Rheological Optimization and Performance Evaluation of 3D Printable Geopolymer Concrete

Rheological Optimization and Performance Evaluation of 3D Printable Geopolymer Concrete

Mohith Raja R

Post Graduate Student, Coimbatore Institute of Technology, Coimbatore-641014, Tamil Nadu, India

M. P Muthuraj

Associate Professor, Department of Civil Engineering, Coimbatore Institute of Technology, Coimbatore – 641014,

Tamil Nadu, India

ABSTRACT – The integration of 3D Concrete Printing (3DCP) with geopolymer technology presents a sustainable alternative to conventional cement-based construction by reducing CO emissions and eliminating formwork. This study investigates the development, optimization, and validation of a high-performance 3D printable geopolymer concrete through a two-phase experimental program.

Phase 1 focused on rheological optimization and mechanical characterization. A geopolymer binder system comprising Fly Ash, GGBS, and Micro Silica was activated using 8M NaOH and NaSiO (ratio = 2.0). Fresh-state properties were evaluated using Flow Table tests to establish the printability window. Design Mix 1 reaching 58.7 MPa compressive strength and 3.40 MPa tensile strength at 3 days.

Phase 2 addressed a critical practical challenge observed during initial printing trials, where premature setting caused nozzle blockage within 1520 minutes. A systematic reformulation of the mix design was undertaken to extend open time while maintaining structural stability. Through iterative trials, Trial Mix13 (Design Mix2) achieved a controlled flow of 165 mm and an extended setting time of 50 minutes. Successful real-time 3D printing validation confirmed stable extrusion, shape retention, and absence of clogging. The optimized mix achieved 48.86 MPa compressive strength and 2.76 MPa tensile strength.

The study establishes a clear correlation between rheological control and mechanical performance, demonstrating the feasibility of durable, high-strength 3D printable geopolymer concrete for sustainable construction applications.

KEYWORDS:

3D concrete printing; geopolymer concrete; rheology; printability; extrudability; buildability; interlayer bond strength; sustainable construction; additive manufacturing.

1. INTRODUCTION

   Additive manufacturing in construction has evolved rapidly with the development of 3D concrete printing (3DCP), enabling automated, formwork-free fabrication of structural components. The performance of 3D printed concrete depends primarily on its fresh-state rheological characteristics, including extrudability, buildability, and open time. Unlike conventional casting, 3D printing requires precise control over yield stress evolution to prevent collapse while avoiding nozzle blockage.

   Geopolymer concrete, synthesized from industrial by-products such as fly ash and GGBS, has emerged as a sustainable alternative to Ordinary Portland Cement due to its reduced carbon emissions and superior early-age strength. However, rapid setting behavior and rheological instability often limit its application in extrusion-based construction.

   Previous studies have investigated printable cementitious systems; however, limited research has systematically correlated rheological optimization with mechanical performance in geopolymer-based 3D printing systems. This study addresses this gap by developing and validating an optimized geopolymer mix capable of achieving extended open time without compromising structural performance.

2. MATERIALS AND METHODS
   1. Materials and Their Properties

      The development of 3D printable fiber-reinforced geopolymer concrete requires careful selection of constituent materials to achieve a balanced combination of extrudability, buildability, and mechanical strength. The materials used in this investigation are described below.

      1. Binder Materials

         The geopolymer binder system consisted of aluminosilicate precursors activated using a sodium-based alkaline solution.

         Fly Ash (FA): Class F fly ash was used as the primary aluminosilicate precursor. Owing to its low calcium content and spherical particle morphology, fly ash improves workability and flowability of the fresh geopolymer mix, which is essential for extrusion- based printing. It primarily contributes to the formation of sodiumaluminosilicatehydrate (N-A-S-H) gel during

         geopolymerization.

         Ground Granulated Blast Furnace Slag (GGBS): GGBS was incorporated to enhance early-age strength and structural build- up. The presence of calcium in GGBS promotes the formation of calciumaluminosilicatehydrate (C-A-S-H) gel in addition to N-A-S-H gel, leading to accelerated reaction kinetics and reduced setting time. This property is beneficial for layer stability and buildability in 3D printing applications.

