This is an outdated version published on 2022-04-18. Read the most recent version.

Physical and mechanical properties of C class fly ash based lightweight geopolymer mortar produced with expanded vermiculite aggregate

Authors

  • Mehmet Kaya Department of Civil Engineering, Yozgat Bozok University, Yozgat (Turkey)
  • Fuat Köksal Department of Civil Engineering, Yozgat Bozok University, Yozgat (Turkey)

DOI:

https://doi.org/10.7764/RDLC.21.1.21

Keywords:

geopolymer, fly ash, vermiculite, flexural strength, compressive strength, thermal conductivity

Abstract

This study presents the physical and the mechanical properties of C class fly ash (FA) based lightweight geopolymer mortars produced with expanded vermiculite (EV) aggregate. The FA was activated with NaOH containing 12%, 14% and 16% sodium by weight. The volumetric ratios of EV/FA in the samples were chosen as 2,4 and 6 in the study. The liquid/solid ratio 0.23, 0.26 and 0.29. Lightweight geopolymer mortar (LGM) samples were produced by mixing FA, EV, NaOH and water in a mixer. The samples placed in molds were exposed to activation temperature of 100°C for 24 hours in the oven. The samples taken out of the oven were demoulded and kept in air curing for 28 days at 20°C±2°C room temperature. After curing, unit weight, apparent porosity, water absorption ratio, ultrasonic pulse velocity (UPV), flexural strength and compressive strength tests were performed on the samples. In addition, the thermal conductivity coefficients of the samples were determined. As a result of the experiment, a compressive strength varying between 0.59 MPa and 3.81 MPa was obtained in lightweight geopolymers samples with a unit weight between 906 kg/m3 and 1477 kg/m3. Expanded vermiculite showed a good performance on thermal conductivity of LGMs and a decrease in thermal conductivity up to the 0.094 W/mK was observed.

Downloads

Download data is not yet available.

References

Aliabdo, Ali A., Abd Elmoaty M. Abd Elmoaty, and Hazem A. Salem. 2016. “Effect of water addition, plasticizer and alkaline solution consti-tution on fly ash based geopolymer concrete performance.” Construction and Building Materials 121:694–703. doi: https://doi.org/10.1016/j.conbuildmat.2016.06.062.

Amran, Y. H. Mugahed, Rayed Alyousef, Hisham Alabduljabbar, and Mohamed El-Zeadani. 2020. “Clean production and properties of geo-polymer concrete; A review.” Journal of Cleaner Production 251:119679. doi: https://doi.org/10.1016/j.jclepro.2019.119679.

ASTM. 2017a. “ASTM C330/C330M -17aStandard specification for lightweight aggregates for concrete masonry units.”

ASTM. 2017b. “Standard specification for lightweight aggregates for insulating concrete.”

ASTM. 2017c. Standard specification for lightweight aggregates for structural concrete. ASTM International.

ASTM, International. 2019. “ASTM C618–19: standard specification for coal fly ash and raw or calcined natural pozzolan for use in con-crete.” Annual Book of ASTM Standards 2019.

Atabey, İsmail İsa, Okan Karahan, Cahit Bilim, and Cengiz Duran Atis. 2020. “Very high strength Na2SiO3 and NaOH activated fly ash based geopolymer mortar.”

Atiş, C. D., E. B. Görür, O. Karahan, C. Bilim, S. Ilkentapar, and E. Luga. 2015. “Very high strength (120 MPa) class F fly ash geopolymer mortar activated at different NaOH amount, heat curing temperature and heat curing duration.” Construction and Building Materials 96. doi: 10.1016/j.conbuildmat.2015.08.089.

Aygörmez, Yurdakul. 2021. “Performance of ambient and freezing-thawing cured metazeolite and slag based geopolymer composites against elevated temperatures.” Revista de La Construcción 20(1). doi: 10.7764/RDLC.20.1.145.

Bhogayata, Ankur, Shemal V Dave, and Narendra K. Arora. 2020. “Utilization of expanded clay aggregates in sustainable lightweight geopol-ymer concrete.” Journal of Material Cycles and Waste Management 22(6):1780–92. doi: 10.1007/s10163-020-01066-7.

