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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39251791
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10.1038/s41598-024-71960-9
Article
Experimental investigation of quarry rock dust incorporated fly ash and slag based fiber reinforced geopolymer concrete circular columns
Waqas Rana Muhammad 1
Alkharisi Mohammed K. m.alkharisi@qu.edu.sa

2
Alsuhaibani Eyad 2
Butt Faheem 1
Shabbir Faisal 1
1 grid.444938.6 0000 0004 0609 0078 Department of Civil Engineering, University of Engineering and Technology, Taxila, 47050 Pakistan
2 https://ror.org/01wsfe280 grid.412602.3 0000 0000 9421 8094 Department of Civil Engineering, College of Engineering, Qassim University, 52571 Buraidah, Saudi Arabia
6 9 2024
6 9 2024
2024
14 2095322 5 2024
2 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Manufacturing ordinary Portland cement (OPC) poses significant challenges for sustainable construction practices. OPC manufacturing emits substantial greenhouse gases into the atmosphere and demands extensive raw materials. In pursuit of greener alternatives, researchers explore geopolymer concrete (GPC), a revolutionary material that entirely replaces OPC, comprising industrial wastes/by-products activated through an alkaline solution. The study aims to investigate the feasibility of incorporating quarry rock dust (QRD) into GPC production for environmentally sustainable structural applications. Circular columns (200 mm diameter, 1000 mm length) were formulated using GPC blends with fly ash, slag (SG), and QRD as a partial SG replacement. The structural performance of these columns, with and without steel fiber reinforcement, was evaluated under varied loading conditions. Results show that QRD is a valuable ingredient in GPC for structural concrete elements, offering performance comparable to traditional OPC concrete. Furthermore, the incorporation of steel fibers significantly enhances the peak axial loads, displacement response, and overall performance of GPC columns with or without QRD. Fiber-reinforced GPC columns demonstrated approximately 8–10% higher ultimate load capacity than equivalent OPC columns. Eccentricity was found to significantly reduce ductility, but fiber reinforcement offers substantial ductility improvements (25–55%).

Keywords

Geopolymer concrete
Quarry rock dust
Fly ash
Slag
Steel fiber
Circular columns
Structural behavior
Ductility
Ultimate loading
Subject terms

Civil engineering
Structural materials
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The Earth's climate is undergoing significant transformations due to the escalating levels of pollution and the unrestrained emission of carbon dioxide (CO2) into the atmosphere. In response, the global research community has intensified its focus on fostering sustainable practices, promoting environmentally conscious lifestyles, and developing technologies that safeguard the planet. Mirroring this trend, the construction industry is actively seeking eco-friendly materials that reduce CO2 emissions and champion environmental sustainability. Ordinary Portland cement (OPC) stands as the most extensively utilized binding material in worldwide construction activities, with annual production reaching approximately 4 billion tons1,2. However, the production of OPC generates a substantial carbon footprint, releasing roughly one ton of CO2 into the atmosphere for every ton of OPC manufactured. Additionally, the OPC production process necessitates a vast quantity of raw materials, consuming approximately 1.6 tons for each ton of production. The significant CO2 emissions and extensive resource consumption associated with OPC manufacturing pose a challenge to the principles of sustainable construction1.

The growing demand for infrastructural development fuels an ever-increasing need for OPC concrete production. As a result, the construction sector confronts mounting pressure to identify viable alternatives to OPC-based materials that uphold sustainability and environmental standards while offering comparable properties to OPC2. Introducing geopolymer concrete (GPC) offers a promising solution to promote sustainable construction practices3,4. GPC has established itself as an innovative construction material with the potential to produce OPC-free concrete suitable for diverse construction applications5,6.

Fly ash (FA), a by-product of coal combustion, is a highly sought-after material for GPC production due to its favorable chemical composition, namely a high abundance of silica (Si) and alumina (Al)7,8. To achieve optimal engineering properties, heat curing is often recommended for FA-based GPC. This process generally involves temperatures ranging from 80 to 100 °C, necessary to activate the FA, which exhibits low reactivity at ambient temperatures9,10. Research demonstrates that elevated temperature curing of GPC can lead to mechanical properties superior to those observed in OPC concrete11,12. Wallah and Rangan13 documented an enhancement in mechanical properties of GPC under heat curing. Additionally, Assi et al.14 explored the partial substitution of FA with OPC to eliminate the need for heat curing, facilitating the practicality of GPC in field applications. While this blended GPC exhibits robust performance without thermal curing, its reliance on OPC deviates from the goal of a purely waste-derived construction material.

