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Article

Use of Lightweight Sintered Fly Ash Aggregates in Concrete at High Temperatures

by
Klára Křížová
*,
Jan Bubeník
and
Martin Sedlmajer
Faculty of Civil Engineering, Brno University of Technology, 602 00 Brno, Czech Republic
*
Author to whom correspondence should be addressed.
Buildings 2022, 12(12), 2090; https://doi.org/10.3390/buildings12122090
Submission received: 24 October 2022 / Revised: 18 November 2022 / Accepted: 24 November 2022 / Published: 29 November 2022
(This article belongs to the Special Issue Rehabilitation and Reconstruction of Buildings)
Browse Figures
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Abstract

:
This study addresses the issue of the resistance to high temperatures of lightweight concrete lightweighted with sintered fly ash aggregate. Lightweight concretes with different amounts of lightweighting and their properties after loading temperatures of 600, 800 and 1000 °C were investigated. In particular, the effect of high temperature on the mechanical properties of the concrete was determined on the test specimens, and the effect on the microstructure was investigated by X-ray diffraction analysis and scanning electron microscopy. It was found that there is an increase in compressive strength between 0 and 21% up to 800 °C, where the increase in strength decreases with increasing degree of lightening. At 1000 °C, the internal structure of the lightweight concrete destabilized, and the compressive strength decreased in the range of 51–65%. After loading at 1000 °C, the scanning electron microscope showed the formation of spherical-shaped neoplasms, which significantly reduced the internal integrity of the cement matrix in the lightweight concrete due to the increase in their volume. It was found that the lightweight concretes with higher lightweighting showed significantly less degradation due to higher temperature.

1. Introduction

The construction of sustainable and environmentally friendly structures and infrastructures has been one of the most important challenges in the construction industry for many years [1]. Consequently, the on-site replacement or partial replacement of traditional components such as Portland cement and natural quarried aggregates with alternative materials is considered an effective way to reduce the environmental impact of cement and concrete production (e.g., References [2,3,4,5,6,7]). Lightweight concrete (LWC) is increasingly applied in the modern construction industry, which makes extensive use of various types of both natural and artificial lightweight aggregates for production [8]. Lightweight concrete is a more energy efficient and environmentally friendly construction material [9] compared to ordinary concrete (OC) in the event that, for example, sintered fly ash aggregate is used for the production of LWC. Sintered fly ash aggregate is, in addition to other additives, primarily produced from fly ash, which is taken as a waste product generated in the power industry. From this point of view, it is possible to consider the LWC produced in this way as a more environmentally friendly building material. It is the use of manufactured aggregates (LWA) in LWCs that could be a suitable alternative to natural aggregates. The most common aggregates are based on fired sintered clays (expanded clay) or sintered fly ash (Agloporite) [8]. Various studies in the literature have investigated the mechanical properties of concrete made from sintered fly ash aggregates (e.g., References [9,10,11,12]). The large-scale utilization of fly ash for the production of fly ash aggregates, which is practiced in many countries, can reduce the increasing consumption and limit the depletion of natural resources of aggregates [1,7,13] and significantly reduce the environmental burden arising from their extraction. To give an idea, the production of 1 ton of natural aggregate is responsible for 0.129 tons of emissions [7] released into the atmosphere.
Due to its advantageous properties, including low bulk density, good thermal insulation properties and fire resistance [4,11], lightweight concrete has a good prerequisite for use as a structural and non-structural building material [4]. Concrete structures are affected by various physical, chemical and mechanical processes in the event of fire loads, leading to degradation of the material parameters of concrete and spalling of the concrete surface layers [14,15]. Concrete consists not only of a solid matrix (aggregate, hardened cement paste and chemically bound water), but also of capillary pores partly filled with liquid water (free or adsorbed on the pore walls), and partly with a gaseous mixture of dry air and water vapor. At high temperatures, therefore, a number of corresponding phenomena are induced, starting with the evaporation of free water. This generally triggers an increase in the heat capacity of the material and a loss of energy due to the consumption of latent heat [16]. The general fact that explosive spalling of the surface layers of concrete structures mainly occurs due to the accumulation of internal pressure in the concrete structure and the associated release of stored energy, leading to crack formation and concrete damage, has been demonstrated by several experiments (e.g., References [3,14,17]). The spalling effect is usually explained by one of two theories: (1) an increase in pore pressure and (2) thermal stresses, which may individually occur or together. In the first case, a thermal-hygral process accompanied by evaporation of water takes place under an increase in pore pressure. If the stress resulting from the pore pressure exceeds the tensile strength of the concrete, the concrete surface layers are cracked off. The second theory is a thermomechanical process controlled by thermal stresses induced by a thermal gradient. The resulting high compressive stresses, localized at the surface of the concrete, exceed the strength of the concrete, and spalling may occur. Typically, debonding is attributed to the combined action of the two mechanisms (thermal-hygral and thermomechanical). Thermochemical spalling has also been found to be related to the mechanisms of calcium hydroxide decomposition and calcium oxide rehydration [15,18].
With regard to the melting temperature of the different types of natural aggregates, the use of lightweight sintered aggregates is recommended for the production of concrete with fire resistance requirements. These types of aggregates are not prone to shrinkage, which could disturb the microstructure by forming cracks in the transit zone (ITZ). This phenomenon is particularly noticeable in the case of inappropriately used types of quartz aggregates or granite. When combining several types of aggregates, it is optimal to use aggregates with the same thermal expansion [19]. From the aggregate point of view, in general, an increase in aggregate grain volume occurs due to increasing temperature in the concrete structure. Natural aggregates release carbon dioxide from calcite at temperatures around 700–1000 °C, so the mineralogical composition of the aggregate influences the nature of aggregate behavior under thermal loading. Limestone aggregates show a more favorable behavior because they are able to consume more heat for endothermic decomposition compared to siliceous aggregates. On the other hand, siliceous aggregate is affected by the modification transformations of quartz accompanied by volumetric changes and the associated development of internal pressure in the ITZ [20].
This study investigates the partial replacement of natural aggregates of coarse fractions by artificial aggregates based on sintered fly ash (Agloporit). The aim is to reduce the impact of the negative effect of explosive spalling when concretes are exposed to high temperatures, while maintaining sufficient mechanical properties of LWC. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed to clarify the trends in mechanical properties during exposure to high temperatures.

