The Use of Recycled Ceramics and Ash from Municipal Sewage Sludge as Concrete Fillers

Abstract

The main aim of the research was to evaluate the feasibility of using recycled ceramics and ash from municipal sewage sludge as concrete fillers. As part of the study, standard cylindrical and cubic samples were investigated. The samples consisted of waste ceramic aggregate fractions 0–4 mm and 4–8 mm, which were sourced from used products manufactured by a sanitary fittings factory, as well as ash from one of the Polish sewage treatment plants. The chemical composition and morphology of recycled materials used to produce concrete were examined. The research itself focused on determining the strength properties of the produced composites under both normal conditions and after initial heat treatment. Microstructural tests of the produced composites were also carried out. The results demonstrated that selected recycled materials can successfully replace materials previously used in concrete production. The obtained strength results do not differ significantly from the strength of concrete made of traditional materials. Research has confirmed the possibility of using waste materials as concrete fillers.

1. Introduction

The main principles in the waste management “hierarchy”, according to the Waste Act [1], include waste prevention, preparation for reuse, and recycling. The goal of the National Municipal Sewage Treatment Program in Poland [2] is to reduce the discharge of insufficiently treated sewage. This program outlines actions for equipping agglomerations with a population equivalent of more than 2000 inhabitants with sewage systems and municipal sewage treatment plants. The use of collective sewage systems and treatment plants operating previously and related to the above program is associated with the receipt of huge amounts of waste generated in the sewage treatment process in Poland. According to the Central Statistical Office [3], in 2021, Poland had 173,500 km of active sewage networks. The dry mass of sludge from industrial and municipal sewage treatment plants amounted to 1,025,800 tons.

One of the available methods of neutralizing this type of waste and its significant reduction is thermal conversion, which includes incineration, co-incineration, and gasification. This method can achieve substantial reductions in waste mass and volume, with incineration potentially reducing waste volume by up to 90% and mass by up to 65% [4]. According to [3], 221,800 tons of dry mass were obtained in Poland using the thermal conversion method. Across EU countries, thermal methods manage approximately 18% of municipal sewage sludge [5]. Among the European countries, the Netherlands neutralizes 100% of municipal sewage sludge using the thermal method. Belgium is in second place with a result of 90% [6].

As a result of the use of municipal sewage sludge incineration, despite a significant reduction in mass and volume, we obtain significant amounts of ash. This ash is increasingly used, among others, in construction as a cement supplement. The PN-EN 450-1 [7] standard classifies fly ash as a fine-grained dust obtained during the combustion of coal dust with pozzolanic properties, significant amounts of SiO₂ and Al₂O₃, and a content of reactive SiO₂ of at least 25% by mass.

Globally, there is a growing body of research on the use of sewage sludge ash (SSA) generated from sewage waste incineration. Sewage waste is characterized by a different chemical composition, which results in ash of a different composition. For instance, ref. [8] investigated the use of bottom ash from the incineration of municipal waste and treated sewage in self-compacting concrete. Samples containing ash and treated sewage showed higher compressive strength values compared to samples with ash and tap water. However, the highest compressive strength values were obtained by the base samples. The authors [9] used an additive (SSA) in the amount of 10–40% as a substitute for cement in concrete based on Portland cement CEM I. The obtained results confirmed that replacing 10% of cement with SSA has a beneficial effect on compressive and tensile strength in bending, total porosity, ability to transport chloride ions and resistance to sulphate ions. Higher amounts of SSA clearly affected the deterioration of these properties. In a comprehensive literature review [10], the potential uses of SSA in concrete production were assessed. Due to the structure and chemical composition, it was assessed that this material can be used as a raw material for the production of cement clinker, lightweight aggregates, fine aggregates, filler aggregates, and—in ground form—as a cement component. Another study [11] found that adding SSA to concrete mixtures significantly decreased workability. They suggested using SSA in combination with slag and fly ash, which resulted in obtaining fluidity similar to that of the pure reference mortar. The inclusion of slag and fly ash in sludge ash significantly increased the strength and durability of sludge ash cementing materials, improving their resistance to water absorption and chloride migration. Further studies [12,13] examined the calcination process’s effects on the physicochemical properties and pozzolanic activity of aluminum-rich sludge ash. The optimum pozzolanic activity of calcined sludge ash was obtained using temperatures of 800 °C and 900 °C. The compressive strength and hydration degree increase with time and become better than the control mortars. The best mechanical effects were obtained by replacing 5% of calcined sewage sludge. In [14], the effect of SSA on the fluidity and strength of ultra-high-strength concrete (UHPC) was investigated. The results showed that the compressive strength of UHPC samples with 10% SSA after 7 and 28 days of curing was slightly higher than that of the base samples. SSA addition increased both autogenous and curing shrinkage. The content of SSA contributed to the reduction of hydration heat release and the increase of pore volume of the modified concrete. In [15], the possibility of synthesizing zeolite from SSA as a foaming additive in warm mix asphalt (WMA) was investigated. The obtained zeolite and commercial zeolite were added to WMA. The authors determined the characteristics of the modified WMA. The overall performance of the WMA mixture with zeolite derived from SSA was better than that of the WMA mixture with commercial zeolite and comparable to that of hot mix asphalt (HMA). In this study, AW-SSA was found to likely possess latent hydraulic properties, positively influencing late strength development. In [16], an acidic extraction method for phosphorus from SSA was used. Phosphorus-free ash (AW-SSA) was used as a partial replacement for sand or cement in concrete mix. With 10% cement replacement, the mortar had a similar 28-day compressive strength to the base one. It was found that AW-SSA probably has a latent hydraulic character, positively influencing late strength development. The compressive strength of the mortar with 10% cement replacement after 42 days was higher than the reference by about 5 MPa. In the paper [17], phosphorus was removed from SSA by electrodialytic separation (EDS), and the treated ash (SSA-EDS) was tested as a clay substitute (0–50%) in brick production. In masonry clay, mass loss occurs during firing due to the carbonate content. Using SSA-EDS, a decrease in mass loss after firing was observed. Due to the high iron oxide content, SSA is red in color, which can act as a coloring agent, allowing to obtain the desired red bricks. After firing at 1000 °C, all brick samples had apparent density and open porosity within the acceptable range of bricks used in construction. In [18], the behavior of cement pastes containing sewage sludge ash (SSA) at elevated temperatures (600–1000 °C) was evaluated. The results showed that the paste containing 10% SSA (S10) showed higher residual compressive strength than the ordinary cement paste (S0). Elevated temperatures caused decomposition of hydrate products, coarsening of pore structures, and formation of cracks. The addition of SSA promoted greater formation of gelenite at 900–1000 °C, and such a phase can fill the pores in the matrix. Therefore, S10 showed less coarsening of the pore structure than S0 at elevated temperatures. In the articles [19,20,21], the effect of SSA addition on the properties of cement mortars was determined. The studies identified the mechanisms behind some of the beneficial effects of SSA on mortar strength development through a comparative study with fine sewage sludge ash (FSSA). The results of this study indicate that the presence of SSA accelerates the rate of heat release of cement hydration. Replacing cement with SSA or FSSA up to 10% did not cause significant changes in the pore structure of the pastes. The formation of brushite in cement mortars with SSA or FSSA contributes to the increase in the strength of mortars after a longer setting period. It was found that the addition of SSA caused the formation of more ettringite in mortars in water immersion conditions and the mortars expanded already before the 7th day of curing. The studies showed that replacing 15% of Portland cement with SSA and its fractions: coarse (SSAC) and medium (SSAM) increased the compressive strength. The article [22] assessed the experimental results available in the world literature on SSA leaching and its application in the production of cement clinker, lightweight aggregates, mortars, concrete, blocks, road surfaces, geotechnics, and ceramics. The data analysis indicates that the mobility of heavy metals in many of these construction products may be significantly limited. This can be attributed to the effects of Portland cement stabilization and additional binding materials. High-temperature treatment used in the production of lightweight aggregates and ceramics is also beneficial. The restrictions on the use of SSA occur in areas with high rainfall and in strongly acidic environments. In [23], the properties of mortars with the addition of ash from septic tank sludge (SA) were investigated. Cement and sand were used in mass proportions of 1:3. The percentage additions of sludge ash were 5–30% in relation to the cement mass. The obtained results indicate that the addition of SA improves the general properties of mortars, and the addition of 20% SA was considered the most optimal. In the works [24,25], washed aggregate sludge (WAS) from gravel pits together with sewage sludge (SS) from sewage treatment plants were analyzed. The waste materials were mixed, ground, and formed into granules, preheated and sintered in a rotary kiln. The sediment from water treatment was also used as an aggregate, replacing expanded clay up to 50% by weight. Among other things, water absorption, compressive strength, swelling index, and density of the aggregate prepared in this way were tested. According to the authors, the obtained aggregate can be used as lightweight aggregate (LWAC) in lightweight insulating concretes and lightweight prefabricated structures. In [26], the authors used fly ash (SSA) as a substitute for cement and sand in the production of concrete blocks intended for breakwaters. Base blocks and those containing sewage sludge ash showed similar strength properties. All the above-mentioned positive properties of ashes resulting from the incineration of sewage sludge confirm their use in the construction industry.

