Science of Sintering, 52 (2020) 149-162doi: 612.086.3; 622.785; 676.017.2; 669.018.29Performances of Vermiculite and Perlite Based ThermalInsulation Lightweight ConcretesAnja Terzić1*), Jovica Stojanović2, Ljubiša Andrić2, Ljiljana Miličić1,Zagorka Radojević1Institute for Testing of Materials - IMS, VojvodeMišića Bl. 43, 11000 Belgrade,Serbia2Institute for Technology of Nuclear and other Mineral Raw Materials,Franchetd’Esperey 86, 11000 Belgrade, Serbia1Abstract:This experimental study was conducted with an aim to investigate the effect of theelevated temperature on the mineral phase composition, microstructure and mechanicalproperties of the thermal insulation lightweight concretes. The first group of experimentalconcretes was based on the expanded vermiculite and expanded perlite used as lightweightaggregates (in 65 wt%) in combination with either ordinary Portland cement or refractorycalcium aluminate cement. The mix-design of the second group of concretes comprisedstandard quartz aggregate, vermiculite or perlite as aggregate replacement (25 wt%) andbinder (PC or CAC). A total of 10 concrete mix-designs were fabricated in form of40 40 160 mm samples which were submitted to heat-treatment at 400 , 600 , 800 and1000 C upon standard 28-days period of curing and hardening. The changes in crystallinityand mineral phase composition induced by temperature were monitored by X-ray diffractiontechnique. Microstructural visualizations of the non-fired and fired concrete samples wereconducted by scanning electron microscopy accompanied with EDX analysis. The resultsindicated that despite the decrease in compressive strengths upon firing, investigatedlightweight concretes can be categorized both as thermal insulators and structural materials.Keywords: Sintering; Mineral phase composition; SEM; Mechanical properties,Construction materials.1. IntroductionModern civil engineering is continuously imposing new requests regarding thereduction in the weight of structural elements, as well as energy-efficiency and fire safety [13]. One of the solutions for these demands is the application of the lightweight concretes.These materials are characterized by low density and low thermal conductivity; and thereby agood thermal insulation [4-7]. Apart from suitable thermal characteristics, lightweightconcretes are often designed with available and affordable raw materials of primary orsecondary origin (e.g. expanded clay or vermiculite, perlite, pumice, coal ash, etc.) whichplaces them in group of low-cost building materials [8, 9].Aggregates characterized by specific weigh less than 1120 kg/m3 are generallyconsidered as lightweight resources for the concrete production [3]. Vermiculite and perlite*)Corresponding author:[email protected]

150A. Terzić et al.,/Science of Sintering,52(2020)149-162are among the most frequently used lightweight aggregates in civil engineering. Vermiculiteas a mica-like mineral with shiny “flakes” is formed during biotite hydrothermal alternationor phlogopite weathering [10]. It belongs to the phyllosilicate group of minerals. The lamellarstructure enables its lubricating properties which are being manifested at high temperatures.Thereby, vermiculite can be used as fireproof material or as lightweight porous filler for heatinsulating [11, 12]. The chemical composition of vermiculite is not altering during the thermalexpansion (only chemically bonded water is removed). The bulk density of thermallyexpanded vermiculite is at least 10 times smaller than its original volume, its thermalconductivity is low (0.04–0.12 W/mK) and melting point is relatively high (1240–1430 C)[13]. Perlite is amorphous siliceous volcanic glass which is usually formed during obsidianhydration. Perlite has relatively high water content and its volume can significantly expandunder the effect of heat [14]. Upon heating above 870 C, the volume of perlite can increase 4to 20 times of its original volume. As a consequence, the expanded perlite is characterized byporous structure, high water absorption, low density and good thermal insulation [15-17].The extensive studies of the application of vermiculite and perlite in various buildingcomposites highlighted