2. Materials and Methods

The composition of the LWC lightweight concrete mixes was based on the replacement of part of the natural aggregate with artificially produced lightweight sintered fly ash aggregate Agloporit. Cement CEM II/B-M (S-LL) 32.5 R, was selected as the binder. The aggregate was composed of natural aggregate (sand) in the 0/4 mm fraction and coarse aggregate in the 4/8 mm fraction, while the fly-ash-based artificial aggregate was used in the 4/8 mm fraction.

2.1. Materials

2.1.1. Binder

Portland blended cement CEM II/B-M (S-LL) in strength class 32.5 (according to EN 197-1 [21]) was selected. The cement was taken from a silo for bulk material storage. The main properties of the cement used are show in Table 1 and Table 2.
To control the input parameters, the volumetric density of the cement was determined by pycnometric test (according to ČSN 72 2113 [24]).

2.1.2. Natural Aggregate

Excavated aggregate from a loose, uncovered landfill was selected as natural aggregate of fine fraction in 0/4 mm grain size. The aggregate was subjected to moisture testing according to EN 1097-5 [25], EN 933-1 sieve analysis [26] and determination of bulk density by pycnometry according to EN 1097-6 [27]. Subsequently, the aggregate was weighed by mass to an accuracy of Figure 1.

2.1.3. Sintered Fly Ash Aggregate

In the Czech Republic, there is no large-scale producer of sintered aggregates based on sintered fly ash compared to the surrounding states. Due to the fact that there currently are no real openings of new deposits of natural aggregates in the Czech Republic and in view of the global trend of reducing the extraction of natural aggregates, the production of alternative aggregates is necessary. The production of aggregates from fly ash is thus a suitable and possible alternative to the currently most commonly produced lightweight clay-based aggregate in the Czech Republic. For the purpose of the experiment, Agloporit aggregate from company Svoboda a syn s.r.o. was used. The quality and mineralogical composition of the produced aggregate mainly depends on the quality of the input raw materials. When using common fly ash from suppliers in the Czech Republic, the amorphous phase, mullite and silicon compounds are the most represented. SiO2 and Al2O3 predominate in about 53% and 30%, respectively. The main properties of aggregate used are shown in Table 3 and Figure 2 shown detail of concrete with Agloporite.
Table
Table 3. Main properties of used aggregate.
Table 3. Main properties of used aggregate.
PropertiesAgloporiteCoarse Natural Aggregate
Value
Particle dry density [kg/m3]1100–13002600–2700
Coefficient of thermal conductivity [W/(m·K)]0.12–0.161.4–1.7
Compressive strength (EN 13055 [28]) [MPa]5–9- *
Absorbency [%]28–32<2
* Compressive strength is not determined for natural aggregates. The degree of strength of the aggregate grain mainly depends on the origin/formation of the aggregate.

