Abstract
This study evaluated the strength properties of concrete produced with palm bunch ash–calcined anthill clay (PBA-CAC) as pozzolans. Two groups of palm bunch ashes were produced: ashes generated by burning only palm bunches (PBA) and ashes obtained by blending palm bunches and anthill clay at elevated temperatures (PBA-CAC). The PBA and PBA-CAC satisfied the requirements of Class C pozzolans. The concrete constituents were batched by mass, and the cement–fine aggregate–coarse aggregate ratio was 1:2:4. The cement content was partially substituted with PBA and PBA-CAC at 5%, 10%, 15%, 20%, and 25%, with the 0% specimen serving as the control. The concrete cubes were cured for 7, 14, 28, 56, and 90 days, whereas the concrete cylinders and beams were cured for 7, 28, 56, and 90 days. The 28th day strength values of the control specimens exceeded those of the PBA and PBA-CAC concrete specimens. By the 90th day of curing, the strength values of the specimens produced with 5% PBA and 5% PBA-CAC exceeded those of the control specimen. The PBA-CAC specimens generally had higher values of compressive strength, splitting tensile strength, and flexural strength than PBA specimens containing the same amount of pozzolan.
1 Introduction
Since the 20th century, the production of concrete has significantly increased worldwide. In many developing nations, including Nigeria, the demand for concrete is rising owing to factors such as industrialisation, urbanisation, increased economic activities, and continuous growth in the human population. The global demand for concrete is predicted to continue to rise [1]. Cement is the binding constituent of concrete, and a typical concrete mixture contains approximately 10%–15% of cement by volume [1]. More cement will need to be manufactured to satisfy the increasing demand for concrete construction. However, two critical issues are associated with using cement to construct concrete structures. First, cement production and concrete construction are associated with adverse environmental impacts, such as pollution and greenhouse gas emissions. Among all the constituent materials in concrete, cement is the most significant emitting source of greenhouse gases. Cement manufacturing generates approximately 8% of global emissions [2]. Second, in some countries, cement is expensive because of the significant demand for concrete and the high energy costs of manufacturing cement. In Nigeria, cement prices have significantly increased in recent years, which has generally increased the financial construction cost of concrete structures. Sustainable development aims to balance the environmental and economic needs of humans for the benefit of present and future generations. Conventional concrete is not a sustainable construction material [3], and the current concrete construction practice is considered unsustainable [4]. Therefore, applying approaches that can make concrete eco-friendly and cost-effective is imperative.
Attempts have been made to seek alternative approaches to minimise the environmental issues associated with concrete and cement production and economise cement usage. This mainly involves utilising large amounts of industrial waste in concrete as a partial Portland cement replacement (that is, as a filler or supplementary cementitious material [SCM]). Partially replacing an amount of cement with waste products is beneficial, as it improves the sustainability of concrete and conserves natural resources. Some agricultural and industrial by-products and wastes, such as rice husk ash and fly ash, have been identified and applied as SCMs in concrete, as they exhibit considerable pozzolanic properties. Other agro-based by-products, such as palm bunch ash (PBA), have been investigated as a pozzolan in concrete. Mbadike [5] conducted experimental tests on PBA and reported that the total sum of silicon dioxide, aluminium oxide, and iron oxide in the PBA was 72.73%, which satisfied the minimum value of 70% specified for pozzolanic materials in ASTM C618-05:2005. PBAs used as pozzolan in concrete are typically sieved to very fine particles with maximum sizes of 150–600 μm [5, 6]. The values of the compressive strength of concrete produced with PBA are generally lower than those of normal concrete at earlier ages, that is, at <28 days [5, 6], [7–9]. Ettu et al. [6] reported that at 50 days, the compressive strengths of PBA–cement concrete at 5%–10% substitution levels were comparable with that of control concrete. The compressive strengths of the 5% and 10% replacement contents at 90 days were higher than the control strength by 8.5% and 3.4%, respectively. Typically, the optimum compressive strength of PBA concrete is obtained at 5%–15% replacement levels [5, 6, 9].
