A laboratory study is performed to evaluate the influence of elevated temperature on the strength and microstructural properties of high strength concretes (HSCs) containing ground pumice (GP), and blend of ground pumice and metakaolin (MK) mixture. Twelve different mixtures of HSCs containing GP and MK were produced, water-to-binder ratio was kept constant as 0.20. Hardened concrete specimens were exposed to 250 C, 500 C and 750 C elevated temperatures increased with a heating rate of 5 C/min. Ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete samples were measured on unheated control concrete and after air-cooling period of heated concrete. The crack formation and alterations in the matrix, interface and aggregate of HSCs were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and polarized light microscope (PLM) analyses. XRD, SEM and PLM analyses have shown that, increasing target temperature result with decrease in mechanical properties i.e. Upv, fc, ffs and fsts values. Elevated temperature also results with crack formation, and increasing target temperature caused more cracks. Alterations in the matrix, interface and aggregate were, also observed by these analyses. The experimental results indicate that concrete made with MK + GP blend together as a replacement of cement in mass basis behaved better than control concrete made with cement only, and concrete containing only GP as a cement replacement.
Construction and Building Materials 124 (2016) 244–257 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat The influence of elevated temperature on strength and microstructure of high strength concrete containing ground pumice and metakaolin M Saridemir a, M.H Severcan a, M Ciflikli b, S Celikten a, F Ozcan a, C.D Atis c,⇑ a Department of Civil Engineering, Nigde University, 51240 Nigde, Turkey Department of Geology Engineering, Nigde University, 51240 Nigde, Turkey c Department of Civil Engineering, Erciyes University, 38039 Kayseri, Turkey b h i g h l i g h t s Influence of elevated temperature on mechanical properties of HSC is examined The changes in microstructure of concrete were examined by XRD, SEM and PLM Increase in temperature result with decrease in mechanical properties of concrete High temperature caused cracks and alterations in microstructures of materials Under elevated temperature concrete containing GP and MK blend behaved better a r t i c l e i n f o Article history: Received 22 July 2015 Received in revised form 18 July 2016 Accepted 22 July 2016 Keywords: High strength concrete Elevated temperature Microstructure Interface a b s t r a c t A laboratory study is performed to evaluate the influence of elevated temperature on the strength and microstructural properties of high strength concretes (HSCs) containing ground pumice (GP), and blend of ground pumice and metakaolin (MK) mixture Twelve different mixtures of HSCs containing GP and MK were produced, water-to-binder ratio was kept constant as 0.20 Hardened concrete specimens were exposed to 250 °C, 500 °C and 750 °C elevated temperatures increased with a heating rate of °C/min Ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete samples were measured on unheated control concrete and after air-cooling period of heated concrete The crack formation and alterations in the matrix, interface and aggregate of HSCs were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and polarized light microscope (PLM) analyses XRD, SEM and PLM analyses have shown that, increasing target temperature result with decrease in mechanical properties i.e Upv, fc, ffs and fsts values Elevated temperature also results with crack formation, and increasing target temperature caused more cracks Alterations in the matrix, interface and aggregate were, also observed by these analyses The experimental results indicate that concrete made with MK + GP blend together as a replacement of cement in mass basis behaved better than control concrete made with cement only, and concrete containing only GP as a cement replacement Ó 2016 Elsevier Ltd All rights reserved Introduction In recent years, high strength concretes (HSCs) containing natural pozzolanas, which are either in raw or calcined condition, such as metakaolin, zeolite, volcanic tuff and diatomite, are used widely in the world The columns, shear walls, foundations, bridges, skyscrapers, nuclear and power structures are among the major areas for high strength concrete applications The ⇑ Corresponding author E-mail address: cdatis@erciyes.edu.tr (C.D Atis) http://dx.doi.org/10.1016/j.conbuildmat.2016.07.109 0950-0618/Ó 2016 Elsevier Ltd All rights reserved application fields of high strength concretes are expanding in time, since they show extraordinary structural performance, protect the environment, save energy by using pozzolanas [1,2] Moreover, the natural pozzolanas provide more advantage i.e the reduction of cost, reduction of heat leak, decrease of permeability and increase of chemical resistance, since they intensify the microstructure of concrete when used as cement replacement material [3] They also provide extra strength in the concrete by reacting with the cement hydration product Ca(OH)2 to form extra calcium-silicate-hydrate (C-S-H) gels particularly in transition zone [4,5] 245 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Over the past several decades, due to its high pozzolanic properties, the influence of using MK as supplementary cementing materials in concrete on the mechanical and durability related properties of concrete was studied by numerous researchers [6–10] MK is an ultrafine