Emerging Insights into the Durability of 3D-Printed Concrete: Recent Advances in Mix Design Parameters and Testing

Abstract

Although 3D-printed concrete (3DPC) offers advantages such as faster construction, reduced labour costs, and minimized material waste, concerns remain about its long-term durability. This review examines these challenges by assessing how the unique layer-by-layer manufacturing process of 3DPC influences key material properties and overall durability. The formation of interfacial porosity and anisotropic microstructures can compromise structural integrity over time, increasing susceptibility to environmental degradation. Increased porosity at layer interfaces and the presence of shrinkage-induced cracking, including both plastic and autogenous shrinkage, contribute to reduced durability. Studies on freeze–thaw performance indicate that 3DPC can achieve durability comparable to cast concrete when proper mix designs and air-entraining agents are used. Chemical resistance, particularly under sulfuric acid exposure, remains a challenge, but improvements have been observed with the inclusion of supplementary cementitious materials such as silica fume. In addition, tests for chloride ingress and carbonation reveal that permeability and resistance are highly sensitive to printing parameters, material composition, and curing conditions. Carbonation resistance, in particular, appears to be lower in 3DPC than in traditional concrete. This review highlights the need for further research and emphasizes that optimizing mix designs and printing processes is critical to improving the long-term performance of 3D-printed concrete structures.

1. Introduction

Three-dimensional (3D) printing, also known as additive manufacturing, is a technology that creates objects by sequentially depositing layers of material based on a digital model. In concrete manufacturing, this process involves extruding layers of concrete to form structures without the need for traditional formwork, vibration, or compaction [1]. The adoption of 3D printing in concrete construction presents numerous benefits, including significant reductions in construction time, labor costs, and material waste, thereby offering substantial social, economic, and environmental advantages [2,3,4,5,6,7]. For instance, 3D printing technology can reduce construction production times by approximately 50% to 70%, addressing critical housing shortages by enabling rapid provision of new homes [3,4]. An example of this is the social housing development at Grange Close in Dundalk, Ireland. Economically, 3D-printed construction can significantly cut labor costs by 50% to 80% compared to traditional methods [6,8]. Environmentally, this method can reduce construction waste by 30% to 60%, owing to increased precision and reduced use of materials, thereby also lowering associated carbon emissions [3,7].

Despite these benefits, 3D-printed concrete (3DPC) introduces specific challenges that can negatively affect its performance compared to conventionally cast concrete. The layer-by-layer deposition technique inherently leads to issues such as increased porosity, reduced interlayer bonding strength, and heightened susceptibility to shrinkage cracking [9,10,11]. Moreover, the directional nature of material deposition (anisotropy) can result in variations in mechanical properties, such as tensile strength and crack resistance, potentially compromising structural integrity under load and environmental exposure [12]. Specifically, the anisotropic properties observed in 3D-printed concrete significantly impact compressive and dynamic strength, varying according to printing direction due to differences in interlayer bonding and material alignment [13,14,15]. These drawbacks can collectively undermine the overall durability of 3D-printed concrete structures.

Durability is a critical parameter in evaluating the long-term performance and structural integrity of concrete structures and becomes even more essential for 3DPC due to the unique characteristics of the printing process. Issues such as increased porosity and weaker mechanical properties can accelerate deterioration mechanisms like freeze–thaw cycles, chloride ingress, carbonation, and chemical attacks, necessitating a thorough understanding of durability performance [11,12,16]. Figure 1 illustrates the key factors influencing the durability performance of 3D-printed concrete, which were selected as core criteria for evaluating long-term material performance in this study [17]. The diagram identifies four critical domains: (1) concrete mix proportion, which directly affects the mechanical properties, workability, and long-term stability of the printed structure; (2) printing process parameters, such as nozzle speed, layer height, and interlayer time, which significantly impact bond strength and structural integrity; (3) concrete porosity and permeability, which govern the ease with which moisture, gases, and aggressive substances penetrate the matrix, thus affecting durability; and (4) aggressive ions, type and concentration, including chlorides, sulfates, and other deleterious agents that can trigger chemical attacks, corrosion, and degradation over time. These criteria were selected based on their recurring significance across the reviewed literature and their interconnected influence on printed concrete structures’ performance and service life.

This review aims to comprehensively address these durability challenges associated with 3D-printed concrete by synthesizing existing research and experimental evidence. It evaluates key deterioration mechanisms, including porosity, shrinkage, freeze–thaw resistance, sulfate/acid resistance, chloride penetration, and carbonation. Through quantitative summaries, comparative analyses, and critical discussions, this review identifies current knowledge gaps and highlights areas requiring further research and standardization. Ultimately, this comprehensive understanding is crucial to advancing 3DPC technology, ensuring its reliability and acceptance as a durable and sustainable construction material.