Anthills and ant nests are a unique group of soils formed when ants tunnel clay subsoil. The transported soil may be deposited underground, above the ground surface, in trees, or under rocks. Anthills have high clay contents and are composed of rubble, soil, ant excretions, and decomposed plants and animals [10]. Significant amounts of anthills are found in tropical Africa, and using them in concrete may significantly decrease the cement ratio in concrete mixes and reduce construction costs [11]. Elinwa [12] demonstrated that anthill clay calcined to 800 °C and ground into fine particles is suitable as a partial replacement of cement. The compressive strength of the mixture produced with 10% replacement content was higher than the control strength at curing durations exceeding 60 days [12]. Kamau et al. [11] reported that an anthill contained the required chemical composition for pozzolans, and the compressive strengths of concrete produced with the anthill as a partial substitute of cement satisfied structural application requirements. Zhou et al. [10] observed that the combined Al2O3, SiO2, and Fe2O3 contents in ant nests calcined at 600 and 800 °C reached 83.37% and 88.76%, respectively, which satisfied the minimum ASTM requirement of 70%. The optimum compressive strength of the ant-nest-added concrete was obtained at 10% ant-nest content, and the early-age strength increased rapidly. Recently, Snellings et al. [13] comprehensively reviewed the performance of emerging and future SCMs, including calcined clays and natural pozzolans. They reported that calcined clays are among the most promising SCMs in blended cement composites.
Although binary blending has benefits, ternary blending (in which Portland clinkers and two SCMs are blended) can further improve the properties of cement-based products. Researchers have investigated the possibility of developing ternary blended systems to further decrease the quantity of cement and improve the performance of blended cement composites. The two SCMs are typically prepared separately, blended in specified proportions, and subsequently blended with the cement during mixing [14–16]. Advancements in separate grinding and mixing technology in the cement industry have made the production of ternary blended cement possible and more convenient. To the best of our knowledge, the ternary blending of PBA and calcined anthill clay (CAC) in Portland cement-based composites has not been investigated extensively.
In this study, the strength properties of concrete containing PBA–CAC as SCMs were investigated. Specifically, the compressive strength, flexural strength, and splitting tensile strength of PBA concrete and PBA–CAC concrete were determined and compared. The ternary mixing of PBA and CAC in concrete further improved their individual characteristics as SCMs and increased the strength of the resulting concrete. The novelty of this study lies in blending and calcining palm bunches and anthill clay simultaneously at elevated temperatures to produce a pozzolan for concrete construction. Palm bunches and anthills are abundant in tropical regions, and utilising them for engineering applications is reasonable from the perspectives of waste management and sustainable construction. This study serves as a reference for the investigation of blended PBA–CAC concrete and provides insights into the optimisation of concrete strength using PBA–CAC as an SCM. The successful application of PBA and CAC in ternary combinations with cement for concrete production will help reduce the volume of cement used in civil engineering projects.
2 Materials and methods
2.1 Materials
Cement: Portland-limestone cement (Grade 42.5) produced in accordance with NIS 444-1:2003 [17] was used as the cementing material. The cement was dry and lump-free. The properties of the cement were as follows: density = 1,440 kg m−3, specific gravity = 3.1, fineness = 2%, initial setting time = 120 min, and final setting time = 300 min.
Fine aggregate: Otammiri River sand was used as the fine aggregate. Preliminary test revealed that the sand had a water absorption of 0.46, fineness modulus of 2.93, specific gravity of 2.63, and bulk density of 1,590 kg m−3. Figure 1 shows the grain size distribution of the fine aggregate.
Grain size distribution of the river sand
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Coarse aggregate: The coarse aggregate was 10–14 mm crushed granite obtained from a rock quarry. The crushed granite exhibited a specific gravity of 2.68, bulk density of 1,680 kg m−3, aggregate crushing value of 25.55%, Los Angeles abrasion value of 11.48%, and aggregate impact value = 29.27%.