         Micro Silica (Silica Fume): Amorphous micro silica conforming to ASTM C1240 was incorporated at 510% of the total binder mass. Due to its ultra-fine particle size, micro silica enhances particle packing density and reduces pore structure, thereby improving cohesiveness in the fresh state and compressive strength in the hardened state.

      2. Fine Aggregate

         Manufactured sand (M-sand) was used as fine aggregate. The material was obtained by crushing hard granite rock and conformed to Zone II grading requirements. The uniform particle size distribution and absence of organic impurities contributed to consistent workability and mechanical performance. Coarse aggregate was intentionally excluded to ensure smooth extrusion and prevent nozzle blockage during printing.

      3. Alkaline Activators

         The alkaline activator solution was prepared using sodium hydroxide (NaOH) and sodium silicate (NaSiO).

         Sodium Hydroxide (NaOH): Analytical-grade NaOH pellets were dissolved in distilled water to prepare an 8M solution. The solution was prepared 24 hours prior to mixing to allow for temperature stabilization and complete dissolution.

         Sodium Silicate (NaSiO): Commercially available sodium silicate solution was used to enhance silicate availability and improve polymerization reactions. The NaSiO/NaOH mass ratio was maintained at 2.0 for all trial mixes to ensure consistent geopolymerization behavior.

      4. Chemical Admixture

         A polycarboxylate ether (PCE)-based high-range water-reducing admixture (Master Glenium 1466) was used to improve flowability without increasing water content. The admixture facilitated controlled rheological behavior required for extrusion while maintaining adequate structural build-up after deposition.

3. MIX DEVELOPMENT AND OPTIMIZATION

   The experimental program was designed to develop a stable and extrusion-compatible geopolymer mix suitable for 3D printing applications. The investigation was conducted in a systematic sequence comprising mix proportioning, fresh-state characterization, specimen fabrication, and hardened-state performance evaluation. Initially, a printable geopolymer base mix was optimized to achieve controlled rheological behavior. Subsequently, fiber reinforcement was incorporated, and the mechanical performance of the printed composite was evaluated.

   1. Mix Proportioning and Optimization Strategy

      An iterative trial-based approach was adopted to achieve the target rheological and mechanical performance requirements necessary for extrusion-based 3D printing. The optimization process focused on balancing extrudability, buildaility, and open time while ensuring adequate early-age strength development.

      1. Target Rheological Requirements

Based on findings reported in prior studies, specific fresh-state performance criteria were established to ensure printability. The flow spread measured using the flow table test was maintained within a range corresponding to approximately 110130% (165185 mm diameter), providing sufficient deformability for extrusion while retaining structural stability after deposition.

In addition, the setting characteristics were controlled to maintain an initial setting time between 2045 minutes and a final setting time below 90 minutes. This range ensured adequate open time for uninterrupted printing operations while promoting rapid structural build-up for layer stability and buildability.

4.3 Printing Simulation and Parametric Optimization

Following the development and rheological optimization of the geopolymer mixes, the study progressed to controlled extrusion trials to translate laboratory-scale material behavior into practical printability performance. This phase aimed to optimize extrusion parameters and establish a stable printing regime for the shortlisted geopolymer concrete mix.

A major challenge in extrusion-based additive manufacturing lies in balancing competing material requirements. As reported in prior studies, printable cementitious systems must exhibit sufficient flowability to permit smooth extrusion through the nozzle while simultaneously developing adequate structural build-up to retain geometric stability after deposition. Excessively low yield stress results in filament spreading and loss of dimensional accuracy, whereas high stiffness promotes nozzle clogging and discontinuous extrusion. Similarly, mixtures designed primarily for high compressive strength often possess reduced workability, adversely affecting extrudability and interlayer bonding.

Printing trials were performed using the optimized geopolymer mix to fabricate simple geometries such as straight wall segments and cylindrical columns. The printed elements were evaluated qualitatively and visually based on extrusion continuity, filament uniformity, shape retention, surface finish, and maximum achievable build height without global instability. Through iterative refinement of printing parameters, a combination of operational conditions was identified that enabled continuous extrusion, stable filament geometry, and acceptable dimensional accuracy. This parameter range defines the printability window of the developed geopolymer mix and serves as a guideline for subsequent fabrication of mechanical test specimens.

