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Article

Self-Compacting Concrete with Artificial Lightweight Aggregates from Sugarcane Ash and Calcined Scheelite Mining Waste

by
Jemima Tabita F. de Sousa
1,
Marcos Alyssandro S. dos Anjos
2,3,*,
José Anselmo da S. Neto
3,
Evilane C. de Farias
4,
Fernando G. Branco
5 and
Cinthia Maia Pederneiras
6
1
Department of Civil Engineering, Federal University of Rio Grande do Norte (UFRN), Natal 59078-970, Brazil
2
Department of Civil Engineering, Federal Institute of Education, Science and Technology of Paraíba (IFPB), João Pessoa 58015-435, Brazil
3
Department of Materials Engineering, Federal University of Paraíba (UFPB), João Pessoa 58051-900, Brazil
4
Department of Civil Engineering, Federal Institute of Education Science and Technology of Rio Grande do Norte (IFRN), Natal 59078-970, Brazil
5
ISISE ARISE Department of Civil Engineering, University of Coimbra, 3000-370 Coimbra, Portugal
6
c5Lab Sustainable Construction Materials Association, 2795-242 Amadora, Portugal
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(1), 452; https://doi.org/10.3390/app15010452
Submission received: 20 November 2024 / Revised: 23 December 2024 / Accepted: 3 January 2025 / Published: 6 January 2025
Browse Figures
Versions Notes

Abstract

:
Self-compacting concrete (SCC) is a relevant technology and an alternative to conventional concrete in complex structures due to its exceptional workability. The rheological parameters demonstrated by SCC provide high fluidity and cohesion, resulting in high mould-filling capability and segregation resistance, as well as optimising concreting processes and reducing costs. In view of this, self-compacting lightweight concrete (SCLC) has emerged as a possible alternative as it combines the benefits of SCC and lightweight aggregate concrete (LWAC). In the production of LWC, the most widely used lightweight aggregate in the world, and also in Brazil, is still expanded clay; however, Brazilian production is restricted to the southeast region. In this context, previous studies have verified the feasibility of producing lightweight aggregates from the sintering of industrial waste and regional raw materials (Rio Grande do Norte/Brazil), such as sugarcane bagasse ash (SBA), scheelite mining residue (SMR), and local clay. Therefore, this study evaluates the influence of three lightweight aggregates, analysing their performance in comparison with SCLC produced with commercial lightweight aggregate (expanded clay). The concretes studied were subjected to characterisation tests in a fresh state; fluidity, apparent viscosity, visual stability, and passing ability were assessed through slump flow tests, flow time (T500), visual stability index, and J-ring, respectively, as well as measurement of the fresh specific mass. In the hardened state, tests were carried out to determine the compressive strength at 7 and 28 days, the dry specific mass, the chloride ion diffusion coefficient, and the thermal conductivity. The new concretes had density values ranging from 1.94 to 2.03 g/cm3 and compressive strength values at 28 days between 26.11 and 36.72 MPa. The results obtained show that it is feasible to produce SCLC with unconventional lightweight aggregates based on sugarcane bagasse waste and scheelite mining waste.

1. Introduction

Self-compacting lightweight concrete (SCLC) combines the unique advantages of self-compacting concrete (SCC) and lightweight aggregate concrete (LWAC), making it a versatile material for applications such as tall buildings and large-span structures. This combination reduces the self-weight of structures and improves their seismic performance, while maintaining the exceptional workability and mould-filling capability characteristic of SCC [1,2]. SCC achieves compaction under its own weight without the need for vibration thanks to its high viscosity and cohesion, which also prevent segregation during handling and placement [3,4,5,6]. Incorporating lightweight aggregates into SCC further enhances its properties by reducing density, lowering transportation costs, and improving onsite workability, as well as thermal and acoustic performance for building occupants [7].
Several lightweight and recycled aggregates, such as expanded clay [8], dolomite [9], marble and granite [10], expanded polystyrene [11], ground granulated blast-furnace slag, and metakaolin [12], have been successfully used in SCLC. The performance of lightweight concrete depends heavily on the mineralogy, production methods, and porosity of the aggregates. These factors influence workability, mechanical strength, density, and thermal conductivity, as well as the transition zone between the aggregate and cement matrix [2,13,14]. While substantial research has explored the properties of SCLC using various aggregate types [14,15,16,17,18,19,20,21,22,23], the development of sustainable alternatives remains a growing priority in the construction industry.
In Brazil, the production of lightweight aggregates is largely limited to a single manufacturer in São Paulo, which uses clay as the sole raw material in a rotary kiln process. This geographical concentration hinders the widespread adoption of lightweight aggregates in regions far from the Southeast, such as the North and Northeast [24]. Simultaneously, the increasing scarcity of natural resources has driven the search for sustainable materials. Two promising byproducts generated in Brazil’s Northeast—sugarcane bagasse ash (SBA) and scheelite mining residue (SMR)—offer a viable alternative for producing artificial lightweight aggregates [25,26].
SBA, a byproduct of renewable energy production, is generated in significant quantities due to Brazil’s extensive sugarcane industry, with the 2022/2023 harvest reaching 615.84 million tons, 45.6 million of which were from the northeast, representing an increase of 10.8% [25,26]. This generates a substantial amount of bagasse ash, which is currently underutilized. Similarly, in the mining of scheelite (CaWO4) used for tungsten production, 99.2% of the extracted material becomes waste. In the municipality of Currais Novos, RN, approximately 10 million tons of this waste have already been accumulated, with 14 local mining companies contributing to this total. Previous studies have demonstrated the technical feasibility of using SBA and SMR for producing lightweight aggregates; however, their application in SCLC remains unexplored.
This study represents a pioneering investigation into the development of SCLC incorporating unconventional lightweight aggregates produced from SBA and SMR. These aggregates, sourced from the northeast region, offer a sustainable alternative to traditional materials like expanded clay, which is widely used but regionally limited. By reducing CO2 emissions, logistics costs, and delivery times, this work supports the use of industrial byproducts and promotes sustainable practices in civil construction. The fresh and hardened properties of SCLC produced with these novel aggregates, as well as their durability, are systematically evaluated. This approach advances sustainable waste management while contributing to the development of environmentally friendly construction materials tailored to regional needs.