Water: Portable water from a tap in a laboratory at the Federal University of Technology, Owerri, was used for concrete mixing and curing.
Anthill soil: Anthill was collected from a site at Aboh Mbaise in Imo State, Nigeria. The anthill was collected in the dry season and temporarily stored in sack bags.
Palm bunch: De-fruited palm bunches were obtained from a palm-oil-processing mill in Owerri, Imo State, and transported to the laboratory for experiments.
2.2 Production of PBA and PBA-CAC
Two groups of palm bunch ashes were produced. One group comprised ashes generated by burning only palm bunches. In the second group, PBA and anthill were blended at elevated temperatures. It was ensured that the palm bunches and anthill soil were maintained in a dry state before burning. For the first group, the palm bunches were loaded into a furnace and subjected to open-air burning at 650 °C for 30 min. No special heating process was adopted because the researchers wanted to use a method that would be easily adopted for local construction by rural dwellers. For the second group, anthill lumps were crushed manually into powder and sieved to obtain anthill clay. Palm bunches were loaded into the furnace, and finer particles of the anthill (anthill clay) were added to the heating palm bunches almost immediately to heat both materials. The blended mixture was also heated in the open air at 650 °C for 30 min, similar to the palm bunches of the first group. The mixing ratio of the palm bunch to anthill clay by mass was approximately 1:10. After heating both groups of ashes (PBA and PBA-CAC), they were cooled in a shed for two days. The cooled ashes were pulverised to obtain particles of 650 µm size. Physical properties (particle density, specific gravity, and autoclave expansion) of the PBA and PBA-CAC were determined. The autoclave expansion test was performed according to the specifications of ASTM C151-05:2010 [18]. In addition, the chemical compositions of the PBA and PBA-CAC were determined using energy-dispersive X-ray (EDX) analysis (EDX-720, Shimadzu, Kyoto, Japan). The size of the collimator was 10 mm, the spin was set to off, and the analysis was conducted in air.
2.3 Production of concrete specimens
The constituent materials of the concrete were batched by mass. 1:2:4 was adopted as the cement–fine aggregate–coarse aggregate ratio; this mix ratio was selected to target compressive cube strength of 21 N mm−2 [19, 20]. This target strength is suitable for producing plain concrete, lightweight-aggregate reinforced concrete, and normal-weight reinforced concrete subjected to low loads. The control concrete was produced by mixing the cement, sand, gravel and water. For the PBA and PBA-CAC concrete, the cement content was partially substituted with PBA and PBA-CAC, respectively, in percentages of 5%, 10%, 15%, 20%, and 25%. The sand and cementitious materials were first mixed to a constant colour. Next, crushed granite was added with continuous mixing. Finally, water was progressively poured into the mixture to produce a workable cementitious composite. The concrete specimens were produced at environmental temperature of 29 ± 2 °C and relative humidity (RH) of 55% ± 5%.
First, cube, cylinder, and beam moulds were used for casting the fresh concrete to produce concrete specimens for compressive strength, splitting tensile strength, and flexural strength tests, respectively. The cubes, cylinders, and beams had dimensions of 150 mm × 150 mm × 150 mm, Φ150 mm × 300 mm, and 150 mm × 150 mm × 600 mm, respectively. The fresh concrete samples were retained in the moulds for 24 h, after which they were de-moulded and completely immersed in open water tanks filled with water for curing (Fig. 2). This procedure was carried out at water curing temperature of 20 ± 1 °C to maintain moist conditions over a period. Three concrete specimens were prepared for each mix proportion. The cubes were cured for 7, 14, 28, 56, and 90 days, whereas the concrete cylinders and beams were cured for 7, 28, 56, and 90 days. The total number of cured concrete cubes, cylinders, and beams were 180, 108, and 108, respectively.