Taking a broader approach to the issue of environmental protection, especially the shrinking resources of natural aggregates and the increasing amounts of stored waste that can be used as recycling aggregates, it was decided to use waste ceramic aggregate in addition to the above-described ash for concrete. In [27,28], crushed ceramic insulators were used as a substitute for coarse natural aggregate in geopolymer concrete. Concrete modified in this way was characterized by higher compressive and bending strength, with a 21% reduction in thermal conductivity. When using 50% replacement of fine aggregate with aggregate from ceramic insulators in plaster mortars, comparable results of compressive strength were obtained in comparison to control mortars. According to analyses of [29], improvement of durability and mechanical properties of mortars is obtained by using a maximum of 40% replacement of sand aggregate with ceramic fines. In [30], information was collected on the effect of replacing sand in concrete using granite dust, ceramic waste, glass aggregates, and plastic aggregates. Among other things, compressive strength, workability, modulus of elasticity, chloride ion penetration, and abrasion were determined. It was found that replacing sand in the range of 15–45% with recycled aggregates has a positive effect on the level of water absorption of concrete samples. In [31], the authors indicate that already at the production stage, about 20–30% of ceramic tiles do not meet the assumed material properties and end up in landfills. In their studies, they replaced up to 35% of coarse and fine aggregate in the concrete mix with appropriately crushed ceramic aggregate. It was found that replacing aggregate with crushed waste ceramic tiles partially increased the strength parameters. In [32,33], traditional coarse and fine aggregate was replaced with crushed sanitary ceramics or in quantities ranging from 10–50%. The highest compressive, tensile, and bending strengths of concrete were obtained with a 20% share of ceramic waste. The possibility of using aggregates from crushed sanitary ceramics as full-value substitutes for natural aggregates in mortars and concrete was confirmed in [34]. The procedure for aggregate production (grinding, division of grains into two groups—fine and coarse-grained and determination of their proportions) and design of concrete mix were described. The tested concrete was characterized by high strength and high abrasion resistance. Particularly favorable results were obtained in the case of using alumina cement and samples subjected to thermal loads up to 1000 °C. The possibility of using sanitary ceramics in ordinary concretes was also confirmed in [35], in which concrete mixes were designed using CEM32.5R cement and only aggregate from crushed sanitary ceramics of various fractions.

2. General Characteristics of the Samples and Properties of the Ash Used in the Tests

2.1. General Description

In order to verify the influence of sewage sludge ash in concrete containing recycled ceramic aggregate, standard cylindrical and cubic samples were prepared. The samples consisted of waste ceramic aggregate fractions of 0–4 mm and 4–8 mm, which was obtained from used products manufactured by the Reybud sanitary fittings factory in Rejowiec Fabryczny, ash obtained from thermal treatment of sewage sludge from the Płaszów II sewage treatment plant in Krakow and Portland cement Cem I 42.5 R from Ożarów SA Cement Plant in Ożarów. A total of four series of samples were made: base samples and three series containing ash as a cement supplement at levels of 5%, 10%, and 15% by mass. Table 1 presents the proportions of materials used to prepare the samples.