2.1.4. Water

Drinking water was used from the normal water supply system. The water was weighed by weight and added during the mixing of the concrete at certain stages. The water was used as pre-wetting water for the artificial aggregate (process water) and as mixing water to achieve the desired consistency of the fresh concrete.

2.1.5. Additives

Liquid chemical superplasticizing additives were added to the concrete to reduce the amount of mixing water and to achieve the desired consistency of fresh concrete.

2.1.6. Admixture

To supplement the cement used as a binder, an admixture in the form of silica fume was added to the concrete. Silica fume is characterized by its unique properties where it is able to thicken the microstructure of the concrete, resulting in an increase in the mechanical properties and durability of the concrete.

2.1.7. Fibers

Generally, fibers in the form of dispersed reinforcement are added to concretes produced to increase fire resistance. In this experiment, polypropylene (PP) fibers of 6 mm length were used. The PP fibers were weighed on a digital balance to the nearest 0.1 g and added to the dry mix (cement, silica fume, aggregate) in the mixer to achieve good distribution in the mix.

2.1.8. Formulas

The design of the LWC mix is a more complex process than that of ordinary concrete because more design parameters need to be determined, such as the water absorbed during concrete mixing, the dosage of other raw materials, etc. In the experiment, four concrete recipes were formulated to meet the lightweight concrete specification of EN 206 [30]. A reference recipe (REF 0) containing only natural aggregates with a maximum grain size of 8 mm was chosen for comparability. The other recipes (REC 1, REC 2 and REC 3) were designed with a combination of both natural and artificially produced Agloporit lightweight aggregate. To achieve similar properties, the remaining raw materials (cement, admixture, silica fume, water, 0/4 mm fine natural aggregate) were dosed at a constant rate. Depending on the ratio of aggregates, the ratio of process water needed for the pre-wetting of Agloporite was adjusted. On the basis of previous experience, the following concrete recipes were proposed as shown in Table 4.
The production of concrete using Agloporit lightweight artificial aggregate is no different from the production of concrete with natural aggregate. The production itself can locally take place in laboratory mixers or in larger quantities at a concrete plant with specialized equipment for storing and pre-wetting Agloporit. The concrete was mechanically mixed using a laboratory mixer with forced circulation type HMB–150. The same dosing procedure for individual components was always used for mixing concrete of all recipes. This method ensured the uniformity of the production process.

2.2. Methods

Standard tests on fresh and hardened concrete were performed on the produced concrete. The LWC testing does not differ from that of ordinary concrete and is carried out in accordance with the prescribed standard procedures. The physical and mechanical properties of the concrete were determined at 28 days of age. The fire resistance tests were identically carried out at 28 days of age under 600, 800 and 1000 °C loading. After the tests, samples were taken for microstructure analysis of the concretes by XRD and SEM.

2.2.1. Consistency of Fresh Concrete

Fresh concrete (REF 0, REC 1, REC 2 and REC 3) was tested for consistency in accordance with EN 12350-2 [31]. The principle of the test is the compaction of fresh concrete into the shape of a truncated cone. The distance the concrete has fallen after the truncated cone has been lifted indicates the consistency of the concrete. The concrete consistency test by cone settlement (Abrams) is based on placing a test cone on a flat surface. The cone is first filled to 1/3 of the volume and compacted with 25 punctures of the piercing rod. This is repeated for the next 2 layers until the cone is fully filled with concrete. After the cone is removed, the sitting height is measured within 5 to 10 s; i.e., the difference between the original and seated height. The resulting cone should be symmetrical and intact. This test determines the consistency of concrete at S1, S2, S3, S4 and S5.

2.2.2. Bulk Density of Fresh Concrete

Bulk density of fresh concrete (REF 0, REC 1, REC 2 and REC 3) was produced according to the procedure specified in EN 12350-6 [32]. The principle of determining the bulk density (kg/m3) of fresh concrete is based on the compaction and weighing of concrete in a rigid watertight container of known volume.

2.2.3. Bulk Density of Hardened Concrete

The bulk density of hardened concrete on the monitored concretes (REF 0, REC 1, REC 2 and REC 3) was carried out according to EN 12350-7 [33]. The principle of the test is to determine the mass (kg) and dimensions of the test samples (m3), and the value of the bulk density of hard concrete in kg/m3 is determined from the empirical relationship of the ratio of mass (m) to volume (V).