Concrete specimens fully immersed in a water tank
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
2.4 Testing of concrete
After curing, the concrete cubes, cylinders, and beams were subjected to compressive, splitting tensile, and flexural strength tests, respectively, in a universal testing machine. The concrete specimens were crushed under the following environmental conditions: temperature = 29 ± 2 °C; RH = 55% ± 15%. For the compressive strength tests, the cubes were brought out from the tank and dried under ambient conditions. Subsequently, they were placed in contact with the platens of the universal testing machine. Three concrete cubes were used for each mix proportion and group. The compressive load was applied at a constant loading rate for each cube until the cube disintegrated (Fig. 3a). The value of the compressive strength was determined by dividing the crushing load by the cube cross-sectional area, and the average of three cubes for each category was used for analysis.
Tests on concrete specimens: (a) compressive strength; (b) splitting tensile strength; (c) flexural strength
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Each beam specimen used for the flexural strength test was positioned in the equipment such that loading was applied to the topmost surface, along two lines at spacings of 200 or 130 mm (Fig. 3(c)). The specimen was positioned such that its axis precisely aligned with that of the loading device. This ensured that surface of the sample was in contact with the load-applying and supporting blocks. Approximately 3%–6% of the estimated ultimate load was steadily amplified until failure occurred. The maximum load applied to each sample was determined, and the flexural strength was computed by dividing the maximum load by the cross-sectional area (that is, the length multiplied by the width).
3 Results and discussion
3.1 Material characterisation
The particle density, specific gravity, and autoclave expansion of the PBA were 0.558 g cm−3, 0.56, and 0.65%, respectively, whereas those of the PBA-CAC were 0.798 g cm−3, 0.80, and 0.43%. The PBA-CAC had a higher particle density and specific gravity than the PBA possibly because of the attachment of calcined anthill clay to the PBA. The autoclave expansion results showed that both ash specimens conformed to the maximum value of 0.8% recommended for SCMs in ASTM C618-03:2023 [21]. Moreover, the PBA-CAC exhibited better resistance to expansion and potential cracking than the PBA. Significant expansion and volume changes of pozzolans and cementitious binders disrupt the hardened concrete mass, which affects the durability of structures when such cementitious materials are used [22].
The EDX analysis results are presented in Table 1. ASTM C618-03:2023 [21] states that the SiO2 + Al2O3 + Fe2O3 content should be ≥ 70% for Classes N and F pozzolans and ≥50% for Class C pozzolans. The SiO2 + Al2O3 + Fe2O3 contents of the PBA and PBA-CAC pozzolans were 51.14% and 65.488%, respectively. Thus, both the PBA and PBA-CAC were Class C pozzolans according to ASTM C618-03:2023 [21]. The SO3 content of PBA was within the prescribed limits (less than 3%), and SO3 was not detected in PBA-CAC. In addition, the losses of ignition of PBA and PBA-CAC were 2.714 and 0.402, respectively, which satisfied ASTM C618-03:2023 [21] requirements (maximum of 6% for Class C pozzolans).
Results of EDX analysis of PBA and PBA-CAC samples
| Oxide | Percentage content (by mass) | |
| PBA | PBA-CAC | |
| SiO2 | 44.217 | 52.236 |
| CaO | 30.514 | 24.952 |
| K2O | 4.565 | 3.638 |
| Al2O3 | 3.610 | 9.864 |
| MgO | 3.388 | 2.742 |
| P2O5 | 2.701 | 0.983 |
| Fe2O3 | 3.313 | 3.388 |
| Na2O | 1.286 | 0.987 |
| MnO | 0.788 | 0.218 |
| ZnO | 0.635 | 0.311 |
| BaO | 0.168 | 0.063 |
| TiO2 | 0.131 | 0.216 |
| SO3 | 1.970 | – |
| *LOI | 2.714 | 0.402 |
*LOI denotes loss on ignition.
