Performances of vermiculite and perlite based thermal insulation lightweight concretes

This experimental study was conducted with an aim to investigate the effect
 of the elevated temperature on the mineral phase composition, microstructure
 and mechanical properties of the thermal insulation lightweight concretes.
 The first group of experimental concretes was based on the expanded
 vermiculite and expanded perlite used as lightweight aggregates (in 65 wt%)
 in combination with either ordinary Portland cement or refractory calcium
 aluminate cement. The mix-design of the second group of concretes comprised
 standard quartz aggregate, vermiculite or perlite as aggregate replacement
 (25 wt%) and binder (PC or CAC). A total of 10 concrete mix-designs were
 fabricated in form of 40?40?160 mm samples which were submitted to
 heat-treatment at 400?, 600?, 800? and 1000?C upon standard 28-days period
 of curing and hardening. The changes in crystallinity and mineral phase
 composition induced by temperature were monitored by X-ray diffraction
 technique. Microstructural visualizations of the non-fired and fired
 concrete samples were conducted by scanning electron microscopy accompanied
 with EDX analysis. The results indicated that despite the decrease in
 compressive strengths upon firing, investigated lightweight concretes can be
 categorized both as thermal insulators and structural materials.


Introduction
Modern civil engineering is continuously imposing new requests regarding the reduction in the weight of structural elements, as well as energy-efficiency and fire safety [1][2][3]. 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 a good thermal insulation [4][5][6][7]. Apart from suitable thermal characteristics, lightweight concretes are often designed with available and affordable raw materials of primary or secondary origin (e.g. expanded clay or vermiculite, perlite, pumice, coal ash, etc.) which places them in group of low-cost building materials [8,9].
Aggregates characterized by specific weigh less than 1120 kg/m 3 are generally considered as lightweight resources for the concrete production [3]. Vermiculite and perlite are among the most frequently used lightweight aggregates in civil engineering. Vermiculite as a mica-like mineral with shiny "flakes" is formed during biotite hydrothermal alternation or phlogopite weathering [10]. It belongs to the phyllosilicate group of minerals. The lamellar structure 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 heat insulating [11,12]. The chemical composition of vermiculite is not altering during the thermal expansion (only chemically bonded water is removed). The bulk density of thermally expanded vermiculite is at least 10 times smaller than its original volume, its thermal conductivity 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 obsidian hydration. Perlite has relatively high water content and its volume can significantly expand under the effect of heat [14]. Upon heating above 870 °C, the volume of perlite can increase 4 to 20 times of its original volume. As a consequence, the expanded perlite is characterized by porous structure, high water absorption, low density and good thermal insulation [15][16][17].
The extensive studies of the application of vermiculite and perlite in various building composites highlighted that these raw materials have certain advantages as well as disadvantages. For instance, the expended vermiculite used in 30-60 % replacement levels contributed to the heat resistance and thermal stability of the cement mortar, despite the increase in water absorption and compressive strength deterioration [18]. Gypsum plasters with the addition of 10 and 20 % expanded vermiculite also exhibited decrease in the 28 days compressive 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's modulus [20]. Also, 55 % reduction in the compressive strength of clay bricks upon firing at 900 °C was induced by the 10 % addition of vermiculite [21]. The compressive strengths of concretes were reduced as the replacement levels of natural sand with vermiculite increased from 5 to 10 % [22][23][24]. Similar effect occurred in self-compacting mortars for hightemperature application when 10, 20, 30 and 40 % of raw vermiculite was incorporated in the mix-design [25]. Vermiculite is often being mixed with other types of light-weight aggregates such as polystyrene [26], or combined with geopolymer to create lightweight composite panels with improved thermal properties [27]. Similar to vermiculite addition, the expanded perlite (10 %) combined with 20 % of natural pozzolan improved the mechanical properties of concrete, while 10 % of pozzolan and 10 % of the perlite led to a reduction in the rate of NaCl corrosion [28]. Lightweight geopolymer concretes with fly ash, pumice and perlite produced compressive strengths up to 10-50 MPa as their unit weights changed between 1250 and 1700 kg/m 3 [29]. Thermal conductivity and the over-all durability of concretes were substantially improved with the use of perlite [30][31][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 calcium aluminate cement were alternated as binders. Thus prepared composite building materials were compared with another group of thermal insulation concretes in which standard quartz sand has been replaced by either vermiculite or perlite (in 25 wt%). The changes in physicomechanical properties, mineral composition and microstructure of the designed concretes were monitored in the thermal range from 20° to 1000 °C.