Bingöl, Şinasi, Cahit Bilim, Cengiz Duran Atiş, Uğur Durak, Serhan İlkentapar, and Okan Karahan. 2020. “An investigation of resistance of sodium meta silicate activated slag mortar to acidic and basic mediums.” Revista de La Construcción. doi: 10.7764/rdlc.19.1.127-133.

Bouguerra, A. 1999. “Temperature and moisture dependence on the thermal conductivity of wood-cement-based composite: experimental and theoretical analysis.” Journal of Physics D: Applied Physics 32(21):2797.

Bouguerra, A., A. Ledhem, F. de Barquin, R. M. Dheilly, and M. Quéneudec. 1998. “Effect of microstructure on the mechanical and thermal properties of lightweight concrete prepared from clay, cement, and wood aggregates.” Cement and Concrete Research 28(8):1179–90. doi: https://doi.org/10.1016/S0008-8846(98)00075-1.

Brooks, Robert, Mozhgan Bahadory, Fernando Tovia, and Hossein Rostami. 2010. “Properties of alkali-activated fly ash: high performance to lightweight.” International Journal of Sustainable Engineering 3(3):211–18. doi: 10.1080/19397038.2010.487162.

Castel, A. 2017. “Chapter 14 - Bond between steel reinforcement and geopolymer concrete.” Pp. 375–87 in Handbook of Low Carbon Con-crete, edited by A. Nazari and J. G. Sanjayan. Butterworth-Heinemann.

Çelikten, Serhat. 2021. “Mechanical and Microstructural Properties of Waste Andesite Dust-Based Geopolymer Mortars.” Advanced Powder Technology 32(1):1–9. doi: 10.1016/j.apt.2020.10.011.

Chi, Hiep Le, Petr Louda, Totka Bakalova, and Vladim’ir Kovačič. 2019. “Preparation and Mechanical Properties of Potassium Metakaolin Based Geopolymer Paste.” Advanced Engineering Forum 31:38–45. doi: 10.4028/www.scientific.net/AEF.31.38.

Colangelo, F., G. Roviello, L. Ricciotti, V. Ferrándiz-Mas, F. Messina, C. Ferone, O. Tarallo, R. Cioffi, and C. R. Cheeseman. 2018. “Mechani-cal and Thermal Properties of Lightweight Geopolymer Composites.” Cement and Concrete Composites 86:266–72. doi: https://doi.org/10.1016/j.cemconcomp.2017.11.016.

Davidovits, J. 1991. “Geopolymers: Inorganic Polymeric New Materials.” Journal of Thermal Analysis and Calorimetry (37):1633–56.

Davidovits, J. 2008. Geoplolymer Chemistry and Application. France: institute Geopolymer Saint-Quentin.

Davis, W. R. 1984. “Hot-Wire Method for the Measurement of the Thermal Conductivity of Refractory Materials.” Compendium of Thermo-physical Property Measurement Methods. 1:231.

Demirboğa, Ramazan, and Rüstem Gül. 2003. “The Effects of Expanded Perlite Aggregate, Silica Fume and Fly Ash on the Thermal Conduc-tivity of Lightweight Concrete.” Cement and Concrete Research 33(5):723–27. doi: https://doi.org/10.1016/S0008-8846(02)01032-3.

Epa U. 1995. AP-42: Compilation of Air Emissions Factors. Research Triangle Park NC: US Environmental Protection Agency.

Fan, Fenghong, Zhen Liu, Guoji Xu, Hui Peng, and C. S. Cai. 2018. “Mechanical and Thermal Properties of Fly Ash Based Geopolymers.” Construction and Building Materials 160:66–81. doi: https://doi.org/10.1016/j.conbuildmat.2017.11.023.

Gencel, Osman, Aliakbar Gholampour, Hayrettin Tokay, and Togay Ozbakkaloglu. 2021. “Replacement of Natural Sand with Expanded Ver-miculite in Fly Ash-Based Geopolymer Mortars.” Applied Sciences 11(4). doi: 10.3390/app11041917.

Gonzo, Elio E. 2002. “Estimating Correlations for the Effective Thermal Conductivity of Granular Materials.” Chemical Engineering Journal 90(3):299–302. doi: https://doi.org/10.1016/S1385-8947(02)00121-3.