Several studies demonstrate the improved mechanical properties of Class F FA subjected to heat curing for 24–48 h. However, due to the low reactivity of FA at ambient temperatures, this curing method yields less favorable results, hindering the geo-polymerization process. Furthermore, the practical limitations of heat curing make it unsuitable for on-site and field applications of GPC. Much of the existing research on GPC has concentrated on fundamental engineering aspects, including material characterization and the influences of the source material's physical, mechanical, and chemical properties2. Conversely, investigations into the behavior of structural elements composed of GPC remain relatively limited. Farhan et al.15 demonstrated that ambient-cured GPC columns frequently failed around their mid-height, suggesting potential benefits from the inclusion of steel fibers to enhance ductility and load-bearing capacity. Shaikh and Hosan16 explored the mechanical properties of steel fiber-reinforced GPC under ambient and elevated temperatures, concluding that a 0.75% fiber fraction offers optimal results. Sumajouw et al.17 studied the behavior of FA-based GPC columns (175 mm × 175 mm × 1500 mm) subjected to various loading eccentricities and concluded from experimental findings that these columns could be utilized in structural applications. Chang18 analyzed the shear response of reinforced GPC beams. The test outcomes suggested that the post-cracking response of GPC beams closely resembled that of OPC beams. Andalib et al.19 assessed the structural performance of GPC beams fabricated using binary mixes of palm oil fuel ash (POFA) and FA. They observed that the failure pattern of GPC beams resembled that of conventional concrete beams. Test results demonstrated a resemblance in the response of GPC beams to conventional concrete beams. Both exhibited closely similar cracking patterns and loading capacities. Albitar et al.20 explored FA and lead smelter slag-based GPC columns. The findings revealed that blended GPC columns composed of FA and lead smelter slag exhibit structural behavior identical to conventional concrete columns.

Youssf et al.48 studied the behavior of FA and SG based GPC mixes blended with lightweight fine aggregates. They developed eleven different geopolymer mixes with different proportions of various lightweight fine aggregates. The authors investigated the materials performance and structural application of the developed GPC mixes. A number of tests were conducted to evaluate the fresh, mechanical and durability performance of the lightweight fine aggregates incorporated GPC mixes. Further they cast reinforced concrete slabs by using the proportions of the optimum GOC mixes and tested under a 4-point bending loading regime.

Elemam et al.49 explored the possibility of using the clay brick powder (CBP) and fine clay brick (FCB) as partial replacement of slag and aggregates respectively in GPC mixes. They assessed the performance of the developed GPC mixtures by studying the mechanical, durability and microstructural properties. Fly ash was also used as a partial replacement for slag in normal geopolymer concrete. The results revealed that CBP and FBP can be used in GPC mixtures as partial replacement of slag and fine aggregates respectively to produce high-strength concrete. It was further recommended that structural performance of the optimized mixes may be assessed by testing the reinforced concrete structural members.

FA and slag (SG) have been traditionally used in the production of GPC. However, limitations in their supply raise concerns for the widespread commercialization of GPC. Stringent regulations on coal-fired power plants have reduced the availability of FA in certain regions. Additionally, the global supply of SG remains limited relative to the increasing demand for concrete in construction activities. To successfully facilitate the adoption of GPC, reduce costs, and meet infrastructure needs, it is crucial to identify alternative and sustainable source materials.

Many researchers have provided correlations to predict the tensile strength and flexural strength of concrete from compressive strength. Similarly, the standard codes of practice also provide such correlations. For example, the Eqs. (1), (2) and (3) are suggested by the ACI 318-14, Eurocode BS EN 1992-1-1:2004 and Australian standard AS 3600, 2009 correspondingly to calculate the tensile strength of OPC mixes.1 fct=0.56fc′

2 fct=0.30fc2/3forfc≤50MPa

3 fct=0.36fc′

The tensile strength of GPC mixes has been estimated by Lee and Lee50 and Sofi et al.51 using the formulas suggested by ACI 318-08 (Eq. 1) and Eurocode (Eq. 2), respectively. It was discovered that the measured values were lower than the estimated strength values. In order to predict the split tensile strength of FA and SG based GPC, Lee and Lee50 and Sofi et al.51, respectively, proposed Eqs. (4) and (5) based on the experimental results.4 fct=0.45fc′

5 fct=0.48fc′

Simple formulas (Eqs. 6 and 7, respectively) are provided by ACI 318-14 and AS 3600, 2009 to estimate the flexural strength of the OPC mixtures. Validation of the application of these formulas to the estimation of GPC mixtures' flexural strength is nevertheless required.6 fct.f=0.62fc′

7 fct.f=0.60fc′

Previous research investigations have also suggested empirical relations for GPC as a means of estimating flexural strength. When Diaz-Loya et al.52 assessed the flexural strength of heat-cured fly ash-based GPC using ACI 318–14 (Eq. 6) and AS 3600 (Eq. 7), they discovered that the estimated values were lower than the test findings. Diaz-Loya et al.52 developed a relationship (Eq. 8) to forecast the flexural strength of fly ash-based GPC mixes based on their experimental results.8 fct.f=0.69fc

The flexural strength of ambient cured fly-ash based GPC was also predicted by Nath and Sarker53 using the relationships found in ACI 318–14 (Eq. 6), AS 3600 (Eq. 7), and Diaz-Loya et al. (Eq. 8). Their measured values were found to be greater than the estimated values. On the other hand, compared to the values predicted by the other equations, the estimated values given by AS 3600 (Eq. 7) were closer to their measured values. In order to better suit their results, Nath and Sarkar53 additionally proposed an empirical connection (Eq. 9) based on their experimental test data, which is as follows:9 fct.f=0.93fc

Quarry rock dust (QRD), a residual material from aggregate production, has been employed in conventional concrete for some time and is experiencing renewed interest21,22. Limestone, a primary source of calcium in cement production, is already a common component in concrete, both as an aggregate and a partial cement substitute23–26. Interestingly, QRD exhibits a similar chemical composition to SG27,28. The improper disposal or lack of recycling of QRD poses significant environmental and health risks due to the release of fine dust particles. Therefore, innovative recycling strategies for this waste material are urgently needed.

