Preliminary Investigation on Ceramic Waste Aggregate in Fly Ash-Based Geopolymer Concrete

Abstract

The increasing generation of ceramic waste from manufacturing defects, construction activities, and demolition operations poses significant environmental and waste management challenges worldwide. This study presents a preliminary investigation into the incorporation of ceramic waste aggregates (CW) as partial and full replacement for natural coarse aggregates in fly ash-based geopolymer concrete (GPC) under water-curing conditions. Five mix compositions were prepared with ceramic waste aggregate replacement levels of 0%, 20%, 40%, 60%, and 100%. Fresh and hardened properties were evaluated using flow table and early-age compressive strength tests at 7 and 14 days. The 20% replacement mix achieved the best compressive strength value of 5.52 MPa at 14 days, slightly exceeding the control GPC mix (5.09 MPa) among the limited mixtures investigated in this preliminary study. However, higher replacement levels resulted in reduced compressive strength, which may be associated with increased porosity, weaker aggregate–matrix bonding, and limitations related to the adopted water-curing regime. Workability remained within acceptable flow ranges for most mixes, although reduced flowability was observed for the 40% replacement. The comparatively low strength values obtained across all mixtures may largely be associated with the absence of heat curing and the inclusion of additional water to improve workability, both of which likely limited the geopolymerization efficiency. Based on the comparatively low compressive strength values obtained, the investigated mixtures, in their current form, are only suitable for low-strength or non-structural applications rather than structural concrete applications. Overall, this study provides preliminary insights into the influence of ceramic waste coarse aggregates on the workability and early-age compressive strength behavior of fly ash-based geopolymer concrete under the adopted experimental conditions. Further optimization of the curing regimes, mix design parameters, and long-term mechanical and durability performance is necessary before broader engineering applicability can be established.

1. Introduction

Historically, a wide range of materials have been utilized in the built environment to satisfy diverse construction requirements [1]. Among these materials, concrete has become the most widely used due to its strength, durability, adaptability, and ability to support the increasing demand for infrastructure such as buildings, highways, and bridges [2,3,4,5,6,7]. Currently, concrete is the second most consumed material globally after water [8,9,10,11,12,13]. However, conventional concrete relies heavily on Ordinary Portland Cement (OPC), whose production contributes significantly to global CO₂ emissions—approximately one ton of CO₂ per ton of OPC produced [14,15,16]. Consequently, the environmental impact and energy-intensive nature of OPC production have become major concerns in the construction industry [17,18,19,20].

To address these concerns, geopolymer concrete (GPC) has emerged as a sustainable alternative to OPC-based concrete [21,22]. Initially developed by Davidovits [23], GPC utilizes aluminosilicate-rich industrial byproducts, such as fly ash and ground granulated blast furnace slag (GGBS), activated using alkaline solutions [24,25]. These alternative binders can significantly reduce carbon emissions while providing satisfactory mechanical and durability performance [24,25,26,27]. The primary constituents of GPC include binders, alkaline activators, aggregates, and admixtures [28,29]. During geopolymerization, a three-dimensional aluminosilicate network, consisting mainly of Si–O–Al–O bonds, is formed within an alkaline medium [30,31,32].

The performance of GPC is influenced by several factors, including precursor type, alkaline activator composition, curing conditions, and aggregate characteristics [32,33,34,35,36,37,38,39]. Fly ash and GGBS are commonly preferred precursor materials due to their availability and engineering performance [29]. Likewise, the type and concentration of alkaline activators significantly affect the dissolution rate, setting behavior, and compressive strength development [33,34,40,41,42,43,44]. Previous studies have reported that heat curing at approximately 60 °C combined with sodium silicate and sodium hydroxide activators may substantially improve early-age strength development in fly ash-based GPC systems [33,34,35]. Aggregates further influence the strength and durability of GPC depending on factors such as grading, shape, texture, and moisture condition [45], while admixtures such as superplasticizers can improve the workability and mixture consistency [46,47]. Previous studies have also reported stress–strain behaviour, optimized mix design approaches, and durability performance associated with geopolymer concrete systems [48,49,50]. In addition to its engineering performance, a life cycle assessment conducted by [51] revealed that GPC outperforms OPC in terms of global warming potential and other air emission-related impacts, such as acidification potential. The diverse performance and sustainability advantages of GPC in construction applications are well-documented in the literature [28].

In parallel, ceramic products are extensively used in the construction industry, particularly in wall and floor tiles, household ceramics, and sanitary ware, with the tile sector accounting for approximately 39% of the total ceramics production [52,53]. However, ceramic waste generated from manufacturing defects, off-specification products, construction, and demolition activities has become a growing environmental concern [53,54,55,56,57]. The primary sources and classification of ceramic waste have been widely reported in the literature [53]. According to [53], approximately 30% of daily ceramic production is discarded, of which 3–7% is considered non-recyclable. The same report indicated that global ceramic tile production reached 13,056 million m² in 2016, alongside the annual production of approximately 200 million sanitary ware units. In Europe, ceramic waste contributes to approximately 48% of construction and demolition waste, yet only 17% is reused, while the remaining 83%—equivalent to nearly 850 million tons annually—is landfilled [53]. This increasing volume of ceramic waste highlights the urgent need for sustainable waste management and recycling strategies [54,55,58,59].

The incorporation of ceramic waste as an aggregate in concrete has been identified as a potential recycling strategy [53]. Since aggregates constitute a major proportion of concrete volume, their characteristics strongly influence concrete performance and durability [60,61]. Ceramic waste aggregates have been reported to possess favorable properties, such as chemical resistance, abrasion resistance, and low thermal expansion [62,63]. Their utilization may therefore reduce dependence on natural aggregates while supporting waste valorization and circular economy practices within the construction industry [53,64,65,66].