Görhan, Gökhan, and Gökhan Kürklü. 2014. “The Influence of the NaOH Solution on the Properties of the Fly Ash-Based Geopolymer Mor-tar Cured at Different Temperatures.” Composites Part B: Engineering 58:371–77. doi: https://doi.org/10.1016/j.compositesb.2013.10.082.

Government of India MoM. 2019. “Part-III: Mineral Reviews.”

Gowda, S. Prathik, and M. Latha. 2017. “Environmental Concrete-Geopolymer of Construction.” Int. J. Curr. Res 9:62358–60.

Hajimohammadi, Ailar, Tuan Ngo, and Jitraporn Vongsvivut. 2019. “Interfacial Chemistry of a Fly Ash Geopolymer and Aggregates.” Journal of Cleaner Production 231:980–89. doi: https://doi.org/10.1016/j.jclepro.2019.05.249.

Hanjitsuwan, Sakonwan, Sitchai Hunpratub, Prasit Thongbai, Santi Maensiri, Vanchai Sata, and Prinya Chindaprasirt. 2014. “Effects of NaOH Concentrations on Physical and Electrical Properties of High Calcium Fly Ash Geopolymer Paste.” Cement and Concrete Composites 45:9–14. doi: https://doi.org/10.1016/j.cemconcomp.2013.09.012.

Hassan, Ahmed, Abdel-Hamid I. Mourad, Yasir Rashid, Najif Ismail, and Mohammad Shakeel Laghari. 2019. “Thermal and Structural Per-formance of Geopolymer Concrete Containing Phase Change Material Encapsulated in Expanded Clay.” Energy and Buildings 191:72–81. doi: https://doi.org/10.1016/j.enbuild.2019.03.005.

Hosan, Anwar, Sharany Haque, and Faiz Shaikh. 2016. “Compressive Behaviour of Sodium and Potassium Activators Synthetized Fly Ash Geopolymer at Elevated Temperatures: A Comparative Study.” Journal of Building Engineering 8:123–30. doi: https://doi.org/10.1016/j.jobe.2016.10.005.

Hwang, Chao-Lung, and Meng-Feng Hung. 2005. “Durability Design and Performance of Self-Consolidating Lightweight Concrete.” Construc-tion and Building Materials 19(8):619–26. doi: https://doi.org/10.1016/j.conbuildmat.2005.01.003.

Kaur, Mandeep, Jaspal Singh, and Manpreet Kaur. 2018. “Microstructure and Strength Development of Fly Ash-Based Geopolymer Mortar: Role of Nano-Metakaolin.” Construction and Building Materials 190. doi: 10.1016/j.conbuildmat.2018.09.157.

Kaya, Mehmet, and Fuat Köksal. 2021. “Effect of Cement Additive on Physical and Mechanical Properties of High Calcium Fly Ash Geopol-ymer Mortars.” Structural Concrete 22: E452–E465.

Kaya, Mehmet, Mucteba Uysal, Kemalettin Yilmaz, and Cengiz Duran Atis. 2018. “Behaviour of Geopolymer Mortars after Exposure to Ele-vated Temperatures.” Medziagotyra 24(4). doi: 10.5755/j01.ms.24.4.18829.

Kaya, Mehmet, Mücteba Uysal, Kemalettin Yilmaz, Okan Karahan, and C. Duran Atis. 2020. “Mechanical Properties of Class C and F Fly Ash Geopolymer Mortars.” Gradjevinar 72(4). doi: https://doi.org/10.14256/JCE.2421.2018.

Khadka, Suraj D., Priyantha W. Jayawickrama, Sanjaya Senadheera, and Branimir Segvic. 2020. “Stabilization of Highly Expansive Soils Containing Sulfate Using Metakaolin and Fly Ash Based Geopolymer Modified with Lime and Gypsum.” Transportation Geotechnics 23:100327. doi: https://doi.org/10.1016/j.trgeo.2020.100327.

Kim, Yun Yong, Byung-Jae Lee, Velu Saraswathy, and Seung-Jun Kwon. 2014. “Strength and Durability Performance of Alkali-Activated Rice Husk Ash Geopolymer Mortar.” The Scientific World Journal 2014.

Köksal, F., M. A. Serrano-López, M. Şahin, O. Gencel, and C. López-Colina. 2015. “Combined Effect of Steel Fibre and Expanded Vermicu-lite on Properties of Lightweight Mortar at Elevated Temperatures.” Materials and Structures 48(7):2083–92. doi: 10.1617/s11527-014-0294-7.