2. Materials and Methods

2.1. Materials

To produce the aggregates, waste from the cities of Arês and Currais Novos in Rio Grande do Norte, Brazil, were used. These wastes were the by-products of sugar and alcohol activities in the case of sugarcane bagasse ash (SBA) and mining in the case of scheelite mining residue (SMR). The aggregates used in this study were selected based on previous research by Leal de Souza et al. [25] and Souza et al. [26]. The selection was made based on the characteristics and results presented by the authors, considering the technical feasibility and accessibility for larger-scale production. Figure 1 illustrates the waste materials used.
Three types of calcined lightweight aggregates were produced for the study: two using sugarcane bagasse ash at two percentages, SBA50 and SBA90, and one aggregate sintered with scheelite mining residue and rice husk ash, SMR37. All aggregate formulations used red clay from the regional ceramic industry, originating from the municipality of Assú in Rio Grande do Norte, Brazil, along with water for mixture homogenization. Table 1 presents the proportions of the constituent materials for each aggregate and the sintering temperatures.
To produce SBA50 and SBA90 aggregates, the sugarcane biomass residue was processed by drying in an oven at 110 ± 5 °C for 24 h, grinding in a Willey knife mill (model SL-31) at 1750 rpm, using 2 kg of SBA for 0.20 h, and sieving to obtain fractions no larger than 0.30 mm, as per Leal de Souza et al. [25]. To produce SMR37, the scheelite residue was processed similarly to the SBA, with the difference being the use of a three-phase ball mill and sieving with an opening of 0.15 mm, according to the methodology used by Souza et al. [26]. A schematic illustration of the procedure used to prepare aggregates from waste is described in Figure 2.
The constituent materials of each aggregate were mixed dry, then placed in a mortar mixer with water. The mixture was subsequently pelletized, limiting the aggregate diameter to approximately 9.5 mm. After air-drying and oven-drying, with each process lasting 24 h, the raw aggregates were sent for sintering in a muffle furnace (Figure 3). In this sintering stage, the aggregates were subjected to temperatures of 1175 °C and 1150 °C for SBA and SMR aggregates, respectively, for 15 min, with heating rates of 10 °C/min and 8 °C/min. Approximately 45 dm3 of each sintered aggregate was produced.
After sintering, the produced lightweight aggregates were characterized in terms of expansion index, bulk density, unit weight, 24 h water absorption, and crushing strength. Table 2 presents the characteristics of the aggregates developed in this study.
After the production and characterization of the lightweight aggregates, the production of self-compacting lightweight concrete (SCLC) used the materials described in Table 3.
Portland cement of type CP V ARI, classified by [27], was chosen because it does not contain mineral additions that could influence the performance of the lightweight self-compacting concrete and because it yields results at early ages. Metakaolin, marketed as BZ, was used as an additive to improve cohesion and mechanical performance and to reduce the environmental impact of the concrete. Table 4 presents the physical characteristics of the materials used.
For the fine aggregate, a quartz sand from riverbed deposits with a maximum diameter of 1.2 mm and a fineness modulus of 1.3, with a specific mass of 2.63 g/cm3 and unit mass of 1.51 g/cm3, was used. For the coarse aggregate, in addition to the aggregates (SBA50, SBA90, and SMR37), commercial expanded clay from Cinexpan Indústria e Comércio (São Paulo/SP, Brazil) was used as the reference aggregate. The expanded clay had a specific mass of 0.97 g/cm3, unit mass of 0.54 g/cm3, water absorption of 14%, and crushing strength of 2.23 MPa.
The water used in the mixtures came from the local utility responsible for distribution in the state of Rio Grande do Norte/RN (CAERN), Brazil. To ensure the required fluidity properties for self-compacting lightweight concrete, MasterGlenium SCC 160, a superplasticizer based on polycarboxylate ether with a solid content between 38% and 42%, was used as the chemical additive.

2.2. Mix Proportions

Four mixes of self-compacting lightweight concretes were produced. The first mix used commercial expanded clay aggregate (SCLC-EC), the second used SBA50 aggregate (SCLC-SBA50), the third used SBA90 aggregate (SCLC-SBA90), and the fourth mix used SMR37 aggregate (SCLC-SMR37). The production of the SCLC was carried out using aggregates in a saturated surface-dry condition, maintaining the same water-to-cement ratio (W/C) for all compositions. While keeping the w/c ratio constant, the amount of superplasticizer was adjusted according to the behaviour of each aggregate in the mixture to meet the self-compacting parameters.
After determining the compositions for each mix and evaluating the performance of the aggregates in the mixture, the material consumption was defined in kg/m3 based on the specific masses of the materials used, as shown in Table 5.
After determining the material quantities, 12 cylindrical test specimens (100 × 200 mm) were produced for each mix. After casting, the specimens were kept under ambient curing for 24 h, covered with plastic film to prevent water loss, then demoulded and placed in wet curing. The samples were removed from wet curing 24 h before testing and left to air-dry until the test date.