Previous studies have investigated the incorporation of ceramic waste into geopolymer systems in different forms, including ceramic powder, fine aggregates, natural sand replacement, and precursor materials [67,68,69,70,71,72]. In addition, the studies reported in [73,74] investigated ceramic waste aggregates in geopolymer concrete systems; however, limited research has focused specifically on the behavior of ceramic waste coarse aggregates in fly ash-based geopolymer concrete under adopted water-curing regimes. Therefore, this study presents a preliminary experimental investigation evaluating different levels of ceramic waste coarse aggregate replacement in fly ash-based geopolymer concrete under the adopted water-curing regime to identify feasibility trends, limitations, and future optimization requirements. This study further discusses the influence of the adopted curing regime and mix composition on the observed mechanical performance while identifying areas requiring further investigation and optimization.

The objectives of this study were designed to guide the preliminary development and evaluation of ceramic waste aggregate-based geopolymer concrete (CWAGPC), which are as follows:

• To preliminarily investigate the incorporation of ceramic waste as coarse aggregate in fly ash-based geopolymer concrete.
• To formulate and prepare CWAGPC with varying levels of ceramic waste coarse aggregate replacement.
• To evaluate the influence of ceramic waste aggregates on the workability and early-age compressive strength behavior of CWAGPC.
• To examine the effect of different ceramic waste aggregate replacement levels on the overall mix performance.
• To discuss the potential sustainability implications associated with the utilization of ceramic waste aggregates in geopolymer concrete.

2. Materials and Methods

To develop and evaluate CWAGPC, this research project methodology is structured into four main stages that include (1) a calculation of the required materials and quantities, (2) the material specifications, (3) the laboratory work, and (4) the experimental program. The process of formulating, designing, and analyzing CWAGPC is summarized in the flowchart in Figure 1.

2.1. Calculation of Materials

CWAGPC was produced using fly ash as the binder and an alkaline activator (NaOH and Na₂SiO₃), combined with natural sand and aggregates, ceramic waste aggregates, and a superplasticizer. Following the calculation method and mix design ratio proposed by [75], the material quantities were initially determined for 1 m³ of concrete and subsequently scaled based on the total volume required for the experimental program. Natural coarse aggregates were replaced with ceramic waste aggregates at varying proportions.

2.1.1. Fixing the Alkaline Activator Solution (AAS) Content

In this study, the alkaline activator solution (AAS) content for CWAGPC was selected based on the methodology reported by [75], considering aggregate size, workability requirements, and consistency during the geopolymer mixture preparation. A total AAS content of 200 kg/m³ was adopted for the experimental program. The aggregate size-based water content values, reported in the referenced study [75] were considered only as a preliminary guideline during the mixture proportioning. However, it is acknowledged that geopolymer concrete proportioning is additionally influenced by activator chemistry, precursor reactivity, geopolymer–solid ratio, and curing conditions [32,33,34,35,36,37,38,39].

Figure 1. Flowchart of the research methodology.

Figure 1. Flowchart of the research methodology.

2.1.2. Selection of Alkaline Activator Solution to Fly Ash Ratio (AAS/FA)

The alkaline activator solution-to-fly ash ratio (AAS/FA) adopted in this study was based on the mix design approach reported by [75], which utilized conventional concrete design principles as a preliminary reference framework for geopolymer concrete proportioning. Although geopolymer concrete proportioning and performance are influenced by factors such as precursor chemistry, activator concentration, curing regime, and geopolymer–solid ratio [32,33,34,35,36,37,38,39], an AAS/FA ratio of 0.4 was adopted in this study to maintain consistency with the referenced experimental methodology [75]. The ACI based strength relationship reported in the referenced study [75] was considered during the preliminary mix proportioning process adopted in the present study.

2.1.3. Calculation of Binder Content

Use the following selected AAS/FA ratio:

Binder content (Bc) = AAS/(AAS/FA) = 200/0.4 = 500 kg/m³

2.1.4. Determination of Activators Content

NaOH and Na₂SiO₃ were used as alkali activators.

Considering, Ratio of Na₂SiO₃ to NaOH (R) = 1.5

Therefore, the mass of the AAS is as follows:

     = Mass of (Na₂SiO₃ + NaOH)
            = Mass of (R × NaOH + NaOH)
= Mass of NaOH (R + 1)

Therefore, the mass of NaOH (M NaOH) is as follows:

              = Mass of ASS/(R + 1)
     = 200/(1.5 + 1)
= 80 kg/m³

Therefore, the mass of Na₂SiO₃ (M Na₂SiO₃) is as follows:

= R × M NaOH = 1.5 × 80 = 120 kg/m³

2.1.5. Calculation of Water Content in the AAS and the Water/GPC Solid Ratio

To determine the water–geopolymer–solid ratio, it is necessary to evaluate the total water content in the alkaline activator solution (AAS), which is obtained by summing the water contents of both the NaOH and Na₂SiO₃ solutions.

Considering the following:

SNaOH = percentage of solids in NaOH solution (45.5%)

SNa₂SiO₃ = percentage of solids in Na₂SiO₃ solution (36%)

Here, the total mass of the NaOH solution (MNaOH) = 80 kg/m³.

Therefore, the mass of water in the NaOH solution is as follows:

= MNaOH − (S NaOH × MNaOH)                  
                   = MNaOH (1 − S NaOH) = 80 (1 − 0.455) = 43.6 kg/m³

Therefore, the solid pellets in the NaOH solution are as follows:

                   = MNaOH − Mass of water in NaOH solution
= 80 − 43.6 = 36.4 kg/m³              

Again, the total mass of the Na₂SiO₃ solution (MNa₂SiO₃) = 120 kg/m³.