2. Durability Properties

2.1. Porosity

Porosity is one of the main factors that influences the durability of concrete [16]. Porosity is generally determined by the following characteristics: air/void pores, capillary pores, dead-end pores, and gel pores [16]. Conventionally cast concrete typically has a homogenous pore distribution, whereas 3DPC exhibits varying porosity, with the highest porosity found at the layer interfaces [17,18]. This can be explained by the construction process, with cast concrete being vibrated, reducing porosity, and 3DPC being used as printed [16]. The increased porosity at interlayer boundaries in 3DPC is worsened by anisotropic features from the directional layer-by-layer process. This can weaken mechanical strength and reduce durability [16,18], as shown in Figure 2.

Specifically, anisotropy introduces elongated and interconnected voids at layer interfaces. These pores align along the printing path. They are more open and better connected than the randomly distributed, isotropic pores in cast concrete. X-ray microcomputed tomography (X-CT) imaging has shown that interfacial zones in 3DPC contain pores with more irregular, crack-like shapes, and lower tortuosity, which enhance connectivity and allow fluids to migrate more easily along the printing direction [19,20]. This directional pore structure facilitates the transport of aggressive agents such as chlorides, sulfates, and CO_2, leading to faster deterioration. These characteristics stand in clear contrast to the isolated and more uniformly shaped pores in vibrated cast concrete. This contrast is further illustrated in Figure 3, which compares pore structure visualization between cast concrete and 3D-printed concrete using high-resolution 3D CT analysis [21]. Cast concrete displays a well-dispersed distribution of spherical pores with minimal connectivity, whereas the 3D-printed sample clearly shows a dense accumulation of elongated, interconnected pores along the interlayer zone, indicating higher directionality and permeability [22].

These voids increase the permeability of the material and allow easier ingress of aggressive agents such as chlorides and sulfates. As a result, deterioration accelerates and overall durability decreases [16,20,23]. Despite the significance of this relationship, research explicitly examining how anisotropy-driven porosity variations impact long-term durability remains limited. Additives and admixtures such as nano-silica can be incorporated into 3DPC mixes to reduce porosity and subsequently improve durability [24].

Many different test procedures can study the porosity of concrete. Vacuum saturation and mercury intrusion porosimetry (MIP) are used to study the global porosity, and the interlayer porosity is studied by the spatial method, which uses optical or electronic microscopy and X-CT [25]. The advantage of the MIP test is its ability to characterize a wide range of pore sizes. However, there is a wide range of disadvantages, including damage to the microstructure caused by the injection of the mercury, cutting printed sections into small samples, and the impact of the drying procedure to remove pore water, which can heavily impact the pore structure through cracking and decomposition of hydrates [25]. The X-CT test is nondestructive, which can make it more viable.

Figure 4 below shows the process of the X-CT test. The equipment uses a high-resolution sample scan to generate a 3D image that shows the spatial distribution of porosity [26]. During scanning, X-rays penetrate the sample while rotating 360 degrees, and the transmitted signals are captured by a detector from multiple angles. These 2D projections are then computationally reconstructed into a 3D model using image reconstruction algorithms. In this study, slices were extracted from critical regions between printed layers and strips, allowing for localized pore segmentation and analysis. This enables a more accurate and detailed evaluation of the pore structure and interlayer heterogeneity, which are essential for understanding the performance and durability of 3D-printed concrete.

Table 1. Analysis of porosity in 3DPC literature: mix design parameters and testing.