Recent research has explored the incorporation of QRD into both GPC and traditional OPC systems. Within these applications, QRD has primarily served as a fine aggregate substitute24–26. In OPC systems, studies have demonstrated that QRD can be successfully used as a partial cement replacement, yielding favorable strength outcomes at substitution levels up to 20%. Investigations into the potential of QRD as a binding agent in FA and SG-based GPC are also underway27. Cohen et al.23 demonstrated the viability of incorporating QRD into FA-based geopolymers at high replacement levels, observing comparable or even improved engineering properties. While this research is promising, comprehensive studies are still needed to fully explore the potential of QRD in GPC, particularly concerning substitutes like limestone dust and dolomite powder. Wagas and Butt investigated the performance of steel fiber-reinforced, square GPC columns incorporating QRD, FA, and SG under eccentric loading28. Their findings demonstrate the suitability of QRD as a GPC material and highlight the significant improvements in loading capacity and ductility provided by steel fiber reinforcement. This study underscores the potential of GPC, with its environmental advantages, as a sustainable construction material. Furthermore, it suggests that steel fiber additions offer notable enhancements to load-bearing characteristics by about 5–7% greater in comparison to the conventional concrete columns28.

While QRD demonstrates potential in both OPC and geopolymer matrices15,28–30, its specific integration within GPC systems requires deeper exploration. Despite the environmental benefits offered by GPC, its broader acceptance is limited by insufficient research on its structural performance. Prior work28 has investigated steel fiber-reinforced FA and SG-based GPC columns with square sections. However, a significant knowledge gap exists regarding the behavior of QRD-integrated fiber-reinforced GPC columns, especially those with circular sections. This study aims to fill this gap, building upon the findings of28 to assess the structural behavior of QRD-incorporated, fiber-reinforced circular GPC columns under both eccentric and concentric loading conditions.

Experimental program

Material properties and mix proportions

The study examined the structural behavior of twenty circular concrete columns (200 mm diameter, 1000 mm length) under concentric (e = 0 mm) and eccentric axial loading (e = 15, 35, and 50 mm). All fiber-reinforced columns incorporated a consistent 0.75% steel fiber volume fraction, aligning with prior research16. Three mixture proportions were used to categorize the specimens: ordinary Portland cement (OPC), geopolymer concrete (GPC), and geopolymer concrete incorporating quarry rock dust (GPCD). Longitudinal reinforcement for each column consisted of six 12 mm deformed bars (450 MPa yield strength), while transverse reinforcement utilized 6 mm closed ties (300 MPa yield strength) spaced at 100 mm intervals. A uniform 25 mm clear concrete cover was maintained for all specimens. Details on the mixing proportions can be found in Table 1, and reinforcement specifics are provided in Fig. 1. All columns were designed to achieve a 28-day compressive strength of 40 MPa.Table 1 The detail of proportions and quantities of design mixes.

S. no	Group ID	Specimen ID	Mix proportions (%)	Mix quantities (kg/m3)	Water	
OPC	FA	SG	QRD	Sand	CA	OPC	FA	SG	QRD	NaOH	Na2SiO3	SP	
1	OPC	OPC-0F-0E	100	–	–	–	640	1201	370	–	–	–			4	170	
OPC-0F-15E	100	–	–	–	643	1206	370	–	–	–			4	170	
OPC-0F-35E	100	–	–	–	640	1201	370	–	–	–			4	170	
OPC-0F-50E	100	–	–	–	643	1206	370	–	–	–			4	170	
2	GPC	GPC-0F-0E	–	50	50	–	643	1206	–	200	200	-	53	107	8	–	
GPC-0F-15E	–	50	50	–	643	1206	–	200	200	-	53	107	8	–	
GPC-0F-35E	–	50	50	–	646	1212	–	200	200	-	53	107	8	–	
GPC-0F-50E	–	50	50	–	643	1206	–	200	200	-	53	107	8	–	
GPC-0.75F-0E	–	50	50	–	643	1206	–	200	200	-	53	107	12	–	
GPC-0.75F-15E	–	50	50	–	644	1208	–	200	200	-	53	107	12	–	
GPC-0.75F-35E	–	50	50	–	643	1206	–	200	200	-	53	107	12	–	
GPC-0.75F-50E	–	50	50	–	647	1214	–	200	200	-	53	107	12	–	
3	GPCD	GPCD-0F-0E	–	50	30	20	644	1205	–	200	120	80	53	107	10	–	
GPCD-0F-15E	–	50	30	20	646	1212	–	200	120	80	53	107	10	–	
GPCD-0F-35E	–	5	30	20	646	1212	–	200	120	80	53	107	10	–	
GPCD-0F-50E	–	50	30	20	646	1212	–	200	120	80	53	107	10	–	
GPCD-0.75F-0E	–	50	30	20	649	1216	–	200	120	80	53	107	12	–	
GPCD-0.75F-15E	–	50	30	20	655	1225	–	200	120	80	53	107	12	–	
GPCD-0.75F-35E	–	50	30	20	649	1216	–	200	120	80	53	107	12	–	
GPCD-0.75F-50E	–	50	30	20	655	1225	–	200	120	80	53	107	12	–	

Fig. 1 The size and reinforcement detail of the test specimens.