2.2. Characteristics of Ash Obtained from the Combustion of Sewage Sludge

The chemical composition and morphology were determined using a Quanta 250 FEG SEM microscope (FEI Company, Hillsboro, OR, USA). The chemical composition of the tested fly ashes was determined using the X-ray fluorescence method with energy dispersive XRF. The mineral composition was determined by X-ray phase analysis XRD. Measurements were performed by the powder method using an X’pertPRO MPD X-ray diffractometer with a PW 3020 goniometer (Malvern Panalytical Ltd., Malvern, UK).

Figure 1 shows an SEM photograph (5000× magnification used) related to the ash under consideration.

According to the PN-EN 450-1 standard [36] for siliceous fly ash, fly ash exhibits pozzolanic reactivity when the sum of reactive oxides—specifically silicon oxide (SiO₂), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃)—exceeds 70% of the material’s mass. The chemical composition of the SSA ash shown in the tests consists mainly of the above-mentioned, structure-forming oxides, which allows us to assume that this material may have the potential to react with calcium hydroxide, and thus to exhibit pozzolanic properties. However, in the tested SSA, the combined mass of these oxides (SiO₂ + Al₂O₃ + Fe₂O₃) is 45.56%, which is below the threshold required for pozzolanic reactivity. Table 2 presents the average chemical composition of the ash in comparison to the requirements outlined in the PN-EN 450-1 standard [36].

By relating the results of the average composition of the entire production year of SSA ash to the information available in the literature [37], where the authors reviewed the chemical composition of SSA from over thirty regions of the world, one can come to the conclusion that the chemical composition of the tested ash differs significantly from the average value determined by the authors of the publication (Table 3).

In the case of the criteria set by the standard [36] regarding the content of phosphates in the tested waste material, their content is significantly exceeded. The content of phosphates shown in the chemical composition of SSA exceeds the limit value by more than 2.5 times, reaching the level of 13.40% of the total ash mass. This fact is important because the presence of P₂O₅ can significantly affect the setting and hardening time of cement binders. The alkali content in siliceous fly ash should not exceed 5% of the total mass. In SSA, the alkali content—expressed as Na₂O_e—is 1.1%, which is below the standard limit. Therefore, the use of this type of material should not result in aggression resulting from the expansive alkali–silica reaction. In the case of testing the content of sulphates expressed as SO₃, the SSA ash showed their content at the level of 1.8% of the ash mass. The determined amount does not exceed the 3.0% limit, which eliminates the occurrence of internal sulphate corrosion. In turn, the content of chloride ions Cl^ࢤ, according to the assumptions of the standard, should not exceed 0.1% of the ash mass. The tested ash did not show any share of soluble chloride ions. Therefore, chloride corrosion and negative impact on reinforcing steel should not be expected in this case either. The content of chloride ions, sulphate ions, and P₂O₅ is presented in Table 4.

In order to determine the qualitative composition of the crystalline phases of SSA, X-ray diffraction studies were carried out. X-ray studies of ash resulting from the combustion of sewage sludge indicate that the main phases present in the material are quartz. Other crystalline phases contained in the SSA ash are iron oxide, which usually occurs in the third oxidation state, forming hematite. Furthermore, the high P₂O₅ content in sewage sludge leads to the presence of a crystalline calcium and magnesium phosphate salt, with the formula Ca₉Mg(PO₄)₇. The source of the high concentration of phosphate ions is the significant share of detergents in municipal sewage. The diffractogram with the peaks of the individual phases, obtained during the study, is presented in Figure 2.

As a result of observation and chemical analysis (SEM–EDS), it was found that the ash consists of grains not only of different shapes, but also of different chemical composition and morphology. Quartz grains characterized by irregular edges are visible. The second phase revealed in the X-ray diffraction pattern is hematite. Typical manifestations of the presence of Fe₂O₃ are spheroidal crystals. The most irregular grains with a porous structure contain minerals derived from calcium and aluminum phosphates, which constitute a significant part of the examined SSA. In addition to the main phases mentioned above, the analyzed chemical composition of individual grains gives grounds to assume that other minerals are also present, such as alkali feldspars and anhydrite. The SEM image with EDS analysis of the micro-area at 500× magnification is presented in Figure 3a–c.

2.3. Characteristics of the Slurries of the Basic Samples and Those Containing SSA Ash (5, 10, and 15%)

The analysis of the chemical composition of hydration products performed by the EDS method in the micro-area indicates a high content of portlandite. The results of the reference cement paste tests confirm the tests performed by X-ray diffraction. The chemical composition determined by the EDS method and X-ray tests are consistent with the presence of portlandite. Among the crystalline phases in the reference paste, calcium carbonate crystallizing in the form of calcite can also be isolated (Figure 4a–c).

The chemical composition of the subsequent pastes in the micro-areas presented in percentage by mass in Figure 5a,b, Figure 6a,b and Figure 7a–c—determined using the EDS method—differs slightly from the composition of the reference paste. This concerns the increased content of phosphates and the Ca/Si ratio, which was significantly reduced in the presence of SSA. However, this does not necessarily indicate the reduction of portlandite in the microstructures of the pastes as a result of its reaction with the components of SSA ash.

This fact is also confirmed by the intensity of the peaks assigned to Ca(OH)₂ shown in the X-ray diagrams in Figure 8.

XRD analysis presented in Figure 8 also reveals phases corresponding to unreacted cement grains. Additionally, characteristic peaks appear for quartz contained in SSA ash, which has not undergone hydration, confirming its low pozzolanic reactivity. Traces of calcium carbonate (CaCO₃) can also be observed, which may indicate progressive carbonation of the paste after 28 days of hydration. Despite the significant amount of P₂O₅ in the composition of fly ash, no crystalline forms of hydrates containing phosphate ions were detected.

