During the concrete manufacturing process (mixing and consolidation), when the water is desorbed, the excess water causes an increase in concrete workability (Mefteh et al., 2013; Poon et al., 2004). The addition of nS resulted in a significant drop in workability, especially at high inclusion levels. The slump flow of concrete was reduced from 789 mm to 661 mm by adding 3% nS. The reduction in slump flow was 16% for the nS-added concrete. Also, the concrete mixtures containing 1.2-3.6% RHA showed approximately 10% lower flow values than that of CON-R mixture. It can be noticed that the use of nS was more influential in decreasing the workability, as compared to RHA. This difference is not surprising since nS has a considerably larger BET specific surface area compared with RHA as shown in Table 1.

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Journal Pre-proof 3.3 Compressive strength Table 5 shows the compressive strength development for different mixtures of geopolymer concretes cured at room temperature. As expected, the replacement of natural limestones with recycled concrete aggregates causes a reduction of the compressive strength. The inclusion of RCAs decreased the compressive strength of geopolymer concrete by approximately 9-13%. In addition to the porous structure of RCAs (as previously discussed in section 1), it is possible to explain the results by using the assumption of “internal bleeding water”, where the recycled aggregate concrete usually has lower strength than the natural aggregate concrete. There is strong evidence that the workability of concrete is affected by the saturated recycled aggregate as discussed earlier. The excess water, which tends to accumulate around the coarse aggregate’ surface, can weaken the concrete (Behera et al., 2014; Poon et al., 2004). The addition of a small amount of nS and RHA can increase the strength at early age of RAGC. The 1nS and 1.2RHA mixtures showed 24% and 33% higher 7-day compressive strength than that of CON-R mixture. The superior performance of the samples containing either the nS or RHA was due to the increase in the density of Si-O-Si bonds of the hydration products. In addition, the amorphous glassy phase of the matrix increased with the incorporation of silica oxide and led to an improvement in the compressive strength of concrete (Pimsaksa et al., 2011). This is in good agreement with the results of solid phase development of the geopolymer paste mentioned previously (Fig. 2). However, when the SiO2/Al2O3 ratio is increased beyond 4.17, a reduction in compressive strength is observed. The excessive amount of SiO2-rich materials adversely affects the matrix structure of geopolymer composite. The formation of geopolymer gel is hindered since the excess silicate delays the evaporation of water during the polycondensation process (Part et al., 2015; Wan et al., 2017). The experimental results indicated that the compressive strength of geopolymer concretes is increased with the increasing curing age. For example, the strength of 2.4RHA mixture was 22.6 MPa at 7 days and increased by 65% from 7 to 28 days. The strength gain for geopolymer concretes is related to the growth of gel phases in the matrix. The strength of geopolymer concretes continued to increase from 28 to 90 days. For example, the 90-day compressive strength of 1.2RHA concrete was 47.5 MPa, which was 25% higher than its 28day compressive strength. Between 28 and 90 days, a major factor responsible for the strength growth was the crystalline phase development in the matrix as can be clearly seen in Fig. 2. In addition to the increase in the degree of crystallinity (Duxson et al., 2007), especially after 90 days of curing, the development of Ca-rich gel phases can also lead to significant improvement in later age strength (Bernal et al. 2011). It is interesting to see that the strength of CON-N concrete (SiO2/Al2O3 = 4.10) was initially lower than that of the SiO2-modified concrete (SiO2/Al2O3 = 4.17), but it was higher when curing was continued for 90 days. A slow transformation of amorphous structure into the crystalline phase due to the presence of extra silica is considered to be the reason behind this deficiency in strength development (De Silva and Sagoe-Crenstil, 2008). 3.4 Tensile strengths The results obtained from tensile strength tests are shown in Table 6. Both the splitting tensile and flexural strengths of geopolymer concrete decreased when natural limestones were substituted with recycled aggregates. The CON-R concrete mixture made from recycled aggregates exhibited an average splitting tensile strength of 2.5 MPa at 28 days—almost 12% lower than that of CON-N concrete. Also, approximately 34% drop in flexural strength was observed for the CON-R mixture. However, based on Tabsh and Abdelfatah (2009), if the 7

