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Article

Advances in Deflocculant Utilisation in Sustainable Refractory Concrete with Refractory Waste

by
Jolanta Pranckevičienė
* and
Ina Pundienė
Institute of Building Materials, Vilnius Gediminas Technical University, Saulėtekio al. 11, 10223 Vilnius, Lithuania
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(2), 669; https://doi.org/10.3390/su17020669
Submission received: 2 December 2024 / Revised: 27 December 2024 / Accepted: 14 January 2025 / Published: 16 January 2025
(This article belongs to the Special Issue High-Value and Resource-Based Utilization of Coal-Based Solid Waste)
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Abstract

:
In the last 10–15 years, the use of waste refractory materials has significantly increased because it is not economically justified to transport such expensive materials to landfills. This work compared the physical and mechanical properties of refractory concrete samples with those of individual deflocculants—polycarboxilate, sodium tripolyphosphate, and a deflocculant mixture. Three refractory concrete compositions with individual deflocculants and a deflocculant mix were created to choose the best main properties of refractory concrete. Five compositions of refractory concrete specimens were created by partial replacement of chamotte aggregate (CA) by refractory concrete waste (RCW) (100, 75, 50, and 25%). Exothermal profile, structure development and spread were determined for fresh refractory concrete pastes. It was found that with an increase in CA replacement level to RCW, the EXO maximum temperature, spread and structure evolution speed decreases. SEM and porosity tests confirmed density, compressive strength results and structural parameters. The study shows that RCW replacement slows the hydration process, particularly at replacement levels above 33%. However, replacement levels of up to 25% improve compressive strength by 13% due to the additional amount of cement minerals in RCW aggregates, which can participate in the hydration process, making it a viable option for applications where enhanced durability is required, such as in non-critical zones of industrial refractory linings.

1. Introduction

In the last decade, the significant increase in reusing waste refractory materials has addressed economic and environmental challenges, allowing for the reduction in the consumption of primary raw materials and the environmental impact associated with their extraction. Additionally, increasing the amount of waste incorporated into the production of new value-added materials reduces the amount of waste in landfills [1,2].
This study aims to evaluate the potential of refractory concrete waste (RCW) as a partial substitute for chamotte aggregates (CA) while improving mechanical and thermal properties. Much waste is generated during the production of refractory concrete, making it difficult to dispose of [3,4]. In order to reuse refractory aggregate waste, the main task is to separate the waste from pollutants, cement, wood, and others. This technological task is challenging and economically complex to justify. RCW is mainly generated in the steel, cement and non-ferrous metal processing industries. A smaller part of RCW comes from other industries related to high-temperature processes, such as petrol treatment power plants, municipal waste or biomass fuel incinerators, and the chemical industry. In the most widely used refractories, at least six types of products generate significant amounts of waste and can be reused. Among them, the following are distinguished: calcined fireclay, calcined andalusite, calcined bauxite, calcined magnesite, calcined dolomite, and refractory products containing carbon. Refractory materials waste is categorised based on magnesium and aluminium oxide content. Refractory materials waste containing magnesium oxide is divided into three types: without carbon, with carbon and with spinel [5]. Refractory materials waste containing aluminium oxide is classified according to Al2O3 content [6].
Previous research has focused on producing refractory bricks using recycled waste, addressing environmental concerns such as limited landfill space and high energy consumption. However, limited attention has been given to RCW in refractory concretes because RCW is used only as an aggregate replacement, which can affect the hydration processes and mechanical properties at high temperatures. Some research is focused on using the waste refractory brick as a partial/total replacement of fine aggregate for the preparation of concrete [7,8]. In the same way, RCW can be reused as aggregate in newly developed refractory materials. However, the usage ratio of refractory materials and refractory RCW is only approximately 10% of the weight, which does not ensure the full utilisation of the waste. When the proportion of waste in the composition is increased, the mechanical properties of refractory concrete deteriorate further due to the drawbacks of the recycled aggregates, such as their high porosity [9]. Another attempt to reuse waste is grinding it and firing it at approximately 1500 °C in cement production. Nevertheless, that consumes a high level of energy.
The recycling amount of concrete waste is minimal due to all these drawbacks. Furthermore, it is still a challenge to eliminate concrete waste. As noted in the literature, recycling refractory waste is technically challenging due to impurities and porosity. Our study confirms these limitations, showing that untreated RCW introduces higher porosity, affecting refractory concretes’ structural properties. Removing contaminants is vital, as impurities can fundamentally damage newly developed products’ physical and mechanical properties. The main challenge here is experienced workers and proper scrap selection [10,11].
In China, over 35% of refractory materials in the last decade have been successfully recycled and used to produce new materials and dry mixes [12]. Studies have shown that in the composition of refractory materials, waste does not deteriorate the properties of refractory concrete and, in some cases, even improves it. For example, using 10% of waste by weight, which contains a large amount of aluminium oxide, in the composition of the mixture improves the thermal durability and corrosion resistance of concrete. Additionally, the authors emphasise that production costs are significantly reduced [13].
Additionally, when evaluating the suitability of waste refractory materials, the waste’s porosity should be evaluated because, generally, the waste refractory materials have a higher porosity than the unused refractories. Therefore, when waste refractory materials are used, the need for water in the mixture increases, and the physical and mechanical properties of the product obtained deteriorate. The most increased porosity is characterised by the waste refractory materials of chamotte products. It is also necessary to control the amount of impurities with a high pH in waste because they affect the rheological properties of the mixture and the duration of the setting process. The recommended amount of fireclay aggregate replacement into fireclay waste varies depending on the requirements for mixtures (responsibility level) from 10–15 to 80% by weight of fireclay aggregate [14,15,16]. In addition, the framework and filling effect of fine waste concrete aggregate make it an alternative to natural aggregate and allow it to produce mortar and new concrete with recycled aggregates [17,18]. The authors [19] describe that refractory waste is first crushed several times into pieces of ~50 mm, which are dried further. After drying, they are further crushed and sieved to ~8 mm size fractions.
Further sieving and grinding occur if activities are planned to use smaller fractions. It was found that when the amount of waste refractory materials in the concrete composition is increased from 0 to 50% (by weight of unused fireclay aggregate), the need for water in the concrete mixture to maintain the same consistency increases from 5 to 7%. The porosity of concrete samples increases from 17 to 20% by increasing the amount of waste refractory materials in the concrete from 0 to 50%.
The fly ash (FA) can be used in refractory materials to improve chemical composition and properties. As an additional alumina and silica source in refractory materials, coal-burning waste FA can be used and is favourable because of its Al/Si ratio for producing such materials [20]. Due to its physical and chemical characteristics, FA can be successfully added to different refractory materials and high-temperature ceramics to improve thermal characteristics and create lightweight refractory materials. On average, a higher percentage of FA results in reduced density and better insulating performance. The spherical shape of some FA and its pozzolanic properties enhance the mechanical properties of concrete, reduce the heat of hydration, and decrease water consumption [21,22]. FA’s pozzolanic reaction often takes longer than cement hydration [23,24]. Using FA in the cement paste results in a pozzolanic interaction between the amorphous phase of FA and the cement hydration products produced during the cement hydration, which produces more C-A-S-H gel and increases density and strength [25,26].
As we can see from the literature review, blocks of refractory single materials, such as fireclay and magnesite bricks, are mostly successfully processed. Recycling waste refractory concrete is complicated by the difficulty of separating the cement matrix from the aggregates. The properties and applications of such waste are more difficult to predict [27,28]. In some cases, using superplasticisers to reduce the mixing water amount can help solve the issues associated with using waste refractory materials [29]. A significant portion of the waste is generated during the production of concrete. The utilisation of such waste has not been thoroughly studied, as it is unclear how thermally untreated concrete waste will affect the properties of the new concrete. Previous studies indicate that refractory waste does not compromise the properties of concrete and can even improve them. Since some of the cement amounts in all refractory concrete remain unhydrated, some unhydrated cement minerals remain in the RCW and may be involved in the hydration process by reusing RCW. This finding aligns with our results, showing that up to 25% RCW replacement enhances compressive strength by utilising residual cementitious materials in RCW aggregates.
This study aims to evaluate the effect of the chamotte aggregate (CA) replacement by RCW on the properties of refractory concrete with FA addition and to provide recommendations for the more efficient use of RCW in refractory concrete.
To reach the aim of the work, the effect of separately used superplasticisers and their mixture on refractory concrete, prepared with refractory waste RCW replacement of non-treated CA (up to 100% replacement in compositions), on rheological, structure densification process and physical and mechanical properties of refractory concretes after treatment at different temperatures was investigated. It is also appropriate to compare the properties of the developed compositions with the control samples made without RCW replacement.