that these raw materials have certain advantages as well asdisadvantages. For instance, the expended vermiculite used in 30-60 % replacement levelscontributed to the heat resistance and thermal stability of the cement mortar, despite theincrease in water absorption and compressive strength deterioration [18]. Gypsum plasterswith the addition of 10 and 20 % expanded vermiculite also exhibited decrease in the 28 dayscompressive strength; however the 28 days bending strength increased 16 and 35 %,respectively[19]. The addition of vermiculite in plasters led to a reduction in the Young’smodulus [20]. Also, 55 % reduction in the compressive strength of clay bricks upon firing at900 C was induced by the 10 % addition of vermiculite [21]. The compressive strengths ofconcretes were reduced as the replacement levels of natural sand with vermiculite increasedfrom 5 to 10 % [22-24]. Similar effect occurred in self-compacting mortars for hightemperature application when 10, 20, 30 and 40 % of raw vermiculite was incorporated in themix-design [25]. Vermiculite is often being mixed with other types of light-weight aggregatessuch as polystyrene [26], or combined with geopolymer to create lightweight compositepanels with improved thermal properties [27]. Similar to vermiculite addition, the expandedperlite (10 %) combined with 20 % of natural pozzolan improved the mechanical properties ofconcrete, while 10 % of pozzolan and 10 % of the perlite led to a reduction in the rate of NaClcorrosion [28]. Lightweight geopolymer concretes with fly ash, pumice and perlite producedcompressive strengths up to 10-50 MPa as their unit weights changed between 1250 and1700 kg/m3 [29]. Thermal conductivity and the over-all durability of concretes weresubstantially improved with the use of perlite [30-32].In this study, the properties of thermal insulation concretes have been investigated.Expanded vermiculite and perlite were used as lightweight aggregates in a high percentage(65 wt%) in the mix-design of concretes. Ordinary Portland cement and refractory calciumaluminate cement were alternated as binders. Thus prepared composite building materialswere compared with another group of thermal insulation concretes in which standard quartzsand has been replaced by either vermiculite or perlite (in 25 wt%). The changes in physicomechanical properties, mineral composition and microstructure of the designed concreteswere monitored in the thermal range from 20 to 1000 C.2. Materials and Experimental Procedures2.1. Raw materialsPerlite and vermiculite were alternated as aggregates in the thermal insulationconcretes. The grain-size distributions obtained by dry sieve analysis are given in Tab. I:

A. Trzić et al.,/Science of Sintering,52(2020)149-162151Tab. I Grain-size distributions of perlite and vermiculite aggregate.Size classVermiculitePerlite 5 (mm)- 5 0.3 (mm)-0.3 0 (mm)0.37 (wt%)92.85 (wt%)6.78 (wt%)0.34 (wt%)96.54 (wt%)3.13 (wt%)Chemical compositions of a) perlite: SiO2 74.4 %, Al2O3 15.4 %, Fe2O3 1.3 %,MgO 0.13 %, K2O 4.55 %, Na2O 3.38 %; and b) vermiculite: SiO2 41.2 %,Al2O3 14.94 %, CaO 3.95 %, Fe2O3 7.2 %, MgO 25.5 %, K2O 5.1 %, TiO2 1.4 %.Mineral compositions of vermiculite and perlite are provided in Fig 1.Fig. 1. XRD analysis of: a) vermiculite; b) perlite.Vermiculite (Fig. 1a) was identified in proportions larger than 90 % represented by itshigh layer stacking, i.e. intense reflections appearing in series between 2 and 11 . Thereflections located at 28 , and 36 , 48 are also characteristic for vermiculite mineral [13].Talc was present around 31 and 48 . Smectite and chlorite were present in traces. Perlitesample was highly amorphous, which is in agreement with the literature [16]. The onlydetected crystalline phases were muscovite, anorthite and quartz.Thermally induced behavior of the raw materials was monitored by differentialthermal analysis (Fig. 2).Both raw materials showed similar exothermic peaks above 500 C, i.e. vermiculite at557 C (Fig. 2a) and perlite at 552 C (Fig. 2b). The second exothermic effect appeared at1023 C. It is followed by a steep decreasing curve-line, meaning that both vermiculite andperlite were thermally stable up to 1000 C.