Ref.	Mix	Binder Description	Mix Description	Test Method	Curing	Cast Porosity (%)	Printed Porosity (%)
[28]	A	100% OPC	VMA%/C 0.4, HRWR%/C 0.81	MIP	38 days @ 20 °C, 6 days @ 50 °C	13.58	13.11
	B	100% OPC	VMA%/C 0.47, HRWR%/C 0.95	MIP	38 days @ 20 °C, 6 days @ 50 °C	13.74	12.89
	C	100% OPC	VMA%/C 0.4, HRWR%/C 1.52	MIP	38 days @ 20 °C, 6 days @ 50 °C	11.23	11.67
[29]	M1	100% MK	-	XCT	28 days	4.48	2.98
				C + G	90 days	10	14.5
	M2	95% MK 5% Slag	-	XCT	28 days	4.07	1.81
				C + G	90 days	8	10.9
[26]	Control	100% OPC	100% SS	XCT	-	-	0.11
	AT50	90% OPC 10% SF	50% SS 50% AT	XCT	-	-	0.09
	AT100	90% OPC 10% SF	100% AT	XCT	-	-	0.3
[30]	REF	50% OPC 50% GGBS	S/B 1	VS	28 days @RH = 60	-	19.75
					28 days @RH > 95	-	9
	M1	100% OPC	S/B 1.5	VS	28 days @RH = 60	-	4.25
					28 days @RH > 95	-	4.5
	M2	75% OPC 25% GGBS	S/B 1.5	VS	28 days @RH = 60	-	12
					28 days @RH > 95	-	8.5
[31]	P	44.6% OPC 29.1% FA 26.3% LP	S/B 1 W/B 0.3	XCT	-	-	1.1
	H	61.7%OPC 32.7% FA 5.6% SF	S/B 1 W/B 0.2	XCT	-	-	1.8
[27]	REF	100% OPC	-	XCT	28 days @ 20 °C RH > 95%	38.9	30.1
	H012	100% OPC	0.6 g HPMC	XCT	28 days @ 20 °C RH > 95%	39.4	34.2
	H02	100% OPC	1 g HPMC	XCT	28 days @ 20 °C RH > 95%	41.2	36.4
	H03	100% OPC	1.5 g HPMC	XCT	28 days @ 20 °C RH > 95%	45.1	40.6
	S6	94% OPC 6% SF	-	XCT	28 days @ 20 °C RH > 95%	36.2	27.7
	S10	90% OPC 10% SF	-	XCT	28 days @ 20 °C RH > 95%	35.1	25.7
	S16	84% OPC 16% SF	-	XCT	28 days @ 20 °C RH > 95%	33.8	23.2
	S012 + S6	94% OPC 6% SF	0.6 g HPMC	XCT	28 days @ 20 °C RH > 95%	38.5	34
[32]	Cast	70% OPC 20% FA 10% SF	S/B 1.5 W/B 0.25	MIP	28 days	8.16	-
				XCT	-	8.07	-
	1Layer	70% OPC 20% FA 10% SF	S/B 1.5 W/B 0.25	MIP	28 days	-	7.86
				XCT	-	-	5.1
	3 Layer	70% OPC 20% FA 10% SF	S/B 1.5 W/B 0.25	MIP	28 days	-	6.7
				XCT	-	-	5.09
[33,34]	Cast	96% OPC 2% NC 2% SF	0.26% HRWR	XCT	-	4.89	-
	T1	96% OPC 2% NC 2% SF	0.26% HRWR	XCT	-	-	2.52
	T2	96% OPC 2% NC 2% SF	0.26% HRWR	XCT	-	-	2.73
	T3	96% OPC 2% NC 2% SF	0.26% HRWR	XCT	-	-	1.65
[35]	Cast	100% OPC	SS/B 1.5 W/B 0.32	XCT	28 days	14.6	-
	UT5	100% OPC	SS/B 1.5 W/B 0.32	XCT	28 days	-	17.6
	LT14	100% OPC	SS/B 1.5 W/B 0.32	XCT	28 days	-	12.4
[23]	Cast	83% OPC 17%SF	S/B 1.78 W/B 0.47	XCT	Ambient	2.67	-
					Sulfuric Acid	2.24	-
	3DP-6-X	83% OPC 17%SF	S/B 1.78 W/B 0.47	XCT	Ambient	-	1.26
					Sulfuric Acid	-	1.1
	3DP-12-X	83% OPC 17%SF	S/B 1.78 W/B 0.47	XCT	Ambient	-	2.31
					Sulfuric Acid	-	1.97
[36]	Mold	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	VS	28 days	6.05	-
	Lab	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	VS	28 days	-	7.025
	R-Site	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	VS	3 h @ 6.3 m/s airflow, 28 days	-	6.15
	U-Site	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	VS	3 h @ 6.3 m/s airflow, 28 days	-	6.5
[32,37],	M	70% OPC 20% FA 10% SF	S/B 1 W/B 0.35	XCT	90 days @ 20 °C RH > 95%	2.15	1.83
[38]	Cast/BM	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	VS	90 days @ 20 °C, 7 days @ 50 °C	7.5	14.9
	OL2	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	VS	90 days @ 20 °C, 7 days @ 50 °C	-	9
	OL4	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	VS	90 days @ 20 °C, 7 days @ 50 °C	-	8.2
	Z’	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	VS	90 days @ 20 °C, 7 days @ 50 °C	-	8.9
[39]	CM	100% OPC	S/B 1.47 W/B 0.35	MIP	28 days	-	10.2
	C-NS-1%	99% OPC 1% NS	S/B 1.47 W/B 0.35	MIP	28 days	-	7.6
	C-NS-2%	98% OPC 2% NS	S/B 1.47 W/B 0.35	MIP	28 days	-	8.5
	C-MK-5%	95% OPC 5% NS	S/B 1.47 W/B 0.35	MIP	28 days	-	7.45
	C-MK-10%	90% OPC 10% MK	S/B 1.47 W/B 0.35	MIP	28 days	-	6
	C-MK-10%-NS-1%	89% OPC 10% MK 1% NS	S/B 1.47 W/B 0.35	MIP	28 days	-	4.4
	C-MK-10%-NS-2%	88% OPC 10% MK 2%NS	S/B 1.47 W/B 0.35	MIP	28 days	-	5.45