pozzolanas, produced by calcination of purified kaolin clay, heating in the temperature range of 600–900 °C [5,10] The main ingredients of MK are amorphous SiO2 and Al2O3 MK has high pozzolanic activity due to its glassy components Apart from the filling effect in concrete, MK reacts with Ca(OH)2 to produce C-S-H gels in the main bonding phase of concrete [11–13] It is known that Ca (OH)2 is the main element which causes the weakness of interface between the aggregate particles-cementitious materials Thus, it influences the strength, porosity, permeability and durability related properties of concrete [13,14] The replacement of natural puzzolanas with cement, such as MK, consumes Ca (OH)2 and improves the above-mentioned properties of interfacial zone of concrete [13] Concrete, which is one of the most widely used as building materials in the world, has higher resistance to elevated temperature, when compared to other building materials i.e steel and wood Nevertheless, this resistance is valid up to a certain temperature level and exposure duration [15,16] When a certain time is exceeded under elevated temperature, it brings about important physical and chemical changes and resulting in deterioration of concrete such as forming cracks, causing large pores, spalls and reduction of the adherence between the aggregate particlescementitious materials in the concrete [16,17] Therefore, the mechanical properties of concrete are decreased due to these changes The reduction in these properties due to elevated temperatures was also associated with the heating rate of specimens When the specimens are heated up to approximately 250 °C temperature, free water present in the specimens evaporates slowly, and no structural damage occurs in the specimens Nevertheless, rapid heating rate results in higher vapor pressure and causes cracks in concrete [18] When the temperature of specimens reaches approximately at 300 °C, the water in the interface of CS-H gels is evaporated Micro-cracks occur approximately at 300 °C temperature in the cement matrix and the bond between the aggregate particles-cementitious materials [16,19] Therefore, the mechanical properties of the specimens, exposed to higher than 300 °C temperature, gradually decrease due to the crack growth and deterioration of C-S-H gels, when compared to nonheated specimens Previous papers studied on the influence of MK were, in general, on the properties of concrete as a cement replacement material Using MK as a cement replacement in concrete improved mechanical and durability related properties at optimal replacement ratio, which depends on the fineness and properties of MK used The effect of ternary blend of MK and silica fume or fly ash on the properties of HSCs was also studied by many researchers Nevertheless, there are no study investigating the effect of ternary blend of MK and GP on the mechanical and microstructural properties of HSCs exposed to elevated temperatures The aim of this paper was to investigate the effect of 5%GP, 10%GP, 15%GP, 20%GP, 2.5%MK + 2.5%GP, 5%MK + 5%GP, 5%MK + 10%GP, 5%MK + 15%GP, 10%MK + 5%GP, 10%MK + 10%GP and 15%MK + 5%GP (5GP, 10GP, 15GP, 20GP, 2.5MK + 2.5GP, 5MK + 5GP, 5MK + 10GP, 5MK + 15GP, 10MK + 5GP, 10MK + 10GP and 15MK + 5GP) blend, as cement replacement in concrete, on the ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete studied In addition, investigating the influence of elevated temperature on residual Upv, fc, ffs, and fsts values of HSCs containing GP and MK + GP blend was another aim of this work Crack formation, alterations in microstructural properties of cementitious matrix, interfacial zone between the aggregate particles and cementitious materials were also to be investigated by XRD, SEM and PLM analyses Experimental study 2.1 Materials The cementitious materials used in the mixtures were ordinary Portland cement (CEM I 42.5 R) complying with relevant TS EN 197-1 [20], GP and MK complying with relevant ASTM C-618 [21] Portland cement, GP and MK were procured from Nigde cement plant of CIMSA, Nevsehir Mikromin Company and BASFThe Chemical Company in Turkey, respectively The chemical compositions, physical and mechanical properties of the cementitious materials are presented in Table The fine aggregates used in the mixtures were natural sand-I (NS-I) and natural sand-II (NS-II) The coarse aggregates used in the mixtures were crushed limestone-I (CL-I) and crushed limestone-II (CL-II) The aggregates used were compatible with the requirements of TS 706 EN 12620+A1 [22] The particle size, mixing ratio and specific gravity of aggregates used in mixtures are given in Table In addition, the gradations of aggregates used are provided in Table 3, with the standard limits 2.2 Mix proportions Twelve HSC mixtures with 0.2 water binder ratio and 500 kg cementitious materials for a cubic meter were prepared These mixtures include one control concrete (C), four concretes containing up to 20% GP and seven concretes containing up to 20% MK + GP blend The details of mixture proportions of concretes containing GP and MK + GP are given in Table The modified polycarboxylic ether polymers based high range water reducing admixture Table Properties of cement, GP and MK admixtures Component (%) Cement GP MK SiO2 Fe2O3 Al2O3 CaO MgO SO3 Na2O K2O Loss on ignition Physical properties Initial-final setting time (min) Specific gravity Specific surface area (m2/kg) Mechanical properties Compressive Strength (MPa) days days 28 days 25.10 2.30 5.10 61.35 1.50 1.65 0.70 0.95 1.25 55.23 0.83 42.05 0.31 0.45 0.32 0.61 0.20 68.14 2.50 13.94 3.23 1.06 1.24 4.46 2.63 