3.2 Compressive strength
The values of the compressive strength of the PBA and PBA-CAC concrete cubes for the different percentage mixes at the various curing periods are presented in Figs 4 and 5, respectively. Table 2 lists the standard deviations of the compressive strengths of PBA and PBA-CAC concrete cubes. The compressive strength of both set of mixtures increased with curing duration. The 7-day-old specimens yielded the minimum compressive strength values, and the 90-day-old specimens exhibited the maximum values. At 7 days, the control specimens provided the highest compressive strength (10.40 MPa), whereas the specimen containing 25% pozzolan (PBA or PBA-CAC) exhibited the lowest strengths. This trend was also observed for the 14-day-old specimens. The compressive strength values of the 14-day-old control concrete were approximately 87% of those of the 28-day-old concrete. The compressive strengths of the 14-day-old PBA concrete ranged between 78% and 88% of the compressive strengths at 28 days, whereas those of the 14-day-old PBA-CAC concrete ranged between 80% and 88% of its 28-day compressive strength. These trends are similar to those of Zhou et al. [10], who reported that the 14-day compressive strengths of calcined-anthill-clay concrete specimens were 81%–86% of their corresponding 28-day compressive strength values. However, by day 56, our specimens containing 5% PBA and PBA-CAC exhibited more significant increase in compressive strength, with the values exceeding the corresponding value of the control specimen. The 56-day compressive strength results for the 10% replacement samples slightly differed from those of Elinwa [12], who reported that the compressive strength of concrete containing 10% calcined soldier-ant-mound clay exceeded that of the reference mix from 60 days onwards. Kamau et al. [11] observed that the compressive strength of the 56-day 10%-anthill-soil concrete were slightly lower than the compressive strength of the control specimen. In our study, the 56-day compressive strengths of the 10%-PBA and 10%-PBA-CAC concrete specimens were not higher, but similar to those of the control mix (approximately 98.1% and 99.5% of the 56-day control compressive strength, respectively). Kamau et al. [11] reported that the 90-day compressive strength of 5% sample exceeded the control compressive strength of 1:2:3 concrete mix, whereas those of 10%–25% were still lower than the control compressive strength. By the 90th day of curing, the compressive strengths of our samples that contained 10% PBA (24.26 MPa) and 10% PBA-CAC (25.56 MPa) were still comparable to those of the control specimen (24.59 MPa). The percentage increases in the compressive strength of specimens replaced with 5% and 10% PBA between 28 and 90 days were 32.5% and 28.4%, respectively, whereas the corresponding increases for the PBA-CAC specimens were 15.2% and 23.7%, respectively. Increases of 28.1% and 21.1% in compressive strength for 5%- and 10%-anthill-soil-replaced specimens between 28 and 90 days, respectively, have been reported in a previous study [11]. In our study, the 90-day-old compressive strengths of the specimens produced with 5% PBA and 5% PBA-CAC were 2.9% and 5.7% higher than that of the control specimen. It should be noted that in our study, the specimens produced with 20% and 25% pozzolans exhibited compressive strengths that were consistently lower than the control compressive strength, even up to the 90th day. Zhou et al. [10] reported similar findings, in which mortar specimens produced with calcined anthill clay at 20% and 30% replacement levels exhibited the lowest compressive strength.