2.3. Test Programme

Self-compacting lightweight concrete (SCLC) does not yet have specific standards or guidelines for handling. Therefore, the characterization of SCLC in its fresh state has been conducted following the same parameters and standards as conventional self-compacting concrete. Thus, to evaluate fluidity, apparent viscosity, visual stability, and passing ability, tests were performed according to the parameters established by [28], as described in Table 6.
Following this, the fresh bulk density was determined by weighing the metal moulds before and after concrete placement. The difference in weight, divided by the volume of the mould, provides the bulk density of the fresh concrete.
Studying the characteristics of hardened SCLC is essential to assess performance at different ages and ensure compliance with standard requirements. After hardening, dry bulk density was determined at 7 and 28 days. Four samples were measured, and the average was considered as the final value.
Axial compressive strength tests were performed following the recommendations of [31] at 7 and 28 days. Four samples were used for each age, and the average was considered. The same test specimens from the dry bulk density test were used. Twenty-four hours before the tests, the concrete specimens were sulphur-capped to level the faces, ensuring uniform load distribution in the mechanical press. The compressive strength test was conducted using a Shimadzu AG-X 300 kN press with a loading rate of 0.30 MPa/s, following the specifications of [31,32].
In addition to bulk density and compressive strength, the efficiency factor (EF) was another parameter used to evaluate the SCLC. The EF is determined by the ratio of compressive strength at 28 days to the bulk density of the concrete. For each mix, the EF was calculated using the compressive strength values at 28 days and the average dry bulk density values.
The diffusion coefficient of chloride ions was assessed using the method established in [33], following the specifications of [34]. This test provides information on the resistance to chloride ion penetration in concrete by determining the diffusion coefficient. As it is accelerated by an electric current, it is possible to characterise the concrete in up to 72 h, depending on the quality of the concrete tested. The concretes were tested 186 days after casting, with specimens measuring 50 ± 5 mm in height and 100 ± 2 mm in diameter and 4 specimens for each design. The specimens were prepared by wet curing up to 28 days and outdoor curing up to the date of the test. The specimens were placed in a saturated Ca(OH)2 solution 24 h beforehand to ensure that the chlorides penetrated the samples predominantly by diffusion. To carry out the test, the specimens were exposed to an anodic solution (0.3 M NaOH) on the upper side and a cathodic solution (10% NaCl) on the lower side, and then subjected to an initial voltage of 30 V, which could increase depending on the passing current, in accordance with the specifications of [34]. Figure 4 illustrates the schematisation of the chloride ion diffusion test by migration in a non-stationary regime.
At the end of the test, the specimens were broken and, by spraying a silver nitrate solution (0.1 M AgNO3) on the broken faces, it was possible to observe the depth of penetration of the ions by colorimetric process due to the precipitation of the silver solution on the affected area. Subsequently, the depth of penetration of each trace was measured based on the NT Build 492 guidelines [33].

3. Results and Discussion

3.1. Fresh Properties

The self-compacting properties of the concretes were determined according to the standards set by [28,29,30]. The fluidity of the concretes was measured using the slump flow test based on the flow measurements. According to [28], concretes can be classified by their flow as SF1 (between 550 and 650 mm), SF2 (between 660 and 750 mm), or SF3 (between 760 and 850 mm), with each class having a specific application.
As shown in Figure 5, the SCLC-EC and SCLC-SBA50 concretes were classified as SF2, suitable for most reinforced concrete applications, while the SCLC-SBA90 and SCLC-SMR37 concretes were classified as SF1, recommended for structures with low reinforcement rates, self-compacting concretes that require pumping, and/or structures that demand little horizontal flow.
The difference in flow between SCLC-EC and the other concretes can be attributed to the low density of the expanded clay aggregate and its vitreous surface, resulting in easier flow of the cementitious matrix due to the rolling action of the aggregates [17].
Similarly, although the aggregate in SCLC-SBA90 had a higher density than the aggregate in SCLC-SMR37, the greater flow of SCLC-SBA90 can be explained by its water absorption. The [35] states that ideal water absorption values for lightweight aggregates typically range from 5% to 25%. Thus, while the SBA90 aggregate had a water absorption of 15%, which facilitated better homogenization between the aggregate and the cementitious matrix and was closer to the expanded clay’s absorption of 14%, the SMR37 aggregate had a water absorption rate of 2.5%, which made it more difficult to bond quickly with the cement paste.
In Figure 6, the results of the flow time test (T500), which measured the apparent viscosity of the concrete, are presented. Apparent viscosity is related to the consistency of the mixture and directly influences the concrete’s resistance to flow.
According to the results, all the concretes were classified as VS2, making them suitable for most common applications. This classification also indicates a thixotropic effect, meaning the concrete exerted less pressure on the forms and had improved resistance to segregation, as per the definitions established by [28].
In agreement with Barnat-Hunek et al. [21], Figure 7 shows a relationship between the increase in fine aggregate consumption and the viscosity of the concretes. As the amount of sand in the mixture increased, the flow time also increased, a result of the density differences between the aggregates and the cementitious matrix [27,28].
In Anjos et al. [29], it was observed that, despite the concretes having the same viscosity classification, mixtures with higher amounts of mineral additions, particularly metakaolin, exhibited higher viscosity values. In the present study, although the amount of metakaolin was the same for all mixes, a directly proportional relationship between the increase in viscosity and the amount of fines in the mixture was noted.
Additionally, Almawla et al. [30] and Nahhab and Ketab [23], found an inverse relationship between the volumetric amount of lightweight aggregates and flow time in their studies. However, in the results obtained in this study, the opposite was observed: increasing the amount of aggregates in the mix increased the flow time. This behaviour was consistent across all mixtures, except for SCLC-SMR37, which can be explained by the low absorption rate of the aggregates that prevented complete homogenization with the matrix.
Figure 8 illustrates the behaviour of the concretes during the slump flow test, which provided the visual stability index. This qualitative parameter evaluated concrete’s resistance to segregation and/or bleeding. According to the observations, all the mixtures were classified as VSI 0, as per [29], meaning that all the mixtures flowed uniformly without evidence of segregation or bleeding, with aggregates visible to the edge of the flow.
Finally, passing ability, which evaluates the concrete’s ability to flow around obstructions, was assessed using the J-ring test. Figure 9 presents the passing ability results for the SCLCs.
All the concrete mixes were classified as PJ2, meaning they were suitable for most applications and for structural elements with reinforcement spacing between 60 and 80 mm, according to the parameters established by [1]. Figure 10 shows the performance of each mix during the test, indicating that all mixtures experienced slight aggregate accumulation in the centre, though no signs of segregation or bleeding were observed at the edges.
The last characteristic evaluated in the fresh state was bulk density. Figure 11 shows the average fresh bulk density values of the concretes studied, along with their respective standard deviations.
Due to the high absorption of the aggregates in the SCLC-SBA50 and SCLC-SBA90 mixes, with absorption rates of 25% and 15%, respectively, the fresh bulk density values exceeded the normative limit for lightweight self-compacting concretes. These higher values were associated with the amount of water absorbed by the aggregates during the saturation period. Additionally, it can be observed that the mixes with the lower volume of lightweight aggregate per cubic meter corresponded to higher bulk density values. These results align with the findings of Nepomuceno et al. [13], which indicated that reducing the volume of lightweight aggregate led to an increase in concrete density, establishing an inversely proportional relationship between these factors.
Based on the results of the fresh concrete characterization tests, one may conclude that all mixtures met the requirements established in the standards, demonstrating the feasibility of using the new aggregates for producing SCLC when compared to commercial expanded clay aggregates. It is worth noting that the mix with the SBA50 aggregate, despite the differences in characteristics compared to the commercial aggregate, showed consistent performance, with values close to those of the SCLC-EC mix.