Note: Ordinary Portland Cement (OPC), Viscosity-Modifying Agent (VMA), High-Range Water Reducer (HRWR), Mercury Intrusion Porosity (MIP), Metakaolin (MK), X-ray Computed Topography (XCT), Capillary + Gel (C + G) Porosity, Silica Fume (SF), Silica Sand (SS), Antimony Tailings (AT), Ground Granulated Blast-Furnace Slag (GGBS), Sand/Binder (S/B), Relative Humidity (RH), Vacuum Saturation (VS), Water/Binder (W/B), Fly Ash (FA), Limestone Powder (LP), (HPMC), Calcium Carbonate (CaCO₃), Gypsum (G), Polyvinyl Alcohol (PVA) Fibers.

demonstrates that porosity is highly dependent on both the mix composition and curing regime. Printed concrete generally exhibits higher porosity at layer interfaces compared to cast concrete. However, this disadvantage can be mitigated by incorporating supplementary cementitious materials (SCMs). For instance, the inclusion of 10–16% silica fume reduced porosity significantly, with values decreasing from 30.1% in the reference mix to as low as 23.2%. Similarly, a combination of metakaolin and nano-silica (C-MK-10%-NS-1%) yielded a 56.9% reduction in porosity compared to the control mix made with 100% OPC. Notably, porosity levels in 3DPC mixes ranged from a minimum of 0.09% to a maximum of over 40%, depending on the materials and testing methods used. These findings affirm that properly optimized mixes can lead to 3DPC having equal or even lower porosity than conventionally cast concrete, directly enhancing its long-term durability.

The results found in papers, including porosity testing, are displayed in Figure 5 and Figure 6. The bar chart shows the percentage decrease in porosity of the optimal mix compared to the reference mix. All the papers showed varying levels of improvement in terms of reducing porosity, excluding Bekaert et al. [30]. Bekaert et al. [30] compared printed mixes with different percentages of ground granulated blast-furnace slag (GGBS) under different curing conditions. The paper found that M2 (75% OPC and 25% GGBS) had a 180% increase in porosity compared to M1 (100% OP) [30]. Shafiq et al. [39] found the greatest decrease in porosity, with a 56.9% decrease when comparing CM (100% OPC) and C-MK-10%-NS-1%. The study found that adding metakaolin (MK) and nano silica (NS) can greatly improve the porosity of a printed sample. Du et al. [31] also looked at reducing the amount of ordinary Portland cement (OPC) used in the binder by supplementing it with fly ash (FA), limestone powder (LP), and silica fume (SF). The results found a 39% decrease in porosity for P (44.6% OPC 29.1% FA 26.3% LP) compared to H (61.7%OPC 32.7% FA 5.6% SF). This shows that reducing the portion of OPC decreases the porosity [31]. Liu et al. [27] also found that reducing OPC decreases the porosity. Comparing a reference mix (100% OPC) with S16 (84% OPC, 16% SF) showed that S16 had a 22.9% decrease in porosity [27]. The literature shows that decreasing the proportion of OPC used in the binder and replacing it with nano-clays and silica decreased the porosity of 3DPC.

According to the literature, 3DPC has a lower porosity than cast concrete. De la Flor Juncal et al. [38] found this exception when comparing the overlap between adjacent printed layers of concrete. The study used a concrete mix of 55% OPC, 30% MK, 12% CaCO₃, and 3% G. It found that an overlap of 4mm (Figure 7) had the lowest porosity of the printed samples; however, when compared to the cast sample, there was a 9.3% increase in porosity.