2. Materials and Methods

2.1. Characteristics of Raw Materials

Alumina cement “Gorkal-40” is produced in Trzebinia, Poland by GÓRKA CEMENT Sp. and meets the essential requirements of the EN 14647 standard [30]. Chemical composition of cement: Al2O3—41.6%; CaO—38.8%; SiO2—4.4%; Fe2O3—13.7%; MgO—1.16%; SO3—0.34%; mineralogical composition: main phases—CA(CaO·Al2O3), CA2(CaO·2Al2O3), C12A7(12CaO·7Al2O3), C4AF (4CaO·Al2O3·Fe2O3) (Figure 1); special characteristics: fire resistance—at least 1270 °C, specific surface according to the Blaine method—0.32–0.37 m2/g, specific density—3200–3300 kg/m3; bulk density—1150 kg/m3, hydraulic characteristics: initial setting in 1–4 h, final setting time—no later than 2 h after initial setting time; mechanical characteristics: compressive strength after 24 h—60–100 MPa, bending strength after 24 h—7–10 MPa.
Chamotte aggregate (CA) is made by crushing chamotte bricks from Vilnius, Lithuani with a 1920 kg/m3 density. Chemical composition of CA: Al2O3—35.1%; SiO2—59.7%; Fe2O3—1.9%; MgO—0.8%; TiO2—1.6%; CaO—0.7%; Na2O + K2O—0.2%; mineralogical composition: main phases—mullite (3AlO3·2SiO2) and quartz (SiO2). The same 0/5 mm fraction of CA was used in the study. The bulk density of this fraction aggregate is 930 kg/m3, respectively. CA water absorption, as determined in the laboratory, was 5.97%. The open porosity of CA is 23.21%. The granulometric composition of CA is presented in Figure 2.
Milled chamotte aggregate (MCA) of 0/0.5 mm fraction was prepared from crushed CA aggregate. MCA was milled in the laboratory ball mill and sifted afterwards through the sieves, as indicated in Figure 1, MCA specific surface—0.4 m2/g, bulk density—1120 kg/m3.
The deflocculating agents such as “Castament FS 20” (PCE-20) by BASF, Beiersdorf, Germany from the group of polycarboxylic ethers and sodium tripolyphosphate (ST) Na5P3O10 were applied to decrease the water-cement ratio (W/C) of the composites, PCE-20 belongs to polycarboxylates—water-soluble dispersant polymers in powder form. Sodium tripolyphosphate is an anhydrous inorganic compound soluble in water as a white powder. Water for preparing the concrete mix meets the EN 1008:2002 standard [31].
Used refractory lining elements, consisting of refractory concrete, were crushed to produce refractory concrete waste. The resulting aggregate (RCW) was then sieved through a 5 mm mesh (see Figure 3).
Chemical composition of RCW: Al2O3—36.4%; SiO2—48.64%; Fe2O3—4.26%; MgO—0.87%; TiO2—1.28%; CaO—8.35%; Na2O + K2O—0.19%; mineralogical composition: main phases—mullite (3AlO3·2SiO2) and quartz (SiO2). The 0/5 mm fractions of RCW aggregate were used in refractory concretes. The bulk density of this fraction is 850 kg/m3. The water absorption of RCW aggregate was determined in the laboratory to be 9.72%. The open porosity of CA is 30.92%. The granulometric tests are presented in Figure 3.
Aluminium silicate comprised mainly of the FA used in this investigation, which came from a coal-fired power station Łaziska, Poland. The main chemical components of FA were SiO2 (57.03%), Al2O3 (31.71%), Fe2O3 (1.66%) and CaO (1.12%). Traces of MgO, Na2O, K2O and other minor oxides were also detected. Its unburned carbon content was indicated by its 5.2% loss on ignition at 1000 °C. FA’s main crystalline phases, according to the X-ray diffraction (XRD) study, are quartz (SiO2) and mullite (3AlO3·2SiO2) (Figure 4).
FA had a bulk density of 794 kg/m3, a specific density of 2200 kg/m3 and a specific 0.371 m2/g surface area. Most FA particles are spherical, have a porous structure, and range in diameter from 80 to 200 µm on average. FA falls into one of two classes: “C” or “F”. While the amounts of SiO2, Al2O3 and Fe2O3 in “Class C” range from 50% to 70%, the amounts in “Class F” are greater than 70%. Because of its higher calcium content, which differentiates it from other types, FA is categorised as “Class C”. The size of FA particles does not exceed 300 μm. Particle diameter at 10% was 11.53 μm, 50%—71.11 μm, and 90%—160.82 μm. The mean particle diameter was 80.93 μm (Figure 5).