2.4. Heat Treatment of Samples

In order to determine the effect of high temperatures on concrete parameters, five samples from each group (0, 5, 10, and 15% ash) were heated to 300, 450 °C, and 600 °C. After reaching the target temperature, the samples were kept in the furnace for 24 h, where they cooled down to ambient temperature. The samples were heated in a PK 1100/5 chamber furnace (Figure 9a). The furnace controller allows for any temperature increase during individual tests and its monitoring in real time. The heating elements of the furnace are Kanthal A1 resistance wire in the shape of spirals. During the heating process, the temperature in the furnace chamber was measured, as well as the temperature reached on the outer surface of selected samples. The samples were protected during the test with steel shields that limited the possibility of damaging the furnace chamber (Figure 9b).

During thermal exposure, the temperature distribution was designed to simulate conditions similar to those of an actual fire in buildings. Therefore, the standard temperature-time distribution according to reference [38] was used as the basis. The standard temperature–time curve is described by Formula (1).

T = 20 + 345log (8t + 1)(1)

• T—temperature [°C]

• t—time [min]

The increase in thermal load was carried out according to the standard temperature-time curve. After reaching the assumed temperature of 300 °C, 450 °C, or 600 °C, the samples were heated for another 60 min to equalize the temperature in the entire sample volume. The curve describing the temperature load of concrete samples is shown in Figure 10.

3. Results of Strength Tests of Concrete Samples

Compression tests of concrete samples were carried out using the Controls Advantest 9 system for testing concrete and mortars according to standards [39,40] in the Construction Laboratory of the Faculty of Civil Engineering and Architecture of the Lublin University of Technology. The tests were performed on cubic samples with dimensions of 150 × 150 × 150 mm after 7, 14, 28, 56, and 90 days from their production date and on beam samples with dimensions of 40 × 40 × 160 mm after a 28-day curing period. The results are presented in Figure 11 and Figure 12.

The replacement of cement with sewage sludge ash (SSA) at a rate of 10% resulted in nearly identical strength values compared to the base samples, particularly after a curing period of more than 28 days. In terms of high-temperature exposure after 120 days of curing, the CP10c samples heated to 300–450 °C achieved higher compressive strength values than the base samples.

4. Test Results—Additional Analysis of CP0c and CP10c Samples at 300 °C

The aim of additional phase composition analysis and microstructural tests was to assess the probable causes of the smaller increase in compressive strength of concretes containing 10% ash (CP10c) after exposure to high temperature (300 °C) in comparison to the reference concrete (CP0c).

Phase composition studies were conducted using thermal analysis with a thermogravimetric analyzer (model SDT Q600, TA Instruments, New Castle, DE, USA). The sample mass was approximately 30 mg, and measurements were performed in an air atmosphere with a flow rate of 100 mL/min, covering a temperature range from 20 °C to 1000 °C, with an increase rate of 10 °C/min. Platinum crucibles were used. TG, DTG, and DTA curves were recorded. The tests were carried out on samples of CP0c and CP10c, which had been previously ground entirely, quartered, and reduced to particles smaller than 0.063 mm.

Microstructure observations were performed using a Sigma 500 VP scanning microscope from ZEISS equipped with an EDX elemental composition analyzer (model Ultim Max 40 from Oxford). Images were recorded using a BSD detector. The test samples were polished sections cut from CP0c, CP10c concrete samples and samples exposed to a temperature of 300 °C (CP0c and CP10c). The samples were cut perpendicularly to the formed surface. Then they were dried, filled with resin, and polished. The method of sample preparation for testing was analogous to that in earlier works [41].

Immediately prior to testing, the samples were sputtered with gold. Sample photos of the concrete microscope surfaces taken using an optical microscope are presented in Figure 13.

The photos presented in Figure 13, taken using an optical microscope, show the microstructure of the reference concrete (CP0c) and concrete with 10% fly ash (CP10c). In addition, photos of the surface of samples heated at 300 °C are presented. The visible white aggregate grains are mostly ceramics or the glaze itself, which covers it. Ceramic grains containing bonded fragments of glaze were also observed. The reddish color of the cement matrix of concrete containing fly ash results from the composition and properties of the ash.

4.1. Thermal Analysis

Analyzing the thermograms of concrete samples CP0c and CP10c (Figure 14) in the temperature range of 20–200 °C, endothermic effects were recorded with a maximum transformation at a temperature of about 74 °C related to the release of free water and water from the CSH phase. In the CP10c sample, an additional effect was recorded peaking at 96 °C, which may indicate the presence of a different CSH phase than in the CP0c sample. In the temperature range of 200–400 °C, water bound to the AFm and Aft (ettryngite) phases among others is released. No significant differences between the samples were observed in this temperature range. In the temperature range of 400–450 °C, endothermic decomposition of portlandite occurs, related to the release of water. The maximum of this transformation for both samples was approximately 430 °C. Another recorded thermal effect is the decomposition of carbonates occurring in the temperature range of 450–700 °C. A slightly different shape of the peaks on the DTG curve may indicate different sizes of calcite crystallites. The quantitative results of thermal analysis are given in Table 5 and Table 6.

Analyzing the quantitative results of the tests obtained based on thermogravimetric measurements, it can be observed that the mass losses in the given temperature ranges for the CP10c sample were practically always lower than for the CP0c sample (Table 6). This is primarily related to the 10% lower cement content. However, the difference in the amount of water bound by the CSH phase in the CP0c sample is much greater than 10% compared to the value for the CP10c sample. This may indicate the formation of another, less hydrated form of the CSH phase. The amount of portlandite in the CP10c sample is about 16% lower than in the CP0c sample, which may result from the pozzolanic activity of the fly ash used. The calcite content in the CP10c sample is about 20% lower than in the CP0c sample. The amount of calcite in concrete is related to the amount of portlandite formed in the process of cement hydration and its subsequent carbonation and the possible presence of carbonate aggregates. The smaller amount of calcite in sample CP10c results from the smaller amount of cement and may also result from the lower degree of carbonation caused by the more dense structure of concrete with the addition of ash.