Fig 4.1 3D Printing Simulation

1. 1. MIX DESIGN
   2. Design Mix 1

      Design Mix1 was developed to achieve a balanced combination of extrudability, buildability, and early-age strength suitable for 3D printable geopolymer concrete. The mix was proportioned based on absolute volume method to ensure volumetric consistency and target fresh density.

      Material	Mass in 1 m³ (kg)	Calculation Basis
      GGBS	641.2	(500 / 1320) × 2055
      Fly Ash	256.5	(200 / 1320) × 2055
      Micro Silica	64.1	(50 / 1320) × 2055
      M-Sand	320.6	(250 / 1320) × 2055
      NaOH (8M Solution)	128.2	(100 / 1320) × 2055
      NaSiO Solution	256.5	(200 / 1320) × 2055
      Superplasticizer	6.4	(5 / 1320) × 2055

      Free Water	19.2	(15 / 1320) × 2055
      Total Mass	2055.0 kg

      Table 5.1

      The binder content comprised GGBS (400 g), Fly Ash (300 g), and Micro Silica (50 g), giving a total binder mass of 750 g. The alkaline activator consisted of NaOH solution (100 g) and NaSiO solution (200 g), resulting in a total activator mass of 300 g.

      The Activator-to-Binder (A/B) ratio was 0.40, while the NaSiO/NaOH ratio was maintained at 2.0. These ratios ensured adequate geopolymerization, controlled reaction kinetics, and stable rheological performance suitable for extrusion-based 3D printing.

   3. Design Mix 2

      Design Mix2 exhibited a significantly extended initial setting time of approximately 50 minutes, exceeding the target value of 45 minutes and demonstrating improved workability retention compared to Design Mix1.

      Material	Mass in 1 m³ (kg)	Calculation Basis
      GGBS	640.59	(400 / 1345) × 2154
      Fly Ash	480.45	(200 / 1345) × 2154
      Micro Silica	80.07	(50 / 1345) × 2154
      M-Sand	400.37	(250 / 1345) × 2154
      NaOH (8M Solution)	184.17	(100 / 1345) × 2154
      NaSiO Solution	368.34	(200 / 1345) × 2154
      Total Mass	2154 kg

      Table 5.2

      The total binder content comprised GGBS (400 g), Fly Ash (300 g), and Micro Silica (50 g), resulting in a binder mass of 750 g. The alkaline activator consisted of NaOH solution (115 g) and NaSiO solution (230 g), giving a total activator mass of 345 g.

      The Activator-to-Binder (A/B) ratio was 0.45, while the NaSiO/NaOH ratio was maintained at 2.0. The increased A/B ratio enhanced workability retention and extended open time without altering the silicate balance required for controlled geopolymerization.

      1. EXPERIMENTAL INVESTIGATION
         1. Preliminary Mechanical Evaluation and Printing Trials

            Following the optimization of fresh-state rheological properties, an initial assessment of mechanical performance was conducted to ensure that the shortlisted mixes demonstrating acceptable printability also possessed adequate structural potential. Two mixes were selected for evaluation based on their compliance with the target flowability range (165185 mm), which is critical for maintaining a balance between extrudability and buildability.

            Trial Mix2 exhibited a flow diameter of approximately 180 mm, indicating relatively higher stiffness and improved structural build-up capacity. Trial Mix5 demonstrated a flow value of 175 mm, representing a controlled balance between deformability and shape retention. Standard 100 mm cube specimens were cast for both mixes. The fresh geopolymer concrete was placed in steel

            moulds in three layers and compacted using a vibrating table to eliminate entrapped air and ensure homogeneity. Specimens were demoulded after 24 hours and cured under ambient conditions until testing at 3 days. The ambient curing regime was adopted considering that geopolymer binders gain strength primarily through polycondensation reactions rather than conventional hydration.

            Based on satisfactory preliminary strength results, the optimized mix (Design Mix1) based on Trial Mix- 5 was subjected to extrusion-based 3D printing trials conducted in collaboration with Konface Pvt. Ltd. Printing was performed using a 10 mm nozzle at a print speed of 1520 mm/s under predefined machine parameters. Although initial extrusion was stable, premature stiffening occurred during the printing operation. The material began solidifying within the nozzle after approximately 1520 minutes, leading to blockage ad incomplete specimen formation. This highlighted a critical limitation in open time despite acceptable flow values under laboratory conditions. The failure of the initial printing trial emphasized the need to extend the workable duration of the geopolymer mix without compromising buildability. Consequently, a second phase of mix reformulation was undertaken, focusing on adjusting binder proportions and activator characteristics to achieve a target setting time of approximately 45 minutes while maintaining controlled flow.