2.78 2.54 16.700 2.50 13.150 125–215 3.16 365 32.65 44.80 53.45 GP = Ground pumice, MK = Metakaolin Table The particle size, mixing ratio and specific gravity of aggregates Particle size (mm) Mixing ratio (%) Specific gravity Fine aggregate Coarse aggregate NS-I NS-II CL-I CL-II 0–1 10 2.55 0–5 30 2.47 5–12 25 2.69 12–22 35 2.71 NS = Natural sand, CL = Crushed limestone 246 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Table Total used aggregate grading with standard limits Sieve size (mm) 31.5 22.4 16 11.2 0.5 0.25 0.15 0.63 Passing from sieve (%) A limit B limit C limit Total used aggregate 100.00 98 85 68 48 33 22 15 10 100.00 99 92 79 63 49 37 28 20 13 100.00 100.00 99 90 77 64 52 41 30 20 11 100 100 89.55 71.25 59.60 42.05 32.55 25.38 17.35 8.04 3.76 0.96 called as Glenium 51 was used in the concrete mixtures to maintain desired slump of 80 ± 20 mm 2.3 Mixing, casting, curing, heating and cooling details The mixing, casting and compacting of concretes containing GP and MK + GP blend were performed complying with relevant standard ASTM C192/C192M-14 [23] A power driven rotating pan mixer was used for mixing, and a vibrating table was used in casting and compacting the samples After casting the fresh concrete mixture samples into the molds, they were covered with wet burlaps for 24 h in the laboratory condition Afterwards, hardened specimens were removed from the molds after a day, and were placed in water tank with 24 ± °C temperature, until testing The heating of concrete specimens carried out by exposing them to 250 °C, 500 °C and 750 °C each target temperatures Before elevated temperature testing, specimens were removed from water tank, and conditioned in laboratory condition for a week, then dried for 24 h in an oven at 105 °C The specimens were put in a furnace at room temperature, and temperature was elevated at a rate of °C/min up to target temperatures The specimens were exposed to target temperature for h in steady-state condition Then the power button on the furnace was shut off At the end of heating process, the door of furnace was opened, and the specimens were exposed to slow cooling in the air for 24 h 2.4 Testing procedure and methods The Upv, fc, ffs and fsts values were determined on the control concrete and concretes containing 5GP, 10GP, 15GP, 20GP, 2.5MK + 2.5GP, 5MK + 5GP, 5MK + 10GP, 5MK + 15GP, 10MK + 5GP, 10MK + 10GP and 15MK + 5GP The Upv and fc tests were performed, in accordance with ASTM C 597-09 [24] and TS EN 12390-3 [25], on cubic specimens with a 10 cm side, at the ages of 7, 28 and 56 days In addition, the Upv, fc, ffs and fsts tests were also performed on the same size cubic specimens after exposing them to 250 °C, 500 °C and 750 °C temperatures at 56 days The fc values on the concrete specimens were measured by compression load applied with a rate of 0.10 MPa/s by using a 3000 kN capacity compression machine The ffs and fsts values were obtained by using flexural tensile testing and split tensile strength apparatus The ffs and fsts tests were carried out in accordance with TS EN 12390-5 [26] and TS EN 12390-6 [27], respectively Flexural (ffs) and split tensile strengths (fsts) were measured at 56 days, by using prism specimens with dimension of 10  10  40 cm, and cubic specimens with a 15 cm side, respectively In this study, microscopic analyses of HSCs containing GP and MK + GP exposed to elevated temperature were performed by using a Philips Panalytical EMPYREAN type XRD, Zeiss EVO 40XVP type SEM, and Nikon ECLIPSE E400 Pol type PLM XRD and SEM analyses were used to investigate the changes in the chemical component, mineralogical structure, microstructure and interface between the aggregate particles and cementitious materials of the control concrete and concretes containing 5GP and 5MK + 5GP exposed to 25 °C, 500 °C and 750 °C temperatures These analyses were performed on the small pieces taken from the specimens used for the PLM analysis For the SEM analyses, the small pieces were mounted on the brass stubs using carbon tapes and, were covered with gold PLM analyses were used to investigate the cracks and alterations in the cementitious matrix, interface between the aggregate particles and cementitious materials, and aggregate microstructures on the thin segments taken from cubic sample with a 10 cm side The analyses were carried out on the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP after exposing them to 25 °C, 500 °C and 750 °C temperatures After exposing the concrete specimens to the target temperatures and cooling, they were cut in four equal parts using a rotary saw and   10 cm prism specimens were prepared as shown Fig 1a, b and d One of four equal parts was selected The selected part was divided into two equal parts using a rotary saw and one of the parts was overlaid in the acetone to clean the free particles and pores This cleaned part was embedded in resin to absorb resin in a vacuum desiccator until there is no micro-air bubble in the part The part embedded in resin for the PLM analyses were left to harden in the laboratory condition as seen Fig 1d The hardened parts were adhered to the   0.5 cm size glass to obtain thinner section The adhered parts were cut very thin by a cutting machine to make thin parts   0.03 cm in size by using a sensitive diamond saw as seen Fig 1c and e Then, thin sections were eroded for use in the PLM analysis size The surfaces of thin Table Mixture proportions of concretes containing GP and MK + GP (kg/m3) Mixtures No Meaning Cement kg/m3 GP % C 5GP 10GP 15GP 20GP 2.5MK + 2.5GP 5MK + 5GP 5MK + 10GP 5MK + 15GP 10MK + 5GP 10MK + 10GP 15MK + 5GP Control concrete %5GP %10GP %15GP %20GP %2.5MK + %2.5GP %5 MK + %5 GP %5MK + %10GP %5MK + %15GP %10MK + %5GP %10MK + %10GP %15MK + %5GP 500 475 450 425 400 475 450 425 400 425 400 400 10 15 20 2.5 10 15 10 MK % 2.5 5 10 10 15 Water kg/m3 NS-I (0–1 mm) NS-II (0–5 mm) CL-I (5–12 mm) CL-II (12–22 mm) SP kg/m3 100 255.10 254.13 253.16 252.18 251.21 254.28 253.46 252.49 251.52 252.80 251.83 252.13 537.66 535.61 533.56 531.52 529.47 535.94 534.21 532.16 530.12 532.81 530.76 531.41 489.60 487.74 485.87 484.01 482.14 488.03 486.46 484.60 482.73 485.19 483.32 483.91 690.50 687.87 685.24 682.61 679.98 688.29 686.07 683.44 680.81 684.28 681.64 682.48 15.00 15.00 18.33 20.00 21.67 15.00 15.83 20.00 21.67 18.33 20.83 19.17 GP = Ground pumice, MK = Metakaolin, NS = Natural sand, CL = Crushed limestone, SP = Superplasticizer M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 247 Fig Preparing of thin part for microstructure analyses sections were moistened to increase the quality of images using the microscope camera In this way, the microstructure images of thin sections between the lines seen in Fig 1f were obtained to investigate the cracks and alterations in the cementitious matrix, interface between the aggregate particles and cementitious materials, and aggregate microstructures Test results and discussion 3.1 Ultrasound pulse velocity and compressive strength The normalized Upv and fc values of HSCs containing GP and MK + GP at the ages of 7, 28 and 56 days are given in Table Besides, the effects of GP on the Upv and fc values of HSC are shown in Figs 2a and 3a in 3D graphs, and also the effects of MK + GP on the Upv and fc values of HSC are shown in Figs 2b and 3b in 3D graphs at the ages of 7, 28 and 56 days As shown in Figs 2a and 3a, the Upv and fc values of concrete containing 5% GP increases at all ages, while these values for concrete containing 10%, 15% and 20% GP decrease These figures shows that, the Upv and fc values of the control concretes varied between 5.38–5.44 km/s and 75.50–81.35 MPa, while these values for concretes containing GP ranged between 5.28–5.45 km/s and 65.13–84.19 MPa, respectively, depending on the curing time and replacement level of GP The effect of MK + GP on the Upv and fc values of concrete can obviously be observed from 248 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Table The normalized fc, fsts and ffs values of HSCs Mixtures Upv (km/s) C 5GP 10GP 15GP 20GP 2.5MK + 2.5GP 5MK + 5GP 5MK + 10GP 5MK + 15GP 10MK + 5GP 10MK + 10GP 15MK + 5GP fc (MPa) fsts (MPa) ffs (MPa) days 28 days 56 days days 28 days 56 days 56 days 56 days 1.00 1.00 0.99 0.99 0.98 1.01 1.01 1.00 1.00 1.01 1.01 1.00 1.00 1.00 1.00 0.99 0.98 1.01 1.01 1.00 1.00 1.01 1.01 1.00 1.00 1.00 0.99 0.99 0.97 1.01 1.01 1.00 0.99 1.01 1.00 1.00 1.00 1.03 0.97 0.93 0.86 1.09 1.10 1.04 1.00 1.08 1.03 1.00 1.00 1.03 0.97 0.93 0.85 1.08 1.09 1.02 0.98 1.06 1.03 0.99 1.00 1.03 0.95 0.92 0.84 1.07 1.08 1.03 0.97 1.07 1.03 0.99 1.00 1.02 0.99 0.97 0.95 1.05 1.07 1.02 0.99 1.05 1.02 0.99 1.00 1.04 0.95 0.90 0.83 1.05 1.09 1.02 0.94 1.05 0.99 0.95 fc = Compressive strength, fsts = Splitting tensile strength, ffs = Flexural strength 5.44-5.46 5.42-5.44 5.40-5.42 5.38-5.40 5.36-5.38 5.34-5.36 5.32-5.34 5.30-5.32 5.28-5.30 5.26-5.28 5.46 5.44 5.42 5.40 5.38 5.36 5.34 5.32 5.30 5.28 5.26 56 28 10 15 84-85 82-84 80-82 78-80 76-78 74-76 72-74 70-72 68-70 66-68 64-66 (a) f c , MPa U pv , km/s (a) Time, days 20 84 82 80 78 76 74 72 70 68 66 64 56 28 GP, % 10 15 Time, days 20 GP, % (b) 5.48-5.50 5.44-5.46 5.50 5.42-5.44 5.48 5.40-5.42 5.46 5.38-5.40 5.44 5.36-5.38 5.42 56 5.40 5.38 28 5.36 Time, days 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5 MK+GP, % Fig The Upv values of HSCs: a) containing GP and b) containing MK + GP 86-88 84-86 82-84 88 80-82 86 78-80 84 f c , MPa U pv , km/s (b) 5.46-5.48 76-78 82 74-76 80 56 78 28 76 74 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5 Time, days MK+GP, % Fig The fc values of HSCs: a) containing GP and b) containing MK + GP Figs 2b and 3b The concretes containing MK + GP had higher Upv and fc values than the control concretes at the same ages, except for 20% replacement level of MK + GP The Upv and fc values of concretes containing MK + GP ranged between 5.39–5.50 km/s and 75.62–87.84 MPa as seen in these figures, respectively The highest Upv and fc values were obtained between 5.44–5.50 km/s and 82.77–87.84 MPa for the concretes containing 5MK + 5GP In the Upv and fc values of HSCs containing GP and MK + GP at the ages of 7, 28 and 56 days are evaluated, it was concluded that the concrete containing MK + GP shown better performance than that of concrete containing only GP Rashiddadash et al [28] investigated the fc values of the polypropylene fiber and steel fiber reinforced concretes containing MK and GP They prepared concrete containing 10%GP and 15% GP, 10%MK and 15% MK, 7.5% MK + 7.5% GP, and a control concrete They reported that the fc values varied between 18–44 MPa and 16.7–37.6 MPa for control concretes and concretes containing GP, respectively They observed that the early and long-term fc values of concretes containing GP were lower than that of the control concretes, depending on the replacement level of GP However, they reported the fc values varied between 18.7 and 46.5 MPa for concrete containing MK were higher than that of the control concretes They concluded that, due to high fineness and reactivity of MK, the concretes containing MK had relatively higher fc development than that of the control concrete and concrete containing GP A high temperature furnace