The control group, 'OPC,' comprised four conventional concrete specimens. One specimen was tested under concentric loading, while the remaining three underwent eccentric loading at the specified eccentricities. The 'GPC' group consisted of eight circular geopolymer columns formulated solely with FA and SG as the primary binders. In the 'GPCD' group, a partial replacement (20%) of SG with quarry rock dust was implemented, while maintaining a constant FA content (50%). Both the GPC and GPCD groups included four fiber-reinforced specimens (0.75% volume fraction) and four non-fiber-reinforced specimens. Within each group, concentric and eccentric loading conditions were applied to both fiber-reinforced and non-reinforced columns (details in Table 2). The columns' longitudinal reinforcement includes 6 No. 12 mm diameter bars, while the transverse reinforcement comprises 6 mm diameter bars spaced at 100 mm c/c. Grade 60 deformed steel bars were utilized for reinforcing the columns. A three-part code was used for specimen labeling. The first segment identifies the group (OPC, GPC, GPCD), the second denotes the fiber fraction (0% or 0.75%), and the third indicates the loading eccentricity (e.g., 50E for a 50 mm eccentric load). For example, GPCD-0.75F-50E signifies a specimen from the GPCD group, containing a 0.75% fiber fraction, and tested under a 50 mm eccentric load.Table 2 Description of column specimen groups, including reinforcement details and loading eccentricities.

Group no	Group ID	Column ID	Fiber fraction (%)	Longitudinal reinforcement	Transverse reinforcement	Eccentricity
e (mm)	
1	OPC	OPC-0F-0E	–	6N12	D6@100-mm	0	
OPC-0F-15E	–	6N12	D6@100-mm	15	
OPC-0F-35E	–	6N12	D6@100-mm	35	
OPC-0F-50E	–	6N12	D6@100-mm	50	
2	GPC	GPC-0F-0E	–	6N12	D6@100-mm	0	
GPC-0F-15E	–	6N12	D6@100-mm	15	
GPC-0F-35E	–	6N12	D6@100-mm	35	
GPC-0F-50E	–	6N12	D6@100-mm	50	
GPC-0.75F-0E	0.75	6N12	D6@100-mm	0	
GPC-0.75F-15E	0.75	6N12	D6@100-mm	15	
GPC-0.75F-35E	0.75	6N12	D6@100-mm	35	
GPC-0.75F-50E	0.75	6N12	D6@100-mm	50	
3	GPCD	GPCD-0F-0E	–	6N12	D6@100-mm	0	
GPCD-0F-15E	–	6N12	D6@100-mm	15	
GPCD-0F-35E	–	6N12	D6@100-mm	35	
GPCD-0F-50E	–	6N12	D6@100-mm	50	
GPCD-0.75F-0E	0.75	6N12	D6@100-mm	0	
GPCD-0.75F-15E	0.75	6N12	D6@100-mm	15	
GPCD-0.75F-35E	0.75	6N12	D6@100-mm	35	
GPCD-0.75F-50E	0.75	6N12	D6@100-mm	50	

The first group comprised specimens created from OPC concrete exclusively, whereas the second group featured columns created using FA and SG as source materials in a 50%-50% proportion. Within the third group, the ideal SG content was substituted with QRD to evaluate the impact of QRD on the mix. Low calcium FA was the selected source material due to its ability to expedite the polymerization process and enhance the geopolymer's microstructure in this experimental study. To produce geopolymer mixes under ambient conditions, SG was employed as a partial substitute for FA in the mix. QRD was obtained from the residue collected at the base of aggregate crushing plants and processed by heating in an oven at 200°C for 24 h, followed by mechanical grinding to achieve an average particle size of 90 µm. The GPC mixture proportions were derived from prior studies, as illustrated in Table 128,31,32. Every concrete mixture was formulated to attain a desired compressive strength of 40 MPa after 28 days. Ensuring consistency, the industrial wastes, FA, SG, and QRD used in all mixtures were sourced from identical origins (power plant, steel industry, and rock quarries, respectively) to prevent result discrepancies resulting from material source variations. X-Ray Fluorescent (XRF) analysis was employed to assess the chemical composition of FA, SG, and quarry dust, as presented in Table 3.Table 3 Chemical composition of materials used in this study.