2.2. Methods of Testing and Sample Preparation

The device MPC 227 of the company METTLER TOLEDO (Greifensee Switzerland, electrode InLab 730, measurement range of 0—1000 μS/cm) was used for investigations of the electric conductivity of pure aluminate cement and the >0.025 fraction of RCW compositions with water.
Refractory concrete paste fluidity was determined using the mini-slump test on the samples immediately after paste mixing at the chosen temperature. For the mini-slump test, the paste was poured into a cone (70 mm in height, 60 mm in upper diameter and 70 mm in bottom diameter) and set on a glass plate, which, after each measurement, was rinsed by water whose temperature corresponded to the temperature selected for the tests. Lifting the cone allowed the paste to spread. The pastes were dropped 15 times on a flow table, and the diameter was measured in two directions. The values used were the arithmetic means of these measurements. The average duration of each measurement was kept at 15 s. A minimum of 105 ± 5% flow was maintained in all mixes.
The temperatures of the exothermic effect of refractory concrete pastes were determined based on the methodology developed by Alcoa. Temperature development, which results from the exothermic (EXO) reaction of the aluminate cement hydration using various fillers, was followed according to the methodology [32] using laboratory-designed equipment (Figure 6). Fresh AC paste (1.5 kg) was poured into the laminate mould (100 × 100 × 100 mm), then a thermocouple of type T was placed in the centre of the mould. The mould was compacted for 1 min on the vibrating table and placed in a steel box coated with a layer of expanded polystyrene (50 mm) from inside. A computer recorded the temperature change every minute.
Observations of changes in the structure of refractory concrete samples were carried out with the help of the Pundit 7 device, measuring the speed of propagation of the ultrasonic pulse in the samples after treatment at different temperatures. The non-destructive ultrasound pulse velocity test was chosen because it is possibly the most advanced method due to its clear physical basis, accuracy, and ease of use. This method is used to monitor the setting, hardening behaviour and development of the structure of cement paste and refractory concrete [33]. The ultrasonic pulse velocity (UPV) method was applied using the ultrasonic pulse indicator Pundit 7 (Proceq SA, Schwerzenbach, Switzerland) with the two 54-kHz standard cylindrical transducers (transmitter and receiver) to evaluate the structure development of the samples. Fresh pastes were set between two ultrasonic transducers operating at 10 pulses per second and a frequency of 54 kHz. The transducers were pressed against the samples at the two strictly opposite points. The value of UPV in the samples was tested after hardening, drying and firing. Vaseline was used to ensure good contact. The UPV was calculated from the following equation:
UPV = l τ ⋅ 10 6
where l is the length of the tested mortar sample (distance between cylindrical heads), and τ is the time of pulse spread.
Observations of changes in the structure of refractory concrete samples were carried out with the help of the same Pundit 7 device, measuring the speed of propagation of the ultrasonic pulse in the samples after treatment at different temperatures.
The fired samples’ structure analysis was conducted at 1100 °C using a scanning electron microscope SEM EVO 50 EP (Carl Zeiss, SMT, Oberkochen, Germany, resolution 1.5 nm).
The preparation of concrete samples and the evaluation of their primary physical and mechanical properties (compressive strength, density and deformation) were conducted according to EN 1402-6 guidelines [34]. The cold crushing strength (CCS) tests were performed using the ALPHA 3-3000 S (FORM+TEST Seidner & Co. GmbH, Riedlingen, Germany) testing machine, with results presented as arithmetic averages of six individual measurements.
The apparent porosity was determined by water absorption, considering the volume of the prepared specimens.
For macrostructure investigation of the cementitious matrix with different deflocculants and its blend, 3 compositions, namely K1, K2 and K3, were prepared (Table 1). The amount of water and deflocculant was kept the same as in refractory concrete compositions to avoid water and deflocculant amount influence factors. They were used in structural investigation tests after burning at 1100 °C.
Seven compositions of refractory concrete were prepared (Table 1). Physical and mechanical properties were tested on 70 × 70 × 70 mm samples prepared using a 20-L Hobart mixer (Hobart corp., Troy, OH, USA). Initially, the dry components of the concrete mix were mixed for 3 min, then ¾ of the total water was poured into the dry concrete mix and mixed for 2 min (56 rpm). Then, the remaining water was added and mixed for 3 min more. The samples were formed under slight vibration, then covered with a polyethene sheet, and hardened for 72 h in normal curing conditions at 20 ± 1 °C. After 3 days of curing, the samples were dried at 105 ± 5 °C for 48 h in the electric furnace. Afterwards, they were held for 5 h at each investigated temperature (800, 1100 °C) in the electronically controlled furnace and finally cooled. The heating rate was 5 °C/h, holding at the highest temperature for 5 h, and finally cooled down to 20 °C at the cooling rate of 60 °C/h.
Thermal cycles with heating to 800 °C and cooling between two water-cooled metallic plates were performed according to the technique described in [35] with samples measuring 40 × 40 × 160 mm.