Journal Pre-proof recycled aggregates derived from high-quality concrete (≥ 50 MPa compressive strength) were used, the same splitting tensile strength as the natural aggregate concrete could be obtained. The results indicated that incorporating SiO2-rich materials in geopolymer concrete had little effect on the splitting tensile strength except at the SiO2/Al2O3 ratio of 4.17 where both the compressive and splitting tensile strengths were significantly improved. The increase in the splitting tensile strength was only up to 8% for both 1nS and 1.2RHA concretes. Unlike the compressive strength, the results show that the flexural strength of SiO2-modified RAGCs was lower than that of CON-N concrete. It is possible that the tensile fracture passed through the recycled aggregates because both the geopolymer paste and the interfacial transition zone were stronger. A similar type of failure can be observed even when the geopolymer concrete is mixed with natural coarse aggregates (Sarker et al., 2013), which indicates that the strength in tension of geopolymer concrete is primary controlled by the strength of aggregates. This may explain why the improvement in flexural strength through the addition of SiO2-rich materials is marginal. 3.5 Porosity and water absorption The results of permeability tests performed at 28 days are shown in Figs. 4 and 5. Compared with CON-N mixture, the porosity and water absorption of CON-R concrete increased by 6.2% and 3.7%, respectively. The presence of porous old mortar in the recycled aggregates leads to both higher porosity and water absorption. This agrees well with the previous work on heat-cured geopolymer concrete (Nuaklong et al., 2016). Although the porosity of RAGC can decrease with the use of SiO2-rich materials, it tends to increase with the increasing SiO2/Al2O3 ratios from 4.17 to 4.33. For example, the porosity of 1nS mixture was 11.0% and increased to 18.3% and 19.7%, respectively, for 2nS and 3nS mixtures. The results also indicate that the water absorption of geopolymer concrete is reduced with the addition of either nS or RHA. The percentage of water absorption was almost halved by changing the SiO2/Al2O3 ratio of the mix from 4.10 to 4.17. In comparison to RHA, nS was less effective in reducing the porosity of geopolymer concrete. Due to its high surface energy, the nS has a tendency to agglomerate which creates a source of weakness in the composite (Kong et al., 2007). This phenomenon, called “formation of the weak zones”, is also a possible reason for a significant decrease in the compressive strength observed for the nS-added specimens with high SiO2/Al2O3 ratios (> 4.17). 3.6 Fire resistance As fire safety is one of the important aspects in designing concrete structures, it is necessary to study the fire endurance of concrete and the factors affecting it. This research is aimed to evaluate the spalling damage and the residual compressive strength of geopolymer concrete after exposed to ISO 834 standard fire (ISO 834, 1999) with different heating durations. 3.6.1 Visual observation and spalling damage In addition to the internal cracking, concrete can also suffer spalling caused by fire action. A sudden rise in vapor pressure inside the concrete due to heat from fire results in the spalling of concrete; however, if the pressure can rapidly be released into the air, the tendency for spalling will be less (Sarker et al., 2014). Also, the restraining effect of the aggregate is one of the factors contributing to concrete spalling. Since the matrix is bonded to aggregate, its shrinkage under the restraint by the surrounded aggregates will induce some internal stress 8