Therefore, the mass of the water in the Na₂SiO₃ solution is as follows:

= MNa₂SiO₃ − (SNa₂SiO₃ × MNa₂SiO₃)           
                       = MNa₂SiO₃ (1 − S Na₂SiO₃) = 120 (1 − 0.36) = 76.8 kg/m³

Therefore, the solid portion in the Na₂SiO₃ solution is as follows:

            = MNa₂SiO₃ − Mass of water in Na₂SiO₃ solution
= 120 − 76.8 = 43.2 kg/m³                          

Therefore, the total water content (Wc) in the GPC mix is as follows:

Mass of water in (NaOH + Na₂SiO₃) = 43.6 + 76.8 = 120.4 kg/m³

Therefore, the water–GPC–solid ratio is as follows:

= Total Water content/(Fly Ash + Solids in NaOH solution + Solids in Na₂SiO₃ solution)
= 120.4/(500 + 36.4 + 43.2) = 0.21                                                                                            

2.1.6. Determination of Total Aggregates

Using the absolute volume method, the total aggregate volume was determined, including both fine (≤4.75 mm) and coarse (4.75–20 mm) aggregates.

Let

The total volume of concrete = Vc

Volume of total aggregates = VTA

Volume of binder content (Fly Ash) = VB

Volume of NaOH = V NaOH

Volume of Na₂SiO₃ = V Na₂SiO₃

Volume of entrapped air = Va

The relationship is as follows:

Vc = VTA + VB + V NaOH + V Na₂SiO₃ + Va

where

VB = Bc/GB,

V NaOH = M NaOH/G NaOH,

V Na₂SiO₃ = MNa₂SiO₃/G Na₂SiO₃

Here,

• GB = specific gravity of fly ash (2.2)
• G NaOH = specific gravity of NaOH solution (1.451)
• G Na₂SiO₃ = specific gravity of Na₂SiO₃ solution (1.35)
• Entrapped air (Va) is assumed as 2%.

Assuming 1 m³ of concrete, then

0.98 = VTA + VB + V NaOH + V Na₂SiO₃

Thus, the volume of the total aggregates is as follows:

$$\mathrm{VTA} = 0.98 - [ \left({\displaystyle \frac{500}{2.2}}+{\displaystyle \frac{80}{1.451}}+{\displaystyle \frac{120}{1.35}}\right)\times {\displaystyle \frac{1}{1000}}]=0.609 {\mathrm{m}}^3$$

The fine and coarse aggregate content were determined in accordance with the combined aggregate grading criteria specified in the DIN 1045 standards [76]. The adopted aggregate grading distribution consisted of 28% of 20 mm passing, 32% of 12 mm passing, 20% of 6 mm passing, and 20% of 4.75 mm passing, corresponding to the DIN “A” recommended grading curve reported in the referenced study [75]. The specific gravity of fine aggregates (4.75 mm passing) was considered to be 2.63, while the specific gravities of coarse aggregates (20 mm, 12.5 mm, and 6.3 mm passing) were taken to be 2.73, 2.76, and 2.61, respectively, based on the referenced methodology reported in [75]. Accordingly, an average coarse aggregate specific gravity value of 2.7 was adopted during the mix proportioning for the quantification of aggregate volume and mass. In the present study, both natural and ceramic waste aggregates were used as coarse aggregate constituents. However, the actual density, porosity, water absorption, and combined aggregate grading characteristics associated with the incorporation of ceramic waste aggregates were not experimentally determined within the scope of this preliminary investigation, and may therefore have influenced the effective aggregate volume, workability behavior, and concrete yield.

Therefore,

Mass of Fine Aggregates (Sand), (M FA) = (20% × VTA) × G FA × 1000

$$=\left({\displaystyle \frac{20}{100}}\times 0.609\right)\times 2.63\times 1000 = 320 \mathrm{kg}/{\mathrm{m}}^3$$

Similarly,

Mass of Coarse Aggregates, (M CA) = (80% × VTA) × G avg (CA) × 1000

$$=\left({\displaystyle \frac{80}{100}}\times 0.609\right)\times 2.7\times 1000 = 1315 \mathrm{kg}/{\mathrm{m}}^3$$

where G denotes the specific gravity of aggregates.

2.1.7. Use of Superplasticizer (SP) Dosage

Based on the experimental findings of [75], a superplasticizer dosage of 1% of the binder content was found to be effective in improving the workability of geopolymer concrete mixtures. Accordingly, the same SP dosage was adopted in this study.

SP Dosage = 1% × Binder content (Fly Ash), Bc = 1% × 500 kg/m³ = 5 kg/m³

2.1.8. Summary of the Calculation

The calculated material quantities required for producing 1 m³ of geopolymer concrete are summarized in Table 1, and formed the basis of the mix design adopted in this study. Five mix conditions were investigated by varying the proportion of natural aggregates (NA) and ceramic waste aggregates (CW) within the coarse aggregate fraction. Mix A (100% NA) served as the control mixture, while Mixes B–E incorporated 20%, 40%, 60%, and 100% CW replacement, respectively.

For each mix condition, six 100 × 100 × 100 mm³ concrete cube specimens were prepared for compressive strength testing at 7 and 14 days. A shrinkage factor of 1.5 was considered during batch quantity calculations to account for material loss and handling during specimen preparation. The actual material quantities used for producing the six cube specimens for each mix condition are summarized in Table 2. In this study, ceramic waste aggregate replacement was performed on a mass basis relative to the natural coarse aggregate content. It is acknowledged that differences in the density and water absorption characteristics between natural aggregates and ceramic waste aggregates may have influenced the effective aggregate volume and mixture behavior.

Table 1. Summary of Material Quantities for 1 m³ of Concrete.

Material	AAS	AAS/Fly Ash Ratio	Fly Ash	NaOH Solution	Na2SiO3 Solution	Total Water in AAS	Water/GPC/ Solid Ratio	Fine Aggregates	Coarse Aggregates	Super- Plasticizer
Quantity (kg/m3)	200	0.4	500	80 Water: 43.6 Solid: 36.4	120 Water: 76.8 Solid: 43.2	120.4	0.21	320	1315	5

Note: The reported water–geopolymer–solid ratio was calculated based on the water contained within the alkaline activator solution prior to the addition of extra water for workability adjustment.

Table 1. Summary of Material Quantities for 1 m³ of Concrete.