152A. Terzić et al.,/Science of Sintering,52(2020)149-162Fig. 2. DTA analysis of: a) vermiculite; b) perlite.2.2. Preparation of the experimental samplesTen concretes prepared for the experiment were based on Portland cement (PC; CEMI 42.5R, Lafarge) or calcium aluminate cement (CAC; Istra 40, Calucem). PC comprised thefollowing oxides:SiO2 21.82 %, Al2O3 6.59 %, CaO 62.36 %, Fe2O3 4.75 %, MgO 2.17 %, K2O 0.78 % and Na2O 0.19 %. The chemical composition of CAC was asfollows: SiO2 3.85 %, Al2O3 40.35 %, CaO 37.50 %, Fe2O3 16.28 %, MgO 0.96 %,K2O 0.15 %, Na2O 0.08 % and MnO 0.2 %. Quartz sand (SiO2 98 %; characteristicsaccording to DIN EN 191-1; Uberwacht FMPA, Baden Wurttenberg), vermiculite and perlitewere alternated as aggregates. The mix designs are provided in Tab. II.Tab. II Mix-designs of the experimental concretes.SamplePC (%) CAC (%) Quartz sand 30Vermiculite (%)65652525Perlite (%)65252525-Dry components, i.e. cement and aggregates, were homogenized in a laboratory panmixer for 120 s. The quantity of water was fixed at 10 % in all mixtures. The green mixtureswere poured into steel prismatic molds (40 40 160 mm) and then sealed in polyethylenebags to be preserved at 20 2 C and 95 5 % humidity for the following 48 hours. Uponremoving molds the samples maintained under the same conditions during the next 5 days.Until 28th day, the samples were stored at 20 2 C and 65 5 % humidity.Fully solidified 28-days-old samples were submitted to thermal treatment in alaboratory furnace at following temperatures: 400 , 600 , 800 and 1000 C. The rate ofheating was 100 C/h with 2 hours of delay upon reaching the targeted temperature.

A. Trzić et al.,/Science of Sintering,52(2020)149-1621532.3. Instrumental analysesRheology of green samples, i.e. workability (in mm) of green mixture was estimatedvia slump test using a flow table (ASTM C230). Bulk density (in kg/m3) was calculated as aquotient of concrete sample’s mass and its volume. Water absorption (in %) was determinedfrom the weight difference between dry and water-saturated samples previously immersed inboiling water for 2 hours. Compressive strength (in MPa) of hardened concrete samples wastested on an Amsler laboratory hydraulic press in accordance with SRPS EN 1015-12:2016.Tests for compressive strength were conducted on halves of experimental prisms (40 40 mmcross-sectional area). Testing was conducted on the 28-days-old solidified samples and onsamples after heating at 400 , 600 , 800 and 1000 C.Differential thermal analysis was conducted on pulverized samples of vermiculite andperlite. The testing temperature range was 25 -1000 C. Samples were placed in an aluminapan and heated at a constant heating rate of 10 C/min in a static air flow.The X-ray diffraction analysis was employed on vermiculite, perlite and pulverizedconcrete samples. The XRD patterns were obtained on a Philips PW-1710 automateddiffractometer using a Cu tube operating at 40 kV and 30 mA. The instrument was equippedwith a diffracted beam curved graphite monochromator and a Xe-filled proportional counter.The diffraction data were collected in 2θ Bragg angle range from 4 to 65o, counting for 1 s(qualitative identification) at every 0.02 o step. The divergence and receiving slits were fixed1 and 0.1, respectively. The analysis was conducted at 20 C in a stationary sample holder.The morphology of non-polished crushed concrete samples was analyzed on a JEOLJSM-6610LV (JEOL, Japan) scanning electron microscope (SEM) connected with an INCAenergy-dispersion X-ray analysis unit; EDX analytical system. An acceleration voltage of20 kV was used. The samples were coated with carbon.3. Results and discussionThe mineral phase compositions of the PC-P65, PC-V65, CAC-P65 and CACV65samples after heat treatment at 600 C are presented in Fig. 3.The phase composition of the PC-P65 sample (Fig. 3a) is as follows: Ca3SiO5 (C3S alite, JCPDS-49-0442 ), β-Ca2SiO4 (β-C2S - belite, JCPDS-49-1673), CaSiO3 (W wollastonite, JCPDS-84-0655), SiO2 (Q - quartz, JCPDS-46-1045), Ca(OH)2 (CaH portlandite, JCPDS-72-0156), CaCO3 (K- calcite, JCPDS-47-1743), Fe1-xS (Py - pyrrhotite,JCPDS-75-0601) and Fe3O4 (M - magnetite, JCPDS-89-0951). The most abundant phaseswere cement minerals alite and belite, and mineral calcite. Quartz was less abundant, while allremaining phases were detected in traces. The crystallinity degree of all present phases waslow. Alite and belite, as the main products of Portland cement hydration, accompanied bysmaller amounts of portlandite were still present after the heat treatment at 600 C. Therebythe created hydration-bonds between cement particles in the concrete samples were stillactive. The reflections corresponding to alite and belite phases had the highest crystallinity(up to 25 arbitrary units – a.u.). These reflections are situated in 25 -35 section, but they aresignificantly overlapped and superposed with other mineral phases. Wollastonite as a calciuminosilicate mineral (CaSiO3) appeared in the mineral phase composition of PC-P65 as a resultof limestone (calcite) reactions induced by the increasing temperature. Pyrrhotite, an ironsulfide mineral, is similar to pyrite, however pyrrhotite is weakly magnetic. Its presence isprobably related to magnetite. Powder Diffraction File/Cards.Joint Committee on Powder DiffractionStandards (JCPDS),Swarthmore, PA.