Journal Pre-proof around the aggregate. Together, the increasing pore pressure and the rising thermal stress combined as the driving force for the pop-outs of concrete elements (Pan et al., 2012). As can be seen in Fig. 6, the visual observation indicates that the surface color of geopolymer concrete changes from dark brown to red after 30 min exposure to hightemperature. Based on the work of Sarker et al. (2014) on the fire resistance of fly ash geopolymer concrete, the temperatures at which the samples turned red were approximately 800-1,000 °C and that the change in color of geopolymer specimens was due to the presence of iron oxide in fly ash. In addition, the CON-R concrete showed a higher mass loss than the CON-N concrete as evident by the test results shown in Table 7, there is a possibility of a further build-up of internal pressure within the CON-R concrete due to the evaporation of excess water in RCAs that may cause damage to the concrete structure. After 60 min of heating at the furnace air temperature of around 900 °C, surface cracking occurred for all concretes. The results were similar to those found by Wongsa et al. (2018). This is probably due to the migration of evaporable water from the inside of concrete, escaping rapidly into the air (Kong et al., 2007). The evidence supporting this assumption is the ongoing mass loss of concrete. Fire damage increased with the increasing exposure duration. The geopolymer concretes decomposed after 90 min fire exposure as shown in Fig. 7. The spalling resistance of concrete was judged by the size of the remaining fire-damaged specimens. Generally, the degree of spalling appears to be lower in concrete with higher permeability (Cree et al., 2013). This is because the moisture can escape more easily through the porous structure than through the denser structure, thus reducing the vapor pressure in the composite. Despite the higher porosity of CON-R concrete compared with CON-N concrete, the degree of spalling appears to be similar. It is possibly due to the decomposition of the Portland cement products adhered on the surface of the recycled aggregates that may lead to the disintegration of concrete upon fire exposure. The results showed that there was no obvious increase in spalling through a reduction in the permeability of the RAGC by incorporating either nS or RHA in the mix. 3.6.2 Residual compressive strength As shown in Table 8, the compressive strengths of geopolymer concretes drop considerably when they are exposed to fire. It has been shown by Kong and Sanjayan et al. (2008) that the geopolymer matrix can undergo progressive shrinkage after being exposed to high temperatures, whereas the aggregates expand. This leads to an extensive crack development in the concrete composite, which is the main source of strength deterioration. Figs. 8 and 9 present the percentage of compressive strength retained for different mixtures of geopolymer concrete after exposure to high temperature for different periods of time. Replacing limestones with 100% recycled concrete resulted in an increasing residual strength after 30 min fire exposure. The CON-R concrete showed an outstanding residual compressive strength of approximately 34%. It is possible that the thermal expansion of the recycled aggregates upon heating was reduced due to the presence of the adhered old cement composite. On the basis of this assumption, the superior strength after high-temperature exposure of the CON-R concrete was achieved through the decreasing thermal incompatibility between the geopolymer matrix and the aggregates as described by Kou et al. (2014). The results indicate that adding either nS or RHA in RAGC decreases the post-fire residual strength. The SiO2-modified concretes showed more strength loss than CON-R concrete. For example, the compressive strength of CON-R, 2nS and 2.4RHA concretes was reduced by 66%, 74% and 74%, respectively, after 30 min of exposure to ISO standard fire. The lower retained strength of RAGCs containing either nS or RHA could be due to their denser concrete structure since a sufficient number of small pores in the composite is required to facilitate the escape of moisture during heating (Kong et al., 2007). It can be seen that increasing the SiO2/Al2O3 ratio from 4.17 to 4.33 did not show a significant effect on the 9

Journal Pre-proof residual strength of RAGCs after 30 min exposure to elevated temperature. For example, the compressive strengths of all RHA-added concretes (1.2RHA, 2.4RHA and 3.6RHA mixtures) were reduced by 73-74% of their ambient temperature strength. Similar findings are observed for fly ash-based geopolymers containing silica fume (Duan et al., 2017). The fire damage of geopolymer concretes increases with further increases in the heating duration. The strength of all samples dropped progressively when they were heated longer for a period of 60 min. This may be attributed to the increasing shrinkage of the matrix that increases the thermal stresses around the aggregates, thus leading to more internal cracking. The results show that there is no difference in the average post-fire residual strength between CON-N concrete and CON-R concrete. Both samples had the same average retained strength of 13% after a fire exposure duration of 60 min. It is believed that calcium hydroxide, which is the hydration product in the Portland cement mortar adhered to the RCAs, starts to decompose at this stage and leads to the deterioration in bonding between the matrix and coarse aggregates. In view of the influence of SiO2-rich materials on the compressive strength of RAGCs after 60 min exposure to high temperature, it can be seen that the residual strength was reduced, especially in the case of nS-added RAGCs. The loss of compressive strength was about 91% for 1nS mixture. This was probably because the voids in the concrete were filled with unreacted SiO2-rich particles, thus reducing the escape routes for the internal pressure release. The accumulated vapor pressure caused, therefore, more damage due to internal cracking in the geopolymer concrete (Kou et al., 2014). However, it is not clear why the residual strength of RAGCs increased when the SiO2/Al2O3 ratio was increased from 4.17 to 4.26, but decreased slightly when it was increased from 4.26 to 4.33. By comparing between the residual strength of the nS-added RAGC and the RHA-added RAGC, it can be noticed that the latter tends to be higher. This may be because the moisture can escape more easily through the RHA-filled voids than through the nS-filled ones, thus reducing the fire damage of the geopolymer concrete. Note that the temperature reached about 1,000 °C after 90-min fire exposure. All concrete specimens decomposed and thus no further information can be derived.