2.2.4. Compressive Strength of Concrete

The compressive strength of the concrete was carried out on test specimens with dimensions 150 × 150 × 150 mm in accordance with the requirements of EN 12390-3 [34]. The test was carried out on a series of 3 specimens after 28 days of concrete maturation (REF 0, REC 1, REC 2 and REC 3). The compressive strength test is based on the principle of destructive failure of the test sample. The stress at which the sample breaks is determined. The compressive strength of concrete (MPa) is calculated from an empirical relationship based on the ratio of the stress (kN) to the test area of the sample (mm2).

2.2.5. Temperature Load

In this experiment, the natural state of concrete under normal atmospheric conditions was simulated, and therefore, the concrete specimens (REF 0, REC 1, REC 2 and REC 3) were left in air for 21 days before thermal loading.
Special attention needs to be paid to the thermal load test, as these are structural concretes that are subjected to a combination of multiple types of loads over time. Several types of tests are used to test heat-resistant materials [8]. According to Rydval and Simunek [35], these tests can be divided into two basic categories:
  • Hot state testing–testing for the duration of the thermal loading. The test has the greatest predictive value as it is a combination of pressure and thermal loading.
  • Ambient temperature testing–testing after cooling to laboratory temperature. The advantage of this type of test is that it can be used with common laboratory equipment.
In the case of this experiment, the second category of specimen testing, ambient temperature testing, was chosen; i.e., the residual strengths of the concrete were determined.
The entire thermal loading test process was carried out in the shortest possible time period to ensure identical conditions (mechanical properties in the same time) so that a realistic comparison between the reference recipe REF 0 and the LWC recipes REC 1 to REC 3 could be made. Three test specimens from each recipe were placed in the furnace system for a total of 12 cubes with dimensions 150 × 150 × 150 mm. The individual test pieces were arranged in the system so that each test piece of the respective recipe had the same position and were not influenced; for example, all test pieces from one recipe were close to the burner, etc. During the temperature loading, the temperature profile inside the test samples was not monitored. However, as showed in Figure 3, Figure 3a shows the cover of the temperature chamber with concrete panels, which were also made of LWCs. In this case, it was a one-sided thermal stress (fire simulation). In the chamber, a temperature of 1000 °C was reached in 95 min and the temperature on the heated side of the panel was in the range of 850–900 °C, inside the panel; i.e., 50 mm from the heated side measured the temperature at 150–200 °C, and on the surface of the LWC panel the temperate was between 80–95 °C. In this way, heat flow was ensured around all the test specimens and around the surface of the test specimens. The homogeneity of the temperature field in the furnace system was ensured by two heating burners and the temperature was sensed by temperature sensors located outside the test specimens in the furnace system. The load temperature curve that was used in accordance with EN 1363-1 [36] and ISO 834 [37] is shown in Figure 4.
After the heat treatment, the samples were left in their natural environment, without thermal shock, for approximately 24 h. This was followed by weighing to evaluate weight changes and compressive loading to determine compressive strength. The same procedure was chosen for thermal loading for maximum temperatures of 800 and 1000 °C.

2.2.6. X-ray Diffraction Analysis (XRD)

XRD is a method used to determine the structure of crystalline substances and is based on the principle that the dimensions of the crystal lattice are comparable to the wavelength of X-rays. XRD analysis was used to determine the crystalline changes of the investigated LWC at increasing temperatures. The test samples for XRD were obtained from the fragments of the test specimens after compressive strength determination, which were then ground in a planetary mill to a fine powder. A Panalitical Empyrean instrument and HighScore evaluation software were used in the evaluation.

2.2.7. Scanning Electron Microscopy (SEM)

Scanning electron microscopy is an effective tool for monitoring the microstructure of developed lightweight concrete at extreme magnification. The test specimens were post-processed by SEM to determine the effect of high temperature on the microstructure of the lightweight concrete under investigation. The evaluation was carried out using a Tescan MIRA 3XMU electron microscope.

3. Results

As part of the experimental work, a series of tests were carried out to determine the behavior of the proposed lightweight concrete during its production, curing and heating to a high temperature.