used in this study to heat, cubic concrete specimens with a 10 cm a side, up to 750 °C temperature is shown in Fig 4b The Upv and fc values of concretes containing GP and MK + GP exposed to 250 °C, 500 °C and 750 °C temperatures are normalized according to the Upv and fc values obtained from unheated (25 °C) specimens at the age of 56 days, and these values are presented in Table In addition, the changes in the Upv and fc values of concretes containing GP and MK + GP exposed to 250 °C, 500 °C and 750 °C temperatures, compared with unheated counterpart control concrete samples which is shown in Figs and as 3D graphs 249 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Fig a) Broken specimen in the normal temperature, b) electric furnace for high temperature, and c) broken specimen after high temperature Table The normalized Upv and fc values of HSCs exposed to elevated temperatures Mixtures Upv (km/s) C 5GP 10GP 15GP 20GP 2.5MK + 2.5GP 5MK + 5GP 5MK + 10GP 5MK + 15GP 10MK + 5GP 10MK + 10GP 15MK + 5GP fc (MPa) 25 °C 250 °C 500 °C 750 °C 25 °C 250 °C 500 °C 750 °C 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.81 0.81 0.80 0.80 0.79 0.81 0.81 0.80 0.79 0.81 0.80 0.78 0.54 0.57 0.54 0.54 0.53 0.58 0.59 0.56 0.53 0.57 0.55 0.54 0.26 0.26 0.25 0.24 0.23 0.26 0.26 0.24 0.24 0.26 0.25 0.25 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.99 0.98 1.00 0.99 1.05 0.99 0.99 0.96 0.97 1.00 0.94 0.96 0.85 0.87 0.87 0.90 0.96 0.88 0.90 0.90 0.87 0.89 0.86 0.87 0.42 0.42 0.42 0.40 0.42 0.40 0.42 0.39 0.41 0.42 0.42 0.42 Upv = Ultrasound pulse velocity, fc = Compressive strength 5.6 5.2 4.8 4.4 4.0 3.6 3.2 2.8 2.4 2.0 1.6 1.2 f c , MPa U pv , km/sn (a) 5.2-5.6 4.8-5.2 4.4-4.8 4.0-4.4 3.6-4.0 3.2-3.6 2.8-3.2 2.4-2.8 2.0-2.4 1.6-2.0 1.2-1.6 (a) 25 250 10 15 20 500 750 Temperature, ºC 85 80 75 70 65 60 55 50 45 40 35 30 25 500 750 10 15 Temperature, ºC 20 GP, % 25 250 500 750 Temperature, ºC 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5 MK+GP, % Fig The effect of high temperature on the Upv values of HSCs: a) containing GP and b) containing MK + GP 85-90 80-85 75-80 70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-40 30-35 (b) f c , MPa 5.2-5.6 4.8-5.2 4.4-4.8 4.0-4.4 3.6-4.0 3.2-3.6 2.8-3.2 2.4-2.8 2.0-2.4 1.6-2.0 1.2-1.6 (b) U pv , km/s 25 250 GP, % 5.6 5.2 4.8 4.4 4.0 3.6 3.2 2.8 2.4 2.0 1.6 1.2 80-85 75-80 70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-40 30-35 25-30 90 85 80 75 70 65 60 55 50 45 40 35 30 25 250 500 750 Temperature, ºC 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5 MK+GP, % Fig The effect of high temperature on the fc values of HSCs: a) containing GP and b) containing MK + GP 250 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 The Upv values of heated concretes containing GP and MK + GP reduce gradually as the temperature increase, as shown in Fig 5a and b in 3D graphs Furthermore, the GP and MK + GP contents of concretes have no significant effect on the decrease of Upv values when compared to strength of control Portland cement concrete as shown in these figures As the temperature increases, the Upv values of concretes decrease gradually together with the growth and increase of the amount of pores and space within the concrete As shown in Fig 6a and b, when concrete specimens are exposed to 250 °C, while the average fc values of concretes containing GP show no change, the average fc values of concretes containing MK + GP reduce about 3% It is reported that, near such temperatures, the fc values of concretes decrease due to the occurrence of micro cracks caused by evaporation of water and the growth of pore structure inside the concrete [18] When the temperature reached to 500 °C, while the average fc values of concretes containing GP decrease about 11%, the average fc values of concretes containing MK + GP reduce about 12% Some researchers stated that loss in fc is generally associated with the dehydration of C-S-H gel and the volumetric expansion due to changing shape of the chemical compound Ca(OH)2 to CaO that is known to happen between 500 °C and 600 °C [18,29,30] Moreover, in these temperatures, the adherence between the aggregate particles and cementitious materials is impaired, due to the cementitious materials shrinkage resulting from the loss of water together with the expansion of the aggregates [30] The fc loss in the concrete increased substantially when the temperature was reached to 750 °C and over temperatures due to the disintegration of C-S-H gel and increase of the macro cracks [19,31,32] In this study, as shown in Fig 6a and b the greatest fc loss was observed at 750 °C temperature At this temperature, while the average loss in the fc values of concrete containing GP was about 58%, the average loss in the fc values of concrete containing MK + GP was 59% compared to fc before heating It can be seen from Fig 6a and b that the replacements of GP and MK + GP with cement have no significant influence on the fc loss occurring due to elevated temperatures The ffs and fsts tests on the concretes containing GP and MK + GP at the age of 56 days were performed on prism specimens with dimension of 10  10  40 cm, and cubic specimens with a 15 cm side The ffs and fsts test results of concretes containing GP and MK + GP are shown in Figs and 8, each result being average of three concrete specimens Besides, the normalized ffs and fsts values of these concretes are also provided in Tables and It can be seen from the results that GP has not enhanced the ffs and fsts values compared to the results of the control concrete, except for 5% GP content, and it could be