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Journal Pre-proof 4. Conclusions It should be emphasized again that the primary objective of this investigation is to compare the performance of the recycled aggregate geopolymer concrete (RAGC) made from rice husk ash (RHA) with those made from the commonly used nano-SiO2 (nS). However, understanding the influence of SiO2-rich materials on the properties of RAGC would be helpful before discussing the comparative effectiveness of nS and RHA for improving the performance of RAGC. The results are summarized as follows: 1. The use of recycled concrete as coarse aggregates in the geopolymer composite causes a reduction in strength and an increase in permeability. At the same time, it also increases the workability of fresh concrete. The geopolymer concretes suffer a considerable loss of compressive strength as a result of fire exposure. The inclusion of recycled concrete aggregates helps decrease the strength loss of geopolymer concretes during the first 30 min of fire testing, and thereafter shows no effect. After 90 min of heating, both the natural and recycled aggregate geopolymer concretes can no longer maintain their dimensional stability. 2. Based on the experimental results, the strengths of RAGCs increased with the increase of the SiO2/Al2O3 mole ratio up to 4.17. The SiO2-modified RAGCs with appropriate proportions, 1nS and 1.2RHA mixtures, exhibited the same 28-day compressive strength as that of concrete made from natural aggregates. Moreover, they demonstrated higher compressive strength at all ages compared with normal concrete made from recycled concrete aggregates. The increase in the silica content also decreased porosity and, therefore, water absorption. However, it is important to note that the tendency for improving the performance of RAGCs at ambient temperature is limited when the SiO2/Al2O3 ratio is larger than 4.17. 3. It is important to note that by incorporating SiO2-rich materials in the RAGC, the workability of fresh concrete was reduced and the hardened concrete showed a reduced residual strength after exposure to high temperature. However, the inclusion of either nS or RHA did not show a significant effect on spalling of geopolymer concretes. There is little difference in the remaining piece of fire-damaged specimens between normal concrete made from recycled concrete aggregates and SiO2-modified concrete. 4. By comparing the properties of the nS- and the RHA-added geopolymer concretes at ambient temperature, the use of RHA as an alternative to nS for improving the performance of RAGCs appears to be quite attractive. When the RHA or nS was added into RAGCs, with optimum dosage, both mixtures showed comparable improvement in strengths and permeability. The spalling resistance of the RHA-added mixtures is very similar to that of the nS-added mixtures. However, some differences were also observed because of the ultra-fine particle size of nS. The reduction in slump flow was much smaller in the RHA-added concrete compared to the nS-added concrete. In addition, the former tends to have a higher post-fire residual strength. 5. By adding RHAs to the RAGC, the normal range of consistency and strength adequate for most cast-in-situ concrete members—i.e., the slump flow of 703-721 mm and the 28-day compressive strength of 36.0-38.1 MPa—can be produced. However, further studies are still required on the durability of SiO2-modified concrete under various exposure conditions.

