Research progress on 3D printed geopolymer materials

2.1. Binder systems and mix design

The selection of materials for 3D printed geopolymer composites plays a decisive role in ensuring the printability, mechanical properties, and long-term durability of the structures [3]. Typically, these composites are produced by alkali-activating aluminosilicate precursors, supplemented with various aggregates, admixtures, and fibers to tailor the fresh and hardened behavior.

2.1.1. Precursor materials and activators

Geopolymer binders, produced by alkali-activating aluminosilicate precursors (fly ash, slag, metakaolin), are attractive for 3D printing due to lower environmental impact and high early strength. Rapid setting and limited thixotropy hinder continuous extrusion and interlayer bonding, but tuning activator concentration and adding retarders such as sodium gluconate can extend open time without sacrificing long-term strength; Gao et al. [4] reported delayed hydration/hardening, reduced yield stress, increased shear-thickening, a drop in 3-day strength, and largely unchanged or slightly improved 28-day strength. Shilar [5] surveyed binder systems and their influence on hardened properties, offering comparative guidance for selecting formulations (Table 1; Fig. 1).

Table 1Impact of various binders on the hardened properties of geopolymer [5]

Binders used	Molarity (M)	Particle size (μm)	Major chemical composition	Compressive strength (MPa)	Tensile strength (MPa)	Flexural strength (MPa)	Bulk density (kg/m3)
Metakaolin	8	10-50	SiO2, Al2O3	40	4	8	1200
Fly Ash	10	5-20	SiO2, Al2O3, Fe2O3	35	3.5	7	1100
GGBS	12	5-25	SiO2, Al2O3, CaO	45	4.5	9	1250
Rice Husk Ash	6	5-40	SiO2	30	3	6	1000
Silica Fume	8	1-5	SiO2	50	5	10	1300
Red Mud	10	10-50	Al2O3, Fe2O3, SiO2, CaO	35	3.5	7	1200
Clay	8	5-20	Al2O3, SiO2, MgO, Fe2O3	30	3	6	1100

Fig. 1The impact of different binders on the compressive, tensile, and flexural strength properties [5]

2.1.2. Enhancing printability and rheological control

Printability, as defined by Nerella et al. [6], is the capacity to pump, extrude, and retain geometry after deposition. It is governed by rheology – especially yield stress, plastic viscosity, and structural build-up rate – which can be tuned via solid-to-liquid ratio, particle packing, and viscosity modifiers; for example, ~0.1 % hydroxypropyl methylcellulose (HPMC) increased yield stress and shape retention while reducing slump flow. Short fibers (PVA, cellulose) act as micro-reinforcements that raise green strength and limit deformation during multilayer stacking. Robust printability requires coordinated control of reaction kinetics (Fig. 2) [7], mix design, environment, and advanced monitoring/prediction to achieve defect-free, high-performance geopolymer prints.

Fig. 2Reaction mechanism of the metakaolin-based geopolymer system [7]

2.2. 3D Printing technology for geopolymers

Fig. 3 illustrates the primary steps involved in the extrusion-based 3D printing of geopolymer, which include mixing, pumping, extrusion, and layer-by-layer construction [8]. Typically, the prepared mixture is conveyed to the extruder through the pumping mechanism, where it is deposited sequentially to form the intended geometry.

Fig. 3Typical printing process for extrusion-based 3D printed geopolymers [8]

Beyond extrusion, key technologies for geopolymer AM include: SLS, which laser sinters powders into strong parts across many materials but is costly and complex for construction; FDM, which melts and extrudes filaments at low cost with recyclability but lower resolution and surface finish; SLA, which photopolymerizes resins with high precision and quality, suited to detailed parts rather than large structures; and binder jetting, which selectively deposits a binder on a powder bed and is evolving for niche uses. Building on these, construction employs gantry, robotic-arm, and crane-type printers; Table 2 summarizes machine types, structures, pros/cons, and typical applications [9].

3.1. Applications of 3D-printed geopolymer composites

Additive manufacturing enables complex, multifunctional geopolymer structures beyond casting. A representative advance is geopolymer/graphene oxide (GO) nanocomposites: Zhong et al. [10] demonstrated extrusion printing where GO tunes rheology for stable, continuous deposition. As Fig. 4 shows, GO nanosheets form a reinforcing network that enhances printability and, after sintering, imparts electrical conductivity for conductive/sensing components.

Scaling up this technology, Bong et al. [11] successfully fabricated a large, multi-layered staircase with complex geometry, demonstrating that optimized geopolymer mixtures can be employed for structural and architectural components in real-world construction. This example, shown in Fig. 5, highlights the potential of extrusion-based 3D printing to produce sizable, load-bearing elements with controlled layering and precision.

Fig. 4Illustration of the 3D-printing process and some 3D-printed structures. The colors of the printed samples turned from brownish to blackish when the GO loading increased [10]

Fig. 53D-printing of a staircase using the optimum 3D-printable geopolymer mixture (M5): a) 3D model of the staircase; b) during 3D-printing process; c) 3D printed staircase [11]

a)

b)

c)

Material composition also plays a crucial role in printability and final properties. Xia et al. [12] studied geopolymer powders with different slag-to-fly ash ratios, revealing that mixtures with high fly ash content lacked sufficient reactivity at ambient temperatures, resulting in unsuccessful prints. The experimental results presented in Fig. 6 underscore the importance of tailoring raw materials to balance workability and mechanical strength for effective 3D printing.

Beyond these cases, the integration of nanoparticles and tailored geopolymer chemistries promises multifunctional composites with enhanced mechanical durability, thermal stability, and potential catalytic or adsorption functionalities. The combination of AM’s geometric freedom and geopolymer’s intrinsic material properties thus paves the way for innovative applications spanning construction, environmental remediation, energy storage, and smart sensing devices.

Fig. 6Powder-based 3D printed green cubes using geopolymer powders with different slag/fly ash ratios [12]

3.2. Challenges in 3D printing geopolymer materials

Despite progress, several obstacles limit adoption of 3D-printed geopolymers. Materials: high viscosity and long setting times complicate extrusion and demand tight control of temperature and flow; nanoparticles can destabilize rheology and reduce printability. Structure: layerwise fabrication causes anisotropy and weak interlayer interfaces, hindering overhangs; powder routes relax geometry limits but suffer low green strength, slower rates, and higher cost. Rough surfaces, dimensional drift, and post-processing needs persist, while elevated-temperature curing heightens shrinkage/cracking risk. Practicality: raw-material costs and safe handling of alkaline activators raise economic and safety burdens. Fully realizing the technology will require formulation optimization, better printing/curing strategies, and safer, energy-efficient, cost-effective processing.