Material	AAS	AAS/Fly Ash Ratio	Fly Ash	NaOH Solution	Na2SiO3 Solution	Total Water in AAS	Water/GPC/ Solid Ratio	Fine Aggregates	Coarse Aggregates	Super- Plasticizer
Quantity (kg/m3)	200	0.4	500	80 Water: 43.6 Solid: 36.4	120 Water: 76.8 Solid: 43.2	120.4	0.21	320	1315	5

Note: The reported water–geopolymer–solid ratio was calculated based on the water contained within the alkaline activator solution prior to the addition of extra water for workability adjustment.

Table 2. Summary of Material Quantities by Mass Used for Producing Six 100 mm Concrete Cube Specimens for Each Mix Condition.

Mixing Type	Fly Ash (Kg)	AAS Solution (Kg)	Fine Aggregates (Sand) (Kg)	Coarse Aggregates		Super- Plasticizer (Kg)	Extra Water Added for Workability Adjustment (Kg)	Remarks (Varying Percentage of Coarse Aggregates for Distinct Mixing Type)
				Natural Aggregates (NA) (Kg)	Ceramic Waste Aggregates (CW) (Kg)
A	4.5	1.8	2.88	11.90	0	0.045	0.69	NA—100% CW—0%
B	4.5	1.8	2.88	9.60	2.4	0.045	0.69	NA—80% CW—20%
C	4.5	1.8	2.88	7.2	4.8	0.045	0.69	NA—60% CW—40%
D	4.5	1.8	2.88	4.8	7.2	0.045	0.69	NA—40% CW—60%
E	4.5	1.8	2.88	0	11.90	0.045	0.69	NA—0% CW—100%

Note: The additional water listed in the table was incorporated separately from the alkaline activator solution to improve workability during the mixture preparation.

Table 2. Summary of Material Quantities by Mass Used for Producing Six 100 mm Concrete Cube Specimens for Each Mix Condition.

Mixing Type	Fly Ash (Kg)	AAS Solution (Kg)	Fine Aggregates (Sand) (Kg)	Coarse Aggregates		Super- Plasticizer (Kg)	Extra Water Added for Workability Adjustment (Kg)	Remarks (Varying Percentage of Coarse Aggregates for Distinct Mixing Type)
				Natural Aggregates (NA) (Kg)	Ceramic Waste Aggregates (CW) (Kg)
A	4.5	1.8	2.88	11.90	0	0.045	0.69	NA—100% CW—0%
B	4.5	1.8	2.88	9.60	2.4	0.045	0.69	NA—80% CW—20%
C	4.5	1.8	2.88	7.2	4.8	0.045	0.69	NA—60% CW—40%
D	4.5	1.8	2.88	4.8	7.2	0.045	0.69	NA—40% CW—60%
E	4.5	1.8	2.88	0	11.90	0.045	0.69	NA—0% CW—100%

Note: The additional water listed in the table was incorporated separately from the alkaline activator solution to improve workability during the mixture preparation.

2.2. Materials

2.2.1. Fly Ash

This study utilized commercially sourced fly ash, commonly referred to in the UK as pulverized fuel ash (PFA), as the primary aluminosilicate binder for geopolymer concrete production. The fly ash was supplied as Cemex Fly Ash 450-N through Conserv^®, Middlesbrough, North Yorkshire, UK. According to the supplier product information, the material is a by-product generated from the combustion of higher-ranking coals for electricity production and consists predominantly of fine spherical aluminosilicate particles. The fly ash is reported by the supplier to contain approximately 50% silica and 26% alumina, and to exhibit normal fineness characteristics, with a Loss on Ignition (LOI) value below 7%. Detailed experimental chemical characterization of the fly ash used in the present study, including X-ray fluorescence (XRF) analysis, was beyond the scope of this preliminary investigation.

2.2.2. Alkaline Activator Solution

In this study, the alkaline activator solution was prepared using a combination of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) solutions. High-purity NaOH pellets (97–100.5%), supplied by APC Pure UK, were dissolved in distilled water within a fume cupboard to prepare a 16 M solution with approximately 45.5% solid content, ensuring safe handling during solution preparation. Following preparation, the NaOH solution was allowed to cool naturally to ambient laboratory temperature for approximately 3 h prior to mixing, in order to maintain consistency during specimen preparation. After cooling to ambient temperature, the NaOH solution was mixed with a commercially available sodium silicate solution (75Tw variant) supplied by Inoxia Ltd., Cranleigh, UK. The sodium silicate solution was characterized by a solid content of 35–37%, a density of approximately 1.36 ± 0.015, and low viscosity. The alkaline activator solution adopted in this study consisted of 80 kg/m³ of NaOH solution and 120 kg/m³ of the Na₂SiO₃ solution, corresponding to a Na₂SiO₃/NaOH ratio of 1.5.

A 16 M NaOH solution was selected based on the methodology reported in [75], which utilized the same activator concentration for fly ash-based geopolymer concrete systems. The selected molarity was considered for promoting the dissolution of aluminosilicate materials and supporting geopolymerization in the fly ash-based system adopted in this preliminary investigation.

2.2.3. Aggregates

Ceramic waste aggregates and natural aggregates were utilized as coarse aggregate constituents in this study, while standard sand was used as the fine aggregate. The ceramic waste aggregates were supplied by McCarthy Marland (Recycling) Ltd., Bristol, UK and comprised mixed ceramic waste materials, including discarded sanitary ware and tile waste. The heterogeneous nature of the ceramic waste, including variations in ceramic composition, porosity, glaze content, and surface morphology, may have contributed to variability in the aggregate behavior and the concrete performance.

The supplied ceramic waste materials were initially greater than 20 mm in size and were subsequently reduced manually using an impact hammer. The crushed materials were placed in sacks and sieved to obtain aggregates with a nominal maximum size of 20 mm. During the preparation process, visual inspection was conducted to remove excessively flaky particles and unwanted fines where possible. However, the manual crushing process may have introduced variability in particle shape, surface texture, and fines content, which could have influenced the workability and mechanical performance of the geopolymer concrete mixtures.