3.2. Mechanical Properties

For the hardened-state properties, the results of the mechanical compressive strength test are discussed. The concretes were subjected to axial compressive strength tests at 7 and 28 days. Figure 12 presents the average compressive strength values and the respective standard deviation ranges for each mix.
As shown in Figure 12, the SCLC-EC sample exhibited greater variation in compressive strength values compared to the other mixtures studied. This behaviour can be explained by the intrinsic nature of the commercial lightweight aggregates used (expanded clay), which have greater porosity and can present variable mechanical properties, such as crushing resistance, depending on their microstructure and production process. Previous studies [2,13] have highlighted that this variability is a characteristic challenge of the use of lightweight aggregates in structural concretes.
Another potential factor is the influence of the surface texture of commercial aggregates. Aggregates with lower uniformity may affect the homogeneity of the cement matrix during mixing, creating localized zones of lower or higher density, which directly reflects on the dispersion of the strength results. In addition, the rheological behaviour of SCLC-EC, combined with the lower water absorption of expanded clay (14%, according to Table 2), may have reduced the efficiency of the interaction between the aggregate and the cement paste, impacting the paste–aggregate transition.
Despite the greater variability, SCLC-EC presented average strength values that exceeded the minimum limits established by ACI 213 R-03 for structural lightweight concrete. This result reinforces its technical feasibility and highlights the importance of considering the variability of commercial aggregates when designing lightweight concretes. Future studies could focus on the detailed analysis of the aggregate microstructure and the refinement of the mixing process to mitigate these effects.
Based on the results obtained, all concretes, even at 7 days, met the minimum compressive strength requirements for structural concrete as per [36,37,38], which sets a minimum strength of 20 MPa, and for lightweight structural concrete as per [35], which requires a minimum strength of 17 MPa. At 28 days, the strength values significantly exceeded the minimum required for lightweight concretes, showing considerable strength gains.
In the work of Angelin et al. [31], it was found that as the amount of larger lightweight aggregate (Dmax > 10 mm) increased, the compressive strength decreased, supporting a previous study [28]. Therefore, a solution to mitigate the influence of larger aggregate diameter is to partially replace the larger lightweight aggregate with a smaller lightweight aggregate (less than 10 mm). However, this may increase the concrete bulk density in most cases [2,32].
Since the aggregates used in the mixes had a standardized maximum diameter of 9.5 mm, this behaviour was not observed in the results of the present study. However, the influence of these factors became noticeable only after 7 days. According to Angelin et al. [31], lightweight aggregates do not affect compressive strength at early ages, as the cementitious matrix is responsible for withstanding mechanical stresses. In this regard, the volume paste-to-aggregate (Vp/Vs) ratio directly affects compressive strength, as a lower ratio (<0.60) results in a larger volume of mortar and a smaller volume of coarse aggregate [13].
Figure 13 presents a relationship between the volume of lightweight aggregate per cubic meter, the mechanical strength from 7 to 28 days, and the water absorption of the lightweight aggregates.
It can be noticed that the highest compressive strength gains corresponded to mixes with larger volumes of aggregate per cubic meter and lower aggregate water absorption values, as seen in the SCLC-EC and SCLC-SMR37 mixes. The other mixtures had lower strength gains, particularly SCLC-SBA50, which, despite having a higher volume of lightweight aggregate compared to SCLC-SBA90, had a higher water absorption value than SCLC-SBA90. High water absorption within the aggregate can directly influence its crushing strength, making the aggregate more brittle and contributing to the lower performance of the concrete in terms of compressive strength.
Thus, in agreement with Bogas et al. [2], in addition to the proportion of lightweight aggregates in the mix and their water absorption, other specific characteristics of the aggregates, such as crushing strength, can influence the performance of SCLC. A directly proportional relationship between the compressive strength of the studied concretes and the crushing strength of each aggregate was observed, where the mix with the strongest aggregate (SCLC-EC) showed the highest strength gain, while the mix with the weakest aggregate (SCLC-SBA50) showed the lowest strength gain at 28 days.
Conventionally, in normal-density concrete, compressive strength is primarily related to the water-to-cement ratio (w/c) and the amount of binder in the mix [27,33]. However, this relationship cannot be applied effectively to concretes with lightweight aggregates due to the difficulty of determining how much of the mixing water will be absorbed by the aggregates. In this study, the water-to-cement ratio (w/c) was not a significant variable, as the same ratio was maintained in all mixtures. Furthermore, the internal water absorbed by each aggregate during the pre-saturation process was not considered in the calculations, following the premise that this water remains retained within the aggregates and is not released during mixing, as supported by [35].
Therefore, the lightweight aggregate becomes the main factor influencing not only the strength of the concrete, but also all its other properties [27]. Supporting the compressive strength results, Figure 14 shows the self-compacting lightweight concrete specimens after failure, allowing for observation of the aggregates’ behaviour within the cement matrix in terms of dispersion and homogeneity.
It is evident in all the concretes that, unlike conventional concrete, the fracture surface passed through the aggregates, reaffirming the lightweight aggregates’ influence as the limiting factor in compressive strength. According to previous studies [34,35], this occurs due to the improvement of the paste–aggregate transition zone and because there is no significant difference between the elastic moduli of the paste and the lightweight aggregate, leaving the lightweight aggregate as the weakest element in the mixture.
Parallel to this, dry bulk density was measured at 7 and 28 days of curing, along with the compressive strength test. Figure 15 presents the dry bulk density values at 7 and 28 days of curing and the average dry bulk density for each studied mix.
From the results, it is noted that only SCLC-SBA90 showed a bulk density above the limit defined by [38], which is 2.00 g/cm3. However, this behaviour can be explained by the higher amount of lightweight aggregate in the SCLC-SBA90 composition, as seen in Table 5, which led to greater water absorption. One possible solution would be to increase the aggregate size [23], as larger lightweight aggregates have lower bulk density and lower water absorption [18]. Moreover, considering that the variation is not significant, previous studies [20,23,31,36] have found that bulk densities greater than 2.03 g/cm3 still met the classification for lightweight self-compacting concrete, as the other requirements were fulfilled.
For instance, Bogas et al. [2] reported compressive strength values between 22.5 MPa and 30 MPa at 28 days for expanded clay-based self-compacting lightweight concrete, with densities ranging from 1.80 to 1.95 g/cm3. In comparison, our SCLC mixes, such as SCLC-EC and SCLC-SMR37, achieved compressive strengths up to 36.72 MPa and densities between 1.94 and 2.03 g/cm3, demonstrating strong performance with unconventional aggregates like SBA and SMR.
Additionally, studies by Santis and Rossignolo [24] and Lotfy et al. [32] reported similar results for expanded clay and pumice-based lightweight concretes, with compressive strengths around 30 MPa and densities of approximately 1.85 g/cm3. Our SCLC mixes not only matched but exceeded these values, reinforcing the potential of SBA and SMR as viable alternatives for producing high-performance self-compacting lightweight concrete.
Finally, the efficiency factor (EF) was evaluated, which is the ratio between compressive strength and concrete bulk density. In this study, the efficiency factor was calculated for the concretes at 28 days, using the average dry bulk density value. Figure 16 presents the efficiency factor for each SCLC mix.
Based on the results, it can be observed that the efficiency factor for the self-compacting lightweight concretes ranged from 13 to 18 MPa·dm3/kg, with the highest values found in SCLC-SMR37 and SCLC-EC, which were the mixes that exhibited the highest compressive strength and lowest bulk density. Previous studies have found efficiency factors ranging from 22 to 28 MPa·dm3/kg [31]. However, in both studies, different aggregate sizes were used, in addition to partial replacement of lightweight aggregate with normal coarse aggregate, which increased the compressive strength of the concretes and, consequently, the efficiency factor.
According to Santis and Rossignolo [24], the efficiency factor is an important parameter in structural design, and concretes with efficiency factors above 25 MPa·dm3/kg can be considered high-performance concretes. In this study, all the mixes were classified as structural concretes, meeting the compressive strength and bulk density parameters defined by [35].