3. Experimental Part

3.1. Fresh Paste Properties

3.1.1. Electrical Conductivity and pH

Electrical conductivity and pH measurements were carried out to determine whether the RCW additive contained unhydrated cement minerals. For comparison, pure cement suspension tests and MCA were performed. Measurements were performed immediately after mixing the suspension (1 min) and then after 15, 30, 45 and 60 min. The results of the measurements are presented in Figure 7. The observations show that the electrical conductivity of RCW is approximately 1.5–3 times lower than that of pure cement suspension. MCA shows approximately 4–5 times lower electrical conductivity than was observed for RCW.
Additionally, it can be observed that MCA suspension electrical conductivity values change very little during testing. In contrast, electrical conductivity values significantly increase over time in pure cement and RCW suspensions. This increase indicates that cement minerals dissolve and ions come into suspension. This test indicates that some amount of cement minerals in RCW has remained unhydrated and, during treatment, was activated and could participate in hydration reactions.
The pH tests of the suspensions (Figure 8) showed that cement suspension pH is higher (10–11) than in RCW suspension (8.6–9.5). In contrast, as expected, in MCA suspension, pH values are near neutral—7.4–7.6. This research also reveals that some of the unharmed cement remains in waste.

3.1.2. Exothermic Reaction

In order to find out how the RCW additive affects cement hydration, samples of four compositions were formed, in which the amount of RCW addition at the expense of CA from 0 to 100% was changed. The amount of deflocculants mix (FS-30 and NT) remained constant. For comparison, just RCW with a constant amount of water (as in the KFN composition) was used without the addition of deflocculants. Signs of hydration of RCW appear only 22 h after mixing with water. As can be observed (Figure 9), the temperature and time of the EXO peak change as the amount of RCW additive increases. The maximum EXO is reached in the control sample in 10 h after mixing with water. The temperature reaches 52 °C. When 25% of the CA is replaced with RCW additive, the maximum EXO is reached after 10.6 h, and the temperature at that time was 49.5 °C. These results are in confidence with [36].
Further increasing the amount of RCW additive to 50 and 100% EXO reaches the same maximum in both cases at 20 and 23 h, but the temperature at that time is different: with 50% of RCW additive, the temperature reaches 46 °C and with 100% only 44 °C. It can be noticed that the RCW additive significantly slows down the hydration time of cement, but very little influences the EXO maximum temperature. Generally, by entirely replacing CA with RCW, the decrease in EXO maximum temperature reaches just 15%, whereas the time to reach EXO maximum temperature extends to 2.3 times. Thus, such observation allows us to conclude that the amount of 50% RCW is less correct hydration of cement, whereas a higher amount of RCW can be used when the hydration time of concrete is not limited.

3.1.3. Spread

An important fluidifying effect of the separately and together used PCE-20 and ST is reflected in the spread test (Figure 10). The mini-slump tests, performed immediately after the paste mixing, show that deflocculant PCE-20 has a slightly higher spread than deflocculant ST. An important fluidifying effect of the mix of deflocculants is manifested by a significant increase in the measured paste spread diameters. A defloculant mix increases spread diameter to 10–15% higher than in the refractory concrete paste with separately used deflocculants. The same results are presented in research [37,38]. When the RCW is used in composition, the spread values depend on the RCW amount used. The higher the CA replacement level to RCW, the lower the paste spread. It can be related to up to 1.3 times higher than the CA porosity of the RCW aggregates. It decreases the amount of free water in the paste because a part of it is quickly adsorbed in RCW aggregates. When the replacement of RCW is the highest, the spread value decreases up to 30% compared to KNF concrete paste [36,39]. The same trends can be seen when comparing the water absorption results of CA and RCW. Water absorption of RCW is approximately 38% higher than CA.

3.1.4. Ultrasonic Pulse Velocity

Many researchers [40] suggested that the ultrasonic method can be used very effectively to monitor the hydration and formation of the structure of cement paste. The authors proposed to describe the hydration process in 3 steps. 1—when UPV does not change—beginning of hydrate formation occurs (induction period, typically (3–4) h); 2—UPV sharply increases—massive precipitation of hydrates with a progressive transition from amorphous to crystallised forms, the mixture stiffens (quickly structure compaction period, until 24 h); 3—UPV slowly increases and becomes stable, then cement skeletons approach their final stiffness (slowly structure compaction period, follow up 24 h). The study of the formation of the structure of fresh refractory concrete paste containing PCE-20, ST and deflocculants mix (PCE-20 and ST) in the early curing period (during 24 h) is presented in (Figure 11).
Results show that the induction period is the shortest in fresh refractory concrete paste KNF—approximately 0.8 h. The induction period in the KFN and KF refractory concrete pastes is slightly longer—1.4 h. These results agree with the results obtained in the investigation of the refractory concrete pastes EXO profiles and reflect the influence of the water amount in the refractory concrete paste. The substitution of CA to RCW (from 25 to 100%) in the compositions delayed the structure development and densification (no significant changes in UPV) during the induction period. Increasing the replacement of CA to RCW prolongs the induction period, respectively, until 2.1 h, 3.5 h and 8.9 h, compared to the KNF composition induction period. The retardation of cement minerals dissolution can explain this prolongation of UPV because of water decreasing in solution. Porous RCW aggregates can adsorb a significant amount of water and decrease free water in refractory concrete paste. This effect can decrease ion transition in refractory concrete paste [41,42].
A sharp increase in the UPV during the development of hydrates occurs during the next 9–22 h. As we can see, deflocculant mix in KFN paste, compared to paste where separately was used PCE-20 and ST, intensifies structure densification during massive precipitation of hydrates, and that is reflected in UPV values after 9–10 h, whereas in KN and KF pastes after 13–15 h. With an increased RCW replacement level, structure densification extends to 20–22 h. After 24 h of curing, the highest UPV values were reached in specimen KNF and the lowest in the refractory concrete specimens RCW75 and RCW100.
To better interpret the obtained results of EXO profiles, reflected cement hydration activity and physical structure compaction measured by UPV, the relationship between the EXO maximum reaching time and sharp structure densification start time is presented in Figure 12.