Raw materials
Perlite and vermiculite were alternated as aggregates in the thermal insulation concretes. The grain-size distributions obtained by dry sieve analysis are given in Tab  Vermiculite ( Fig. 1a) was identified in proportions larger than 90 % represented by its high layer stacking, i.e. intense reflections appearing in series between 2° and 11°. The reflections 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. Perlite sample was highly amorphous, which is in agreement with the literature [16]. The only detected crystalline phases were muscovite, anorthite and quartz.
Thermally induced behavior of the raw materials was monitored by differential thermal analysis (Fig. 2).

Preparation of the experimental samples
Dry components, i.e. cement and aggregates, were homogenized in a laboratory pan mixer for 120 s. The quantity of water was fixed at 10 % in all mixtures. The green mixtures were poured into steel prismatic molds (40×40×160 mm) and then sealed in polyethylene bags to be preserved at 20 ± 2 °C and 95 ± 5 % humidity for the following 48 hours. Upon removing molds the samples maintained under the same conditions during the next 5 days. Until 28 th 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 a laboratory furnace at following temperatures: 400°, 600°, 800° and 1000 °C. The rate of heating was 100 °C/h with 2 hours of delay upon reaching the targeted temperature.

Instrumental analyses
Rheology of green samples, i.e. workability (in mm) of green mixture was estimated via slump test using a flow table (ASTM C230). Bulk density (in kg/m 3 ) was calculated as a quotient of concrete sample's mass and its volume. Water absorption (in %) was determined from the weight difference between dry and water-saturated samples previously immersed in boiling water for 2 hours. Compressive strength (in MPa) of hardened concrete samples was tested 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 mm cross-sectional area). Testing was conducted on the 28-days-old solidified samples and on samples after heating at 400°, 600°, 800 ° and 1000 °C.
Differential thermal analysis was conducted on pulverized samples of vermiculite and perlite. The testing temperature range was 25°-1000 °C. Samples were placed in an alumina pan 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 pulverized concrete samples. The XRD patterns were obtained on a Philips PW-1710 automated diffractometer using a Cu tube operating at 40 kV and 30 mA. The instrument was equipped with 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 65 o , counting for 1 s (qualitative identification) at every 0.02 o step. The divergence and receiving slits were fixed 1 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 JEOL JSM-6610LV (JEOL, Japan) scanning electron microscope (SEM) connected with an INCA energy-dispersion X-ray analysis unit; EDX analytical system. An acceleration voltage of 20 kV was used. The samples were coated with carbon.