concluded from the results that the higher amount of GP in the mixture leads to decrease in the ffs and fsts values However, it can be seen from the results that MK + GP blend has enhanced the ffs and fsts values up to 20% MK + GP content compared to the results of the control concrete, and concretes containing GP The highest ffs and fsts values were obtained as 10.31 MPa and 5.70 MPa from concrete containing 5MK + 5GP mixture, while the lowest ffs and fsts values were obtained as 8.91 MPa and 5.29 MPa from a concrete containing 5%MK and 15%GP blend together On the other hand, as the temperature increase, the ffs and fsts values of concretes containing GP and MK + GP exposed to 250 °C, 500 °C and 750 °C temperatures decrease gradually as shown in Figs and 8, in 3D graphs Normalized ffs and fsts values of these concretes are presented in Table 7, with respect to the ffs and fsts values of unheated concrete at the age of 56 days It is observed that GP and MK + GP contents of concretes have no important effect in a decrease of ffs and fsts values as shown in these figures and table When the temperature reached to 250 °C, 500 °C and 750 °C, while the average ffs value of concrete containing GP decrease about 6%, 20%, and 60%; the average ffs value of concrete containing MK + GP decrease about 9%, 23%, and 66% compared to none heated concrete ffs value, respectively Similarly, in these temperatures, while the average fsts values of concrete containing GP decrease about 9%, 22%, and 67%; the average fsts (a) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 f fs , MPa 5.5-6.0 5.0-5.5 4.5-5.0 4.0-4.5 3.5-4.0 3.0-3.5 2.5-3.0 2.0-2.5 1.5-2.0 1.0-1.5 (a) f sts , MPa 3.2 Flexural and splitting tensile strengths 25 250 500 Temperature, ºC 750 10 15 20 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 25 250 750 GP, % 10 15 20 25 250 500 Temperature, ºC 750 2.5+2.55+5 5+10 5+15 10+510+1015+5 MK+GP, % Fig The effect of high temperature on the fsts values of HSCs: a) containing GP and b) containing MK + GP 10.0-11.0 (b) f fs , MPa f sts , MPa 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 500 Temperature, ºC GP, % 5.5-6.0 5.0-5.5 4.5-5.0 4.0-4.5 3.5-4.0 3.0-3.5 2.5-3.0 2.0-2.5 1.5-2.0 1.0-1.5 (b) 9.0-10.0 8.0-9.0 7.0-8.0 6.0-7.0 5.0-6.0 4.0-5.0 3.0-4.0 2.0-3.0 9.0-10.0 8.0-9.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 7.0-8.0 6.0-7.0 5.0-6.0 25 250 500 Temperature, ºC 750 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5 MK+GP, % Fig The effect of high temperature on the ffs values of HSCs: a) containing GP and b) containing MK + GP 251 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Table The normalized fsts and ffs values of HSCs exposed to elevated temperatures Mixtures C 5GP 10GP 15GP 20GP 2.5MK + 2.5GP 5MK + 5GP 5MK + 10GP 5MK + 15GP 10MK + 5GP 10MK + 10GP 15MK + 5GP fsts (MPa) ffs (MPa) 25 °C 250 °C 500 °C 750 °C 25 °C 250 °C 500 °C 750 °C 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.91 0.89 0.91 0.92 0.90 0.90 0.92 0.92 0.90 0.92 0.90 0.89 0.77 0.77 0.76 0.77 0.75 0.79 0.78 0.78 0.78 0.79 0.80 0.79 0.33 0.33 0.34 0.33 0.29 0.34 0.33 0.32 0.32 0.33 0.34 0.31 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.94 0.89 0.93 0.94 0.92 0.91 0.94 0.94 0.95 0.92 0.94 0.90 0.75 0.77 0.78 0.78 0.78 0.80 0.78 0.81 0.83 0.71 0.81 0.80 0.34 0.34 0.34 0.33 0.33 0.33 0.35 0.33 0.35 0.34 0.34 0.35 fsts = Splitting tensile strength, ffs = Flexural strength value of concrete containing MK + GP decrease about 11%, 22%, and 67% As shown Figs and 8, the greatest ffs and fsts losses were observed at 750 °C temperature The explanation for this decrease is the disintegration of C-S-H gels and macro cracks as stated in compressive strength section the matrix and interface were doubled with increasing temperature The increases of cracks and pores are caused to decrease in the fc value It is concluded that the reduction in the fc value is a natural results of deterioration, large pores and crack formation in the concrete specimens due to elevated temperature Similar conclusions and reports are revealed in the literature [31,36,37] 3.3 XRD analyses after elevated temperatures 3.5 PLM analyses after elevated temperatures X-ray diffraction (XRD) analyses were performed on powdered samples obtained from control concrete and concretes made with 5GP and 5MK + 5GP after exposing them to 25 °C and 750 °C temperatures as shown in Fig 9a, b and c Besides, the chemical component and mineralogical structures of these concretes were determined by X-ray fluorescence and XRD semi-quantitative analysis as seen in Tables and 9, respectively The results of analyses indicated that the major mineralogical structures were calcite and quartz from the aggregates as the main impurity in the concretes, as well as some traces of feldspar and dolomite Moreover, the analyses indicated that other mineralogical structures were Ca (OH)2, C2S, C3S and C-S-H from the cementitious materials The reduction in the XRD peak for mineralogical structures of concretes was observed when the temperature was 750 °C The reduction in the calcite and quartz may be due to transformation of amorphous phase of SiO2 and lime of calcite [33,34] The dehydration caused by the decomposition of Ca(OH)2, C2S, C3S and C-S-H gel plays a dominant role in the reduction [35] The reduction in the XRD peaks for the control concrete was higher than the concretes containing GP and MK + GP at 750 °C temperature as shown in Fig These situations are supported by the chemical components and mineralogical structures of concretes as seen in Tables and Microstructures of matrix, interface