3.2. Concrete Properties

3.2.1. Density

Tests on the properties of concrete were performed with two batches of refractory concrete, which differed according to the deflocculant used and the amount of the RCW in the refractory concrete composition. Specimens KF were produced using only the PCE-20 deflocculant additive. Specimens KN samples were produced using ST deflocculant. KNF specimens were prepared using a mixture of ST and PCE-20 at deflocculant ratios of 1:1. RCW25–RCW100 compositions were prepared using a mixture of ST and PCE-20 at deflocculant ratios of 1:1. Refractory concrete density tests (Figure 13) show that the use of a mixture of ST and PCE-20 deflocculants together increases the density of the specimen by up to 3.8%, compared to refractory concrete with PCE-20 and ST used separately. This result reflects that the water amount used for mixing the KNF specimens was the lowest (Table 1) between the studied KF and KN compositions. The highest increase in density after thermal treatment is observed in samples with a mixture of ST and PCE-20 deflocculants. When using larger amounts of deflocculants, as in KFN, the water demand in the mixtures decreases, and the density increases accordingly. The density of the refractory concrete samples with the lower amount of RCW (25% and 50% from CA) decreases slightly compared to KFN samples [43,44]. However, with an increase in the proportion of RCW to 75 and 100%, the density of samples decreased by up to 4% because of the lower bulk density of RCW aggregates (~850 kg/m3) compared to the bulk density of CA aggregate (~980 kg/m3). The same observations are presented in research studies [45,46].

3.2.2. Macrostructure and Microstructure

To explain the differences in the density properties of KF, KN and KNF samples, a cement matrix was prepared with a separate admixture of deflocculants. The K1 sample, using PCE-20, displayed a non-homogeneous macrostructure with numerous voids and pores (0.3–10 µm). This irregularity likely contributed to its lower compressive strength and higher porosity than samples with optimised deflocculant mixtures (Figure 14a) [47].
Macrostructure analysis of the K2 sample with ST showed that the structure is more homogeneous than that of the K1 sample, and the number of pores and cracks is less visible. Pores vary from 0.1 to 4 µm length (Figure 14b). This observation is also proved in [48,49].
Observed macrostructure analysis of the K3 sample with a blend of ST and PCE-20 allows us to observe a very solid, homogenous structure; pores are almost invisible and mostly 0.1–0.2 µm in size, and there are no cracks (Figure 14c) [41,42,50].
The observation (Figure 15a) of the K1 sample reveals that the voids and coarse pores are visible in the structure. The length of voids reaches 5 µm, and the width reaches 0.5–1 µm. The microstructure is not homogenous because many small voids are visible in the structure in addition to large voids. The size of hydrate crystals is small, and this observation is similar to those of the studies [37,51].
Significantly coarser hydrated crystals are observed in the K2 sample structure (Figure 15b). The structure is more densified, and fewer voids are observed. The same tendencies were observed in the research [49]. The structure of K3 samples (Figure 15c) looks the densest, and crystals are visibly coarser than in K1 and K2 samples [42,50].
As can be concluded, the deflocculants can change the macrostructure and microstructure of samples, the hydration products’ size, and the amount of pores and cracks in the structure. The blend of deflocculants creates a denser macrostructure, reflected in density and compressive strength results.

3.2.3. Compressive Strength

Compressive strength is the most critical property of refractory concrete. The different deflocculant types, their mix and RCW amounts influence compressive strength value after treatment at different temperatures as presented in Figure 16. Tests after hardening showed that the compressive strength of the KNF sample is up to 23% higher than samples with separately used deflocculants. This beneficial effect is observed in several research studies [16,52]. The lowest CA replacement to the RCW level in the composition is slightly increased compressive strength. This phenomenon can influence an additional amount of cement minerals in RCW aggregates. When there is enough water for the cement dissolution process, cement minerals from RCW can pass freely into the solution.
As is observed in conductivity research, some increase in EC in a short time proves the cement dissolution process [53]. With increased RCW replacement in composition, the compressive strength is up to 39% lower than in the reference KNF sample, which may be due to less free water in the paste that stops cement dissolution. Due to the RCW additive’s high porosity, the specimens’ strength may decrease significantly [54,55].
After drying, the strength of samples and structure densification of all compositions increase due to water evaporation, especially in RCW25 composition. That could be influenced by the additional hydration of the cement minerals in the RCW additive [56]. This assumption is confirmed by EC research (Figure 7), witnessing that some amount of cement minerals in RCW has remained unhydrated and, after mixing with water, can participate in hydration reactions. The increase in strength during the drying process is characteristic of compositions with a blend of deflocculants [57] due to the additional hydration of not-reacted cement minerals in the presence of components with a high amount of amorphous siliceous phase. The siliceous phase in the presence of a blend of deflocculants may create the mineral stratlingite, which, according to research [58], can appear at temperatures of 45–65 °C. An additional possible reason to increase the compressive strength of concrete after drying is CA2 hydration, which is strongly increased when the treatment temperature reaches 40 °C or more [59].
When samples were fired at temperatures of 800 °C and 1100 °C, sintering processes occurred in the samples. Depending on the deflocculant used, the structure and compressive strength of the samples are different. The highest compressive strength of 28 MPa is characterised by the KNF sample, which used a mixture of deflocculants. The KN and KF samples, in which individual deflocculants were used, are characterised by a lower compressive strength of 23 and 22 MPa.
The lowest CA substitution to RCW level in the composition after firing at 800 °C and 1100 °C temperature shows an increase in compressive strength of up to 7% compared to KNF (without RCW) samples. With an increase in the proportion of RCW to 50, 75 and 100%, the compressive strength is up to 38–44% lower than in the reference KNF sample.
Increasing the porous aggregate in the composition has a noticeable effect on both the density of the samples and the compressive strength. However, it can be noticed that using 50% of CA replacement to RCW still shows satisfactory compressive strength results, which fully correspond to the composition of KF characteristics: 40 MPa after firing at 800 °C and 22 MPa after firing at 1100 °C. MS addition in the presence of a blend of deflocculants makes it possible to almost double the strength of concrete samples after firing at 1000 °C. The same results are reported in a study produced from used refractory lining elements [57,60,61,62].
It can be concluded that using a deflocculant mix ensures a higher degree of utilisation of RCW without affecting the properties of refractory concrete. That is an essential factor in the choice of composition. Combining PCE-20 and ST as deflocculants, due to less mixing water in the composition, significantly improves the density (increase up to 3.8%) and compressive strength (up to 23%) of samples, compared to using these additives separately. This finding highlights the potential of optimised deflocculant mixtures in enhancing refractory concrete’s performance [63].