Natural coarse aggregates ranging from 4.75 to 20 mm were blended to achieve a well-graded aggregate distribution. A combined aggregate proportion consisting of 20% fine aggregate and 80% coarse aggregate (1:4.13), incorporating both natural and ceramic waste aggregates, was adopted in accordance with the DIN “A” recommended grading curve reported in the referenced study [75]. However, detailed experimental sieve analysis and combined grading characterization of the aggregate mixtures after ceramic waste replacement were not conducted within the scope of this preliminary investigation. Therefore, the adopted DIN “A” grading curve was used as a referenced mix proportioning framework for aggregate selection.

Prior to mixing, all aggregates were pre-soaked and brought to a saturated surface-dry (SSD) condition to minimize uncontrolled water absorption during mixture preparation. However, detailed experimental measurements of the water absorption capacity and the corresponding moisture correction calculations for the ceramic waste aggregates were beyond the scope of this study. Consequently, variations in the aggregate absorption characteristics may have influenced the effective water content and workability behavior of the mixtures.

2.2.4. Superplasticizer

A polycarboxylate ether-based high-range water-reducing superplasticizer (liquid form) was used to enhance the workability of GPC and CWAGPC across all mixes. It was supplied by Source For Me Ltd., Swansea, UK.

2.3. Laboratory Work

2.3.1. Mixing

Five mix conditions (A–E) were prepared based on the material quantities outlined in Table 2 using a laboratory concrete mixer. Aggregates in a saturated surface-dry (SSD) condition were pre-measured prior to mixing. Mixing commenced with Condition A and continued sequentially through to Condition E, incorporating fly ash, alkaline activator solution (AAS), sand, aggregates, and superplasticizer.

For each mix, half of the coarse aggregates together with additional water was first introduced into the mixer and blended for approximately 10–20 s. This was followed by the gradual addition of fly ash. Subsequently, the AAS and superplasticizer were incorporated, and mixing continued for approximately 1 min. The remaining sand, fly ash, and aggregates were then added, and mixing proceeded for an additional 2–3 min to achieve a homogeneous mixture. The mixing procedure is briefly illustrated in Figure 2.

The fresh CWAGPC exhibited a dark appearance with a cohesive consistency, as shown in Figure 3.

Figure 2. Mixing procedure for ceramic waste aggregate-based geopolymer concrete (CWAGPC).

Figure 2. Mixing procedure for ceramic waste aggregate-based geopolymer concrete (CWAGPC).

Figure 3. Fresh CWAGPC ready for placement.

Figure 3. Fresh CWAGPC ready for placement.

2.3.2. Casting and Curing

Following the mixing process, the fresh CWAGPC was placed into cube molds in two layers and compacted on a vibration table for approximately 10–15 s to reduce any entrapped air and to improve mixture consolidation during casting. The molds were covered and stored at room temperature for 24 h before demolding and subsequent water curing until the designated testing age.

2.4. Experimental Program

2.4.1. Workability Test

The workability of the fresh concrete was evaluated prior to casting using the standard flow table test method in accordance with BS EN 12350-5:2019 [77] to assess the flowability and placement characteristics of fresh CWAGPC. The procedure involved placing fresh CWAGPC into a flow table cone positioned centrally on the table. The cone dimensions consisted of a top diameter of 130 ± 2 mm, bottom diameter of 200 ± 2 mm, and height of 200 ± 2 mm in accordance with BS EN 12350-5:2019 [77]. Once filled and leveled, the cone was lifted vertically to allow the concrete to spread freely. The flow table was then raised and dropped 15 times within approximately 15 s in accordance with the standard procedure. The resulting spread diameter was measured in two perpendicular directions, and the average value was recorded as the flow diameter (Figure 4).

2.4.2. Compressive Strength Test

Compressive strength testing was conducted using a 2000 kN compression testing machine following the general concrete cube testing procedures outlined in BS EN 12390-3 [78]. Three 100 mm cube specimens were tested for each mix condition (A to E) at the specified curing ages. The load was applied continuously at an approximately constant loading rate of 3 kN/s, corresponding to an equivalent stress rate of approximately 0.3 MPa/s, until specimen failure occurred.

3. Experimental Results and Discussion

3.1. Workability

The workability of both the conventional GPC and CWAGPC mixes was evaluated using the flow table test in accordance with BS EN 12350-5:2019 [77]. While this standard was originally developed for testing the consistency of fresh concrete made with Portland cement binders, it remains a widely accepted and reliable method for assessing the flow characteristics of alternative binder systems, including geopolymer-based mixtures [79]. The test is applicable for concrete mixes with a maximum aggregate size not exceeding 63 mm, which aligns with the materials used in this study. The flow table test is sensitive to variations in concrete consistency within an acceptable flow range of 340–620 mm; beyond these limits, alternative consistency assessment methods should be considered [77,80].

Table 3. Flow Test Results.

Mix Type	Flow Diameter 1 (mm)	Flow Diameter 2 (mm)	Average Flow Diameter (mm)
A	604	604	604
B	602	602	602
C	360	360	360
D	560	570	565
E	570	620	595

presents the average flow diameters for each mix, while Figure 5 illustrates the comparative flow behavior of the different mixtures. The control mix (Mix A), representing conventional geopolymer concrete without ceramic waste aggregates, exhibited an average flow diameter of 604 mm, indicating satisfactory workability. Mixes B (602 mm), D (565 mm), and E (595 mm), incorporating 20%, 60%, and 100% ceramic waste aggregate replacement, respectively, also demonstrated acceptable flow values. The observed workability behavior may be associated with the incorporation of additional water (690 mL per batch) and superplasticizer (45 g per batch), which may have improved particle dispersion and reduced internal friction within the mixtures. In addition, the effective liquid content of the mixtures was likely influenced by the water associated with the alkaline activator solution.