3.1. Basic Properties of the Concretes Monitored

The concrete in its fresh state showed good stability and adequate consistency. To determine the workability of the fresh concrete, a standard test method (EN 12350-2 [31]) using the Abrams cone was used. After the test, it was visually assessed that there were no negative manifestations, such as segregation and/or bleeding of the concrete. The design of the individual concrete recipes aimed at achieving the same consistency 60 mm for all concretes (REF 0, REC 1, REC 2 and REC 3), which was achieved, and the consistency corresponded to the cone settlement level S2. The fresh concrete (REF 0, REC 1, REC 2 and REC 3) was further monitored for bulk density. The bulk density for OC with natural aggregate only was found to be 2270 kg/m3 and the range 2010–2190 kg/m3 for LWC corresponding to increasing the ratio between natural and artificial aggregate Agloporit.
Figure 5 shows a decrease in bulk weights with increasing dosage of lightweight aggregate Agloporit while decreasing the dosage of natural aggregate. A similar trend is evident in the case of the compressive strength of concrete (in accordance with EN 12390-3 [34]), where the strength values decrease in direct proportion with increasing Agloporit dosage. The obtained strengths range from 41.6 to 53.4 MPa. After the concrete compressive strength test, the test specimens were broken and the moisture in the internal structure retained by the lightweight aggregate Agloporit was clearly visible, as in Figure 6. This effect is confirmed by studies [5,38] showing the absorption of excess water in the matrix by the lightweight aggregate and that the aggregate acts as a reservoir of water in the individual grains, which causes the internal curing of the concrete.

3.2. Evaluation of Monitored Properties after High Temperature Loading

3.2.1. Comparison of Color Change Due to High Temperature

The primary phenomenon that is visible is the color change of the concrete surface that is exposed to high temperatures. The color spectrum of the concrete surface (REF 0, REC 1, REC 2 and REC 3) conform to the general specifications [8,39].

3.2.2. Surface Changes of Concrete Due to High Temperature

Changes on the surface of the concrete are a concomitant phenomenon during heating of the concrete. The test specimens loaded at 600 °C did not show significant surface changes in the form of cracks. On the other hand, when the temperature was increased to 800 °C, the first visible manifestations occurred, as can be seen in Figure 7a. The most pronounced effect occurred at 1000 °C, when quite extensive damage occurred on the concrete surface and on all the concretes studied (REF 0, REC 1, REC 2 and REC 3). Significant cracks were observed on the surface of the test specimens throughout the specimen; see Figure 7b.

3.2.3. Change in the Bulk Density of Hardened Concrete and Compressive Strength

With the change in moisture content, or loss of water during heating of the concrete, there is also a change in the bulk density of the hardened LWC. In the case of LWC, this change is even more pronounced because the concrete contains not only water for mixing, but also water for the treatment of the artificial aggregate. Table 5 shows a trend towards an almost constant decrease in the bulk density for all concretes (REF 0, REC 1, REC 2 and REC 3), regardless of whether the concrete only contains natural aggregate or also Agloporit aggregate. In the case of the formulation with natural aggregate (REF 0), there was an overall decrease in bulk density between 20 and 1000 °C of 11.4%. For the concretes containing Agloporit aggregate (REC 1, REC 2 and REC 3) this decrease was in the range of 12.1–13.2%.
During the heating of concrete, as the temperature increases, the concrete microstructure undergoes modification transformations at various stages of the temperature process, which results in a reduction in the compressive strength of the concrete. The decrease in compressive strength was especially observed at temperatures of 20–600 °C and 800–1000 °C. However, when the samples were subjected to a temperature of 800 °C, practically all concretes showed an increase in compressive strength, see Figure 8. Since the LWCs contained microsilica, Wollastonite was formed at this temperature stage. The occurrence of Wollastonite was detected by XRD analysis and in the concrete structure it causes the formation of a ceramic bond, which results in an increase in compressive strength. Chapter 4.3 deals more with this topic. Based on the chosen method of thermal loading and the subsequent determination of the compressive strength of the concrete (cube 150 × 150 × 150 mm), the so-called residual strength was determined.
Figure 8 shows the compressive strengths with increasing heating temperature of the test specimens. From the measured results, for the studied concretes (REF 0, REC 1, REC 2 and REC 3), there was a slight increase in compressive strengths up to 800 °C, which decreased with increasing lightweighting with Agloporit aggregate. When the concrete was loaded to 1000 °C (the actual maximum temperature reached in the furnace system was 1019 °C), a significant decrease in compressive strength was observed for all the concretes produced (REF 0, REC 1, REC 2 and REC 3). The compressive strength of the concrete dropped from an initial value of 53.4 MPa to 21.6 MPa (REF 0), from 50.8 MPa to 21.3 MPa (REC 1), and from 46.2 MPa and 41.6 MPa to 21.6 MPa and 20.7 MPa for REC 2 and REC 3, respectively. On average, there is a 60% decrease in the compressive strength of concrete.