2.2. Shrinkage

Overall, 3DPC is susceptible to three different forms of shrinkage: plastic, autogenous, and drying (Figure 8). This is due to the lack of formwork that helps prevent evaporation [16,29,40]. Plastic shrinkage is the moisture loss and contraction before setting [16,40]. It is caused by rapid moisture loss through evaporation. This evaporation rate increases due to the lack of formwork [40]. Plastic shrinkage can greatly reduce the durability of concrete as it can cause cracking and slippage between the printed layers [40]. Autogenous shrinkage refers to the continued reaction caused by cement hydration, leading to decreased volume. The process is known as self-desiccation. It occurs after the plastic shrinkage has ceased [16,40]. Autogenous shrinkage is the most dominant type of shrinkage in terms of 3DPC due to the faster setting time of 3DPC compared to cast concrete [16]. Autogenous shrinkage causes printed sections to deform before hardening and then crack once hardened, which may cause a large loss in durability [16]. Drying shrinkage is caused by water loss after hardening concrete [16,40].

When the same mix is used, 3DPC experiences a similar drying shrinkage level to cast concrete [16]. However, 3DPC often uses admixtures such as geopolymers, which have been found to experience a higher level of drying shrinkage due to the network of micropores formed [41]. The increased cracking rate associated with the shrinkage of 3DPC greatly impacts its durability. There are two main test methods commonly used to assess shrinkage behavior in concrete. ASTM C1581-04, Standard Test Method for Determining Age at Cracking and Induced Tensile Stress Characteristics of Mortar and Concrete under Restrained Shrinkage, is known as the restrained ring test [42]. ASTM C1581 places concrete around a steel ring, as shown in Figure 9. As the specimen shrinks, it induces compressive stress in the steel and tensile stress in the concrete. Cracking is detected based on a measurable reduction in strain on the inner steel ring. This test provides a quantitative way to determine the age at which cracking occurs under restrained conditions and is suitable for evaluating autogenous and drying shrinkage. ASTM C1579-21, Standard Test Method for Evaluating Plastic Shrinkage Cracking of Restrained Fiber Reinforced Concrete (Using a Steel Form Insert), focuses on plastic shrinkage behavior [43]. In this test, specimens are subjected to controlled drying conditions with set temperature, humidity, and wind speed. The extent and severity of surface cracking are observed and recorded. This method is particularly useful for comparing the effects of fiber reinforcement or admixtures on plastic shrinkage performance. Both ASTM standards are comparative in nature and do not define acceptance limits but offer a consistent framework to evaluate shrinkage tendencies under specific conditions [42,43].

Several papers were reviewed, and testing was carried out based on ASTM C1579 [43]. These papers had similar test conditions in terms of temperature of 25 °C and RH of 60%. However, the wind speed varied greatly, with 6.3 m/s [36] and 3.6 m/s [44]. Zhou et al. [45] also examine the drying shrinkage using a test designed for a Chinese standard JC/T C603-2004 [46]. The test used the same RH of 60% and a similar temperature of 20 °C as the previous papers [45]. Jaji et al. [29] also examined drying shrinkage. However, it is not accelerated using wind. Markers are placed along the different faces of the sample, and the change in length due to drying shrinkage is measured after 7, 14, 28, and 90 d [29]. Papachristoforou et al. [47] found the cracking age of samples due to restrained shrinkage in line with ASTM C1581.

Table 2. Analysis of shrinkage in 3DPC literature: mix design parameters and testing.