Koksal, Fuat, Yusa Sahin, and Osman Gencel. 2020. “Influence of Expanded Vermiculite Powder and Silica Fume on Properties of Foam Concretes.” Construction and Building Materials 257:119547. doi: https://doi.org/10.1016/j.conbuildmat.2020.119547.

Li, Yuancheng, Xiaobo Min, Yong Ke, Degang Liu, and Chongjian Tang. 2019. “Preparation of Red Mud-Based Geopolymer Materials from MSWI Fly Ash and Red Mud by Mechanical Activation.” Waste Management (New York, N.Y.) 83:202–8. doi: https://doi.org/10.1016/j.wasman.2018.11.019.

Liang, J. Z., and F. H. Li. 2006. “Measurement of Thermal Conductivity of Hollow Glass-Bead-Filled Polypropylene Composites.” Polymer Testing 25(4):527–31. doi: https://doi.org/10.1016/j.polymertesting.2006.02.007.

Luo, Xin, Jin-yu Xu, and Weimin Li. 2015. “The Preparation of Energy-Absorbing Material by Using Solid Waste.” RSC Adv. 5(12):9283–89. doi: https//doi.org/10.1039/C4RA06092J.

Madheswaran, C. K., G. Gnanasundar, and N. Gopalakrishnan. 2013. “Effect of Molarity in Geopolymer Concrete.” International Journal of Civil and Structural Engineering 4:106–15.

Medri, V., E. Papa, M. Mazzocchi, L. Laghi, M. Morganti, J. Francisconi, and E. Landi. 2015. “Production and Characterization of Lightweight Vermiculite/Geopolymer-Based Panels.” Materials & Design 85:266–74. doi: https://doi.org/10.1016/j.matdes.2015.06.145.

Melo, Miguel Otávio B. C., Luiz Bueno da Silva, Antonio S. Coutinho, Vivian Sousa, and Normando Perazzo. 2012. “Energy Efficiency in Building Installations Using Thermal Insulating Materials in Northeast Brazil.” Energy and Buildings 47:35–43. doi: https://doi.org/10.1016/j.enbuild.2011.11.021.

Mo, Kim Hung, Hong Jie Lee, Michael Yong Jing Liu, and Tung-Chai Ling. 2018. “Incorporation of Expanded Vermiculite Lightweight Aggre-gate in Cement Mortar.” Construction and Building Materials 179:302–6. doi: https://doi.org/10.1016/j.conbuildmat.2018.05.219.

Mohammed, Tarek Uddin, and Md Nafiur Rahman. 2016. “Effect of Types of Aggregate and Sand-to-Aggregate Volume Ratio on UPV in Concrete.” Construction and Building Materials 125:832–41. doi: https://doi.org/10.1016/j.conbuildmat.2016.08.102.

Muñoz-Sánchez, Belén, María José Arévalo-Caballero, and María Concepción Pacheco-Menor. 2016. “Influence of Acetic Acid and Calcium Hydroxide Treatments of Rubber Waste on the Properties of Rubberized Mortars.” Materials and Structures 50(1):75. doi: https://doi.org/10.1617/s11527-016-0912-7.

Mustafa Al Bakri, A. M., H. Kamarudin, M. Bnhussain, A. R. Rafiza, and Yahya Zarina. 2012. “Effect of Na 2 SiO 3/NaOH Ratios and NaOH Molarities on Compressive Strength of Fly-Ash-Based Geopolymer.” ACI Materials Journal 109(5).

Novais, Rui M., L. H. Buruberri, G. Ascensão, M. P. Seabra, and J. A. Labrincha. 2016. “Porous Biomass Fly Ash-Based Geopolymers with Tailored Thermal Conductivity.” Journal of Cleaner Production 119:99–107.

Okoye, F. N., J. Durgaprasad, and N. B. Singh. 2015. “Fly Ash/Kaolin Based Geopolymer Green Concretes and Their Mechanical Properties.” Data in Brief 5. doi: https://doi.org/10.1016/j.dib.2015.10.029.

Onwudili, Jude A., and Paul T. Williams. 2009. “Role of Sodium Hydroxide in the Production of Hydrogen Gas from the Hydrothermal Gasi-fication of Biomass.” International Journal of Hydrogen Energy 34(14):5645–56. doi: https://doi.org/10.1016/j.ijhydene.2009.05.082.