3.2.4. X-ray Diffraction Analysis (XRD)

Samples of the observed concretes (REF 0 and REF 3) at temperatures of 20 (temperature under laboratory conditions) and 1000 °C were used for XRD analysis. The results of the XRD analyses are shown in Figure 9.
The results of XRD analysis confirm the presence of Portlandite (CaOH2) as one of the main constituents of concrete. In the XRD analysis, it was found that during the thermal loading of the lightweight concrete, the Portlandite was completely decomposed and at the same time, due to the amount of silica fume used, Wollastonite was formed due to the temperature, at approximately 800 °C. At temperatures of 1000 °C, there was a significant loss of strength due to the formation of cracks in the ceramic bond by the decomposition of CaCO3. The record continued to show changes in the structure of the aggregate used.

3.2.5. Scanning Electron Microscopy (SEM)

To demonstrate the effect of high temperatures on microstructural changes, images at 500× and 2000× magnification were selected. Representative samples were selected from LWC recipes (REC 2 and REC 3) without temperature loading and an image of the sample after 1000 °C. The differences are evident in Figure 10, Figure 11, Figure 12 and Figure 13.
In Figure 10 and Figure 11, PP fibers can be seen firmly anchored in the cement matrix at 500× and 2000× magnifications. The surface of the cement matrix is smooth with no signs of obvious temperature damage as these samples were not heated. In contrast, Figure 12 and Figure 13 show samples loaded at 1000 °C; only traces of where the fibers were deposited can be seen as the PP fibers have a melting point significantly lower than the loading temperature. The surface of the cement matrix is much rougher, with crystal growths forming a ceramic bond formed at 800 °C. The test specimens loaded above 800 °C represent the formation of non-hydrated phases such as β-C2S, C3S and wollastonite due to the decomposition of CSH at higher temperature as well as the presence of some microcracks. In Figure 13, it is also possible to see the ingrowth of cracks due to high temperatures, which have a significant negative effect on the compressive strength shown in Figure 8.

4. Discussion

4.1. Bulk Density LWC

The results of the hardened concrete bulk density for concrete containing natural aggregate reached 2290 kg/m3, and for LWC concretes with lightweight aggregate Agloporit, a minimum value of 1970 kg/m3 and a maximum value of 2170 kg/m3 was achieved. In the case of OC, the bulk density ranges between 2000 and 2600 kg/m3. From this overview, it is clear that only by using lightweight aggregates is it possible to achieve a reduction in the bulk density of concrete, which is confirmed by the hypotheses of other studies (e.g., [6,40]). According to the EN 206 [30], lightweight concrete is defined by a bulk density between 800 and 2000 kg/m3. The results obtained in this experiment show that this value was not achieved. However, the produced LWC concretes are very close to this limit. By additional modification of the composition, such as further increasing the percentage of lightweight aggregate at the expense of natural aggregate.

4.2. Degradation of Mechanical Properties of LWC

In terms of mechanical properties, the expected conclusion was that with increasing amounts of Agloporit aggregate, the compressive strength degrades and the bulk density of hardened concrete decreases. In the case of the experiment carried out for the lightweighting of concrete using 5, 10 and 15% (by weight), the compressive strength was reduced by 5, 13 and 22% respectively. A similar trend was observed in a study by Kockal and Ozturan [41], where the use of fly ash aggregates instead of natural aggregates (coarse fraction) resulted in a significant reduction in compressive strength from 62.9 MPa to 42.3 MPa. An almost identical hypothesis was observed in the study of Gesoğlu et al. [42], which demonstrated a decrease in the mechanical properties of concretes when lightweight aggregate was used.