4.2. Scanning Electron Microscopy (SEM)

The microstructure of the analyzed concrete samples (C P0c and CP10c) not exposed to high temperature was compact and tight. Relics of Portland cement clinker were observed, and in the CP10c samples, fly ash grains from the combustion of sludge were also present. No other main components of cement, such as silica fly ash or blast furnace slag, were observed, which confirms the use of CEM I cement for the preparation of concrete. Figure 15 shows an example image of the microstructure of the CP0c sample and an enlarged ceramic–paste contact zone.

Both in the CP0c concrete without fly ash and in the concrete with the additive, the ceramic–paste contact zone was well-developed, continuous, and contained only a few air pores or detachments. It was observed that the composition of the CSH phase and its porosity are different in the area of the porous ceramic fragment and the glaze. On the side of the porous ceramic fragment, the CSH phase has less calcium and is more compact, which may result from partial migration of the calcium hydroxide solution into the interior of the porous ceramic. As a result, the water–cement ratio may decrease locally, which has a positive effect on increasing the mechanical strength in this area and the adhesion of the paste to the aggregate. Figure 16 shows the microstructure of the CP0c concrete subjected to a temperature of 300 °C.

Figure 16 shows cracks formed in the ceramic–paste contact zone as a result of the effect of the a temperature of 300 °C. It was observed that the number of cracks in the ceramic–paste contact zone is much greater in the area where the ceramic grain is glazed, compared to the unglazed areas of the ceramics. This is most likely due to the lower adhesion of the paste to the glaze caused by the weaker contact zone than in the areas where the ceramics are porous. The occurrence of additional stresses caused by different thermal expansion of the glaze and ceramics is not excluded here, however, this issue was not analyzed. The tendency for cracks to appear in the glazed areas of the ceramics was also observed in concrete containing 10% ash after thermal exposure (CP10c-300).

4.3. The Influence of Ash on the Thermal Resistance of Concrete

Figure 17 presents example images of the microstructure of CP0c and CP10c concretes exposed to a temperature of 300 °C.

Comparing the microstructure of the CP10c concrete containing fly ash to the reference CP0c concrete without such an additive, heated at 300 °C, numerous cracks were observed in the CP0c concrete. The number of cracks and their opening in the concrete with the fly ash additive (CP10c) were significantly smaller than in the concrete without the additive. Figure 18 shows an example image of the microstructure of the CP10c concrete.

Analyzing the microstructure of the CP10c concrete sample containing the fly ash additive, it can be observed that the composition of the CSH phase around the fly ash grains (marked with yellow arrows in Figure 18) contains less calcium. This may be due to the porosity of the grains of this material, which causes the migration of the calcium ion solution from the contact zone areas to the interior of the grains. This in turn affects the local reduction of water content resulting in the formation of the CSH phase with a slightly different composition and structure and a reduction of the water–cement ratio, which has a positive effect on the mechanical properties of the composite. In addition, it should be noted that phosphorus compounds present in the ash, which are also visible in Figure 18, can disturb the cement binding process, affecting the properties of the composite.

The observed smaller increase in the compressive strength of concretes with the addition of fly ash exposed to a temperature of 300 °C in relation to the reference concretes is most likely due to the increased porosity of ash grains, which leads to a local decrease in the water–cement ratio in the cement matrix and an increase in its mechanical strength, as well as a smaller amount of water bound in the CSH phase and other cement hydration products that decompose as a result of the effect of high temperature.

5. Conclusions

A challenge accompanying the treatment of municipal and industrial sewage is the specific waste generated during technological processing in the form of sewage sludge, which requires appropriate management due to its properties. This sludge includes waste from fermentation chambers and other parts of the installation. With increasing urbanization, the volume of waste produced has become a serious issue. This waste contains—among other components—heavy metals, and pathogenic organisms, which can negatively impact the natural environment and human health. Proper management and disposal are therefore essential. To protect both the environment and human health and to minimize the volume of sludge, it is subjected to thermal treatment. Although ash generated from the combustion of sewage sludge no longer contains viable pathogenic organisms, the presence of heavy metals in its composition remains an issue. The purpose of the present research was to explore potential management methods for sludge generated through the thermal treatment of sewage sludge.

Based on the results obtained, it can be concluded that concrete containing waste ceramic aggregate and ash as a partial cement supplement can be components of the concrete mix. Such components of the concrete mix allow you to obtain concrete with high strength parameters. Using ash as a cement supplement for maturation periods of 56 and 90 days, similar values of compressive strength were obtained for the samples (CP5c and CP10c) compared to the base samples. Phase composition tests partially revealed hydraulic components in the ash in question. The binding process itself is slower than in the base samples. The use of waste ceramic aggregate and ashes from the combustion of waste generated in sewage treatment plants in concrete allows for the reduction of landfilled waste, limiting the use of natural resources and generally protecting the environment.

The results of the tests and analyses of concretes CP0c and CP10c led to the following conclusions:

• Chemical analysis showed that SSA ash consists mainly of silicon, iron and aluminum oxides. Additionally, this material contains a significant amount of phosphates, which results in the extension of the setting times of the tested ecological binders;

• Phase composition analysis showed that the SSA ash is mainly composed of non-reactive silica occurring mainly in the form of crystalline quartz;

• Observation of the microstructure of concrete slurries confirms the presence of a large number of relics of unhydrated cement;

• The contact zone of ceramic aggregate—grout is properly developed, and no major cracks or separations were observed in the unbaked concrete;

• Ceramic aggregates contain glazed areas, where the contact zone with the cement paste seems to be weaker, which is probably due to the very low porosity of these areas of the aggregate. Lower adhesion of the paste to glazed surfaces becomes visible after the impact of high temperatures when more cracks appear in these areas;

• The addition of ash influences the formation of the CSH phase with a different composition and locally reduces the water–cement ratio, which has a positive effect on the resistance of concrete to high temperatures (300 °C).