154A. Terzić et al.,/Science of Sintering,52(2020)149-162Fig. 3. XRD of concrete samples after heat treatment at 600 C: a) PC-P65; b) PC-V65; c)CAC-P65; d) CAC-V65.Both Fe minerals were detected in traces, and they can be related to the impuritiespresent in perlite and small quantity of iron oxide present in Portland cement (usually up to5 %).In the PC-V65 sample (Fig. 3b) the following mineral phases were identified: alite,belite, tricalcium aluminate (C3A - Ca3Al2O6, JCPDS-38-1429), vermiculite (V, JCPDS-160613), muscovite (M - KAl2(Si3,Al)O10(OH)2, JCPDS-06-0263), mica (Mi), quartz,portlandite, calcite, pyrrhotite and magnetite. The most abundant phases were cementminerals, followed by calcite and vermiculite with muscovite, and lesser amounts of quartz.Other phases were hardly traceable. Crystallinity degree of all present phases was extremelylow. The crystallinity of the main cement minerals whose reflections were identified in 25 35 section was lower than in the PC-P65 sample counting up to only 18 a.u. Pyrrhotite andmagnetite, formed in reactions that included impurities from vermiculite and Fe2O3 fromPortland cement were present in traces. Vermiculite as a hydrated laminar mineral(aluminum-iron magnesium silicate) was accompanied by muscovite and mica. Theseminerals originated from the lightweight aggregate used in this concrete.The sample CAC-P65 (Fig. 3c) comprised refractory cement; therefore the phasecomposition was slightly different than those of previous two samples. Two cement minerals– alite (C3S) and tricalcium aluminate (C3A) were the most abundant phases, but theircrystallinity was extremely low (10 a.u. in the 25 -35 area). The hydration bonds in thecement were still present at this point. Portlandite and calcite, also originating from therefractory cement, were less abundant. Pyrrhotite and magnetite were hardly traceable, but

A. Trzić et al.,/Science of Sintering,52(2020)149-162155Fe2O3 (H - hematite, JCPDS-87-1164) was detected in higher amounts. This can be explainedby increased amount of iron oxide in CAC: 16.28 %. Also andradite (An - Ca3Fe2(SiO4)3,JCPDS-89-4378) was identified in relatively high quantity. This nesosilicate mineral ischaracterized by very high hardness (up to 7.5. on Mohs hardness scale).Fig. 4. XRD of the concrete samples after heat treatment at 1000 C: a) PC-P65; b) PC-V65;c) CAC-P65; d) CAC-V65.The sample CAC-V65 (Fig. 3d) comprised the following mineral phases: alite, belite,tricalcium aluminate, vermiculite, muscovite, quartz, portlandite, calcite, andradite, magnetiteand hematite. The cement minerals were matching to those of CAC-P65 regarding theirabundance and crystallinity. Andradite was present in this concrete sample, too. Thevermiculite and muscovite minerals originated from the lightweight aggregate.The mineral phase compositions of the samples PC-P65, PC-V65, CAC-P65 andCAC-V65 after heat treatment at 1000 C are presented in Fig. 4.Upon heat-treatment at 1000 C, the majority of hydraulic bonds in cement werereplaced by ‘chemical’ bonds. This means that new high temperature mineral phases werecreated as a result of the concrete sintering. Namely, in the PC-P65 sample (Fig. 4a), alite(C3S) and belite(β-C2S) were still present as the most abundant crystalline phases. Theintensity of the main alite belite superposed reflection at 33 was 25 a.u. Quartz and Kfeldspar (Kf - KAlSi3O8, JCPDS-89-1455) were identified in small amounts. Pyrrhotite andhematite were detected in traces. Gehlenite (G - Ca2Al2SiO7, JCPDS-89-6887) was detectedas a new mineral phase. Gehlenite is a sorosilicate with a high melting point: 1593 C.The sample PC-V65 (Fig. 4b) had similar mineral phase composition: alite, belite,quartz, gehlenite, K-feldspar, pyrrhotite and hematite. Besides glassy phase, the mostabundant crystal phases were cement minerals. All other phases were present in smallamounts. The intensity of the main alite belite reflection at 33 was 20 a.u. Very small