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Journal Pre-proof Acknowledgement This research is supported by Ratchadapisek Somphot Fund for Postdoctoral Fellowship, Chulalongkorn University and the “Chulalongkorn Academic Advancement into Its 2nd Century Project”, Chulalongkorn University. The second author would like to acknowledge the Chula Engineering Research Fund from Faculty of Engineering, Chulalongkorn University, Thailand. The authors would also like to acknowledge the Fire Safety Research Center (FSRC) of Chulalongkorn University for the fire tests.

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Journal Pre-proof References Adak, D., Sarkar, M., Mandal, S., 2014. Effect of nano-silica on strength and durability of fly ash based geopolymer mortar. Constr. Build. Mater. 70, 453-459. Affandi, S., Setyawan, H., Winardi, S., Purwanto, A., Balgis, R., 2009. A facile method for production of high-purity silica xerogels from bagasse ash. Adv. Powder. Technol. 20 (5), 468-472. ASTM C1611/C1611M-18, 2018. Standard test method for slump flow of self-consolidating concrete, American Society for Testing and Materials. ASTM C39/C39M, 2018. Standard test method for compressive strength of cylindrical concrete specimens, American Society for Testing and Materials. ASTM C496/C496M-17, 2017. Standard test method for splitting tensile strength of cylindrical concrete specimens, American Society for Testing and Materials. ASTM C642-13, 2013. Standard test method for density, absorption, and voids in hardened concrete, American Society for Testing and Materials. ASTM C78/C78M-18, 2018. Standard test method for flexural strength of concrete (using simple beam with third-point loading), American Society for Testing and Materials. Bassani, M., Garcia, J. D., Meloni, F., Volpatti, G., Zampini, D., 2019. Recycled coarse aggregates from pelletized unused concrete for a more sustainable concrete production. J. Clean. Prod. 219, 424-432. Behera, M., Bhattacharyya, S. K., Minocha, A. K., Deoliya, R., Maiti, S., 2014. Recycled aggregate from C&D waste & its use in concrete–A breakthrough towards sustainability in construction sector: A review. Constr. Build. Mater. 68, 501-516. Bernal, S. A., Provis, J. L., Rose, V., De Gutierrez, R. M., 2011. Evolution of binder structure in sodium silicate-activated slag-metakaolin blends. Cem. Concr. Compos. 33 (1), 4654. Bostanci, S. C., Limbachiya, M., Kew, H., 2018. Use of recycled aggregates for low carbon and cost effective concrete construction. J. Clean. Prod. 189, 176-196. Chindaprasirt, P., De Silva, P., Sagoe-Crentsil, K., Hanjitsuwan, S., 2012. Effect of SiO2 and Al2O3 on the setting and hardening of high calcium fly ash-based geopolymer systems. J. Mater. Sci. 47 (12), 4876-4883. Chindaprasirt, P., Kanchanda, P., Sathonsaowaphak, A., Cao, H. T., 2007. Sulfate resistance of blended cements containing fly ash and rice husk ash. Constr. Build. Mater. 21 (6), 1356-1361. Chindaprasirt, P., Rattanasak, U., 2010. Utilization of blended fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash in geopolymer. Waste. Manage. 30 (4), 667-672. Chindaprasirt, P., Rattanasak, U., Taebuanhuad, S., 2013a. Role of microwave radiation in curing the fly ash geopolymer. Adv. Powder. Technol. 24 (3), 703-707. Chindaprasirt, P., Rattanasak, U., Taebuanhuad, S., 2013b. Resistance to acid and sulfate solutions of microwave-assisted high calcium fly ash geopolymer. Mater. Struct. 46 (3), 375-381. Cree, D., Green, M., Noumowé, A., 2013. Residual strength of concrete containing recycled materials after exposure to fire: a review. Constr. Build. Mater. 45, 208-223. De Silva, P., Sagoe-Crenstil, K., 2008. Medium-term phase stability of Na2O–Al2O3–SiO2– H2O geopolymer systems. Cem. Concre. Res. 38 (6), 870-876. Deb, P. S., Sarker, P. K., Barbhuiya, S., 2016. Sorptivity and acid resistance of ambient-cured geopolymer mortars containing nano-silica. Cem. Concr. Compos. 72, 235-245. Duan, P., Yan, C., Zhou, W., 2017. Compressive strength and microstructure of fly ash based geopolymer blended with silica fume under thermal cycle. Cem. Concr. Compos. 78, 108-119. 13