3.3. Durability Properties

To determine the chloride ion diffusion coefficient in the studied concretes, the procedures established by [34] were followed. In this test, chloride ion penetration occurred in a single direction, from bottom to top, driven by the passage of electric current. Figure 17 shows the concrete specimens after the test and the application of silver nitrate (AgNO3) spray to define the chloride ion penetration depth.
Qualitative evaluation showed that the SCLC-SMR37 mix exhibited the smallest chloride ion penetration front, followed by SCLC-EC, SCLC-SBA50, and finally SCLC-SBA90, which showed the greatest penetration depth. However, it was noted that the aggregates in the SCLC-SMR37 mix floated within the paste and deposited on the top part of the specimen, resulting in segregation between the cementitious matrix and the aggregates. This behaviour can be explained by the low absorption and relatively low density of the SMR37 aggregate, making it difficult to homogenize with the paste as it requires more time to absorb the mixture.
Due to the behaviour exhibited by the SMR37 aggregate, in accordance with the normative specifications of [34], the chloride ion diffusion coefficient for SCLC-SMR37 was disregarded as it did not represent the proper behaviour of the mix. Figure 18 presents the chloride ion diffusion coefficients for the self-compacting lightweight concretes at 186 days, after being subjected to wet and air curing.
From the obtained results, a relationship between the diffusion coefficient and the mortar content of each mix can be observed, as per the data in Table 5. It is noted that the mixes with lower diffusion coefficients, except for SCLC-SMR37, had higher mortar contents. According to Gonen and Yazicioglu [19] and Nahhab and Ketab [23], the presence of porous aggregates contributes to increased permeability in concretes. However, a higher mortar content produces a denser cementitious matrix, reducing permeability by improving the matrix’s pore structure, thus enhancing durability performance.
Additionally, there is microstructural improvement due to internal curing provided by the water retained by the lightweight aggregate, which refines the aggregate–matrix interface, reducing microcracks typically found in concretes with conventional aggregates. These microcracks are the main pathways for aggressive agents to percolate.
Real et al. [37] reported that lightweight aggregate itself does not influence the determination of the diffusion coefficient, indicating no clear relationship between aggregate porosity and the diffusion coefficient. However, other factors such as matrix quality, w/c ratio, binder content, and especially pre-saturation of high-absorption aggregates can affect the concrete’s resistance to chloride ion penetration. It is recommended to avoid saturating the aggregates to reduce their contribution to chloride penetration.
As observed in the study and discussed by other authors [19,23,38], this may have occurred due to the low mortar content in the mixture, as well as the higher proportion ratio between coarse aggregate and mortar (Vg/Vm). Alternatively, as Real et al. [37] found in their study, although lightweight aggregate alone does not influence the determination of the diffusion coefficient, lightweight aggregates with high absorption levels can influence the resistance of concrete to the entry of chloride ions. However, when comparing CLAA-RBC50 (25% absorption) with CLAA-RBC90 (15% absorption), we observed that the diffusion coefficient of CLAA-RBC50 was lower, even though it had the aggregate with the highest absorption. However, when compared to CLAA-RBC90, CLAA-RBC50 has a higher mortar content and a lower Vg/Vm ratio, confirming what was stated in previous studies: The higher mortar content produces a denser cementitious matrix, contributing to the reduction in concrete permeation through the improvement of the matrix pore structure, thus contributing to better performance in terms of the durability of the structures.
Considering this, the results obtained allowed for a comparison between the volume of lightweight aggregate per cubic meter and resistance to chloride ion penetration, as shown in Figure 19.
It is evident that, except for the SCLC-SMR37 mix, the higher the volume of lightweight aggregates in the mix, the lower the chloride ion diffusion coefficient. The porosity of the aggregates does not have a significant direct relation with the resistance to chloride ion penetration. Thus, it can be concluded that matrix quality plays a larger role than lightweight aggregates in influencing durability, as the mix with the greatest susceptibility to chloride ion penetration corresponds to the one with the highest aggregate-to-mortar ratio (Vg/Vm) and the lowest mortar content. Table 7 presents a summary of the results obtained in this study.
SCLC-SMR37 presented the best overall performance, particularly in compressive strength, density, and resistance to chloride ion diffusion. SCLC-SBA90 presented competitive results in some parameters, such as compressive strength and apparent density. However, it was disregarded in the chloride ion diffusion analysis due to less satisfactory behaviour compared to SCLC-EC. Based on the overall balance of performance metrics, SCLC-SMR37 is indicated as the most efficient blend.
The effect of aggregates on the properties of self-compacting concrete (SCC) has been extensively studied in the literature, with numerous international studies emphasizing the significant impact of lightweight aggregates on workability, strength, and durability. For example, Gopi et al. [8] emonstrated that the use of expanded clay aggregates in SCC notably enhanced its workability, achieving slump flow values comparable to those observed in the present study for SCLC-EC. Similarly, Ting et al. [28] highlighted that aggregates with lower density, such as those derived from sugarcane bagasse ash (SBA), improve the flowability and passing ability of SCC, aligning well with the results for SCLC-SBA50 and SCLC-SBA90 in this study.
Further studies by Bogas et al. [2] and Lotfy et al. [21,32] underscore the critical role of water absorption characteristics in lightweight aggregates, which directly influence viscosity and compressive strength. In this study, SCLC-SBA90, with its higher water absorption rate, exhibited a moderate increase in flow time (T500) compared to SCLC-EC. This observation is in line with findings by Barnat-Hunek et al. [21], who reported that aggregates with higher water absorption led to increased viscosity, which in turn affected the overall workability of the concrete.
When comparing the compressive strength results, this study’s findings are consistent with those of Santis and Rossignolo [24], who found that mixes with smaller lightweight aggregates demonstrated higher compressive strength gains due to a better interfacial bond between the aggregate and the cement paste. Our study, SCLC-SMR37, which showed the highest strength at 28 days, follows this trend, further emphasizing the importance of aggregate properties in enhancing concrete strength.
Regarding chloride ion diffusion, the research by Real et al. [37] and Nahhab and Ketab [23] highlights that aggregates with lower porosity and water absorption rates significantly improve resistance to chloride ion penetration. This was evident in the performance of SCLC-SMR37 in this study, which demonstrated a lower chloride diffusion rate compared to other mixtures.
Overall, the results of the present study align well with international research, reinforcing the potential of using unconventional aggregates such as SBA and SMR for producing sustainable self-compacting lightweight concrete. These aggregates not only offer environmental benefits, but also demonstrate optimized performance in both the fresh and hardened states.