By contrast, Mix C (40% CW) recorded a substantially lower flow value of 360 mm compared with the other mixtures, despite containing the same quantities of additional water and superplasticizer. Although the measured flow value remained within the acceptable range (340 mm–620 mm) according to the BS EN 12350-5 standard reported in [77,79], the observed reduction in workability may be associated with the porosity, angularity, and water absorption characteristics of the ceramic waste aggregates, which could have contributed to increased internal friction and reduced paste lubrication within the mixture. In addition, variability introduced during the manual preparation and crushing process of the ceramic waste aggregates may also have influenced the fresh mixture behavior. Similar observations regarding the influence of aggregate characteristics on fresh concrete properties have been discussed in previous studies [81].

Interestingly, Mixes D and E, despite containing higher ceramic waste aggregate replacement levels than Mix C, exhibited comparatively improved flowability. This behavior may be associated with differences in aggregate distribution, particle interaction, and mixture consistency at varying replacement levels.

Overall, the results indicate that CWAGPC mixtures can achieve a workability behavior comparable to conventional GPC mixtures under the adopted mixture proportions and water curing regimes in this preliminary investigation.

3.2. Compressive Strength

The compressive strength of both the conventional GPC and CWAGPC mixtures was evaluated after 7 and 14 days of water curing. Each reported strength value represents the average compressive strength obtained from three specimens per mix condition.

Table 4. Compressive Strength Results.

Mix Type	Compressive Strength on 7th Day (MPa)	Compressive Strength on 14th Day (MPa)
A	2.62	5.09
B	3.23	5.52
C	0.5	0.8
D	1.01	1.83
E	1.55	2.02

presents the compressive strength results for five mix types (A–E), where Mix A represents the control mixture (conventional GPC), while Mixes B–E incorporated 20%, 40%, 60%, and 100% replacement of natural coarse aggregates (NA) with ceramic waste aggregates (CW), respectively. The adopted mix design maintained consistent binder content, alkaline activator ratios, and superplasticizer dosage across all mixtures in accordance with the referenced methodology reported in [75].

As shown in Table 4, Mix B exhibited a slightly higher compressive strength at 14 days (5.52 MPa) compared with the conventional GPC mix (Mix A), which recorded 5.09 MPa. The remaining CWAGPC mixes (C, D, and E) demonstrated a substantially lower compressive strength values. Although Mix B showed marginal improvement relative to Mix A, all investigated mixtures exhibited comparatively low compressive strength values and did not achieve the strength levels typically associated with structural-grade concrete, indicating significant performance limitations.

The compressive strength values obtained in the present study, including the control mix, were substantially lower than those reported in the referenced study [75], despite adopting a similar mix proportioning methodology. This discrepancy indicates a significant variation in strength development and highlights the need for further investigation into the underlying influencing factors.

3.2.1. Contributing Factors to the Discrepancy and Low Strength in Control Mix A (Conventional GPC)

Absence of Heat Curing and Use of Water Curing

Mix A was proportioned based on the mix design methodology reported in [75]; however, unlike the referenced study, the present investigation adopted a water-curing regime rather than heat curing. After casting, the specimens were covered with plastic sheets and stored at ambient temperature for 24 h prior to demolding and subsequent submersion in a water-curing regime until the designated testing age. By contrast, the referenced study [75] cured the specimens at 60 °C for 24 h followed by air drying at room temperature. The absence of heat curing in this preliminary investigation likely contributed to the significantly lower compressive strength observed in Mix A and across the other investigated mixtures. Previous studies have supported this assertion: Refs. [39,82] showed that, although ambient curing can achieve reasonable long-term strength, elevated temperature curing significantly enhances early strength development. Ref. [83] also reported that geopolymer concrete cured at ambient conditions exhibited much lower strength—approximately 3 MPa at 7 days and under 18 MPa at 28 days—compared to heat-cured mixes. Similar observations were made by [84]. Ref. [85] emphasized that curing at temperatures between 60 °C and 90 °C accelerates the geopolymerization process, promoting early strength gain, whereas ambient curing slows the reaction and results in a significantly lower strength. Therefore, the use of water curing in this study likely limited the thermal activation of geopolymerization, leading to the markedly reduced compressive strength in Mix A. This limitation also likely affected the performance of all other mixes in this study.

In addition, continuous water curing may have contributed to alkali dilution or leaching from the geopolymer matrix within the fly ash-based geopolymer system adopted in this study. Previous studies have reported that excessive exposure to water during early curing stages may hinder geopolymerization by reducing the alkaline ion concentration within the pore solution. This effect may have further limited the strength development in both the GPC and CWAGPC mixtures under the adopted curing regime [86,87].

Excess Water Content

In contrast to the study by [75], which relied primarily on the water contained within the alkaline activator solution (AAS), this study incorporated an additional 690 mL of water per mix to improve workability during mixture preparation. The inclusion of this additional water increased the effective liquid content and likely altered the overall water–geopolymer–solid ratio of the mixtures. Previous studies have reported that excessive water content may dilute the alkaline activator concentration and adversely affect geopolymerization efficiency, leading to a reduced mechanical performance [88]. Consequently, the additional water incorporated in this study likely contributed to the comparatively low compressive strengths observed in both the GPC and CWAGPC mixtures.

Material Variability

The material properties used in the present study differed from those reported in [75]. The current investigation, conducted in the UK, utilized commercially sourced fly ash referred to as pulverized fuel ash (PFA), together with locally sourced aggregates and superplasticizer materials, whereas the referenced study, conducted in India, utilized Class F low-calcium fly ash. Variations in material properties, including specific gravity, mineral composition, pH of water, and superplasticizer type (polycarboxylate-based versus sulphonated naphthalene-based), may have influenced the reactivity and strength outcomes. Previous studies have reported that such variations can significantly affect geopolymer concrete performance [89,90].

Among these mentioned factors, the primary reason for the discrepancy with Pavithra et al.’s study [75], despite adhering to the mix design parameters, is likely the absence of heat curing, the negative impact of water curing, and the utilization of excess water as a significant contributing factor.