Compressive strength development of PBA concrete with curing age
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Compressive strength development of PBA-CAC concrete
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Standard deviations of compressive strength values
| Pozzolan content (%) | Standard deviation (MPa) | |||||||||
| PBA concrete | PBA-CAC concrete | |||||||||
| 7 d | 14 d | 28 d | 56 d | 90 d | 7 d | 14 d | 28 d | 56 d | 90 d | |
| 0% | 1.6 | 1.8 | 1.9 | 2.0 | 1.9 | 1.6 | 1.8 | 1.9 | 2.0 | 1.9 |
| 5% | 1.8 | 2.3 | 2.0 | 2.1 | 0.8 | 1.8 | 1.1 | 2.0 | 2.1 | 2.3 |
| 10% | 1.5 | 1.1 | 1.8 | 1.7 | 2.1 | 2.2 | 1.6 | 1.8 | 1.9 | 1.6 |
| 15% | 2.2 | 0.9 | 1.4 | 2.2 | 1.7 | 1.8 | 1.9 | 2.1 | 1.5 | 1.9 |
| 20% | 1.8 | 1.8 | 1.7 | 1.2 | 2.0 | 1.7 | 0.8 | 2.1 | 2.3 | 1.7 |
| 25% | 2.0 | 2.2 | 1.9 | 1.4 | 2.3 | 2.1 | 2.0 | 1.8 | 1.6 | 2.4 |
The lower strength values of the pozzolana-blended specimens compared with the control concrete strength at earlier curing durations (up to 28 days) indicated that pozzolanic reaction was still in its early stages; that is, pozzolanic reaction slowly occurred at this stage. Because of the continued pozzolanic reactivity, the concrete containing relatively low amounts of PBA and PBA-CAC developed higher strength with curing age. At later curing ages (56 and 90 days), pozzolanic reaction became more significant; thus, the strength values of the specimens containing 5% and 10% pozzolans increased significantly. Moreover, the strength values of the PBA-CAC samples relatively exceeded those of the samples containing only PBA for the 5% and 10% mixtures. For example, the compressive strength of the 56-day-old 5% PBA-CAC specimen was approximately 1% higher than that of the 56-day-old 5% PBA specimen. The compressive strength value of the 90-day-old 10% PBA-CAC specimen was 5.4% higher than that of the 10% PBA specimen of the same curing age. This trend occurred despite the fact that the 5% and 10% PBA specimens exhibited higher rates of strength increase than the 5% and 10% PBA-CAC specimens between 28 and 90 days. This suggests that at the same percentage replacement, pozzolanic reaction completed earlier in PBA-CAC specimens than in PBA specimens. This earlier reaction might be attributed to the higher proportions of silica and alumina in PBA-CAC than in PBA, which can induce more rapid pozzolanic reactions in PBA-CAC concrete paste than in PBA concrete paste.
Overall, the specimens containing the pozzolans exhibited a slower rate of strength development than the control specimen up to the 28th day because their silica waited for calcium hydroxide, a product of initial cement hydration. Both products then reacted during the secondary hydration process to form C–S–H, which further increased the strength. PBA and PBA-CAC improve the paste matrix and transition zone structure when used as pozzolans owing to their reactivity, which results in the generation of additional C–S–H with increasing curing age. This enhanced reactivity and microstructure improves the strength development of the resulting concrete. However, increased strength is achieved with controlled amounts of PBA and PBA-CAC in the paste matrix; high amounts of PBA and PBA-CAC result in decreased concrete strength because some of the pozzolans do not participate in the pozzolanic reaction. Generally, improved reactivity can be achieved using finer particles of pozzolans.
3.3 Splitting tensile strength
Figures 6 and 7 show the splitting tensile strengths of PBA and PBA-CAC concrete cylinders for various percentage mixes at the different curing durations. Table 3 lists the standard deviations of the splitting tensile strengths of the PBA and PBA-CAC concrete cylinders. The splitting tensile strength increased as the curing duration increased, similar to the development of the compressive strength. The splitting tensile strength of the 28-day-old control specimens was 2.59 MPa, and those of the 5%, 10%, 15%, 20%, and 25% PBA were 2.48, 2.43, 2.15, 2.1, and 2.04 MPa, respectively. The corresponding values for the 5%, 10%, 15%, 20%, and 25% PBA-CAC specimens were 2.5, 2.42, 2.38, 2.26, and 2.11 MPa respectively. At 28 days, the splitting tensile strengths of the samples containing pozzolans decreased with increasing pozzolan content, with the PBA-CAC specimens having slightly higher values than the PBA concrete. The slower pozzolanic reactivity of the PBA and PBA-CAC by this time resulted in the decreased splitting tensile strength of the samples produced with PBA and PBA-CAC.