and aggregate on the thin sections were examined by using polarized light microscope (PLM) 3.4 SEM analyses after elevated temperatures 3.5.1 The effect of elevated temperature on the matrix structure The matrix structures of the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to 25 °C, 500 °C and 750 °C temperatures were examined by using PLM image analyses as seen in Fig 11 The changes of the crack formation and deterioration on the matrix of the specimens were evaluated by these image analyses As seen in Fig 4c, the discoloration in the specimens was observed when the specimens exposed to 500 °C and 750 °C temperatures This discoloration was the cause of altered and oxidized zones in the matrix Nearly no crack was observed on the matrix of the specimens when exposed to 500 °C temperature However, the cracks were observed on the matrix of the specimens when exposed to 750 °C temperature These cracks caused considerable reduction in the mechanical properties of the specimens As shown in Fig 11, the GP and MK + GP contents have not an important effect on the cracks and deterioration occurred on the matrix of these specimens due to elevated temperature Similar observations have been reported by Akca and Zihnioglu [37], Akỗaửzoglu [38] and Ingham [39] Microstructural analyses of concrete specimens were carried out by scanning electron microscope (SEM) The microstructure and interface between the aggregate particles and cementitious materials of the control concrete and concretes containing 5GP and 5MK + 5GP exposed to 25 °C, 500 °C and 750 °C temperatures were examined on the crushed sample surfaces As seen in Fig 10, the changes were emerged in the C-S-H gels, matrix and interface due to the increase of temperature At 25 °C temperature, the internal structure of concretes is compact, and the C-S-H gels are as the shape of block When exposed to 500 °C temperature, internal structure of concretes still compact, but the pores in the Ca(OH)2 and C-S-H gels start to increase When the elevated temperature was 750 °C, the deterioration in the Ca(OH)2 and C-S-H gels emerges in the internal structure of concretes Particularly, when compared to 500 °C temperature, the cracks and pores in 3.5.2 The effect of elevated temperature on the interface structure The interfacial zones are the weakest bond of the concrete and the cracks development commonly reveal in the interfacial zones between the aggregate particles and cementitious matrix [38] The weakening of bonding strength in these zones due to the elevated temperature causes major decrease in the mechanical properties of concrete [38,40] Hence, the bonding strength in the zones has a significant effect on these properties at elevated temperature [38] Because of this, in this part, the interfacial structure between aggregate particles and cementitious matrix is examined The interfacial structure between aggregate particles and cementitious matrix of the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to 25 °C, 500 °C and 750 °C temperatures were examined by using 252 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Fig XRD analyses of HSCs exposed to 25 °C and 750 °C temperatures: a) control concrete, b) concrete containing 5GP and b) concrete containing 5MK + 5GP PLM image analyses as seen in Fig 12 The changes of cracks, spaces and deterioration on the interfacial structure of these concretes were evaluated in these image analyses As seen from the image analyses, no cracks, spaces and deterioration were 253 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Table The chemical components of concretes by X-ray fluorescence Table The mineralogical structures of concretes by semi-quantitative analysis Component (%) C 750 °C C 5GP 750 °C 5GP 5MK + 5GP 750 °C 5MK + 5GP Mineralogy C 750 °C C 5GP 750 °C 5GP 5MK + 5GP 750 °C 5MK + 5GP SiO2 CaO Al2O3 Fe2O3 TiO2 MgO Na2O K2O SO3 LOIa Total 30.43 40.54 4.94 2.13 0.35 1.78 0.63 0.68 0.26 18.13 99.87 27.96 42.62 4.78 2.05 0.31 1.65 0.49 0.59 0.37 19.03 99.85 29.65 40.45 4.85 2.03 0.45 1.99 0.62 0.69 0.47 18.67 99.87 28.84 42.49 3.83 1.57 0.21 2.03 0.43 0.57 0.39 19.54 99.90 29.42 38.84 5.57 2.14 0.36 1.53 0.75 0.75 0.45 20.03 99.84 28.22 42.05 4.85 1.52 0.21 1.22 0.42 0.55 0.36 20.48 99.88 Calcite Quarts Albeit Feldspar C3S C2S C4AF Dolomite Hematite Amorphous Phase Other 52 20 5 1 50 18 5 2 53 21 2 1 51 19 5 2 52 22 4 – 49 21 5 2 3 5 4 a LOI = Loss on MK = Metakaolin ignition, C = Control concrete, GP = Ground pumice, C = Control concrete, GP = Ground pumice, MK = Metakaolin observed between the aggregate particles and cementitious matrix in the concretes not exposed to elevated temperature Besides, as seen from the image analyses of concretes subjected to normal temperature (25 °C) in Fig 12, the bonding strength between the aggregate particles and cementitious matrix was very strong However, the increase at the cracks, pores and deterioration and the decrease at the bonding strength between the aggregate particles and cementitious matrix were observed depending on the increase in the temperature The micro-cracks, micro-spaces, a little 25 oC (a) C Matrix C-S-H deterioration and bonding strength are revealed in the interfacial zones of the concretes when exposed to 500 °C temperature, while the macro-cracks, macro-spaces, significant deterioration and impaired bonding strength are shown in the interface zones of the concretes when exposed to 750 °C temperature The effect of GP and MK + GP contents on the interface structure between aggregate particles and the cementitious matrix decreased as the temperature increased Akca and