3.2.4. UPV

The results of ultrasound tests in samples after hardening, drying and firing show that the mix of deflocculants in the composition increases structure parameters by up to 10% compared to compositions with separately used deflocculants (Figure 17). As the processing temperature increases, the UPV values drop due to the destruction processes (water evaporations, decomposition of hydrates, the appearance of new ones) occurring in the specimens. The lowest CA replacement to RCW (25%) in the composition shows a minimal difference compared to the KNF sample in all temperature ranges. That can be because a lower amount of RCW has a minor impact on the cement dissolution process, and some cement from RCW can also contribute to cement hydration. As the CA replacement by RCW increases, the UIG values drop more significantly due to the porous structure of RCW after firing at 800 °C and 1100 °C. Figure 17 illustrates the progressive drop in UPV values as RCW content increases, highlighting the direct relationship between increased porosity and reduced structural integrity. This trend suggests that RCW’s porous nature limits its application in high-load scenarios unless porosity reduction techniques are implemented. It shows that cement from RCW can poorly participate in the hydration process; free water is too low in composition, cement cannot fully dissolve and it does not create additional bonds with porous aggregate [64,65]. UPV measurements correlate well with all other results (density, strength and shrinkage).

3.2.5. Shrinkage

The shrinkage of the refractory concrete samples was determined after firing at a temperature of 1100 °C (Figure 18). KF composition samples show lower shrinkage, 0.16%, whereas KN composition samples show significantly higher values, 0.24%. KNF samples with a mix of deflocculants show 0.28% shrinkage. These results correlate with the density and compressive strength results.
The lowest CA replacement to RCW level in the composition shows a minimal difference in shrinkage compared to the KNF sample. As the CA replacement by RCW increases, the shrinkage increases to −0.4%. This value is not high and suggests that porous aggregate does not increase the shrinkage of refractory concrete samples [60,66]. However, along with low density and sufficient shrinkage, samples with RCW demonstrate sufficiently high compressive strength values.

3.2.6. Porosity

The pore properties are equally important for refractory materials and concrete. The pore size of refractory concretes is mostly from 1.5 µm to 1.1 µm. [65,67]. Our study [48,64] clearly shows that using different deflocculants strongly influences the pore size and amount. The results concerning porosity (Figure 19) after firing at a temperature of 1100 °C show that a mix of deflocculants reduced the porosity of refractory concrete samples to 19.4% (the difference between compositions with separately used deflocculants varied from 4.0 to 5.3%). Such an effect is observed in research [68]. The lowest CA replacement to RCW level in the composition shows minimal difference in porosity compared to the KNF sample.
Porosity increases up to 21%. As the CA replacement by RCW increases, the porosity tends to increase. The difference between KNF and RCW50–RCW100 samples reaches 10%. Generally, it is known that the total porosity of convenient refractory with high aluminate cement amount varies in the range of 20–23%, whereas the porosity of middle aluminate cement amount concrete varies in the range of 18–20% [67]. These results are presented for concretes with untreated aggregate used as fillers. In our case, the porosity results reflect porous aggregate presence in the composition, which is why our study results are higher than those in the research [69]. However, the porosity results of our sample correlate with density, compressive strength and UPV results.
In conclusion, it can be said that concretes with a higher RCW content are advisable to be used in refractory lining areas where the risk of corrosion is lower compared to areas exposed to more aggressive environments. However, concretes in which the CA replacement level to RCW is up to 25% can also be used in more responsible refractory lining areas. The increase in porosity with higher RCW replacement levels (up to 29.3% at 100% RCW) directly affects compressive strength and thermal resistance. That suggests that compositions with over 50% RCW should be limited to applications with lower mechanical stress requirements.
In Figure 20, the dependence of concrete samples porosity on concrete compressive strength at different CA replacement levels to RCW is described by regression equations. It shows a practically linear dependence between the growth of porosity and the decrease in concrete compressive strength. The coefficient of determination R2, whose value is 0.96, indicates a good correlation between concrete samples’ porosity and concrete compressive strength.

3.2.7. Thermal Shock Resistance

The thermal shock resistance of concrete samples of compositions KNF, RWA25, RWA50, RWA75 and RWA100 (Figure 21) was assessed at a temperature of 800 °C by cyclically heating and cooling of concrete samples previously fired at a temperature of 800 °C. The most significant changes in UPV occurred after the first cycle for controlling KNF concrete with both deflocculant admixtures. The UPV values decreased by up to 20.8%. For concrete samples with RCW, the decrease in UPV is less, and according to an increase in RCW amount, it is as follows: 16.2, 13.3, 16.6 and 18.3%. When the amount of RWC in the sample is increased to 50% of replacement, the samples are less susceptible to destruction during the first cycle (from 20.8 to 13.3%). As we can see, the higher the RCW replacement level (up to 100%), the more the effect of the first thermal cycle on the structure increases.
Further, after three cycles, the study of the samples showed that in the control samples without RWC, the UPV value decreased up to 29.2% from the ultrasound values after firing at 800 °C. After three cycles, previously observed trends persist for concrete samples with RCW. A decrease in UPV is observed in the samples with a lower RWC replacement level, and a further increase in the destruction was observed according to an increase in RCW amount in composition, as follows: 24.1, 20.7, 23.3 and 24.2%. Further, after seven cycles, structure-destroying occurs, and UPV values are further decreasing compared to the ultrasound values after firing at 800 °C and are as follows: 34% (for KNF samples), 29.9, 26.3, 28.8 and 35.7%, according to an increase in RCW amount in composition. Samples with up to 50% RCW replacement exhibit lower UPV loss during thermal cycles due to the ability of porous RCW aggregates to dissipate stress. However, higher replacement levels reduce overall structural integrity, correlating with increased porosity and decreased compressive strength. A specific relationship between concrete sample porosity and structural changes after thermal cycling is visible. KNF samples with the lowest porosity of 20.1% and a relatively denser structure (density 1970 kg/m3) most visibly lose structural integrity and UPV values after the first cycle. Lower loss of UPV values is observed for RWA50 composition samples with porosity values of 22.6% because waste concrete aggregates are more porous than untreated aggregates and better compensate for the tension in the structure. In the case of the highest amount of waste concrete aggregates, the lower compressive strength of samples does not provide enough stress and tension distribution in structures during thermal cycles. The main drop in UPV in all concrete samples occurs during the first three cycles.
In order to confirm the obtained results, photos of the concrete sample (KF, RWA50 and RWA100) surface after burning at 800 °C and after the seventh cycle are presented (Figure 22). This figure shows that KNF samples look densest, and the surface of RWA50 and RWA100 looks more porous before thermal cycles. After seven thermal cycles, large cracks up to 2–3 mm in width form on the surface of samples of KNF, whereas only isolated small cracks develop on the surface of RWA50, and more pronounced cracks appear on the surface of samples of RWA100.
While the findings indicate that RCW can partially replace CA without significant loss of mechanical properties, the higher porosity and reduced density at replacement levels above 50% may limit its application in high-stress environments. Future studies could explore optimising RCW pretreatment methods to reduce porosity and enhance its compatibility with cement matrices.