Splitting tensile strength development of PBA concrete
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Splitting tensile strength development of PBA-CAC concrete
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Standard deviations of splitting tensile strength values
| Pozzolan content (%) | Standard deviation (MPa) | |||||||
| PBA concrete | PBA-CAC concrete | |||||||
| 7 d | 28 d | 56 d | 90 d | 7 d | 28 d | 56 d | 90 d | |
| 0% | 2.1 | 2.2 | 1.9 | 1.8 | 2.3 | 2.0 | 1.8 | 1.5 |
| 5% | 1.9 | 2.0 | 1.6 | 1.7 | 2.1 | 1.9 | 1.6 | 1.1 |
| 10% | 1.5 | 1.8 | 2.1 | 2.4 | 1.9 | 0.9 | 1.9 | 2.0 |
| 15% | 1.7 | 1.1 | 0.7 | 1.9 | 1.4 | 1.6 | 0.6 | 2.2 |
| 20% | 1.6 | 1.9 | 2.1 | 1.2 | 2.0 | 1.5 | 2.1 | 1.4 |
| 25% | 1.8 | 2.3 | 2.0 | 2.3 | 2.2 | 1.8 | 1.7 | 1.8 |
At 56 days of curing, PBA-CAC concrete prepared with 5% cement replacement content (2.89 MPa) exhibited splitting tensile strength comparable to those of the control (2.93 MPa). By the 90th day, the splitting tensile strength values of the 5% PBA (3.11 MPa), 5% PBA-CAC (3.26 MPa), and 10% PBA-CAC (3.10 MPa) exceeded those of the control specimen by 2.3%, 7.2%, and 2.0%, respectively.
The tensile strength of concrete is significantly low in comparison with the compressive strength of concrete. Hence, structural concrete members are typically reinforced with steel reinforcement bars. The direct and splitting tensile strength values of concrete vary between 5% and 13% of the concrete cube compressive strength [23]. In this study, the 28-day splitting tensile strengths of the control, 5%-PBA, 10%-PBA, and 5%-PBA-CAC, and 10%-PBA-CAC specimens were 13.2%, 13.0%, 12.9%, 11.1%, and 11.7%, respectively, of their individual compressive strength values. The 90-day splitting tensile strengths of these specimens were 12.4%, 12.3%, 12.4%, 12.5%, and 12.1%, respectively, of their corresponding compressive strength values. Similar splitting tensile strength–compressive strength percentages were obtained by Kamau et al. [11].
3.4 Flexural strength
The flexural strength of concrete is a critical parameter in the design of concrete roads and pavements. The flexural strengths of the PBA and PBA-CAC concrete beams with the different pozzolan contents at various curing ages are shown in Figs 8 and 9, respectively. Table 4 presents the standard deviation values of the compressive strengths of the PBA and PBA-CAC concrete beams. The development patterns of the specimens in terms of flexural strength were similar to those of the splitting tensile strength.