Ref.	Mix	Binder Description	Mix Description	Test Method	Curing	Shrinkage (%)	Time to Cracking (Days)
[29]	M1	100% MK	-	ASTM C596—Drying Shrinkage	90 days	2.98	-
	M2	95% MK 5% Slag	-	ASTM C596—Drying Shrinkage	90 days	2.86	-
[44]	L1-200	70% OPC 20%FA 10% SF	Layers 1, Length 200 mm	ASTM C1579—Plastic Shrinkage	-	-	-
	L3-200	70% OPC 20%FA 10% SF	Layers 3, Length 200 mm	ASTM C1579—Plastic Shrinkage	-	-	-
	L1-300	70% OPC 20% FA 10% SF	Layers 1, Length 300 mm	ASTM C1579—Plastic Shrinkage	-	-	-
	L3-300	70% OPC 20% FA 10% SF	Layers 3, Length 300 mm	ASTM C1579—Plastic Shrinkage	-	-	-
[47]	CL2	90% OPC 10% SF	50% SS 50% LF	ASTM C1581—Drying Shrinkage	Restrained	-	4.88
	FL2	70% OPC 20%FA 10% SF	50% SS 50% LF	ASTM C1581—Drying Shrinkage	Restrained	-	7.64
	F2	70% OPC 20% FA 10% SF	100% SS	ASTM C1581—Drying Shrinkage	Restrained	-	6.79
	LL2	70% OPC 20% Slag 10% SF	50% SS 50% LF	ASTM C1581—Drying Shrinkage	Restrained	-	5.95
	L2	70% OPC 20% Slag 10% SF	100% SS	ASTM C1581—Drying Shrinkage	Restrained	-	4.95
[45]	T0	100% OPC	100% N	JC/T C603-2004—Drying Shrinkage	90 days	0.15	-
	T15	100% OPC	85% N 15% BT	JC/T C603-2004—Drying Shrinkage	90 days	0.185	-
	T25	100% OPC	75% N 25% BT	JC/T C603-2004—Drying Shrinkage	90 days	0.205	-
	T35	100% OPC	65% N 35% BT	JC/T C603-2004—Drying Shrinkage	90 days	0.22	-
	T45	100% OPC	55% N 45% BT	JC/T C603-2004—Drying Shrinkage	90 days	0.285	-
[36]	Mold	70% OPC 20% FA 10% SF	S/B 1.4, W/B 0.45	ASTM C1581—Drying Shrinkage	28 days	-	-
	Lab	70% OPC 20% FA 10% SF	S/B 1.4, W/B 0.45	ASTM C1581 -Drying Shrinkage	28 days	-	-
	R-Site	70% OPC 20% FA 10% SF	S/B 1.4, W/B 0.45	ASTM C1581—Drying Shrinkage	3 h @6.3 m/s wind, 28 days	-	-
	U-Site	70% OPC 20% FA 10% SF	S/B 1.4, W/B 0.45	ASTM C1581—Drying Shrinkage	3 h @6.3 m/s wind, 28 days	-	-
[48,49,50]	M	90% OPC 10% SF	S/B 1.8, W/B 0.46	Plastic Shrinkage	210 min covered @25 °C, @5 m/s wind	0.14	-
					210 min uncovered @25 °C, @5 m/s wind	0.36	-

Note: Metakaolin (MK), 1 Layer (L1), 3 Layers (L3), Ordinary Portland Cement (OPC), Fly Ash (FA), Silica Fume (SF), Silica Sand (SS), Limestone Filler (LF), Natural (N) Sand, Bauxite Tailings (BT), Sand/Binder (S/B), Water/Band (W/B), Restrained on-Site (R-Site), Unrestrained on-Site (U-Site).

outlines various shrinkage tests, highlighting how additives influence the shrinkage behavior of different mixes. Drying shrinkage ranged from 0.14% in covered samples to 2.98% in mixes made with 100% metakaolin. Restrained shrinkage results showed delayed cracking, with fly ash additions improving cracking resistance by up to 57% compared to control samples. In contrast, high contents of bauxite tailings increased shrinkage by over 47%, due to enhanced surface area and capillary pressure. Uncovered samples subjected to plastic shrinkage showed 61% greater deformation compared to covered ones. These results emphasize the need for effective shrinkage control measures, especially fly ash and slag additions, to mitigate early-age cracking and ensure dimensional stability in 3DPC structures.

Figure 10 illustrates the influence of various experimental parameters on shrinkage reduction in concrete. Zhou et al. [45] investigated the impact of bauxite tailings (BT) on the durability of 3DPC and found that a mix containing 45% BT exhibited the highest shrinkage, whereas 15% BT achieved the best performance among all substituted mixes. Notably, the control mix (with no BT) demonstrated the lowest shrinkage overall. The inclusion of 15% BT increased shrinkage by 23.3%, which was attributed to the increased surface area introduced by the tailings. Moelich et al. [36] examined the effect of early-age shrinkage restraint in 3DPC by comparing unrestrained samples to those mechanically restrained with rods. Results showed that restraining shrinkage at early ages reduced the interlayer bond strength and facilitated increased chloride penetration. Jaji et al. [29] explored the influence of 5% slag addition in a metakaolin-based mix. The study revealed that drying shrinkage was more pronounced in the build-up (vertical) direction than in the horizontal direction for both the control and slag-modified mixes. However, the slag-enhanced mix experienced reduced drying shrinkage compared to the control. Papachristoforou et al. [47] further investigated the effects of slag, fly ash, and limestone additions on shrinkage in a mix containing 10% silica fume. In line with Jaji et al. [29], the inclusion of slag improved shrinkage resistance, with cracking delayed by 22% relative to the control. Among all additives studied, fly ash yielded the most significant improvement, extending the cracking age by 57%. In terms of surface exposure, Figure 10 also indicates that uncovered samples experienced a 61% increase in shrinkage compared to covered specimens. Moelich et al. [44] and Markin et al. [48] both emphasized the high susceptibility of 3DPC to plastic shrinkage. This vulnerability is primarily due to the absence of formwork, which exposes the concrete surface, accelerates evaporation, and intensifies water loss. The resulting capillary pressure buildup contributes to elevated plastic shrinkage levels. Additionally, interlayer slip was observed, primarily caused by the reduced cross-sectional area and increased porosity in printed layers [36,44]. As shown in Figure 11, the presence of a slip plane can undermine structural performance. Combined with early-age cracking, interlayer slip underscores the importance of shrinkage control in enhancing the durability of 3DPC.