Oxides	FA (%)	SG (%)	QRD (%)	
SiO2	54.55	34.20	9.35	
Al2O3	31.93	14.20	11.64	
Fe2O3	3.12	0.76	3.45	
Na2O	0.25	0.30	0.20	
CaO	4.65	37.30	47.13	
SO3	0.40	1.75	0.90	
K2O	0.70	0.50	0.20	
P2O5	0.45	0.05	0.50	
MgO	1.42	0.95	1.25	
TiO2	1.15	0.50	0.85	

The core components of GPC consist of source materials and alkaline activating solution (AAS) excluding the conventional coarse aggregates and sand found in regular concrete. The prevalent AAS utilized in GPC production typically involves a combination of either NaOH and Na2SiO3 or kOH and potassium silicate33,34. AAS holds significant importance in the polymerization process, which commences once the precursors are activated by an AAS created by blending a predetermined proportion of NaOH solution (12 M) and Na2SiO3 solution. Coarse aggregates (CA) with a maximum size of 20 mm, and surface-dry natural sand were used in accordance with ASTM C127-1535 and ASTM C128-1536, respectively (Table 4). Given the higher cohesion and viscosity of the GPC mix34, a naphthalene-based superplasticizer (SP) was employed to enhance the fresh mix's workability. For the production of fiber-reinforced concrete specimens, hooked-end steel fibers conforming to ASTM A82037, Type 1, were employed. These fibers exhibited the following technical specifications: a length of 32 mm, a diameter of 0.53 mm, an aspect ratio of 62, and a tensile strength of 1355 MPa.Table 4 Physical properties of sand and CA.

Entity	Sand	CA	
Relative Density	2.64	2.70	
Bulk Density (kg/m3)	1688	1644	
Fineness Modulus	2.50	7.65	
Water Absorption (% mass)	1.40	1.20	

Preparation of specimens

Concrete mixing was conducted using a 0.15 cubic meter capacity drum mixer. Steel molds were employed for casting the column specimens. Prior to concrete mixing, the AAS was prepared according to predetermined ratios one hour in advance. A consistent mixing procedure was followed for all mixes. Initially, dry ingredients (sand, CA, and binder materials) were combined in the mixer for two minutes. Following this, steel fibers were gradually introduced and mixed for an additional three minutes to ensure uniform distribution. After pre-mixing the dry components, the AAS was added, and if necessary, a superplasticizer. The mixture was blended for a final minute to achieve homogeneity. The fresh concrete was then poured into steel column molds, with compaction achieved using an internal vibrator. Along with the column specimens, six cubes per concrete batch were cast for determining the 28-day compressive strength according to BS EN 12390-3:200938. Demolding occurred 24 h post-casting, and specimens were stored at ambient conditions (25 °C, 70% relative humidity) prior to testing.

Test setup and procedure

Following 28 days of curing, all columns underwent testing under axial loading conditions. A gradual load application was employed at a rate of one kN/s using an electro-hydraulic testing machine with a capacity of 4800 kN. The specimens were subjected to failure loading under controlled conditions. Figure 8 illustrates the testing setup utilized in this study. High-strength steel pins were affixed to the loading plate at both ends of the specimen to generate eccentric loading. The loading setup was uniform for concentrically loaded specimens, excluding the use of the steel pin. To prevent premature overstressing and failure of columns, 75 mm wide and 3 mm thick steel collars were fitted at both ends before testing. Additionally, a thin layer of plaster of Paris was applied to the top and bottom faces of the columns to ensure a level surface for uniform load distribution during testing. Figure 9 displays the instrumentation setup used in the study. Axial deflections of the specimens were measured using two vertically fixed deflection gauges (No 01 and No 02) on the loading plate base. Lateral deflection was assessed using a separate dial gauge (No. 3) positioned laterally to the side of the columns.

After a 28-day curing period, all columns were subjected to axial loading within a 4800 kN capacity electro-hydraulic testing machine. Gradual loading at a rate of one kN/s was applied until specimen failure occurred under controlled conditions. Figure 2a demonstrates the experimental test setup. For eccentric loading, high-strength steel pins were fixed to the loading plate at both specimen ends. Concentrically loaded specimens maintained a similar setup, excluding the steel pins. To mitigate premature failure and localized stress concentrations, 75 mm wide and 3 mm thick steel collars were fitted to both ends of each column prior to testing. Additionally, a thin layer of plaster was applied to the top and bottom column faces to ensure uniform load distribution. Instrumentation for the study is depicted in Fig. 2b. Axial deflections were recorded using two vertically mounted deflection gauges (No 01 and No 02) attached to the loading plate base. A separate dial gauge (No. 3) positioned laterally to the column measured lateral deflection. The column specimen is labeled 4, and the base plate is labeled 5.Fig. 2 Test set-up: (a) Applied loading setup for eccentric axial loading; (b) Experimental setup for measuring deformations. Dial gauges 1 and 2 record axial deformation, gauge 3 measures lateral deformation. Label 4 denotes the column specimen, label 5 indicates the base plate.

All columns were subjected to monotonic loading until failure. The general behavior of the specimens was assessed based on ultimate loading values, failure patterns, and load–displacement relationships. Figure 3 illustrates the three distinct phases of the axial load–displacement response of the tested specimens. The initial phase demonstrates the load-bearing capacity of the column's gross cross-sectional area prior to cracking of the concrete cover. Specimen behavior remains unaffected by steel fibers and transverse reinforcement at this stage. The second phase exhibits a pronounced decline in axial load (Pmax), associated with the spalling of the concrete cover. The third phase represents the load–displacement response post-concrete cover removal. Notably, steel fibers significantly influence the post-peak axial load–displacement behavior, preventing sudden failure by mitigating the downward trend in the load–displacement curve. Each specimen's ductility index, calculated as shown in Fig. 3, was used to quantify the impact of steel fibers on ductility. The ductility index of the tested specimens was computed using the following Eq. 10:10 μ=Δ85Δy

Fig. 3 Ductility index curve39.