Zihnioglu [37] investigated the colour changes, cracks and spalls of HSC exposed to elevated 25 oC (b) 5GP o C-S- 25 C H (c) 5MK+5GP Aggregate Aggregate Interface C-S-H Aggregate Interface Interface Matrix Matrix 500 oC (d) C 500 oC (e) 5GP 500 oC (f) 5MK+5GP Matrix Aggregate GP Aggregate Matrix Inter face Matrix Aggregate Interface C-S-H (g) C 750 oC Space Space Inter Aggregate face Cracks C-S-H Matrix Cracks Inter face C-S-H 750 oC (h) 5GP Inter face Cracks Matrix Crack s Aggregate Space 750 oC (i) 5MK+5GP C-S-H Matrix Aggregate Crack Interface s Fig 10 SEM micrographs of HSCs exposed to 25 °C, 500 °C and 750 °C temperatures Cracks 254 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 Fig 11 The effect of high temperature on the cementitious matrix Fig 12 The effect of high temperature on the interface temperatures They observed that the cracks were on the interface of the specimen heated to 600 °C and the porous were on the interface of specimens heated up to 900 C Akỗaửzoglu [38] reported that the interfacial structure between the aggregate particles and the cementitious matrix was deteriorated as the temperature increased M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 3.5.3 The effect of elevated temperature on the aggregate structure The quartz and crushed limestone were used as an aggregate in the concrete mixtures The oxidation, alteration and 255 cracks in aggregates of the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to 25 °C, 500 °C and 750 °C temperatures were Fig 13 The effect of high temperature on the aggregates Fig 14 The effect of high temperature on the mineralogy 256 M Saridemir et al / Construction and Building Materials 124 (2016) 244–257 investigated by using PLM image analyses as seen in Fig 13 The changes of the oxidation, alteration and cracks on the aggregate particles of these concretes were evaluated by these image analyses Some micro-cracks derived from the formation of the quartzes were appeared by PLM image analyses in the concrete specimens unexposed to elevated temperature (Fig 13) These cracks in the formation of the quartzes gradually increased depending on the increase in temperature Besides, the oxidation and alteration in the quartzes were observed when the concrete specimens exposed to 500 °C and 750 °C temperatures The micro-cracks in the crushed limestone aggregates were not appeared in the concrete specimens unexposed to elevated temperature However, the micro-cracks in the crushed limestone aggregates were appeared in the concrete specimens exposed to 500 °C temperature and expanded in the 750 °C temperature Besides, the oxidation and alteration in the crushed limestone aggregates were generally observed when the concrete specimens exposed to 750 °C temperatures Moreover, biotite, feldspar, ferromagnesium and plagioclase minerals situated in aggregates were appeared as more pronounced in the concrete specimens exposed to elevated temperature as seen in Fig 14 The oxidation occurred in some aggregates are known to be caused from the Fe2O3 minerals Akỗaửzoglu [38] and Ingham [39] revealed similar observations Akỗaửzoglu [38] stated that some cracks were at the surface of the quartzes used in the concrete specimen unexposed to the elevated temperature The author also stated that the increase of cracks, fractures and disintegrations in the quartzes depend on elevated temperatures Moreover, the author stated that the disintegration emerged in almost all of the quartzes in the concrete exposed to 600 °C temperature Ingham [39] stated that actual concrete colour depends on the types of aggregate used in the concrete The author expressed that the colour changes were most evident in the aggregates containing siliceous and were very little in the limestone and granite aggregates Besides, the author stated that the red colour change is a function of (oxidation) iron content in the aggregate and emerged at around 300 °C temperature Conclusions Based on the experimental study and microstructural analyses presented in the paper, the following results can be drawn from the study The enhancements of Upv, fc, ffs and fsts values of concretes containing MK + GP were proved comparatively better than control concrete (except for the concrete containing up to 20% MK + GP) However, no improvements were observed on concretes containing GP compared to control concrete (except concrete containing 5% GP) When regarding the concretes containing GP, and blend of MK + GP, 5% and 5% + 5% replacement were found to be the most effective substitution level for enhancing the concrete strength for all curing time The decrease in the Upv and fc values of concretes was importantly enormous when the concrete specimens exposed to temperatures higher than 500 °C The reason for the decrease in these values are the pores and deterioration in the Ca(OH)2 and C-S-H gels, changes in the morphology and formation of micro-cracks SEM analyses conducted on the concrete specimens confirmed the increase of porosity and deterioration in the Ca(OH)2 and C-S-H gels as the temperature increased PLM image analyses showed that the oxidation, alteration and cracks in the matrix, interface and aggregate structures of all concrete specimens gradually increased depending on exposed to elevated temperatures The crushed limestone aggregates exhibited higher resistance to oxidation, 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MPa and 16.7–37.6 MPa for control concretes and concretes containing GP, respectively They observed that the early and long-term fc values of concretes containing GP were lower than that of the