Author Contributions

Conceptualization, J.S. and P.O.; methodology, J.S., P.O., F.C., B.K., A.P., M.Ż. and A.B.; software, J.S. and F.C. validation, J.S., F.C. and B.K.; formal analysis, J.S., B.K., F.C., M.Ż. and A.B.; investigation, J.S. and F.C.; resources, J.S., P.O., F.C. and A.P.; data curation, J.S.; writing—original draft preparation, J.S., F.C., B.K. and P.O.; writing—review and editing, J.S.; visualization, J.S., F.C. and B.K.; supervision, J.S.; project administration, J.S. and F.C.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Detailed data are available from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. SEM photography—ash from the incineration of sewage sludge.

Figure 2. Diffraction pattern with indicated crystalline phases occurring in the tested SSA ash.

Figure 3. (a) SEM image of the SSA ash microstructure, magnification 500×. (b) EDS analysis of the chemical composition of the SSA ash micro-area, magnification 500×. (c) EDS analysis of the chemical composition of the SSA ash micro-area, magnification 500×.

Figure 4. (a) SEM image of the CP0c paste microstructure, magnification 500×. (b) EDS analysis of the chemical composition of the micro-area of the CP0c paste, magnification 500×. (c) EDS analysis of the chemical composition of the micro-area of the CP0c paste, magnification 500×.

Figure 5. (a) SEM image of the CP5c paste microstructure, magnification 500×. (b) EDS analysis of the chemical composition of the micro-area of the CP5c paste, magnification 500×.

Figure 6. (a) SEM image of the CP10c paste microstructure, magnification 500×. (b) EDS analysis of the chemical composition of the micro-area of the CP10c paste, magnification 500×.

Figure 7. (a) SEM image of the CP15c paste microstructure, magnification 500×. (b) EDS analysis of the chemical composition of the micro-area of the CP15c paste, magnification 500×. (c) EDS analysis of the chemical composition of the micro-area of the CP15c paste, magnification 500×.

Figure 8. Diffraction patterns with indicated crystalline phases occurring in the tested pastes CP0c, CP5c, CP10c, CP15c.

Figure 9. (a) Sample heating furnace. (b) Samples protected by steel shields.

Figure 10. Temperature distribution graph during heating of concrete samples.

Figure 11. Mean compressive strengths and standard deviations of samples with different ash contents measured after 28, 56, and 90 days.

Figure 12. Average compressive strength and standard deviation of samples measured after 120 days, heated at temperatures of 300, 450, and 600 °C.

Figure 13. Sample photos of concrete microsection surfaces (optical microscopy), (a) sample CP0c, (b) sample CP10c, (c) sample CP0c (300 °C), (d) sample CP10c (300 °C).

Figure 14. Thermograms of concrete samples CP0c and CP10c.

Figure 15. Example image of the ceramic–paste contact zone in CP0c concrete.

Figure 16. Microstructure of CP0c concrete exposed to a temperature of 300 °C (cracks in the glaze-paste contact zone are marked).

Figure 17. Comparison of the microstructure of CP0c and CP10c concretes after heating at 300 °C (cracks are marked with arrows) (a) sample CP0c, (b) sample CP10c, (c) sample CP0c, (d) sample CP10c.

Figure 17. Comparison of the microstructure of CP0c and CP10c concretes after heating at 300 °C (cracks are marked with arrows) (a) sample CP0c, (b) sample CP10c, (c) sample CP0c, (d) sample CP10c.

Figure 18. Microstructure of CP10c concrete, clinker relics marked with red arrows, and fly ash marked with yellow arrows.

Table 1

Composition of concrete mixtures.

Ingredients (kg/m3)	Sample Series Designation
	CP0c	CP5c	CP10c	CP15c
Cement (I 42.5 R)	488.0	463.6	439.2	414.8
Ash	0.0	24.4	48.8	73.2
Ceramic aggregate 0–4/4–8 (mm)	997.0/398.0	997.0/398.0	997.0/398.0	997.0/398.0
Water	196.0	196.0	196.0	196.0

Table 2

Chemical composition of SSA determined by XRF method along with the criteria included in the standard [36].

Component	SSA	PN-EN 450-1 [36]
(-)	(% m.)	(% m.)
SiO2	33.03, 24.64	Total (SiO2, Al2O3, Fe2O3) ≥ 70.0
Al2O3	5.72, 9.51
Fe2O3	6.81, 13.15
CaO	25.32	≤10.0
P2O5,total	13.40	≤5.0
On2Oe	1.54	≤5.0
LOI	0.30	A ≤ 5.0
		B ≤ 7.0
		C ≤ 9.0

Table 3

Chemical composition of SSA based on literature.

Component	SSA [37]
	Min–Max	Average
	Quantity (% m.)
SiO2	14.4–65.0	36.1
Al2O3	4.4–34.2	14.2
Fe2O3	2.1–30.0	9.2
CaO	1.1–40.1	14.8
On2Oe	0.01–6.8	0.9
SO3	0.01–12.4	2.8
P2O5	0.3–26.7	11.6
Cl−	-	-
LOI	0.2–41.8	6.1

Table 4

Chemical composition of SSA with criteria included in the standard [36].

Component	SSA	PN-EN 450-1 [36]
(-)	(% m.)	(% m.)
SO3	1.84	≤3.0
Cl−	-	≤0.1
CaOin	-	≤1.5
P2O5 (mg/kg)	13.40	≤100

Table 5

Weight loss of samples during the test.

Sample Symbol	Mass Loss in Approximate Temperature Ranges (%)
	20–200 °C	200–400 °C	400–450 °C °C	450–700 °C	700–1000 °C	0–1000 °C
CP0c	2.8	1.7	0.6	2.6	0.3	8.0
CP10c	2.4	1.5	0.5	2.2	0.3	6.7

Table 6

Phase composition of the tested concretes.