Results and discussion

Ultimate loading capacity

Figure 4 presents the ultimate load values derived from experimental testing, where GPC columns exhibit lower failure load values than their OPC counterparts (without fibers). Concentrically loaded GPC specimens displayed a 12% reduction in load capacity compared to corresponding OPC columns. Failure loads of GPC columns (GPC-0F-0E, GPC-0F-15E, GPCP-0F-35E, and GPC-0F-50E) were 12%, 19%, 23%, and 27% lower than their respective OPC counterparts. The ultimate load values of all columns were significantly affected by applied eccentricity.Fig. 4 Ultimate load-carrying capacities of all columns.

Axial load eccentricity adversely affects the load-bearing capacities of columns. As eccentricity increased from 0 to 50 mm, all columns experienced a substantial decrease in load carrying capacity. Eccentrically loaded GPC specimens (GPC-0F-15E, GPC-0F-35E, and GPC-0F-50E) exhibited ultimate loads 31%, 60%, and 71% lower than the concentrically loaded specimen (GPC-0F-0E). Figure 11 indicates that GPC specimens possess higher ultimate load values than those in the GPCD group. GPCD specimens (GPCD-0F-0E, GPCD-0F-15E, GPCD-0F-35E, and GPCD-0F-50E) showed an average load reduction of 9–20% compared to their respective GPC counterparts.

It is evident from Figure 11 reveals that fiber-reinforced specimens consistently exhibit greater ultimate load values than their non-fiber-reinforced counterparts. Each fiber-reinforced specimen failed at an axial load 20–30% higher than its equivalent non-fiber-reinforced specimen. Fiber-reinforced GPC columns exceeded the load capacities of equivalent OPC columns by approximately 6–8%. This enhanced strength likely stems from the beneficial action of fibers in arresting microcracks and bridging macrocracks. This crack-arresting mechanism persists until the fibers rupture or pull out, potentially contributing to the strength increase. For example, fiber-reinforced GPC specimens (GPC-0.75F-15E, GPC-0.75F-35E, and GPC-0.75F-50E) reached peak loads of 729 kN, 460 kN, and 340 kN, respectively, compared to their non-fiber-reinforced counterparts (570 kN, 335 kN, and 250 kN). Similarly, fiber-reinforced GPCD specimens (GPCD-0.75F-15E, GPCD-0.75F-35E, GPCD-0.75F-50E) showed load capacities of 655 kN, 390 kN, and 265 kN respectively, representing a 30–37% increase when compared to their non-reinforced equivalents.

Structural engineers worldwide utilize multiple design codes such as Eurocode 240, BS 811041, and ACI 31842. For constructing reinforced concrete structures, each with distinct philosophies and methodologies. Some codes are more cost-efficient than others. In this study phase, the ultimate loading capacities of the columns were estimated using prescribed expressions from recognized standards like BS 811041 and ACI 31842. According to ACI 31842, the useful design strength of an axially loaded column is given by:11 Pu=0.85∅0.85fc′Ag-Ast+fyAst

where: fc′ is specified compressive strength of concrete, Ag is gross area of concrete section, and Ast is the total area of longitudinal reinforcement.

According to BS 811041, the design ultimate axial load of an axially loaded column is given by the Eq. 3 as:12 N=0.4fcuAc+0.8Ascfy

where: fcu is characteristic strength of concrete, Ac is net cross-sectional of concrete in a column, fy is characteristic strength of reinforcement, and Asc is area of vertical reinforcement.

Table 5 compares the experimental findings to the predicted ultimate load values from these design code equations. Experimental results consistently exceed the values derived from the code models. BS 811041 was found to be more conservative than ACI 31842, typically underestimating experimental values by 10–15%, while ACI 31842 predictions were 15–25% lower.Table 5 The measured and predicted values of ultimate load.

Mix ID	Ultimate load values (kN)	
Test	ACI 31842	BS 811041	Test/ ACI 318	Test/ BS 8110	
OPC-0F-0E	955	790	820	0.84	0.87	
OPC-0F-15E	722	592	632	0.83	0.89	
OPC-0F-35E	440	355	380	0.83	0.88	
OPC-0F-50E	338	250	275	0.76	0.83	
GPC-0F-0E	880	727	762	0.85	0.88	
GPC-0F-15E	587	505	587	0.88	1.03	
GPC-0F-35E	351	290	353	0.87	1.05	
GPC-0F-50E	262	208	255	0.75	1.02	
GPC-0.75F-0E	1014	764	797	0.76	0.79	
GPC-0.75F-15E	731	550	610	0.76	0.85	
GPC-0.75F-35E	471	343	369	0.75	0.80	
GPC-0.75F-50E	351	252	267	0.74	0.79	
GPCD-0F-0E	802	788	820	0.99	1.03	
GPCD-0F-15E	531	590	631	1.15	1.23	
GPCD-0F-35E	308	354	380	1.21	1.30	
GPCD-0F-50E	219	249	275	1.24	1.37	
GPCD-0.75F-0E	960	815	842	0.85	0.92	
GPCD-0.75F-15E	668	590	647	0.90	0.99	
GPCD-0.75F-35E	401	356	390	0.92	1.01	
GPCD-0.75F-50E	281	250	281	0.94	1.06	