2.3. Freeze–Thaw Resistance

Freeze–thaw resistance in concrete is the capacity to resist the cyclic temperature changes that can cause degradation [18]. Establishing the freeze–thaw resistance of 3DPC contributes to its validity as a construction method, as it can be used in regions with temperatures that fluctuate above and below freezing. Freeze–thaw refers to water freezing and expanding by approximately 9% in volume in concrete pores, causing internal pressures and cracking [16,51], as shown in Figure 12. Freeze–thaw can also occur due to water penetrating the concrete through cracks and joints. This can particularly impact 3DPC. Mohan et al. [52] state that higher porosity at the layer interface can make 3DPC less freeze–thaw-resistant.

The freeze–thaw resistance of a sample is determined by applying F/T (freeze–thaw) cycles. A standard test is ASTM C666/C666M—Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing [53]. The test has two procedures: Procedure A, Rapid Freezing and Thawing in Water, and Procedure B, Rapid Freezing in Air and Thawing in Water. The test states that samples must be lowered from 4 to −18 °C and raised from −18 to 4 °C between 2 and 5 h. The test allows for a temperature variation of ±2 °C at the low temperature of −18 °C and the higher temperature of 4 °C. Samples should be between 75 and 125 mm in width, depth, and diameter and between 275 and 405 mm in length. Lastly, the sample should be left in the apparatus for 36 cycles.

Some of the studies from the papers reviewed are designed in line with the ASTM C666/C666M standard [31,32,53]. Sikora et al. [32] specified that sample measurements were taken after 25 and 50 cycles. Figure 13 shows the breakdown of the cycle. Du et al. [31] took measurements every 25 cycles up to 300 cycles and specified that the cycles lasted between 2 and 4 h with the temperature changing from −18 to 5 °C. A study by Dong et al. [54] also looks at a rapid freezing and thawing test. However, it is designed in line with a Chinese standard GB/T5008 [55]. Similar to the previous study, the measurements are taken every 25 cycles, up to 400 cycles [54]. An alternative test was used by Assaad et al. [51] that closely simulates the natural freeze–thaw cycle, freezing taking 12 h at −20 ± 4 °C and thawing taking a further 12 h at 20 ± 3 °C, with measurement being taken after 56 and 112 cycles. GivKashi et al. [56] state that the study samples used 24 h freezing and thawing cycles with a temperature range of −18 to 24 °C. It was also stated that samples were measured after 50 cycles [56].

Table 3. Analysis of freeze–thaw in 3DPC literature: mix design parameters and testing.