156A. Terzić et al.,/Science of Sintering,52(2020)149-162differences between the diffractograms of PC-P65 and PC-V65 samples can be noticed. Thissuggests that the employed aggregates did not make the prevailing influence on the sinteringof concrete; actually the applied cement type proved to be significantly more influential.The X-ray diffractograms of CAC-P65 and CAC-V65 samples (Fig. 4c, d) showedhigh similarities. However, the identified mineral phases significantly varied from PC-basedsamples due to the differences in the sintering mechanisms of Portland cement and calciumaluminate cement. Both CAC-P65 and CAC-V65 comprised: gehlenite as predominant hightemperature mineral, andradite and pyroxene/diopside (D - Ca(Mg,Fe)(Si,Al)2O6, JCPDS-890831). Diopside is also characterized by its high melting point (1391 C). These phases werethe most abundant. Less abundant were: olivine (O - (Fe,Mg)2SiO4, JCPDS-88-2001) andakermanite (A - Ca2MgSi2O7, JCPDS-87-0046). Depending on the amount of forsterite inolivine, its melting point can be as high as 1900 C. Akermanite is usually associated withgehlenite, and its melting point is approximately 1380 ºC. Magnetite and hematite weredetected in traces.The differences in mineral compositions of PC-Q-V25 and CAC-Q-V25 samplesupon heating at 600 C as a result of the applied type of cement are illustrated in Fig. 5.Fig. 5. XRD of concrete samples after heat treatment at 600 C: a) PC-Q-V25; b) CAC-Q-25.The phase composition of the PC-Q-V25 sample (Fig. 5a) is: alite, belite, tricalciumaluminate, quartz, calcite and magnetite. The most abundant phase was quartz whichoriginated from the prevailing SiO2 aggregate used in this concrete. Cement minerals arerelatively abundant, while all other phases are much lesser present. CAC-Q-V25 sample (Fig.5b) comprised: alite, belite, tricalcium aluminate, quartz, calcite, muscovite and magnetite.Calcium related phases are comparatively more present in this sample than in PC-Q-V25, dueto the higher CaO content in CAC cement.The XRD diffractograms of the PC-Q-P25, PC-Q-V25, CAC-Q-P25 and CAC-Q-V25samples upon heating at 1000 C are given in Fig. 6.The phase composition of PC-Q-P25 (Fig. 6a) sample: C3S - alite, β-C2Si - belite,C3A, quartz, gehlenite, K-feldspar, Ca5(PO4)3(OH) (apatite, JCPDS-49-0442). Apatite (A)belongs to a group of phosphate minerals with a high melting point (1660 C). The mostabundant phases were quartz and gehlenite, followed by cement minerals. K-feldspar wasdetected in minor quantities. Apatite was identified in small quantities. The PC-Q-V25 samplewith vermiculite aggregate comprised a highly similar mineral phase composition (alite,belite, C3A, quartz, gehlenite, K-feldspar, and muscovite) due to the application of the samecement type (Fig. 6b). Upon sintering at 1000 C, the refractory cement based sample CACQ-P25 (Fig. 6c) was composed of the following minerals: alite, belite, C3A quartz, calcite,magnetite, gehlenite, K-feldspar. Quartz and gehlenite were the most abundant phases.