4. Conclusions

The article discusses the use of undervalued refractory waste in the refractory industry, which provides an understanding of the benefits that the aforementioned industry can obtain by recycling waste and turning waste into valuable resources with broad applications. This work assessed the possibility of using refractory concrete waste (RCW) to replace chamotte aggregate (CA) (up to 100% replacement in compositions) in refractory concrete in a deflocculant mixture. EC research and EXO profile tests indicate that some unreacted cement in RCW can, after mixing with water, participate in hydration reactions and enhance the physical and mechanical properties of refractory concrete.
The electrical conductivity of suspensions of pure cement and RCW revealed that some unhydrated aluminate cement remains in RCW. The electrical conductivity of cement suspension is up to 2 times higher than that of RCW suspension and 4 times that of untreated chamotte (MCA) suspension. Cement suspension pH values are (10–11), in RCW suspension (8.6–9.5), and in the MCA suspension, pH values are near neutral—7.4–7.6.
Spread, UPV and EXO profile analysis confirmed the electrical conductivity and pH test results, showing that some unhydrated cement minerals are present in the RCW. Increased replacement of CA to RCW decreases spread by up to 30%, prolongs the structure formation period in the past by 2 times, and prolongs the time of EXO maximum by 2.3 times, respectively, compared to the KNF composition paste. That happens due to the porous nature of RCW, which adsorbs the mixing water and results in the retardation of the dissolution process of cement minerals. However, replacing CA with RCW (up to 25%) does not affect the EXO maximum temperature noticeably; increasing CA replacement to RCW by up to 100% decreases the EXO maximum temperature by 15%. CA replacement to RCW significantly slows the hydration of cement, so its replacement level in refractory concrete should be strictly limited and not exceed 32% in the refractory concrete composition.
Refractory concrete density and UPV studies have shown that due to a decrease in mixing water and structure densification, the density of samples with a mix of deflocculants is higher than in compositions with individual deflocculants during all treatment temperatures. Proportionally to the CA replacement to RCW level in composition, the density of the samples decreases up to 4% due to the higher porosity of RCW and higher amount of mixing water in the composition.
It was found that a mix of deflocculants NT and FS20 in refractory concrete increased the compressive strength more effectively (strength is up to 23% higher) than the individual deflocculants. Due to the additional amount of cement minerals in RCW aggregates, which can participate in the hydration process, the lowest CA replacement to RCW (25%) in the composition increases compressive strength up to 7% during all treatment temperatures. Further, increase in CA replacement to RCW up to 100% due to higher RCW aggregate porosity decreases compressive strength up to 44% compared to a reference sample. However, it can be noticed that using 50% of CA replacement to RCW still shows satisfactory compressive strength results, which fully correspond to the composition of KF characteristics: 40 MPa after firing at 800 °C and 22 MPa after firing at 1100 °C.
UPV results confirm both density and compressive strength and thermal cycle studies. As the processing temperature increases, the UPV values drop due to the destruction occurring in the specimens.
The shrinkage and porosity of the refractory concrete samples with individual deflocculants FS20 and NT after firing at 1100 °C were 0.16% and 20.6% and 0.24% and 20.2%, respectively. Using a mix of deflocculants increases the shrinkage of samples to 0.28% but decreases porosity up to 19.4%. The lowest CA replacement to RCW 25% does not influence the shrinkage and porosity values, whereas higher CA replacement by RCW level increases, the shrinkage increases to −0.4% and porosity up to 29.3%. RCW’s high porosity influences this effect.
Due to a more porous RCW structure, CA replacement to RCW positively influences the concrete’s resistance to sudden temperature changes when CA replacement of RCW does not exceed 50%.
It can be concluded that using a deflocculant mix ensures a higher degree of utilisation of RCW without affecting the properties of refractory concrete. That is an essential factor in the choice of composition. A deflocculant mix enhances refractory concrete density and compressive strength, particularly at lower RCW replacement levels (up to 25%). This approach offers a sustainable alternative for industrial applications, balancing mechanical performance with environmental benefits.
Due to the research limitation, this investigation only considered the effect of deflocculant and RCW amount on the properties of refractory concrete. Future research should focus on RCW pretreatment methods, which would allow the activation of unreacted cement to obtain additional effects of RCW on concrete strength growth. Another promising future research direction should focus on RCW pretreatment methods that would allow further porosity reduction (milling, creating dense surface layers on the RCW aggregate surface, granulation) and exploration of the applications in high-performance refractory systems.