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Journal Pre-proof Pan, Z., Sanjayan, J. G., Kong, D. L., 2012. Effect of aggregate size on spalling of geopolymer and Portland cement concretes subjected to elevated temperatures. Constr. Build. Mater. 36, 365-372. Part, W. K., Ramli, M., Cheah, C. B., 2015. An overview on the influence of various factors on the properties of geopolymer concrete derived from industrial by-products. Constr. Build. Mater. 77, 370-395. Phoo-ngernkham, T., Chindaprasirt, P., Sata, V., Hanjitsuwan, S., Hatanaka, S., 2014. The effect of adding nano-SiO2 and nano-Al2O3 on properties of high calcium fly ash geopolymer cured at ambient temperature. Mater. Des. 55, 58-65. Pimraksa, K., Chindaprasirt, P., Rungchet, A., Sagoe-Crentsil, K., Sato, T., 2011. Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios. Mat. Sci. Eng. A. 528 (21), 6616-6623. Poon, C. S., Shui, Z. H., Lam, L., Fok, H., Kou, S. C., 2004. Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete. Cem. Concre. Res. 34 (1), 31-36. Prasara-A, J., Gheewala, S. H., 2017. Sustainable utilization of rice husk ash from power plants: A review. J. Clean. Prod. 167, 1020-1028. Provis, J. L., 2014. Geopolymers and other alkali activated materials: why, how, and what?. Mater. Struct. 47 (1-2), 11-25. Provis, J. L., 2018. Alkali-activated materials. Cem. Concre. Res. 114, 40-48. Provis, J. L., Myers, R. J., White, C. E., Rose, V., Van Deventer, J. S., 2012. X-ray microtomography shows pore structure and tortuosity in alkali-activated binders. Cem. Concre. Res. 42 (6), 855-864. Sarker, P. K., Haque, R., Ramgolam, K. V., 2013. Fracture behaviour of heat cured fly ash based geopolymer concrete. Mater. Des. 44, 580-586. Sarker, P. K., Kelly, S., Yao, Z., 2014. Effect of fire exposure on cracking, spalling and residual strength of fly ash geopolymer concrete. Mater. Des. 63, 584-592. Shi, C., Li, Y., Zhang, J., Li, W., Chong, L., Xie, Z., 2016. Performance enhancement of recycled concrete aggregate–a review. J. Clean. Prod. 112, 466-472. Shi, X. S., Collins, F. G., Zhao, X. L., Wang, Q. Y., 2012. Mechanical properties and microstructure analysis of fly ash geopolymeric recycled concrete. J. Hazard. Mater. 237, 20-29. Sturm, P., Gluth, G. J. G., Brouwers, H. J. H., Kühne, H. C., 2016. Synthesizing one-part geopolymers from rice husk ash. Construction and Building Materials, 124, 961-966. Tabsh, S. W., Abdelfatah, A. S., 2009. Influence of recycled concrete aggregates on strength properties of concrete. Constr. Build. Mater. 23 (2), 1163-1167. Tho-In, T., Sata, V., Boonserm, K., Chindaprasirt, P., 2018. Compressive strength and microstructure analysis of geopolymer paste using waste glass powder and fly ash. J. Clean. Prod. 172, 2892-2898. Wan, Q., Rao, F., Song, S., García, R. E., Estrella, R. M., Patino, C. L., Zhang, Y., 2017. Geopolymerization reaction, microstructure and simulation of metakaolin-based geopolymers at extended Si/Al ratios. Cem. Concr. Compos. 79, 45-52. Wang, Y., Kalinina, A., Sun, T., Nowack, B., 2016. Probabilistic modeling of the flows and environmental risks of nano-silica. Sci. Total. Environ. 545, 67-76. Wongsa, A., Sata, V., Nuaklong, P., Chindaprasirt, P., 2018. Use of crushed clay brick and pumice aggregates in lightweight geopolymer concrete. Constr. Build. Mater. 188, 1025-1034. Zhuang, X. Y., Chen, L., Komarneni, S., Zhou, C. H., Tong, D. S., Yang, H. M., Yu, W. H., Wang, H., 2016. Fly ash-based geopolymer: clean production, properties and applications. J. Clean. Prod. 125, 253-267. 15

Journal Pre-proof List of figures:

Fig. 1. XRD results of HCF, nS, and RHA.