Visual observation and failure modes

Figures 5, 6, and 7 depict the post-failure conditions of OPC, GPC, and GPCD specimens, respectively. Specimen failure modes were significantly influenced by both the presence of steel fibers and the applied loading pattern. Concentrically loaded columns exhibited a sudden, brittle failure characterized by concrete crushing. Eccentrically loaded specimens, however, displayed typical flexural failure behavior with less abrupt failure. Initially, vertical flexural cracks developed along the length of all eccentrically loaded specimens. The first crack typically appeared within 100 to 200 mm of the column's midpoint, between the mid-central region and the ends. With increasing load, longitudinal cracks extended along the column's height, ultimately leading to failure by concrete crushing. The failure zone in all specimens occurred within a region between the central portion and 300 mm above or below the mid-height. The specific failure mechanism and cracking pattern varied with applied eccentricity; higher eccentricities resulted in the development of more extensive cracking.Fig. 5 Failure mode of all conventional concrete tested columns.

Fig. 6 Failure mode of all GPC tested columns.

Fig. 7 Failure mode of all GPCD tested columns.

In plain (non-fiber reinforced) OPC, GPC, and GPCD columns, concrete cover spalling occurred. The surface concrete of all plain specimens spalled within a 200 to 300 mm region from the ends, while the interior remained intact. Severe surface concrete spalling was observed in all GPC and GPCD columns under both concentric and eccentric loading. In stark contrast, fiber-reinforced GPC and GPCD specimens did not exhibit any cover spalling.

The failure modes of fiber-reinforced GPC and GPCD columns were consistent across both groups. Concentrically loaded fiber-reinforced specimens exhibited brittle failure followed by concrete crushing. However, eccentrically loaded specimens displayed a different failure pattern characterized by the development of vertical flexural cracks. Notably, the concrete cover in these specimens did not spall, although substantial disintegration was observed. Figures 6 and 7 illustrate the post-failure conditions of all fiber-reinforced GPC and GPCD specimens.

Extensive surface cracking was a prominent feature in all fiber-reinforced GPC and GPCD columns. Initial vertical flexural cracks appeared on both sides of the specimens. These cracks propagated along the column's length under increasing load. Further load increase resulted in severe disintegration of the concrete cover and eventual specimen failure. Notably, numerous surface fractures clustered around a third of the column's height in all fiber-reinforced specimens, despite the lack of concrete cover spalling.

Axial load–displacement behavior

Columns across the OPC, GPC, and GPCD groups were subjected to both concentric and eccentric axial loading. Figures 8, 9, and 10 illustrate the axial load-axial displacement responses of all columns within these groups. Concentrically loaded columns in the OPC group (OPC-0F-0E), GPC group (GPC-0F-0E and GPC-0.75F-0E), and GPCD group (GPCD-0F-0E and GPCD-0.75F-0E) exhibited a linear increase in load–displacement curves until reaching maximum axial force. This initial behavior is primarily determined by the compressive strength of the core concrete. At 90% of the ultimate load, fine cracks appeared on the column surfaces. While concrete cover spalling did not occur at peak load, plain columns in all groups experienced an axial load decrease post-failure due to cover loss.Fig. 8 Axial load-axial displacement behavior of all OPC columns.

Fig. 9 Axial load-axial displacement behavior of all GPC columns.

Fig. 10 Axial load-axial displacement behavior of all GPCD columns.

For fiber-reinforced GPC and GPCD columns, only concrete cover disintegration was observed. GPC-0F-0E and GPC-0F-15E specimens experienced a sharp decline in the descending phase of their load–displacement curves prior to their maximum load-bearing capacity. This rapid decline is likely due to immediate spalling of the concrete cover upon failure. A similar pattern was observed in all plain columns without fibers. However, fiber-reinforced specimens in both the GPC and GPCD groups exhibited a gradual decrease in peak axial load until failure point. The presence of steel fibers shifted the columns' behavior from brittle to a more ductile flexural response due to their confining effect. Incorporating fibers in specimens like GPC-0.75F-0E, GPC-0.75F-15E, GPC-0.75F-35E, and GPC-0.75F-50E led to ductility enhancements of 55%, 40%, 35%, and 25%, respectively, compared to their non-fiber-reinforced counterparts.

Figures 11, 12, and 13 show mid-height lateral deformation variations under applied axial loads. Concentrically loaded columns in all groups demonstrated brittle failure, with a sharp drop in peak load at failure. In plain GPC and GPCD columns, a sudden decline in the axial load-lateral displacement curve was observed after reaching the ultimate load, contrasting with fiber-reinforced specimens. Fiber-reinforced specimens in both groups exhibited a gradual, steady decline in load and a prolonged plateau in the post-peak descending phase, indicating an overall ductile failure mode. This could be attributed to steel fibers effectively delaying microcrack formation, bridging cracks, and arresting macrocrack propagation–enhancing both load capacity and ductility15. Notably, plain columns in the GPC and GPCD groups exhibited axial load-lateral displacement behaviors closely resembling those of OPC columns. It can be noted from Figs. 12 and 13 that fiber-reinforced GPC and GPCD specimens displayed greater lateral displacement than their plain counterparts. Steel fibers significantly alter the post-peak load-lateral displacement behavior, preventing sudden failure.Fig. 11 Axial load-lateral displacement behavior of all OPC columns.