Author Contributions

Conceptualisation, J.P. and I.P.; methodology, I.P.; validation, I.P.; formal analysis, J.P.; investigation, I.P.; resources, J.P.; data curation, J.P.; writing—original draft preparation, J.P. and I.P.; writing—review and editing, J.P. and I.P.; visualisation, J.P.; supervision, I.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD pattern of the alumina cement.
Figure 1. XRD pattern of the alumina cement.
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Figure 2. The granulometric composition of CA, MCA and RCW.
Figure 2. The granulometric composition of CA, MCA and RCW.
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Figure 3. Basic scheme of RCW production.
Figure 3. Basic scheme of RCW production.
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Figure 4. XRD pattern of the FA.
Figure 4. XRD pattern of the FA.
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Figure 5. Particle size distribution of FA.
Figure 5. Particle size distribution of FA.
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Figure 6. The equipment for measuring the temperature of the exothermic effect: 1—Computer, 2—Data logger, 3—Temperature sensors, 4—Experimental containers and 5—Test sample.
Figure 6. The equipment for measuring the temperature of the exothermic effect: 1—Computer, 2—Data logger, 3—Temperature sensors, 4—Experimental containers and 5—Test sample.
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Figure 7. The electrical conductivity of cement, RCW and MCA suspensions, measured for 60 min.
Figure 7. The electrical conductivity of cement, RCW and MCA suspensions, measured for 60 min.
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Figure 8. The pH of cement, RCW and MCA suspensions, measured for 60 min.
Figure 8. The pH of cement, RCW and MCA suspensions, measured for 60 min.
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Figure 9. The course of the exothermic reaction of RCW and concrete compositions (KNF, RCW25, RCW50, RCW100) with different amounts of RCW additive.
Figure 9. The course of the exothermic reaction of RCW and concrete compositions (KNF, RCW25, RCW50, RCW100) with different amounts of RCW additive.
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Figure 10. Spread of refractory concrete pastes with varying deflocculants and RCW replacement levels.
Figure 10. Spread of refractory concrete pastes with varying deflocculants and RCW replacement levels.
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Figure 11. Concrete paste structure development over time.
Figure 11. Concrete paste structure development over time.
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Figure 12. The relationship between EXO maximum time and sharp structure densification start time.
Figure 12. The relationship between EXO maximum time and sharp structure densification start time.
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Figure 13. The influence of different deflocculant types, blends and RCW replacement levels on the density of refractory concrete samples after treatment at various temperatures.
Figure 13. The influence of different deflocculant types, blends and RCW replacement levels on the density of refractory concrete samples after treatment at various temperatures.
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Figure 14. View of the macrostructure of samples (a) K1, (b) K2 and (c) K3 with separate and blend of deflocculants PCE-20, ST and blend of PCE-20 and ST.
Figure 14. View of the macrostructure of samples (a) K1, (b) K2 and (c) K3 with separate and blend of deflocculants PCE-20, ST and blend of PCE-20 and ST.
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Figure 15. View of the microstructure of samples (a) K1, (b) K2 and (c) K3 with separate and blend of deflocculants PCE-20, ST and blend of PCE-20 and ST.
Figure 15. View of the microstructure of samples (a) K1, (b) K2 and (c) K3 with separate and blend of deflocculants PCE-20, ST and blend of PCE-20 and ST.
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Figure 16. The influence of different deflocculant types, blends and RCW replacement levels on the compressive strength of refractory concrete samples after treatment at various temperatures.
Figure 16. The influence of different deflocculant types, blends and RCW replacement levels on the compressive strength of refractory concrete samples after treatment at various temperatures.
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Figure 17. Changes in the UPV of concrete samples with different deflocculants and their mixtures after treatment at different temperatures.
Figure 17. Changes in the UPV of concrete samples with different deflocculants and their mixtures after treatment at different temperatures.
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Figure 18. Shrinkage of refractory concrete samples after firing at 1100 °C with varying CA replacement by RCW and different deflocculant compositions.
Figure 18. Shrinkage of refractory concrete samples after firing at 1100 °C with varying CA replacement by RCW and different deflocculant compositions.
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Figure 19. The porosity of refractory concrete samples after firing at 1100 °C with varying CA replacement by RCW and different deflocculant compositions.
Figure 19. The porosity of refractory concrete samples after firing at 1100 °C with varying CA replacement by RCW and different deflocculant compositions.
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Figure 20. Correlation between porosity and compressive strength.
Figure 20. Correlation between porosity and compressive strength.
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Figure 21. UPV measurements in concrete samples after firing at 800 °C and thermal cycles.
Figure 21. UPV measurements in concrete samples after firing at 800 °C and thermal cycles.
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Figure 22. The surface of samples after burning at 800 °C temperature (on the left side) and after thermal cycles (on the right side): (a)—KNF, (b)—RCW50, (c)—RCW100.
Figure 22. The surface of samples after burning at 800 °C temperature (on the left side) and after thermal cycles (on the right side): (a)—KNF, (b)—RCW50, (c)—RCW100.
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Table 1. Compositions of cement paste and refractory concrete with different amounts of RCW, %.
Table 1. Compositions of cement paste and refractory concrete with different amounts of RCW, %.
ComponentsSample Series
K1K2K3KFKNKFNRCW25RCW50RCW75RCW100
Cement10010010020202020202020
CA–––656565483217–
MCA–––12121212121212
RCW –––00017334865
FA–––3333333
ST *–0.10.1–0.10.10.10.10.10.1
PCE-20 * 0.1–0.10.1–0.10.10.10.10.1
Water *11.8 11.510.811.811.510.810.911.612.313.1
* over 100% dry components.
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MDPI and ACS Style

Pranckevičienė, J.; Pundienė, I. Advances in Deflocculant Utilisation in Sustainable Refractory Concrete with Refractory Waste. Sustainability 2025, 17, 669. https://doi.org/10.3390/su17020669

AMA Style

Pranckevičienė J, Pundienė I. Advances in Deflocculant Utilisation in Sustainable Refractory Concrete with Refractory Waste. Sustainability. 2025; 17(2):669. https://doi.org/10.3390/su17020669

Chicago/Turabian Style

Pranckevičienė, Jolanta, and Ina Pundienė. 2025. "Advances in Deflocculant Utilisation in Sustainable Refractory Concrete with Refractory Waste" Sustainability 17, no. 2: 669. https://doi.org/10.3390/su17020669

APA Style

Pranckevičienė, J., & Pundienė, I. (2025). Advances in Deflocculant Utilisation in Sustainable Refractory Concrete with Refractory Waste. Sustainability, 17(2), 669. https://doi.org/10.3390/su17020669

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