4. Conclusions

This study investigated self-compacting lightweight concrete using lightweight aggregates from the reuse of agro-industrial and mining waste, specifically sugarcane biomass ash (SBA) and scheelite mining residue (SMR). An experimental campaign was conducted to compare these unconventional aggregates with commercially available expanded clay aggregates in Brazil. The results highlight the following key points:
  • The results of the slump flow test showed that all concretes studied met the self-compacting characteristics. However, the SCLC-EC and SCLC-SBA50 mixtures demonstrated better performance, making them suitable for most reinforced concrete applications, according to [28].
  • Regarding viscosity measurement, assessed by the T500 test, it was found that an increase in the consumption of both fine and coarse aggregates contributed to higher viscosity in the concretes. The SCLC-SBA90, followed by the SCLC-SBA50, the SCLC-SMR37, and lastly, the SCLC-EC, showed the longest flow time and, consequently, the highest viscosity.
  • In terms of passing ability, all concretes were equally classified as suitable for most applications. However, among the results, the SCLC-EC exhibited a better ability to overcome obstacles due to its self-compacting nature when compared to concretes with non-conventional aggregates, with SCLC-SBA90 displaying the greatest difficulty. However, visual stability indices indicated no evidence of segregation and/or bleeding in the mixtures, where all presented uniform flow and homogeneous behaviour between the paste and aggregate.
  • The SCLC-EC and SCLC-SMR37 concretes showed the lowest specific mass values in the fresh state, while the SCLC-SBA50 and SCLC-SBA90 showed values exceeding the 2.00 g/cm3 limit. The same trend was observed in the determination of air-dried specific mass, except for SCLC-SBA50, which remained within the limit. Nonetheless, the values exceeding the specified limit for lightweight concrete can still be classified as such, given the minimal variation.
  • All studied concretes met the minimum compressive strength values for structural lightweight concrete. The SCLC-EC showed the best performance in terms of strength gain (41.4%); however, the SCLC-SMR37 achieved the highest strength values at 7 days (26.35 MPa) and 28 days (34.15 MPa). Additionally, an inversely proportional relationship was observed between the increase in lightweight aggregate content in the mix and the strength gain from 7 to 28 days, with SCLC-SBA90 and SCLC-SBA50 displaying the lowest strength gains of 23.1% and 18.4%, respectively.
Overall, although the concrete with the commercial aggregate showed a more satisfactory performance in both fresh and hardened properties, the other concretes also met the pre-established requirements and can, thus, be classified as self-compacting lightweight concretes using more sustainable regional lightweight aggregates.

Author Contributions

Conceptualisation: M.A.S.d.A.; Methodology: M.A.S.d.A., J.T.F.d.S. and E.C.d.F.; Validation: M.A.S.d.A., J.T.F.d.S. and E.C.d.F.; Formal analysis: M.A.S.d.A., J.T.F.d.S., E.C.d.F., J.A.d.S.N., F.G.B. and C.M.P.; Investigation: M.A.S.d.A., J.T.F.d.S. and E.C.d.F.; Resources: M.A.S.d.A.; Writing original draft: M.A.S.d.A., E.C.d.F. and J.A.d.S.N.; Writing review and editing: J.A.d.S.N., F.G.B. and C.M.P.; Visualisation: M.A.S.d.A., E.C.d.F., F.G.B. and C.M.P.; Supervision: M.A.S.d.A. and E.C.d.F.; Project administration: M.A.S.d.A. and F.G.B.; Funding acquisition: M.A.S.d.A. and F.G.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors gratefully acknowledge the support of the Fundação de Apoio à Pesquisa do Estado da Paraíba—FAPESQ, National Council for Scientific and Technological Development—CNPq, Instituto Federal da Paraíba—IFPB, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brazil (CAPES), program PVEX 01/2019, process number: 88881.337017/2019-01, FCT/MCTES through portuguese national funds (PIDDAC) under the R&D Unit Institute for Sustainability and Innovation in Structural Engineering (ISISE), under reference UIDB /04029/2020 (https://doi.org/10.54499/UIDB/04029/2020, accessed on 4 January 2024), and under the Associate Laboratory Advanced Production and Intelligent Systems ARISE under reference LA/P/0112/2020 and FCT—Foundation for Science and Technology, I.P., within the scope of the project 2022.03631.PTDC (MSSHBioCem—https://doi.org/10.54499/2022.03631.PTDC, accessed on 4 January 2024).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Waste materials from the sugar, alcohol activities, and the scheelite residue used.
Figure 1. Waste materials from the sugar, alcohol activities, and the scheelite residue used.
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Figure 2. Illustration of the aggregate manufacturing.
Figure 2. Illustration of the aggregate manufacturing.
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Figure 3. Aggregates in the muffle furnace: (a) SBA aggregates; (b) SMR aggregates.
Figure 3. Aggregates in the muffle furnace: (a) SBA aggregates; (b) SMR aggregates.
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Figure 4. Test diagram for determining the diffusion coefficient using the NT Build 492 method [33].
Figure 4. Test diagram for determining the diffusion coefficient using the NT Build 492 method [33].
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Figure 5. Flow of the concretes studied.
Figure 5. Flow of the concretes studied.
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Figure 6. Apparent viscosity of the concretes in the T500 test.