3.2.2. Factors Influencing the Strength of the CWAGPC Mixes

Influence of Aggregate Type and Mix Proportions

The variations in the compressive strength under different mix conditions can largely be attributed to the replacement of natural aggregates (NA) with ceramic waste aggregates (CW) (Figure 6). Mix B, which incorporated 20% CW and 80% NA, exhibited the slightly highest compressive strength of 5.52 MPa at 14 days, surpassing the control GPC mix (Mix A, 5.09 MPa). This improvement suggests that partial substitution of NA with CW enhances strength, potentially due to improved interfacial bonding with the geopolymer binder and more efficient load distribution.

By contrast, Mixes C–E, with higher CW replacement levels (≥40%), showed a marked reduction in strength. This decline is attributed to the porous nature of CW, which increases void content (Figure 7) and reduces compaction efficiency, thereby limiting the material’s capacity to resist compressive forces.

In addition, CW particles sourced from sanitary ware and tiles typically exhibit a glazed and smooth surface layer on one side. This characteristic hinders optimal adhesion with the geopolymer matrix, particularly in mixes with higher CW proportions, as shown in Figure 8. This observation is consistent with the findings reported by [53].

Curing Age

The results demonstrate that the compressive strength increased over time in all mix types subjected to water curing, similar to the behavior of conventional concrete. In the control GPC mix (Mix A), the compressive strength increased from 2.62 MPa on the 7th day to 5.09 MPa on the 14th day. A comparable trend was also observed in the CWAGPC mixes (Types B, C, D, and E), as shown in Figure 9. These findings are consistent with the observations of the previous literature [83], which reported strength gains over time under both ambient and heat curing conditions. Although water curing contributed to the strength development in this study, the overall strength values remained relatively low across all mixes and cannot be classified as structural-grade concrete.

Curing Temperature

Previous studies have demonstrated that the curing temperature significantly influences the strength development of geopolymer concrete [75,83,84,85]. Although both the GPC and CWAGPC specimens in the present study were subjected to water curing, heat curing has generally been reported to promote improved compressive strength development in geopolymer systems. Consequently, all investigated mixtures in this study exhibited comparatively low compressive strength values. Figure 10 presents a comparative illustration between the water-curing regime adopted in the present study and the heat-curing approach reported by Pavithra et al. [75] for Mix A. Since the CWAGPC mixtures were also subjected to water curing in this preliminary investigation, their strength development may likewise have been limited by the adopted curing regime. Nevertheless, a gradual increase in the compressive strength was observed over time under continuous water curing (Figure 9).

3.3. Implications and Sustainability Potential

Among all the CWAGPC mixtures investigated under the adopted preliminary experimental conditions, Mix B (20% CW replacement) demonstrated the most promising performance, slightly surpassing the conventional GPC mix in compressive strength. This observation may be associated with improved particle packing and adequate matrix–aggregate interaction at lower ceramic waste replacement levels. However, higher CW replacement levels (Mixes C–E) resulted in substantially reduced compressive strengths (0.80–2.02 MPa). Based on observational interpretation and hypothesis, this reduction may be associated with weaker aggregate–matrix bonding, increased porosity, variability in aggregate characteristics, and possible limitations in mixture compaction.

Despite the relative improvement observed in CWAGPC (Mix B), all mixtures exhibited compressive strengths significantly below the typical requirements for structural-grade concrete. The primary limitations were the water curing regime adopted in this study and the addition of excess water to improve workability, both of which likely impeded geopolymerization. The absence of heat curing emerged as a critical factor restricting strength development, consistent with the literature reports on geopolymerization kinetics [91,92,93].

To fully realize the mechanical performance of CWAGPC, further optimization is required—particularly in curing regimes, water control, optimized mix design, and 28th day compressive strength value. Nevertheless, the findings of this preliminary investigation suggest that 20% ceramic waste aggregate replacement in geopolymer concrete may be feasible under the adopted experimental conditions, while further investigation is required to evaluate the long-term performance and broader sustainability implications associated with ceramic waste reuse and reduced reliance on natural aggregates. In relation to reducing reliance on natural aggregates, a recent study has also explored high-performance and artificial aggregate systems within alkali-activated and geopolymer materials, highlighting continuing advancements in sustainable concrete technologies [94].

4. Conclusions

This preliminary study developed ceramic waste aggregate-based geopolymer concrete (CWAGPC) and investigated its workability and early-age compressive strength performance in comparison with conventional geopolymer concrete (GPC) under the adopted experimental conditions. Five mix designs were prepared with varying proportions of ceramic waste aggregates replacing natural coarse aggregates in geopolymer concrete at replacement levels of 0%, 20%, 40%, 60%, and 100%. Based on the experimental results obtained in this preliminary investigation, the following conclusions can be drawn:

(1)

Both the conventional GPC and CWAGPC mixtures exhibited a generally acceptable workability behavior under the adopted mix proportions and testing conditions. The control GPC mix (Mix A) achieved a flow value of 604 mm, while the CWAGPC mixtures exhibited flow values ranging between 360 mm and 602 mm. Most mixtures remained within the acceptable flow range (340–620 mm) specified in BS EN 12350-5. However, Mix C (40% ceramic waste aggregate replacement) demonstrated comparatively lower flowability (360 mm), which may have been influenced by aggregate-related variability and mixture behavior under the adopted experimental conditions.

(2)

The compressive strength results under water-curing conditions demonstrated variable performance among the investigated mixtures. At 14 days, the 20% ceramic waste aggregate replacement mixture (Mix B) achieved the best compressive strength value (5.52 MPa), slightly exceeding the control GPC mixture (5.09 MPa) under the limited mixture tested within the scope of this preliminary investigation. By contrast, higher ceramic waste replacement levels (40–100%) resulted in substantially lower compressive strength values ranging between 0.80 MPa and 2.02 MPa.

(3)

Although the compressive strength increased with the curing time for all investigated mixtures, the overall strength development remained comparatively low under the adopted water-curing regime during the investigated testing period. The comparatively reduced early-age development observed in this study may be associated with several interacting factors, including the adopted water-curing regime, absence of heat curing, incorporation of additional water for workability adjustment, material variability, and the heterogeneous characteristics of the ceramic waste aggregates. Nevertheless, the observed increase in strength between 7 and 14 days suggests that continued curing may contribute to further strength development over longer curing durations, although this was not evaluated within the scope of the present preliminary investigation.