            Subsequent trials (TM-6 to TM-13) were conducted to refine the rheological response. Trial Mix13 (Design Mix2) achieved the desired performance criteria, with a controlled flow diameter of 165 mm and an actual setting time of approximately 50 minutes. The mix exhibited stable cohesiveness without segregation or deformation. To validate its structural suitability, three cube specimens and three cylindrical specimens were cast using Design Mix2. All specimens retained geometric stability during casting and curing, confirming improved rheological control and mix homogeneity.

            Fig 6.1 3d Printing trials

         2. Specimen Details

            To evaluate the mechanical performance of the developed 3D printable geopolymer mixes, cube and cylindrical specimens were cast for compressive and split tensile strength testing. Cube specimens: 100 mm × 100 mm × 100 mm, cylindrical specimens: 100 mm diameter × 200 mm height

            Compressive strength test and Split Tensile Test of the specimens is conducted on 3rd day.

            Mix ID	Cubes (Compressive)	Cylinders (Split Tensile)
            Design Mix1	3	3
            Design Mix2	3	3

            Table 6.1

         3. Compressive Strength Test (IS 516:1959)

            The compressive strength of Design Mix- 1.

            Cube No.	Weight (kg)	Peak Load (kN)	Compressive Strength (MPa)
            Cube 1	2.13	649.6	64.9
            Cube 2	2.18	582.0	58.2
            Cube 3	2.15	530.2	53.0

            Average Compressive Strength = 58.7 MPa

            Table 6.2

            The compressive strength of Design Mix- 2.

            Cube No.	Weight (kg)	Peak Load (kN)	Compressive Strength (MPa)
            Cube 1	2.13	490.2	49.0
            Cube 2	2.18	465.2	46.5
            Cube 3	2.15	511.0	51.1

            Average Compressive Strength = 48.9 MPa

         4. Split Tensile Strength Test (IS 5816:1999) The Split Tensile Strength of Design Mix- 1.

            Table 6.3

            Cylinder Number	Weight (kg)	Peak Load (kN)	Tensile Strength (MPa)
            Cylinder 1	2.50	112.2	3.57
            Cylinder 2	2.54	96.9	3.46
            Cylinder 3	2.48	89.5	3.19

            Average Tensile Strength: 3.40 MPa

            Table 6.4

            The Split Tensile Strength of Design Mix- 2.

            Cylinder Number	Weight (kg)	Peak Load (kN)	Tensile Strength (MPa)
            Cylinder 1	2.50	90.8	2.89
            Cylinder 2	2.54	86.5	2.75
            Cylinder 3	2.48	83.0	2.64

            Average Tensile Strength: 2.76 MPa

            Table 6.5

      2. CONCLUSIONS

         This study investigated the development and optimization of a geopolymer concrete mixture suitable for extrusion-based 3D printing applications.

         Initial printing trials performed using Design Mix1 revealed premature stiffening of the material during the printing process. The mixture began to set within 1520 minutes, which resulted in nozzle blockage and interruption of the printing operation. This indicated that the mix possessed insufficient open time for continuous extrusion.

         Subsequent iterative optimization of the mixture (TM-6 to TM-13) was therefore conducted to improve workability retention while maintaining adequate buildability and shape stability during printing. The final optimized mixture, Trial Mix13 (Design Mix2), achieved a controlled flow diameter of approximately 165 mm and an extended setting time of about 50 minutes, which falls within the desirable range for extrusion-based concrete printing. Using this optimized mix, the geopolymer concrete was successfully extruded through the printing nozzle as a continuous filament, demonstrating stable material deposition and satisfactory layer formation without clogging or segregation. Mechanical evaluation showed that the optimized mix achieved an average compressive strength of approximately 48.9 MPa at 3 days, indicating strong early-age mechanical performance.

         Future work will involve printing structural specimens using the optimized geopolymer mix and evaluating their mechanical performance. The properties of the 3D printed specimens will be compared with conventionally cast specimens to understand the influence of the layered printing process. Further studies will also focus on improving interlayer bonding and optimizing printing parameters for enhanced structural performance.

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