Fig. 2. XRD patterns of geopolymer pastes: (a) Control mixture; (b) 1nS mixture; and (c) 1.2RHA mixture at 7, 28 and 90 days.

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Fig. 3. Mean values for slump flow of geopolymer concretes.

Fig. 4. 28-day porosity of geopolymer concretes.

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Fig. 5. 28-day water absorption of geopolymer concretes.

Fig. 6. Geopolymer concretes after 30-, 60-, and 90-min fire exposure.

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Fig. 7. Remaining pieces of fire-damaged specimens after 90-min fire exposure.

Fig. 8. Residual compressive strength (%) after 30 min exposure to elevated temperature.

Fig. 9. Residual compressive strength (%) after 60 min exposure to elevated temperature.

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Journal Pre-proof List of tables: Table 1 Chemical compositions and physical properties of source materials. Compositions (%) HCF nS SiO2 42.4 99.8 Al2O3 21.3 0.05 CaO 15.7 Fe2O3 13.2 MgO 2.3 K2O 2.0 Na2O 0.9 TiO2 0.5 0.03 P2O5 0.2 MnO 0.1 SO3 1.0 LOI (%) 0.4 <1 Specific gravity 2.54 BET specific surface area 3.2 200 (m2/g)

RHA 81.6 1.7 6.5 0.9 8.9 0.2 2.3 2.06 16.3

Table 2 Properties of aggregates used in this study. Properties Aggregate gradation Specific gravity (SSD) Water absorption (%) Dry-rodded unit weight (kg/m3) Los Angeles abrasion test (% loss)

Coarse aggregates Limestone RCA Gap-graded (4.5-9.5 mm) 2.65 2.26 0.61 5.90 1,511 1,241 33.9 37.1

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Fine aggregate River sand Well-graded 2.63 1.07 1,764 -

Journal Pre-proof Table 3 Proportioning of geopolymer concrete mixtures (kg/m3). Mixture CON-N CON-R 1nS 2nS HCF 450 450 450 450 Powder nS 4.5 9.0 RHA Na2SiO3 135 135 135 135 Alkali activator NaOH 135 135 135 135 River sand 500 500 500 500 Aggregate Limestone 1,150 RCA 970 970 970 SiO2/Al2O3 4.10 4.10 4.17 4.26 Mole Na2O/SiO2 0.22 0.22 0.21 0.21 ratio Na2O/Al2O3 0.90 0.90 0.90 0.90 Unit weight (kg/m3) 2,318 2,206 2,197 2,159

Table 4 Testing of hardened geopolymer concrete. Properties Sample size Compressive strength

Cylinder (Ø 10 cm × 20 cm)

Splitting tensile strength Flexural strength

Cylinder (Ø 10 cm × 20 cm) Prism (7.5× 7.5 ×30 cm) Cube (10 cm) Cube (10 cm) Cylinder (Ø 10 cm × 20 cm)

Porosity Water absorption Residual strength

Testing age 7, 28 and 90 days 28 days

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3nS 450 13.5 135 135 500 970 4.33 0.21 0.90 2,193

1.2RHA 450 5.4 135 135 500 970 4.17 0.21 0.90 2,197

2.4RHA 450 10.8 135 135 500 970 4.26 0.21 0.90 2,179

Number of Method of testing replications 3 ASTM C39 (2018) 3

28 days

3

28 days

3

28 days

3

28 days

3

ASTM C496 (2017) ASTM C78 (2018) ASTM C642 (2013) ASTM C642 (2013) ASTM C39 (2018)