Pan, Zhu, Zhong Tao, Yi-Fang Cao, and Richard Wuhrer. 2018. “Measurement and Prediction of Thermal Properties of Alkali-Activated Fly Ash/Slag Binders at Elevated Temperatures.” Materials and Structures 51(4):108. doi: https://doi.org/10.1617/s11527-018-1233-9.

Provis, John L. 2018. “Alkali-Activated Materials.” Cement and Concrete Research 114:40–48. doi: https://doi.org/10.1016/j.cemconres.2017.02.009.

Puligilla, Sravanthi, Xu Chen, and Paramita Mondal. 2019. “Does Synthesized C-S-H Seed Promote Nucleation in Alkali Activated Fly Ash-Slag Geopolymer Binder?” Materials and Structures 52(4):65. doi: https://doi.org/10.1617/s11527-019-1368-3.

Qu, Fulin, Wengui Li, Zhong Tao, Arnaud Castel, and Kejin Wang. 2020. “High Temperature Resistance of Fly Ash/GGBFS-Based Geopoly-mer Mortar with Load-Induced Damage.” Materials and Structures 53(4):111. doi: https://doi.org/10.1617/s11527-020-01544-2.

Rattanasak, Ubolluk, and Prinya Chindaprasirt. 2009. “Influence of NaOH Solution on the Synthesis of Fly Ash Geopolymer.” Minerals Engi-neering 22(12):1073–78. doi: https://doi.org/10.1016/j.mineng.2009.03.022.

Safari, Zrar, Rawaz Kurda, Botan Al-Hadad, Faraydon Mahmood, and Mucip Tapan. 2020. “Mechanical Characteristics of Pumice-Based Geopolymer Paste.” Resources, Conservation and Recycling 162:105055. doi: https://doi.org/10.1016/j.resconrec.2020.105055.

Sarker, Prabir Kumar. 2011. “Bond Strength of Reinforcing Steel Embedded in Fly Ash-Based Geopolymer Concrete.” Materials and Struc-tures 44(5):1021–30. doi: https://doi.org/10.1617/s11527-010-9683-8.

Schackow, Adilson, Carmeane Effting, Marilena V Folgueras, Saulo Güths, and Gabriela A. Mendes. 2014. “Mechanical and Thermal Proper-ties of Lightweight Concretes with Vermiculite and EPS Using Air-Entraining Agent.” Construction and Building Materials 57:190–97. doi: https://doi.org/10.1016/j.conbuildmat.2014.02.009.

Sengul, Ozkan, Senem Azizi, Filiz Karaosmanoglu, and Mehmet Ali Tasdemir. 2011. “Effect of Expanded Perlite on the Mechanical Properties and Thermal Conductivity of Lightweight Concrete.” Energy and Buildings 43(2):671–76. doi: https://doi.org/10.1016/j.enbuild.2010.11.008.

Shao, Ning-ning, Yan-bo Zhang, Ze Liu, Dong-min Wang, and Zuo-tai Zhang. 2018. “Fabrication of Hollow Microspheres Filled Fly Ash Based Foam Geopolymers with Ultra-Low Thermal Conductivity and Relative High Strength.” Construction and Building Materials 185:567–73. doi: https://doi.org/10.1016/j.conbuildmat.2018.07.077.

Shoukry, H., M. F. Kotkata, S. A. Abo-EL-Enein, M. S. Morsy, and S. S. Shebl. 2016. “Enhanced Physical, Mechanical and Microstructural Properties of Lightweight Vermiculite Cement Composites Modified with Nano Metakaolin.” Construction and Building Materials 112:276–83. doi: https://doi.org/10.1016/j.conbuildmat.2016.02.209.

Shubbar, Ali Abdulhussein, Monower Sadique, Hayder Kamil Shanbara, and Khalid Hashim. 2020. “The Development of a New Low Carbon Binder for Construction as an Alternative to Cement.” Pp. 205–13 in Advances in Sustainable Construction Materials and Geotechnical Engineering. Springer.

Stefanidou, Maria, Marc Assael, Konstantinos Antoniadis, and Gregory Matziaroglou. 2010. “Thermal Conductivity of Building Materials Employed in the Preservation of Traditional Structures.” International Journal of Thermophysics 31(4):844–51. doi: https://doi.org/10.1007/s10765-010-0750-8.