Fig. 12 Axial load-lateral displacement behavior of all GPC columns.

Fig. 13 Axial load-lateral displacement behavior of all GPCD columns.

Ductility of specimens

Ductility was evaluated using a displacement ductility ratio index (µ)43–45. Figure 14 presents the ductility indices for all tested columns. Applied eccentricity significantly impacted column ductility; as eccentricity increased from 0 to 15, 35, and 50 mm, ductility notably decreased. For example, GPC specimens under 15-, 35-, and 50-mm eccentricities (GPC-0F-15E, GPC-0F-35E, and GPC-0F-50E) exhibited ductility reductions of 18%, 26%, and 35%, respectively, compared to the concentrically loaded counterpart (GPC-0F-0E). This pattern of decreasing load capacity and ductility with increasing eccentricity was consistent across all groups.Fig. 14 The ductility index of all tested columns.

Plain (non-fiber-reinforced) GPC and GPCD columns exhibited lower ductility indices than their OPC counterparts. For concentrically loaded plain columns, GPC and GPCD specimens showed ductility reductions of 13% and 22%, respectively, compared to the OPC group. A similar trend was observed in eccentrically loaded specimens. Furthermore, GPC columns consistently demonstrated slightly higher ductility than their GPCD counterparts.

The incorporation of steel fibers significantly enhanced the ductility of both GPC and GPCD columns. In GPC specimens (GPC-0.75F-0E, GPC-0.75F-15E, GPC-0.75F-35E, and GPC-0.75F-50E), ductility increased by 55%, 40%, 35%, and 25%, respectively, compared to their non-fiber-reinforced counterparts. A similar pattern was seen in GPCD columns. This enhancement is attributed to steel fibers' ability to bridge and arrest crack propagation15,29,30,46,47, leading to a more gradual failure mode.

Conclusions

The study investigated an innovative approach to GPC by replacing SG with QRD, obtained from local crushing plants, and utilizing steel fibers to create a fiber-reinforced geopolymer matrix. The performance of this new geopolymer matrix was comprehensively evaluated by testing circular columns under varying loading conditions. Adopting this strategy holds potential benefits for sustainable construction, including the preservation of natural resources, minimization of environmental impact, and reduction of CO2 emissions. Key conclusions from the study are as follows:Loading patterns significantly influence failure modes in columns. Concentric loading leads to sudden, brittle failure, while eccentric loading results in gradual flexural failure. Flexural cracks in eccentrically loaded specimens typically initiate 100-200mm from the column's midpoint.

Fiber reinforcement plays a crucial role in altering failure mechanisms and improving performance. It prevents concrete cover spalling and consistently increases ultimate load capacity by 20–30%. Fiber-reinforced GPC columns can even outperform equivalent OPC columns by 6–8%.

Experimental results show that real-world column performance exceeds estimations from design codes like BS 8110 and ACI 318. BS 8110 underestimates by 10–15%, while ACI 318 underestimates by 15–25%.

Fiber-reinforced GPC and GPCD columns exhibit distinct failure modes based on loading type. Under concentric loads, they fail suddenly with concrete crushing. However, under eccentric loads, they experience flexural failure without concrete cover spalling. These columns display consistent cracking behavior, with initial cracks appearing 100-200mm from the column's midpoint. Fiber reinforcement enhances ductility by bridging cracks and hindering propagation, resulting in a gradual decline in load and prolonged post-peak plateau.

Eccentricity significantly reduces ductility across all column types. Plain GPC and GPCD columns show lower ductility compared to OPC columns, but fiber reinforcement enhances their ductility by 25–55%, depending on eccentricity levels.

Overall, this study demonstrates the significant potential of GPC, particularly when incorporating sustainable QRD and performance-enhancing steel fibers. This combination offers a promising alternative to conventional OPC in structural circular column applications. The utilization of waste materials like QRD alongside the structural benefits of fiber reinforcement creates a pathway for both environmentally conscious and structurally sound construction practices. Further research on optimizing mix designs and investigating long-term performance would be valuable for broader adoption of this approach in the construction industry.

Acknowledgements

The researchers would like to thank the Deanship of Graduate Studies and Scientific Research, Qassim University for financial support (QU-APC-2024-9/1).

Author contributions

Conceptualization, F.S. and R.M.W.; Methodology, F.B., M.K.A.; Validation, R.M.W. and M.K.A.; Investigation, F.S., M.K.A. and E.A.; Supervision, F.B. and R.M.W.; Resources, M.K.A. and E.A.; Project administration, F.B., M.K.A. and E.A.; Writing—original draft, F.S. and R.M.W.; Writing—review & editing, M.K.A. and E.A.; Visualization, F.B.; data curation, M.K.A., E.A. and R.M.W.; All authors have read and agreed to the published version of the manuscript.

Data availability

Raw data supporting the conclusions of this article will be made available by the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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