3.6RHA 450 16.2 135 135 500 970 4.33 0.21 0.90 2,190

Journal Pre-proof Table 5 Compressive strength development of geopolymer concretes. Compressive strength (MPa) Mixture SiO2/Al2O3 7 days S.E. 28 days S.E. 90 days CON-N 4.10 18.9 ±0.3 37.4 ±2.8 48.7 CON-R 4.10 17.2 ±0.4 33.9 ±3.0 42.6 1nS 4.17 21.4 ±0.2 38.2 ±0.3 47.3 2nS 4.26 19.1 ±0.2 36.5 ±0.3 44.7 3nS 4.33 18.6 ±0.5 34.2 ±3.0 44.2 1.2RHA 4.17 22.9 ±1.0 38.1 ±0.3 47.5 2.4RHA 4.26 22.6 ±1.3 37.2 ±0.2 46.4 3.6RHA 4.33 21.0 ±0.2 36.0 ±0.3 47.5 Note: S.E. = Standard error of mean.

S.E. ±0.7 ±0.6 ±1.1 ±2.4 ±0.2 ±0.7 ±1.0 ±0.9

Table 6 Tensile strength of geopolymer concretes. 28-day tensile strength (MPa) Splitting tensile strength S.E. Flexural strength CON-N 4.10 2.8 ±0.1 5.3 CON-R 4.10 2.5 ±0.1 3.5 1nS 4.17 2.7 ±0.3 3.8 2nS 4.26 2.6 ±0.2 3.6 3nS 4.33 2.6 ±0.2 3.5 1.2RHA 4.17 2.7 ±0.1 3.8 2.4RHA 4.26 2.5 ±0.1 3.5 3.6RHA 4.33 2.4 ±0.1 3.4 Note: S.E. = Standard error of mean. Mixture

SiO2/Al2O3

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S.E. ±0.1 ±0.1 ±0.5 ±0.3 ±0.2 ±0.1 ±0.2 ±0.1

Journal Pre-proof Table 7 Mean values for mass loss of geopolymer concretes after fire exposure (%). Mixture CON-N CON-R 1nS 2nS 3nS 1.2RHA SiO2/Al2O3 4.10 4.10 4.17 4.26 4.33 4.17 % mass loss (30 6.2 8.7 8.8 8.9 8.8 8.7 min) % mass loss (60 10.2 10.2 9.8 10.3 10.4 10.3 min) % mass loss (90 min) Note: The specimens decomposed after 90-min fire exposure.

2.4RHA 4.26 8.6

3.6RHA 4.33 8.6

10.8

9.6

-

-

Table 8 28-day compressive strength of geopolymer concretes after exposure to elevated temperature. Mixture SiO2/Al2O3 Strength of concrete after fire exposure ± standard error (MPa) 0 min 30 min 60 min 90 min CON-N 4.10 37.4±2.8 10.5±0.2 4.9±0.1 CON-R 4.10 33.9±3.0 11.5±0.7 4.4±0.1 1nS 4.17 38.2±0.3 10.3±0.1 3.4±0.2 2nS 4.26 36.5±0.3 9.5±0.6 4.4±0.1 3nS 4.33 34.2±3.0 8.9±1.5 3.4±0.5 1.2RHA 4.17 38.1±0.3 10.3±1.5 4.2±0.3 2.4RHA 4.26 37.2±0.2 9.7±1.4 4.8±0.4 3.6RHA 4.33 36.0±0.3 9.7±0.7 4.3±0.6 Note: The specimens decomposed after 90-min fire exposure.

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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights    



The properties of recycled aggregate geopolymer concrete (RAGC) cured at room temperature were investigated. The use of rice husk ash (RHA) as an alternative to nano-SiO2 (nS) for improving the performance of RAGCs appears to be quite attractive. By adding RHA to the RAGCs, the normal range of consistency and strength adequate for most cast-in-situ concrete members can be produced. The spalling resistance of the RHA-added RAGCs is similar to that of the nS-added RAGCs. The residual compressive strength of the RHA-added RAGCs tends to be higher than that of the nS-added RAGCs.