4.3. Mineralogical Transformations of LWC during Heating and Their Effect on Compressive Strength

The change in compressive strength of the designed concretes (REF 0, REC 1, REC 2 and REC 3) as a function of loading temperature is graphically depicted in Figure 8. A slight increase in compressive strengths was observed up to the loading temperature of 600–800 °C. A study conducted by Heikal at al [43] reported that LWC concrete containing 10, 15 and 20% (by weight) silica fume (SF) achieved stable strength up to 600 °C. This is justified by the higher consumption of Portlandite (P) during the heating of the concrete and the subsequent hydration of the unhydrated cement clinker. Other studies have attributed this trend to the water released during the ongoing modification transformations, which may just cause additional hydration, i.e., the formation of new CSH, and thus increase the compressive strength of the concrete [8]. Generally, heating of the cementitious mortar results in evaporation of water, which leaves the concrete in the form of water vapor. Free water is the first to leave [43] and then chemically bound water [19]. Phase transformations can occur due to increasing concrete temperature, which affects the mineralogical composition of the cement, the ratio of Ca and SiO2. According to the theory of Hartmann et al. [44], when the Ca/SiO2 ratio approaches 1, the optimum conditions for the formation of Tobermorite gel (about 150 °C) and for the formation of Xonolite and Hillebrandite [19] occur. In the experiment conducted, the most interesting results were obtained after thermal loading of the concretes at 800 °C. Compared to the expected decrease in compressive strength, there was an increase in compressive strength for all concretes studied (REF 0, REC 1, REC 2 and REC 3). This statement can be seen in Figure 8. Available research [20,45] has explained this trend by the conversion of the hydraulic bond to a ceramic bond. This results in partial sintering leading to an increase in the compressive strength of the concrete. Since the designed concretes (REF 0, REC 1, REC 2 and REC 3) contained SF, the results obtained for the compressive strengths at 800 °C support another hypothesis. The study conducted by Heikal at al. [43] demonstrated the presence of Wollastonite at the heating temperature of 800 °C for the concretes containing SF; this phenomenon was also demonstrated in the experiment and is shown in Figure 9, from the XRD analysis. High pozzolanite of SF form more additional products from the pozzolanic reaction, not only from the de-composition of CSH from cement hydration, which decomposed at 800 °C, but also form CSH formed from SF and P. Wollastonite as a ceramic product, thus causing an increase in the compressive strength of the concrete [43]. In the experiment at loading above 800 °C, i.e., at a maximum temperature of 1000 °C, a significant decrease in compressive strengths occurred (e.g., References [43,46,47]), due to the decomposition of calcite to form CaO and CO2. For this reason, the concretes (REF 0, REC 1, REC 2 and REC 3) exhibited strengths approximately one third of the original values.

5. Conclusions

In this study, the mechanical properties and microstructure of lightweight concretes with lightweight artificial aggregate based on sintered fly ash were investigated after thermal loading to temperatures of 600, 800 and 1000 °C. With regard to the evaluation of the results obtained from this work, the following conclusions were drawn:
  • With Agloporit lightweight aggregate it is possible to achieve a lower bulk density of hardened LWC compared to ordinary concretes. The bulk density decreases in direct proportion to the increase in the lightweight aggregate content.
  • The LWC compressive strength values were approximately 5 to 22% lower (depending on the amount of lightweight aggregate used) compared to the control concrete with natural aggregate only.
  • Silica fume was contained in all the concretes studied, regardless of the type of aggregate used, and its morphological transformations during heating of the concrete to 800 °C caused an increase in strength compared to the compressive strength values at 600 °C. This was confirmed by XRD analysis.
  • The SEM analysis confirmed the ongoing changes in the microstructure during heating of the concrete and at 1000 °C; incipient cracks were detected causing destruction of the mechanical properties of the concrete.
  • At a maximum temperature load of 1000 °C, there was no explosive cracking of the LWC surface layers. Only color changes and a relatively significant crack formation on the surface of the concretes studied were visible.
  • As expected, the compressive strength of LWC rapidly decreased after 1000 °C and did not reach even half of the compressive strengths without thermal loading.
Based on the evaluation of the results, it is clear that it is possible to produce high quality lightweight concrete using lightweight aggregates based on sintered fly ash.

Author Contributions

K.K.: Conceptualization, Investigation, Writing–original draft, Formal analysis. J.B.: Writing–review and editing, Visualization, Resources. M.S.: Validation, Formal analysis, Methodology, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

The contribution was prepared with the financial support of the project GAČR 21-25813S “Study of the influence of organic fibers on the properties of cement composites under extreme loading”.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Non equivalent.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