Results and discussion
The mineral phase compositions of the PC-P65, PC-V65, CAC-P65 and CAC-V65samples after heat treatment at 600 °C are presented in Fig. 3.
The phase composition of the PC-P65 sample ( . The most abundant phases were cement minerals alite and belite, and mineral calcite. Quartz was less abundant, while all remaining phases were detected in traces. The crystallinity degree of all present phases was low. Alite and belite, as the main products of Portland cement hydration, accompanied by smaller amounts of portlandite were still present after the heat treatment at 600 °C. Thereby the created hydration-bonds between cement particles in the concrete samples were still active. 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 are significantly overlapped and superposed with other mineral phases. Wollastonite as a calcium inosilicate mineral (CaSiO 3 ) appeared in the mineral phase composition of PC-P65 as a result of limestone (calcite) reactions induced by the increasing temperature. Pyrrhotite, an iron sulfide mineral, is similar to pyrite, however pyrrhotite is weakly magnetic. Its presence is probably related to magnetite. Both Fe minerals were detected in traces, and they can be related to the impurities present in perlite and small quantity of iron oxide present in Portland cement (usually up to 5 %).
In the PC-V65 sample (Fig. 3b) the following mineral phases were identified: alite, belite, tricalcium aluminate (C 3 A -Ca 3 Al 2 O 6 , JCPDS-38-1429), vermiculite (V, JCPDS-16-0613), muscovite (M -KAl 2( Si 3 ,Al)O 10 (OH) 2 , JCPDS-06-0263), mica (Mi), quartz, portlandite, calcite, pyrrhotite and magnetite. The most abundant phases were cement minerals, followed by calcite and vermiculite with muscovite, and lesser amounts of quartz. Other phases were hardly traceable. Crystallinity degree of all present phases was extremely low. 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 and magnetite, formed in reactions that included impurities from vermiculite and Fe 2 O 3 from Portland cement were present in traces. Vermiculite as a hydrated laminar mineral (aluminum-iron magnesium silicate) was accompanied by muscovite and mica. These minerals originated from the lightweight aggregate used in this concrete.
The sample CAC-P65 (Fig. 3c) comprised refractory cement; therefore the phase composition was slightly different than those of previous two samples. Two cement minerals -alite (C 3 S) and tricalcium aluminate (C 3 A) were the most abundant phases, but their crystallinity was extremely low (10 a.u. in the 25°-35° area). The hydration bonds in the cement were still present at this point. Portlandite and calcite, also originating from the refractory cement, were less abundant. Pyrrhotite and magnetite were hardly traceable, but  The sample CAC-V65 (Fig. 3d) comprised the following mineral phases: alite, belite, tricalcium aluminate, vermiculite, muscovite, quartz, portlandite, calcite, andradite, magnetite and hematite. The cement minerals were matching to those of CAC-P65 regarding their abundance and crystallinity. Andradite was present in this concrete sample, too. The vermiculite and muscovite minerals originated from the lightweight aggregate.
The mineral phase compositions of the samples PC-P65, PC-V65, CAC-P65 and CAC-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 were replaced by 'chemical' bonds. This means that new high temperature mineral phases were created as a result of the concrete sintering. Namely, in the PC-P65 sample (Fig. 4a), alite (C 3 S) and belite(β-C 2 S) were still present as the most abundant crystalline phases. The intensity of the main alite + belite superposed reflection at 33° was 25 a.u. Quartz and Kfeldspar (Kf -KAlSi 3 O 8, JCPDS-89-1455) were identified in small amounts. Pyrrhotite and hematite were detected in traces. Gehlenite (G -Ca 2 Al 2 SiO 7 , JCPDS-89-6887) was detected as 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 most abundant crystal phases were cement minerals. All other phases were present in small amounts. The intensity of the main alite + belite reflection at 33° was 20 a.u. Very small differences between the diffractograms of PC-P65 and PC-V65 samples can be noticed. This suggests that the employed aggregates did not make the prevailing influence on the sintering of 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) showed high similarities. However, the identified mineral phases significantly varied from PC-based samples due to the differences in the sintering mechanisms of Portland cement and calcium aluminate cement. Both CAC-P65 and CAC-V65 comprised: gehlenite as predominant high temperature mineral, andradite and pyroxene/diopside  The phase composition of the PC-Q-V25 sample (Fig. 5a) is: alite, belite, tricalcium aluminate, quartz, calcite and magnetite. The most abundant phase was quartz which originated from the prevailing SiO 2 aggregate used in this concrete. Cement minerals are relatively 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, due to the higher CaO content in CAC cement.
Similarly, the phase composition of CAC-Q-V25 (Fig 6.d) was C 3 A, quartz, calcite, muscovite, gehlenite. Quartz, gehlenite and muscovite were the most abundant phases. It can be noticed that sintering process was governed by the type of cement -whether it is Portland cement or calcium aluminate cement. Impurities originating from the used aggregates might be involved in the reactions, but the quantities of formed mineral phases were 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, it can be noticed that hydration bond in cement was replaced by chemical bond during this interval which resulted in a number of mineral phases with high melting points.
The physico-mechanical properties of the concretes measured/determined at ambient temperature are provided in Tab. III.
Tab. IIIPhysico-mechanical properties of the experimental concretes.
The consistency (i.e. workability) of the concrete samples that included expanded vermiculite and perlite as aggregates (or aggregate replacements) was drier than consistency of 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 times higher 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 and CACC, but the values were lower than those of concretes with the expanded vermiculite or perlite as aggregates. Bulk densities in dry condition (i.e. bulk densities of the solidified samples measured after 28 days) were lower than bulk density of the standard PC concrete. However, all bulk densities were below 1900 kg/m 3 , which categorized these concretes as lightweight. Furthermore, bulk densities being higher than 800 kg/m 3 refer to the fact that the investigated concretes can be used both as insulation materials and structural materials. The complete replacement of standard aggregate with lightweight aggregate influenced a decrease in the 28-days compressive strengths, i.e. compressive strengths of PC-P65 and PC-V65 were 3 and 6 times lower, respectively, than compressive strength of PCC. When refractory cement (CAC) was used in combination with lightweight aggregates, the decrease in compressive strengths was approximately 4.5 times. The concretes with quartz aggregate and expanded vermiculite/perlite as aggregate replacement exhibited 20 % higher compressive strengths than concreters that comprised lightweight aggregate solely.
The decrease in compressive strengths of investigated concretes induced by the increasing 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 concretes prepared with refractory cement (CAC-P65 and CAC-V65) underwent very small changes with the increasing temperature. The combination PC + quartz aggregate + perlite/vermiculite aggregate replacement exhibited significant decrease in compressive strength, unlike corresponding concretes prepared with refractory CAC cement.    Two samples of CAC-P65 concrete are compared in Fig.8a-d. The sample in Fig. 8b is fully hydrated and solidified at ambient temperature. The other sample (Fig. 8d) was recorded upon firing at 1000 °C. In Fig. 8a, a magnified perlite grain is illustrated. The characteristic lamellar structure of the expanded perlite composed of thin "flakes" can be seen. 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 into laminas by such leaving the vacant spaces between singular flakes. This lamellar composition represents a base of expanded perlite grain porous structure. In Fig. 8b, a characteristic cluster of several perlite grains is noticed. This section has significantly increased porosity in comparison with the rest of the cementitious sample. In Fig. 8c-d, the structure is more homogenous and characterized by the absence of pores due to the sintering. The structure of the CAC-Q-P25 sample is significantly less porous than CAC-P65. Small inclusions of flaky perlite structures are visible in the microphotographs recorded before and after sintering. The changes in the mineral phase composition (e.g. formation of high temperature phases like gehlenite) previously identified by XRD are highlighted by visible differences in the chemical composition of the samples CAC-P65 and CAC-Q-P25 detected by EDX analyses (Tab. IV) prior to and upon sintering.