Figure 6. Apparent viscosity of the concretes in the T500 test.
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Figure 7. Relationship between fine aggregate consumption and the viscosity of the concretes.
Figure 7. Relationship between fine aggregate consumption and the viscosity of the concretes.
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Figure 8. Visual stability index based on the edge of the flow.
Figure 8. Visual stability index based on the edge of the flow.
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Figure 9. Passing ability of the concretes measured by the J-ring test.
Figure 9. Passing ability of the concretes measured by the J-ring test.
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Figure 10. Behaviour of the concretes during the J-ring test.
Figure 10. Behaviour of the concretes during the J-ring test.
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Figure 11. Average fresh bulk density of the SCLCs.
Figure 11. Average fresh bulk density of the SCLCs.
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Figure 12. Compressive strength values at 7 and 28 days for each mix.
Figure 12. Compressive strength values at 7 and 28 days for each mix.
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Figure 13. Relationship between strength gain, aggregate volume, and aggregate absorption.
Figure 13. Relationship between strength gain, aggregate volume, and aggregate absorption.
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Figure 14. SCLC specimens after axial compression testing.
Figure 14. SCLC specimens after axial compression testing.
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Figure 15. Dry bulk density at 7 and 28 days of curing.
Figure 15. Dry bulk density at 7 and 28 days of curing.
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Figure 16. Efficiency factors of the SCLCs.
Figure 16. Efficiency factors of the SCLCs.
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Figure 17. SCLC specimens after silver nitrate (AgNO3) spray.
Figure 17. SCLC specimens after silver nitrate (AgNO3) spray.
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Figure 18. Chloride ion diffusion coefficients in the SCLCs.
Figure 18. Chloride ion diffusion coefficients in the SCLCs.
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Figure 19. Relationship between coarse aggregate volume and chloride ion diffusion coefficient.
Figure 19. Relationship between coarse aggregate volume and chloride ion diffusion coefficient.
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Table 1. Composition of aggregate mixtures (% in mass).
Table 1. Composition of aggregate mixtures (% in mass).
AggregateConstituentsTemperature
SBA5050% sugarcane biomass residue (SBA) + 50% red clay + 56% water1175 °C
SBA9090% sugarcane biomass residue (SBA) + 10% red clay + 77% water1175 °C
SMR3737% scheelite residue (SMR) + 29% rice husk ash + 34% red clay + 30% water1150 °C
Table 2. Characteristics of the artificial aggregates produced.
Table 2. Characteristics of the artificial aggregates produced.
AggregateExpansion Index (%)Bulk Density (g/cm3)Unit Weight (g/cm3)Water Absorption (%)Crushing Strength (MPa)
SBA5039.691.270.6125.001.12
SBA9034.721.760.8015.001.38
SMR3740.381.250.652.501.53
Table 3. Constituents of SCLC.
Table 3. Constituents of SCLC.
MaterialDescription
Portland CementCP V—ARI
MetakaolinCommercial
Fine aggregateQuartz fine granulometry—Commercial
Coarse aggregateCommercial expanded clay; SBA50, SBA90, and SMR37
Mixing waterLocal utility
Chemical additiveSuperplasticizer
Table 4. Characteristics of CP V ARI Portland cement and BZ metakaolin.
Table 4. Characteristics of CP V ARI Portland cement and BZ metakaolin.
CharacteristicCement (CP V—ARI)Metakaolin (MK)
Specific mass (g/cm3)3.112.59
Unit mass (g/cm3)1.010.53
Specific surface area (m2/g)2.7614.51
Total pore volume (cm3/g)4.61 × 10−32.97 × 10−2
Table 5. Consumption of materials for the self-compacting lightweight concretes used in the study (kg/m3).
Table 5. Consumption of materials for the self-compacting lightweight concretes used in the study (kg/m3).
CompositionSCLC-ECSCLC-SBA50SCLC-SBA90SCLC-SMR37
Cement449.31468.74476.48457.53
Metakaolin44.9346.8747.6545.75
Fine aggregate831.22867.17881.49846.43
Lightweight coarse aggregate336.98396.09528.89410.40
Water181.97189.84192.97185.30
Superplasticizer (% of fines)1.000.821.301.30
Water-to-cement ratio0.4050.4050.4050.405
Aggregate-to-fines ratio2.362.452.692.50
Table 6. Tests and properties evaluated for SCLC in the fresh state.
Table 6. Tests and properties evaluated for SCLC in the fresh state.
TestProperty EvaluatedReference
Slump flowFilling ability and segregation resistance[29]
T500Filling ability and apparent viscosity (fluidity)[29]
Visual stability indexPresence of bleeding and/or segregation[29]
J-ringPassing ability[30]
Table 7. Summary of the results obtained.
Table 7. Summary of the results obtained.
PropertySCLC-ECSCLC-SBA50SCLC-SBA90SCLC-SMR37Optimum Mixture
Slump flow (mm) SF2 (660–750)SF2 (660–750)SF1 (550–650)SF1 (550–650)SCLC-EC
T500 (s)VS2VS2VS2VS2Equal
Visual stability Index (VSI)VSI 0 (Stable)VSI 0 (Stable)VSI 0 (Stable)VSI 0 (Stable)Equal
Passing ability (J-ring)PJ2PJ2PJ2PJ2Equal
Fresh bulk density (g/cm3)1.96>2.00>2.001.97SCLC-EC
Compressive strength 28d (MPa)30.4325.7429.0934.15SCLC-SMR37
Dry bulk density 28d (g/cm3)>2.00>2.00>2.00<2.00SCLC-SMR37
Efficiency factor (MPa.dm3/kg)15.6813.1214.5018.74SCLC-SMR37
Chloride ion diffusion (m2/s)1.261.834.240.58SCLC-SMR37
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MDPI and ACS Style