Flexural strength development of PBA concrete with curing age
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Flexural strength development of PBA-CAC concrete with curing age
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00905
Standard deviations of flexural strength values
| Pozzolan content (%) | Standard deviation (MPa) | |||||||
| PBA concrete | PBA-CAC concrete | |||||||
| 7 d | 28 d | 56 d | 90 d | 7 d | 28 d | 56 d | 90 d | |
| 0% | 1.4 | 1.2 | 1.5 | 1.9 | 2.1 | 1.8 | 2.0 | 1.6 |
| 5% | 2.3 | 2.1 | 1.6 | 1.5 | 1.7 | 2.2 | 1.1 | 1.5 |
| 10% | 1.2 | 2.0 | 1.0 | 2.3 | 1.2 | 1.5 | 1.8 | 2.1 |
| 15% | 1.7 | 1.9 | 0.9 | 2.0 | 1.8 | 1.2 | 1.6 | 2.0 |
| 20% | 2.0 | 0.8 | 1.8 | 1.6 | 1.4 | 1.3 | 1.8 | 2.1 |
| 25% | 1.7 | 2.2 | 1.9 | 2.3 | 2.0 | 2.1 | 1.5 | 1.9 |
The 7-day flexural strengths of the PBA and PBA-CAC specimens were 58%–63% and 58%–65% of the 28-day flexural strengths, respectively. The 28-day flexural strengths of the specimens containing pozzolans decreased with increasing pozzolanic content, with the PBA specimens having slightly lower values than the PBA-CAC specimens. The strengths of the 5% PBA concrete and 5% PBA-CAC concrete continued to increase and exceeded the control flexural strength by the 56th and 90th days. This was because the pozzolanic reaction of the 5% pozzolan concrete continued with curing age. The strength values of the 15%, 20%, and 25% PBA and PBA-CAC specimens failed to attain the control strength value, even by the 90th day. Overall, the flexural strength values of the PBA-CAC specimens were higher than the PBA-concrete flexural strengths at different curing ages. This suggests that pozzolanic reaction completed earlier in PBA-CAC specimens than in PBA specimens for the same replacement level.
4 Conclusions
In this study, the strength properties of concrete produced with PBA-CAC pozzolan were investigated. Material characterisation of the PBA and PBA-CAC were performed, and the compressive strength, splitting tensile strength, and flexural strength of PBA and PBA-CAC concrete specimens were investigated. The following conclusions are drawn.
The material characterisation analysis showed that the PBA and PBA-CAC samples can be classified as Class C pozzolans. Both materials satisfied ASTM requirements for SiO2 + Al2O3 + Fe2O3 content, SO3 content, and loss of ignition. In addition, both ash specimens satisfied the expansion limit requirement for SCMs, with the PBA-CAC exhibiting superior resistance to expansion and potential cracking.
The compressive, splitting tensile, and flexural strength properties of the PBA and PBA-CAC concrete increased with curing age, similar to the strength development trend of the control samples. The control specimens had the highest strength values up to the 28th day of curing. By the 90th day, specimens produced with 5% PBA and 5% PBA-CAC consistently had higher strength values than the control sample. The 10% PBA-CAC specimens exhibited higher compressive, splitting tensile, and flexural strengths than the control specimen.
Overall, the PBA-CAC concrete attained higher strength values than PBA concrete at the same curing age and cement replacement percentage. This may be attributed to the higher silica and alumina contents in PBA-CAC than in PBA, which can induce more rapid pozzolanic reactions in PBA-CAC concrete paste than in PBA concrete paste.
PBA and PBA-CAC exhibited pozzolanic properties in the concrete matrix, which resulted in the generation of additional C–S–H with curing age. This reactivity at a later age improved the microstructure of the matrix and increased the strength of the resulting concrete.
Blended cement concrete in which concrete is partially replaced with 5%–10% PBA-CAC can be produced to achieve a target strength of up to 35 MPa if a good concrete mix design and good quality control are applied. Examples of such high-quality control measures include grinding the PBA-CAC to finer particle sizes and adding superplasticisers and water-reducing agents during the concrete-mixing process. This type of blended cement concrete is useful for concrete construction in cases where high early strength is not a critical requirement. Further studies should determine the best calcination temperature ranges, mix ratios, and particle sizes of PBA-CAC for optimal strength to optimise PBA-CAC production. In addition, comprehensive material characterisation of PBA-CAC and analyses of other mechanical and durability properties of PBA-CAC concrete should be conducted.
Funding
This study was supported by the Tertiary Education Trust Fund, Nigeria, under the Intervention for Institution-Based Research awarded to the first author at the Imo State Polytechnic, Umuagwo (now Imo State Polytechnic, Omuma).
Acknowledgement
The contributions of Mr. Kelechi Okonkwo in conducting the laboratory tests are gratefully acknowledged.
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