A. Trzić et al.,/Science of Sintering,52(2020)149-162157Similarly, the phase composition of CAC-Q-V25 (Fig 6.d) was C3A, quartz, calcite,muscovite, gehlenite. Quartz, gehlenite and muscovite were the most abundant phases.Fig. 6. XRD of the concrete samples after heat treatment at 1000 C: a) PC-Q-P25; b) PC-QV25; c) CAC-Q-P25; d) CAC-Q-V25.It can be noticed that sintering process was governed by the type of cement – whetherit is Portland cement or calcium aluminate cement. Impurities originating from the usedaggregates might be involved in the reactions, but the quantities of formed mineral phaseswere low and therefore they could not reduce the quality of the thermal insulation concrete.Upon comparison of the mineral phases present at 600 C and those identified at 1000 C, itcan be noticed that hydration bond in cement was replaced by chemical bond during thisinterval which resulted in a number of mineral phases with high melting points.The physico-mechanical properties of the concretes measured/determined at ambienttemperature are provided in Tab. III.Tab. IIIPhysico-mechanical properties of the experimental concretes.WorkWaterBulk density ofBulk density ofabilityabsorptiongreen samplessolidifiedSample(mm)(%)(kg/m3)samples 5201475CAC-Q-V2514527.6014901460Compressive 4527.1025.30

A. Terzić et al.,/Science of Sintering,52(2020)149-162158The consistency (i.e. workability) of the concrete samples that included expandedvermiculite and perlite as aggregates (or aggregate replacements) was drier than consistencyof PCC and CACC concretes due to the increased porosity and higher requirements for water.Consequently, PC-P65, PC-V65, CAC-P65 and CAC-V65 exhibited approximately 6 timeshigher water absorption values in comparison with PCC and CACC. Similarly, PC-Q-P25,PC-Q-V25, CAC-Q-P25 and CAC-Q-V25 had higher water absorptions than PCC andCACC, but the values were lower than those of concretes with the expanded vermiculite orperlite as aggregates. Bulk densities in dry condition (i.e. bulk densities of the solidifiedsamples measured after 28 days) were lower than bulk density of the standard PC concrete.However, all bulk densities were below 1900 kg/m3, which categorized these concretes aslightweight. Furthermore, bulk densities being higher than 800 kg/m3 refer to the fact that theinvestigated concretes can be used both as insulation materials and structural materials. Thecomplete replacement of standard aggregate with lightweight aggregate influenced a decreasein the 28-days compressive strengths, i.e. compressive strengths of PC-P65 and PC-V65 were3 and 6 times lower, respectively, than compressive strength of PCC. When refractory cement(CAC) was used in combination with lightweight aggregates, the decrease in compressivestrengths was approximately 4.5 times. The concretes with quartz aggregate and expandedvermiculite/perlite as aggregate replacement exhibited 20 % higher compressive strengthsthan concreters that comprised lightweight aggregate solely.The decrease in compressive strengths of investigated concretes induced by theincreasing temperature is illustrated in Fig. 7. The concrete with perlite aggregate (PC-P65)exhibited a lesser strength deterioration than PC-V65. The compressive strengths of concretesprepared with refractory cement (CAC-P65 and CAC-V65) underwent very small changeswith the increasing temperature. The combination PC quartz aggregate perlite/vermiculiteaggregate replacement exhibited significant decrease in compressive strength, unlikecorresponding concretes prepared with refractory CAC cement.50Temperature ( C)40045600Compressive strength (MPa)40800100035302520151050PCCCACCPC-P65 PC-V65 CAC-P65 CAC-V65 PC-QP25PC-QV25CAC-Q- CAC-QP25V25Fig. 7. Compressive strengths of concretes determined after firing at 400 , 600 , 800 and1000 C.The SEM microphotographs ( 500 and 150 recording magnification) of the samplesCAC-P65 and CAC-Q-P25, which showed the smallest compressive strength variations, areprovided in Fig. 8. EDX chemical analyses of the concrete samples are given in Tab. IV.