Sousa, J.T.F.d.; Anjos, M.A.S.d.; Neto, J.A.d.S.; Farias, E.C.d.; Branco, F.G.; Maia Pederneiras, C. Self-Compacting Concrete with Artificial Lightweight Aggregates from Sugarcane Ash and Calcined Scheelite Mining Waste. Appl. Sci. 2025, 15, 452. https://doi.org/10.3390/app15010452

AMA Style

Sousa JTFd, Anjos MASd, Neto JAdS, Farias ECd, Branco FG, Maia Pederneiras C. Self-Compacting Concrete with Artificial Lightweight Aggregates from Sugarcane Ash and Calcined Scheelite Mining Waste. Applied Sciences. 2025; 15(1):452. https://doi.org/10.3390/app15010452

Chicago/Turabian Style

Sousa, Jemima Tabita F. de, Marcos Alyssandro S. dos Anjos, José Anselmo da S. Neto, Evilane C. de Farias, Fernando G. Branco, and Cinthia Maia Pederneiras. 2025. "Self-Compacting Concrete with Artificial Lightweight Aggregates from Sugarcane Ash and Calcined Scheelite Mining Waste" Applied Sciences 15, no. 1: 452. https://doi.org/10.3390/app15010452

APA Style

Sousa, J. T. F. d., Anjos, M. A. S. d., Neto, J. A. d. S., Farias, E. C. d., Branco, F. G., & Maia Pederneiras, C. (2025). Self-Compacting Concrete with Artificial Lightweight Aggregates from Sugarcane Ash and Calcined Scheelite Mining Waste. Applied Sciences, 15(1), 452. https://doi.org/10.3390/app15010452

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