Sukprasert, Sart, Menglim Hoy, Suksun Horpibulsuk, Arul Arulrajah, Ahmad Safuan A. Rashid, and Ramli Nazir. 2021. “Fly Ash Based Geopolymer Stabilisation of Silty Clay/Blast Furnace Slag for Subgrade Applications.” Road Materials and Pavement Design 22(2):357–71. doi: https://doi.org/ 10.1080/14680629.2019.1621190.

Tavman, I. H. 1996. “Effective Thermal Conductivity of Granular Porous Materials.” International Communications in Heat and Mass Trans-fer 23(2):169–76. doi: https://doi.org/10.1016/0735-1933(96)00003-6.

Tekin, Ilker, Osman Gencel, Aliakbar Gholampour, Osman Hulusi Oren, Fuat Koksal, and Togay Ozbakkaloglu. 2020. “Recycling Zeolitic Tuff and Marble Waste in the Production of Eco-Friendly Geopolymer Concretes.” Journal of Cleaner Production 268:122298. doi: https://doi.org/10.1016/j.jclepro.2020.122298.

Top, Soner, Hüseyin Vapur, Mahmut Altiner, Dogan Kaya, and Ahmet Ekicibil. 2020. “Properties of Fly Ash-Based Lightweight Geopolymer Concrete Prepared Using Pumice and Expanded Perlite as Aggregates.” Journal of Molecular Structure 1202:127236. doi: https://doi.org/10.1016/j.molstruc.2019.127236.

Turkish Standard Institution. 2004. “12504-4. Testing Concrete–Part 4: Determination of Ultrasonic Pulse Velocity.” 18.

Turkish Standard Institution. 2019. “Methods of Test for Mortar for Masonry-Part 11: Determination of Flexural and Compressive Strength of Hardened Mortar.”

UEG, Report. 2018. “No Title.” Retrieved (https://wedocs.unep.org/bitstream/handle/20.500.11822/34438/ EGR20ESE.pdf?sequence=25).

Wongsa, Ampol, Vanchai Sata, Peem Nuaklong, and Prinya Chindaprasirt. 2018. “Use of Crushed Clay Brick and Pumice Aggregates in Light-weight Geopolymer Concrete.” Construction and Building Materials 188:1025–34. doi: https://doi.org/10.1016/j.conbuildmat.2018.08.176.

Yılmaz B Erdoğmuş E, Erdoğan Y. 2005. “Investigation of the Effect of Sodium Carbonate Addition on Fly Ash Base Ash and Cements with Wallostonite.” in Symposium on Chemical Additives in Buildings. Chamber of civil engineers.

Yu, Xiao, Linhua Jiang, Jinxia Xu, and Yuheng Zu. 2017. “Effect of Na2SiO3 Content on Passivation and Corrosion Behaviour of Steel in a Simulated Pore Solution of Na2SiO3-Activated Slag.” Construction and Building Materials 146:156–64. doi: https://doi.org/10.1016/j.conbuildmat.2017.04.091.

Yurt, Ümit. 2020a. “An Experimental Study on Fracture Energy of Alkali Activated Slag Composites Incorporated Different Fibers.” Journal of Building Engineering 32:101519. doi: https://doi.org/10.1016/j.jobe.2020.101519.

Yurt, Ümit. 2020b. “High Performance Cementless Composites from Alkali Activated GGBFS.” Construction and Building Materials 264:120222. doi: https://doi.org/10.1016/j.conbuildmat.2020.120222.

Zhang, Zuhua, John L. Provis, Andrew Reid, and Hao Wang. 2015. “Mechanical, Thermal Insulation, Thermal Resistance and Acoustic Ab-sorption Properties of Geopolymer Foam Concrete.” Cement and Concrete Composites 62:97–105. doi: https://doi.org/10.1016/j.cemconcomp.2015.03.013.

Downloads

Published

2022-04-18

Versions

How to Cite

Kaya, M., & Köksal, F. . (2022). Physical and mechanical properties of C class fly ash based lightweight geopolymer mortar produced with expanded vermiculite aggregate. Revista De La Construcción. Journal of Construction, 21(1), 21–35. https://doi.org/10.7764/RDLC.21.1.21