(4)

Among the limited mixtures investigated in this preliminary study, the 20% ceramic waste aggregate replacement mixture demonstrated a comparatively better compressive strength performance relative to the other tested CWAGPC mixtures under the adopted experimental conditions. However, the present findings are insufficient to establish an optimum replacement level or to support a structural engineering application of the investigated mixtures in their current form without substantial optimization of mix design and curing conditions. Based on the comparatively low compressive strength values obtained, the investigated CWAGPC mixtures may presently be more appropriate for low-strength or non-structural applications rather than structural concrete applications.

(5)

The present work should therefore be interpreted as a preliminary experimental investigation into the incorporation of ceramic waste aggregates in fly ash-based geopolymer concrete under water-curing conditions. Additional research involving optimized geopolymer mix design approaches, heat-curing evaluation, long-term strength development, durability assessment, detailed material characterization, microstructural analysis, statistical validation, and environmental performance evaluation is required before the broader engineering applicability of CWAGPC can be established.

5. Recommendations

Based on the findings and limitations of this preliminary investigation, the following recommendations are proposed for future research:

(1)

Future studies should investigate the performance of CWAGPC under alternative curing regimes, particularly heat curing and controlled ambient curing, to evaluate their influence on geopolymerization efficiency, long-term strength development, and durability performance.

(2)

Further optimization of the geopolymer mix design parameters is recommended, including adjustment of the alkaline activator concentration, water–geopolymer–solid ratio, alkaline activator–binder ratio, aggregate proportioning, and superplasticizer dosage to improve both the fresh and hardened concrete performance.

(3)

Long-term mechanical performance evaluation is recommended, including 28-day and later-age compressive strength assessments, together with flexural strength, splitting tensile strength, modulus of elasticity, shrinkage, density, and water absorption measurements.

(4)

Detailed material characterization should be conducted in future studies, including X-ray fluorescence (XRF), scanning electron microscopy (SEM), aggregate porosity analyses, water absorption measurements, and interfacial transition zone (ITZ) evaluations to better understand the interaction between ceramic waste aggregates and the geopolymer matrix.

(5)

Future investigations should include statistical validation of the experimental results through repeated testing, standard deviation analysis, and larger specimen sample sizes to improve reliability and interpretation of the observed performance trends.

(6)

The influence of aggregate grading, particle shape, ceramic surface texture, and compaction behavior on the fresh and hardened properties of CWAGPC should be systematically evaluated using controlled aggregate processing methods.

(7)

Durability-related performance, including permeability, freeze–thaw resistance, sulfate resistance, chemical resistance, and long-term environmental exposure behavior, should be investigated before the broader engineering application of CWAGPC can be considered.

(8)

Further research should investigate whether optimized curing conditions, improved mix design approaches, and enhanced material characterization can improve the mechanical performance of CWAGPC toward potential future structural engineering applications

6. Limitations of the Research

The present study should be interpreted as a preliminary experimental investigation into the incorporation of ceramic waste aggregates in fly ash-based geopolymer concrete under the adopted experimental conditions. Several limitations associated with the material characterization, experimental scope, curing conditions, and performance evaluation should therefore be acknowledged.

(1)

The adopted mix proportioning approach was based on a previously reported heat-cured fly ash-based geopolymer concrete methodology [75], whereas the present investigation evaluated the mixtures under a water-curing regime. Redesign and optimization of the geopolymer mix specifically for water curing were beyond the scope of this preliminary investigation.

(2)

Heat-cured control mixtures and heat-cured CWAGPC mixtures were not included in the experimental program. Consequently, the influence of heat curing on CWAGPC performance was not experimentally evaluated within this study.

(3)

Detailed material characterization, including X-ray fluorescence (XRF), scanning electron microscopy (SEM), aggregate porosity analysis, density measurements, water absorption characterization, and interfacial transition zone (ITZ) analysis, were not conducted within the scope of this preliminary investigation.

(4)

Ceramic waste aggregates were manually crushed and processed, which may have introduced variability in particle shape, grading, fines content, and surface texture. In addition, combined aggregate grading verification for each replacement level was not experimentally evaluated.

(5)

The experimental investigation focused primarily on workability and early-age compressive strength behavior under the adopted curing conditions. Long-term strength development, durability performance, shrinkage, permeability, flexural strength, splitting tensile strength, modulus of elasticity, density, and environmental exposure behavior were not evaluated.

(6)

Statistical analysis of the experimental results was limited to reporting the average values obtained from the tested specimens. Standard deviation, coefficient of variation, and statistical significance analysis were not included within the scope of this study.

(7)

The workability assessment was based on single flow table measurements for each mix condition, and variability associated with repeated testing was not experimentally evaluated.

(8)

The comparatively low compressive strength observed across all investigated mixtures may have been influenced by several interacting factors, including the adopted water-curing regime, additional water incorporated for workability adjustment, material variability, possible compaction limitations, and the heterogeneous characteristics of the ceramic waste aggregates. However, the individual contribution of each parameter could not be isolated within the scope of this preliminary investigation.

(9)

Interpretations regarding the influence of aggregate porosity, angularity, particle packing, water absorption behavior, and aggregate–matrix interaction were presented as observational interpretations and hypotheses based on visual observation and the supporting literature discussion, as detailed quantitative characterization was not performed.

(10)

The comparatively small difference in compressive strength observed between Mix B and the control mixture may fall within experimental variability. Therefore, conclusions regarding the potential performance improvement associated with 20% ceramic waste aggregate replacement should be interpreted cautiously within the limitations of this preliminary investigation.

(11)

The investigated CWAGPC mixtures exhibited comparatively low compressive strength values under the adopted experimental conditions and therefore cannot presently be considered suitable for structural concrete applications without substantial optimization and further validation.