A. Trzić et al.,/Science of Sintering,52(2020)149-162159Tab. IV EDX chemical analyses of the concretes illustrated in the SEM microphotographs.Oxide (%)Al2O3SiO2CaO Fe2O3 TiO2 Na2O K2O23.1053.57 11.79 4.930.55 2.533.16CAC-P65 (20 C)37.5413.96 32.32 13.22 1.28 0.830.85CAC-P65 (1000 C)60.82 8.974.660.40 2.583.70CAC-Q-P25 (20 C) 18.88CAC-Q-P2541.9320.94 25.93 8.320.63 1.081.16(1000 C)Fig. 8.SEM microphotographs of: a and b) CAC-P65 at 20 C; c and d) CAC-P65 at 1000 C;e and f) CAC-Q-P25 at 20 C; g and h) CAC-Q-P25 at 1000 C.Two samples of CAC-P65 concrete are compared in Fig.8a-d. The sample in Fig. 8bis fully hydrated and solidified at ambient temperature. The other sample (Fig. 8d) wasrecorded upon firing at 1000 C. In Fig. 8a, a magnified perlite grain is illustrated. The

160A. Terzić et al.,/Science of Sintering,52(2020)149-162characteristic lamellar structure of the expanded perlite composed of thin “flakes” can beseen. The crystallinity of perlite is extremely low, as it was showed by XRD analysis (Fig 1b).Thin flakes have an amorphous structure with no pores. However, flakes are aligned intolaminas by such leaving the vacant spaces between singular flakes. This lamellar compositionrepresents a base of expanded perlite grain porous structure. In Fig. 8b, a characteristic clusterof several perlite grains is noticed. This section has significantly increased porosity incomparison with the rest of the cementitious sample. In Fig. 8c-d, the structure is morehomogenous and characterized by the absence of pores due to the sintering. The structure ofthe CAC-Q-P25 sample is significantly less porous than CAC-P65. Small inclusions of flakyperlite structures are visible in the microphotographs recorded before and after sintering. Thechanges in the mineral phase composition (e.g. formation of high temperature phases likegehlenite) previously identified by XRD are highlighted by visible differences in the chemicalcomposition of the samples CAC-P65 and CAC-Q-P25 detected by EDX analyses (Tab. IV)prior to and upon sintering.4. ConclusionInfluence of elevated temperature (400-1000 C) on the mineral phase compositions,microstructure and mechanical properties of thermal insulation concretes was investigated inthis study. Two groups of concretes were successfully fabricated: 1) concretes based on theexpanded vermiculite or perlite as lightweight aggregates; and 2) concretes based on quartzaggregate with vermiculite or perlite used as the aggregate replacement. Performances havebeen compared to those of standard-weight concretes based on Portland cement or refractorycalcium aluminate cement as binders and quartz aggregate.The sintering process was governed mainly by the type of cement. Impuritiesoriginating from lightweight aggregates were involved in the high-temperature reactions, butthe quantities of newly formed mineral phases were scarce. Therefore, the employedaggregates did not make the prevailing influence on the sintering of concrete, instead theapplied cement type proved to be significantly more influential. The hydration bonds incement were replaced by chemical bonds during 600 -1000 C interval. At 1000 C, anumber of mineral phases with high melting points were detected (e.g. gehlenite).The bulk densities of investigated concretes were below 1900 kg/m3, whichcategorized these concretes as lightweight. The complete replacement of standard aggregatewith lightweight aggregate influenced a decrease in the 28-days compressive strengths (3-6times for PC concretes and 4.5 times for concretes with refractory cement). The concreteswith quartz aggregate and expanded vermiculite/perlite as aggregate replacement exhibited20 % higher compressive strengths than concreters that comprised lightweight aggregatesolely. The compressive strength of concretes prepared with refractory cement andlightweight aggregate underwent very small changes with the increasing temperature. Theresults indicated that despite the decrease in compressive strengths upon firing, theinvestigated lightweight concretes can be categorized both as thermal insulators and structuralmaterials.AcknowledgmentsThis investigation is financially supported by Ministry of Education, Scienceand Technological Development of the Republic of Serbia.

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Thermal conductivity and the over-all durability of concretes were substantially improved with the use of perlite [30-32]. In this study, the properties of thermal insulation concretes have been investigated. Expanded vermiculite and perlite were used as lightweight aggregates in a