Sample Symbol	Content of Ingredients (%)
	Bound Water			Portlandite	Calcite	Loss on Ignition up to 1000 °C
	HI	HCH	Σ
CP0c	4.5	0.6	5.1	2.5	6.1	8.0
CP10c	3.9	0.5	4.4	2.1	4.8	6,7

References

1. Act of 14 December 2012 on Waste, Journal of Laws of 2018, Item 992, as Amended. 2013; Available online: https://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=wdu20130000021 (accessed on 23 January 2013).

2. National Municipal Wastewater Treatment Program. Master Plan Developed on the Basis of AKPOŚK 2017 Approved by the Management of the Ministry of Environment on September 8, 2017. Available online: https://www.gov.pl/web/wody-polskie/krajowy-program-oczyszczania-sciekow-komunalnych (accessed on 8 September 2017).

3. Central Statistical Office. Environmental Protection. 2021; Available online: https://stat.gov.pl/obszary-tematyczne/srodowisko-energia/srodowisko/ochrona-srodowiska-2021,1,22.html (accessed on 30 November 2021).

4. Rosik-Dulewska, C. Basics of Waste Management; 6th ed. PWN: Warsaw, Poland, 2015; pp. 1-389.

5. Central Statistical Office. Statistical Yearbook of Environmental Protection 2017. Available online: https://stat.gov.pl/obszary-tematyczne/srodowisko-energia/srodowisko/ochrona-srodowiska-2017,1,18.html (accessed on 30 November 2017).

6. Pietrzyk, J. Analysis of the Possibilities of Using Waste from Thermal Processing of Municipal Sewage Sludge. Ph.D. Thesis; Poland, 2018; Available online: https://winntbg.bg.agh.edu.pl/rozprawy2/11428/summ11428.pdf (accessed on 10 October 2024).

7. Standard PN-EN 450-1 Fly Ash for Concrete—Definitions, Specifications and Conformity Criteria; NSAI: Dublin, Ireland, 2012.

8. Taherlou, A.; Asadollahfardi, G.; Salehi, A.M.; Katebi, A. Sustainable use of municipal solid waste incinerator bottom ash and the treated industrial wastewater in self-compacting concrete. Constr. Build. Mater.; 2021; 297, 123814. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.123814]

9. Zdeb, T.; Tracz, T.; Adamczyk, M. Physical, mechanical properties and durability of cement mortars containing fly ash from the sewage sludge incineration process. J. Clean. Prod.; 2022; 345, 131055. [DOI: https://dx.doi.org/10.1016/j.jclepro.2022.131055]

10. Lynn, C.J.; Dhir, R.K.; Ghataora, G.S.; West, R.P. Sewage sludge ash characteristics and potential for use in concrete. Constr. Build. Mater.; 2015; 98, pp. 767-779. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2015.08.122]

11. Guo, S.; Dong, R.; Chang, Z.; Xie, Y.; Chen, G.; Long, G. Performance and microstructure of sustainable cementitious materials mixed by municipal sewage sludge ash, slag, and fly ash. Constr. Build. Mater.; 2023; 367, 130028. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2022.130028]

12. Chang, Z.; Long, G.; Xie, Y.; Zhou, J.L. Pozzolanic reactivity of aluminum-rich sewage sludge ash: Influence of calcination process and effect of calcination products on cement hydration. Constr. Build. Mater.; 2022; 318, 126096. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.126096]

13. Naaman, S.; Rais, Z.; Taleb, M. The effectiveness of the incineration of sewage sludge on the evolution of physicochemical and mechanical properties of Portland cement. Constr. Build. Mater.; 2016; 112, pp. 783-789. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2016.02.121]

14. Gu, C.; Ji, Y.; Yao, J.; Yang, Y.; Liu, J.; Ni, T.; Zhou, H.; Tong, Y.; Zhang, X. Feasibility of recycling sewage sludge ash in ultra-high performance concrete: Volume deformation, microstructure and ecological evaluation. Constr. Build. Mater.; 2022; 318, 125823. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.125823]

15. Zou, F.; Leng, Z.; Cao, R.; Li, G.; Zhang, Y.; Sreeram, A. Performance of zeolite synthesized from sewage sludge ash as a warm mix asphalt additive. Resour. Conserv. Recycl.; 2022; 181, 106254. [DOI: https://dx.doi.org/10.1016/j.resconrec.2022.106254]

16. Ottosen, L.M.; Thornberg, D.; Cohen, Y.; Stiernström, S. Utilization of acid-washed sewage sludge ash as sand or cement replacement in concrete. Resour. Conserv. Recycl.; 2022; 176, 105943. [DOI: https://dx.doi.org/10.1016/j.resconrec.2021.105943]

17. Ottosen, L.M.; Bertelsen, I.M.G.; Jensen, P.E.; Kirkelund, G.M. Sewage sludge ash as resource for phosphorous and material for clay brick manufacturing. Constr. Build. Mater.; 2020; 249, 118684. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.118684]

18. Liu, M.; Zhao, Y.; Xiao, Y.; Yu, Z. Performance of cement pastes containing sewage sludge ash at elevated temperatures. Constr. Build. Mater.; 2019; 211, pp. 785-795. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2019.03.290]

19. Chen, Z.; Poon, C.S. Comparative studies on the effects of sewage sludge ash and fly ash on cement hydration and properties of cement mortars. Constr. Build. Mater.; 2017; 154, pp. 791-803. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2017.08.003]

20. Gu, C.; Shuang, Y.; Ji, Y.; Wei, H.; Yang, Y.; Xu, Y.; Qian, R.; Cui, D.; Zhou, H. Effect of environmental conditions on the volume deformation of cement mortars with sewage sludge ash. J. Build. Eng.; 2023; 65, 105720. [DOI: https://dx.doi.org/10.1016/j.jobe.2022.105720]