Conclusion
Influence of elevated temperature (400-1000 °C) on the mineral phase compositions, microstructure and mechanical properties of thermal insulation concretes was investigated in this study. Two groups of concretes were successfully fabricated: 1) concretes based on the expanded vermiculite or perlite as lightweight aggregates; and 2) concretes based on quartz aggregate with vermiculite or perlite used as the aggregate replacement. Performances have been compared to those of standard-weight concretes based on Portland cement or refractory calcium aluminate cement as binders and quartz aggregate.
The sintering process was governed mainly by the type of cement. Impurities originating from lightweight aggregates were involved in the high-temperature reactions, but the quantities of newly formed mineral phases were scarce. Therefore, the employed aggregates did not make the prevailing influence on the sintering of concrete, instead the applied cement type proved to be significantly more influential. The hydration bonds in cement were replaced by chemical bonds during 600 °-1000 °C interval. At 1000 °C, a number of mineral phases with high melting points were detected (e.g. gehlenite).
The bulk densities of investigated concretes were below 1900 kg/m 3 , which categorized these concretes as lightweight. The complete replacement of standard aggregate with lightweight aggregate influenced a decrease in the 28-days compressive strengths (3-6 times for PC concretes and 4.5 times for concretes with refractory cement). The concretes with quartz aggregate and expanded vermiculite/perlite as aggregate replacement exhibited 20 % higher compressive strengths than concreters that comprised lightweight aggregate solely. The compressive strength of concretes prepared with refractory cement and lightweight aggregate underwent very small changes with the increasing temperature. The results indicated that despite the decrease in compressive strengths upon firing, the investigated lightweight concretes can be categorized both as thermal insulators and structural materials.