References

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Figure 1. Sieve analysis of natural aggregate fraction 0/4 (amphibolite sand) and coarse aggregate fraction 4/8 mm.
Figure 1. Sieve analysis of natural aggregate fraction 0/4 (amphibolite sand) and coarse aggregate fraction 4/8 mm.
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Figure 2. Section of concrete with Agloporite [29].
Figure 2. Section of concrete with Agloporite [29].
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Figure 3. (a) View of the furnace system; (b) view of the furnace at a temperature load of 1000 °C.
Figure 3. (a) View of the furnace system; (b) view of the furnace at a temperature load of 1000 °C.
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Figure 4. Temperature load and temperature course of concrete REC 2.
Figure 4. Temperature load and temperature course of concrete REC 2.
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Figure 5. Mechanical properties of concretes.
Figure 5. Mechanical properties of concretes.
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Figure 6. (a) Sample of LWC with Agloporite aggregate (REC 2); (b) details (blue circle) of moisture inside the LWC structure.
Figure 6. (a) Sample of LWC with Agloporite aggregate (REC 2); (b) details (blue circle) of moisture inside the LWC structure.
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Figure 7. Surface changes in concrete (REC 1). (a) Specimen 1–11 loaded to 800 °C–grey surface color and first appearance of cracks. (b) Specimen 1–3 loaded to 1000 °C–light grey shade with a slightly pinkish-orange tone and a considerable number of surface cracks.
Figure 7. Surface changes in concrete (REC 1). (a) Specimen 1–11 loaded to 800 °C–grey surface color and first appearance of cracks. (b) Specimen 1–3 loaded to 1000 °C–light grey shade with a slightly pinkish-orange tone and a considerable number of surface cracks.
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Figure 8. Overview of mechanical properties of lightweight concrete after high temperature loading.
Figure 8. Overview of mechanical properties of lightweight concrete after high temperature loading.
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Figure 9. XRD results of LWC at 20 and 1000 °C (A—Aragonite, C—Calcite, F—Feldspar, Q—Quartz, M—Mica, P—Portlandite, W—Wollastonite).
Figure 9. XRD results of LWC at 20 and 1000 °C (A—Aragonite, C—Calcite, F—Feldspar, Q—Quartz, M—Mica, P—Portlandite, W—Wollastonite).
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Figure 10. REC 2–20 °C (500×).
Figure 10. REC 2–20 °C (500×).
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Figure 11. REC 2–20 °C (2000×).
Figure 11. REC 2–20 °C (2000×).
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Figure 12. REC 3–1000 °C (500×).
Figure 12. REC 3–1000 °C (500×).
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Figure 13. REC 3–1000 °C (2000×).
Figure 13. REC 3–1000 °C (2000×).
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Table
Table 1. Main properties of CEM II/B-M (S-LL) 32.5 R.
Table 1. Main properties of CEM II/B-M (S-LL) 32.5 R.
Main PropertiesValue
Specific surface [m2/kg] (EN 196-6 [22], Blaine)503
Volumetric density [kg/m3]3020
Compressive strength [MPa]–28 days (EN 196-1 [23])48.7
Tensile strength [MPa]–28 days (EN 196-1 [23])8.4
Table
Table 2. Volumetric density of cement CEM II/B-M (S-LL) 32.5 R.
Table 2. Volumetric density of cement CEM II/B-M (S-LL) 32.5 R.
SpecimentVolumetric Density [g/cm3]
13.0142
23.0856
33.1067
Diameter3.0688
Table
Table 4. Composition of individual concrete formulations.
Table 4. Composition of individual concrete formulations.
ComponentsQuantity per 1 m3 of Concrete [kg]
REF 0REC 1REC 2REC 3
CEM II/B-M (S-LL) 32.5 R375
Silica fume42
Sand 0/4 mm (natural)964
Aggregate 4/8 mm (natural)686515343171
Agloporit 4/8 mm0114229343
Superplasticizer3.3
Fibers (Polypropylene)1.0
WaterTechnological021.743.565.2
Mixing207
Table
Table 5. Change in the bulk density of hardened concrete.
Table 5. Change in the bulk density of hardened concrete.
TemperatureBulk Density of Hardened Concrete [kg/m3]
REF 0REC 1REC 2REC 3
600 °C2170211019901870
800 °C2080198018401730
1000 °C2030190018101710
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Křížová, K.; Bubeník, J.; Sedlmajer, M. Use of Lightweight Sintered Fly Ash Aggregates in Concrete at High Temperatures. Buildings 2022, 12, 2090. https://doi.org/10.3390/buildings12122090

AMA Style

Křížová K, Bubeník J, Sedlmajer M. Use of Lightweight Sintered Fly Ash Aggregates in Concrete at High Temperatures. Buildings. 2022; 12(12):2090. https://doi.org/10.3390/buildings12122090

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Křížová, Klára, Jan Bubeník, and Martin Sedlmajer. 2022. "Use of Lightweight Sintered Fly Ash Aggregates in Concrete at High Temperatures" Buildings 12, no. 12: 2090. https://doi.org/10.3390/buildings12122090

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