21. Monzó, J.; Payá, J.; Borrachero, M.V.; Córcoles, A. Use of sewage sludge ash (SSA)-cement admixtures in mortars. Cem. Concr. Res.; 1996; 26, pp. 1389-1398. [DOI: https://dx.doi.org/10.1016/0008-8846(96)00119-6]

22. Lynn, C.J.; Dhir, R.K.; Ghataora, G.S. Environmental impacts of sewage sludge ash in construction: Leaching assessment. Resour. Conserv. Recycl.; 2018; 136, pp. 306-314. [DOI: https://dx.doi.org/10.1016/j.resconrec.2018.04.029]

23. Ingunza, M.P.D.; Camarini, G.; Costa, F.M.S. Performance of mortars with the addition of septic tank sludge ash. Constr. Build. Mater.; 2018; 160, pp. 308-315. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2017.11.053]

24. González-Corrochano, B.; Alonso-Azcárate, J.; Rodríguez, L.; Lorenzo, A.P.; Fernández Torío, M.; Ramos, J.J.T.; Corvinos, M.D.; Muro, C. Valorization of washing aggregate sludge and sewage sludge for lightweight aggregates production. Constr. Build. Mater.; 2016; 116, pp. 252-262. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2016.04.095]

25. Mañosa, J.; Formosa, J.; Giro-Paloma, J.; Maldonado-Alameda, A.; Quin, M.J.; Chimenos, J.M. Valorization of water treatment sludge for lightweight aggregate production. Constr. Build. Mater.; 2021; 269, 121335. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.121335]

26. Prabhakar, A.K.; Krishnan, P.; Lee, S.S.-C.; Lim, C.S.; Dixit, A.; Mohan, B.C.; Teoh, J.H.; Pang, S.D.; Tsang, D.C.W.; Teo, S.L.-M. et al. Sewage sludge ash-based mortar as construction material: Mechanical studies, macrofouling, and marine toxicity. Sci. Total Environ.; 2022; 824, 153768. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2022.153768] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35151736]

27. Naenudon, S.; Wongsa, A.; Ekprasert, J.; Sata, V.; Chindaprasirt, P. Enhancing the properties of fly ash-based geopolymer concrete using recycled aggregate from waste ceramic electrical insulator. J. Build. Eng.; 2023; 68, 106132. [DOI: https://dx.doi.org/10.1016/j.jobe.2023.106132]

28. Garg, N.; Shrivastava, S. Mechanical, durability and sustainability assessment of rendering mortar with synergistic utilization of recycled concrete and ceramic insulator fine aggregates. J. Build. Eng.; 2023; 76, 107269. [DOI: https://dx.doi.org/10.1016/j.jobe.2023.107269]

29. Garg, N.; Shrivastava, S. A review on utilization of recycled concrete aggregates (RCA) and ceramic fines in mortar applications. Mater. Today Proc.; 2023; 73, pp. 64-73. [DOI: https://dx.doi.org/10.1016/j.matpr.2022.09.226]

30. Rifa, A.; Subhani, S.M.; Bahurudeen, A.; Santhosh, K.G. A systematic comparison of performance of recycled concrete fine aggregates with other alternative fine aggregates: An approach to find a sustainable alternative to river sand. J. Build. Eng.; 2023; 78, 107695. [DOI: https://dx.doi.org/10.1016/j.jobe.2023.107695]

31. Manikandan, K.P.; Nanthakumar, P.; Balachandar, M.; Shankar, D.G.; Vijayakumari, G. Partial replacement of aggregate with ceramic tile in concrete. Mater. Today Proc.; 2023; in press [DOI: https://dx.doi.org/10.1016/j.matpr.2023.05.214]

32. Priyadharshani, S.A.; Sah, R. Durability studies on concrete with ceramic waste as partial replacement to coarse aggregate. Mater. Today Proc.; 2023; in press [DOI: https://dx.doi.org/10.1016/j.matpr.2023.01.171]

33. Sivakumar, A.; Srividhya, S.; Sathiyamoorthy, V.; Seenivasan, M.; Subbarayan, M.R. Impact of waste ceramic tiles as partial replacement of fine and coarse aggregate in concrete. Mater. Today Proc.; 2022; 61, pp. 224-231. [DOI: https://dx.doi.org/10.1016/j.matpr.2021.08.142]

34. Halicka, A.; Ogrodnik, P.; Zegardlo, B. Using ceramic sanitary ware waste as concrete aggregate. Constr. Build. Mater.; 2013; 48, pp. 295-305. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2013.06.063]

35. Ogrodnik, P.; Szulej, J.; Franus, W. The Wastes of Sanitary Ceramics as Recycling Aggregate to Special Concretes. Materials; 2018; 11, 1275. [DOI: https://dx.doi.org/10.3390/ma11081275] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30042367]

36. Standard PN-EN 450-1 Fly Ash for Concrete—Part 1: Definitions, Specifications and Conformity Criteria; NSAI: Dublin, Ireland, 2009.

37. Cyr, M.; Coutand, M.; Clastres, P. Technological and environmental behavior of sewage sludge ash (SSA) in cement-based materials. Cem. Concr. Res.; 2007; 37, pp. 1278-1289. [DOI: https://dx.doi.org/10.1016/j.cemconres.2007.04.003]

38. Standard PN-EN 1991-1-2 Eurocode 1: Actions on Structures, Part 1–2: Actions General—Impacts on Construction in Conditions Fire; The European Union: Strasbourg, France, 2002.

39. Standard PN-EN 12390-3:2011 Concrete Testing. Part 3. Compressive Strength of Samples Designated for Testing; Polish Committee for Standardization: Warszawa Poland, 2019.

40. PN-EN 12390-4:2001 Concrete Testing. Part 4. Compressive Strength. Requirements for Strength Testers; Polish Committee for Standardization: Warszawa Poland, 2020.

41. Chyliński, F.; Kuczyński, K. Ilmenite MUD Waste as an Additive for Frost Resistance in Sustainable Concrete. Materials; 2020; 13, 2904. [DOI: https://dx.doi.org/10.3390/ma13132904] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32605247]