Ref.	Mix	Binder Description	Mix Description	Test Method	No. F/T Cycles	f′c Loss (%)	fr Loss (%)	Mass Loss (%)
[51]	550-0.45	92% OPC 8%SF	HRWR%/B 2.1	12 h @ −20 ± 4 °C 12 h @20 ± 3 °C	112	24.2	33.8	-
	550-0.45 Air	92% OPC 8%SF	HRWR%/B 1.9, AEA%/B 0.08	12 h @ −20 ± 4 °C 12 h @20 ± 3 °C	112	8.5	13.4	-
	550-0.45-SBR	92% OPC 8%SF	HRWR%/B 1.8, SBR%/B 15	12 h @ −20 ± 4 °C 12 h @ 20 ± 3 °C	112	15.6	9.4	-
	650-0.45	92% OPC 8%SF	HRWR%/B 0.95	12 h @ −20 ± 4 °C 12 h @ 20 ± 3 °C	112	17.2	22	-
	650-0.45-Air	92% OPC 8%SF	HRWR%/B 0.9, AEA%/B 0.1	12 h @ −20 ± 4 °C 12 h @ 20 ± 3 °C	112	5.5	9.4	-
	650-0.45-SBR	92% OPC 8%SF	HRWR%/B 0.8, SBR%/B 15	12 h @ −20 ± 4 °C 12 h @20 ± 3 °C	112	10.1	9.4	-
	750-0.35	92% OPC 8%SF	HRWR%/B 1.25	12 h @ −20 ± 4 °C 12 h @ 20 ± 3 °C	112	16.6	21	-
	750-0.35-Air	92% OPC 8%SF	HRWR%/B 1.15, AEA%/B 0.15	12 h @ −20 ± 4 °C 12 h @ 20 ± 3 °C	112	5.2	7.9	-
	750-0.35-SBR	92% OPC 8%SF	HRWR%/B 1, SBR%/B 15	12 h @ −20 ± 4 °C 12 h @ 20 ± 3 °C	112	10.2	10.6	-
[56]	C	100% OPC	SP%/B 1.1	12 h F/T Cycles @ −18 to 24 °C	50	5.7	-	-
	A0.08	100% OPC	SP%/B 1.1, AEA%/B 0.08	12 h F/T Cycles @ −18 to 24 °C	50	4.3	-	-
	A0.10	100% OPC	SP%/B 1.1, AEA%/B 0.1	24 h F/T Cycles @ −18 to 24 °C	50	3.1	-	-
	A0.12	100% OPC	SP%/B 1.1, AEA%/B 0.12	24 h F/T Cycles @ −18 to 24 °C	50	0.4	-	-
[31]	P-O	44.6% OPC 29.1% FA 26.3% LP	S/B 1 W/B 0.3, Cast	2–4 h F/T Cycles @ −18 to 5 °C	300	28.7	48.3	1.4
	P-Z	44.6% OPC 29.1% FA 26.3% LP	S/B 1 W/B 0.2, Printed	2–4 h F/T Cycles @ −18 to 5 °C	300	19.5	41.3	0.2
	H-O	61.7%OPC 32.7% FA 5.6% SF	S/B 1 W/B 0.2, Cast	2–4 h F/T Cycles @ −18 to 5 °C	300	22.7	31.1	0.2
	H-Z	61.7%OPC 32.7% FA 5.6% SF	S/B 1 W/B 0.2, Printed	2–4 h F/T Cycles @ −18 to 5 °C	300	5.9	19.1	0
[32]	Cast	70% OPC 20% FA 10% SF	S/B 1.5 W/B 0.25	ASTM C666	50	2.7	11	-
	1 Layer	70% OPC 20% FA 10% SF	S/B 1.5 W/B 0.25	ASTM C666	50	5.1	9	-
	3 Layer	70% OPC 20% FA 10% SF	S/B 1.5 W/B 0.25	ASTM C666	50	1.3	21	-
[54]	JZ	93% OPC 7% SF	60% FS 40% AS	GB/T5008—Quick Freeze	400	21	-	3.02
	3D-X	93% OPC 7% SF	60% FS 40% AS	GB/T5008—Quick Freeze	400	22.4	-	3.12
	3D-Y	93% OPC 7% SF	60% FS 40% AS	GB/T5008—Quick Freeze	400	24	-	3.56

Note: Compressive Strength (f′c), Flexural Strength (fr), Ordinary Portland Cement (OPC), Silica Fume (SF), High-Range Water Reducer (HRWR), Air-Entraining Agent (AEA), Styrene–Butadiene Rubber (SBR), Superplasticizer (SP), Fly Ash (FA), Limestone Powder (LP), Sand/Binder (S/B), Water/Binder (W/B), Ferrochrome Sand (FS), Aeolian Sand (AS).

provides a comprehensive comparison of freeze–thaw performance across various 3DPC mixes. The compressive strength loss under cyclic freezing ranged from just 0.4% in air-entrained mixes to 28.7% in control mixes without additives. The addition of air-entraining agents (AEA) and styrene–butadiene rubber (SBR) significantly reduced strength loss, in some cases by more than 75%. For example, a mix that lost 24.2% of compressive strength without AEA saw this reduced to only 5.2% with AEA. In terms of mass loss, printed samples generally outperformed cast samples, with printed variants showing losses as low as 0.2%, while cast mixes exhibited up to 1.4% mass reduction. These findings confirm that with appropriate use of admixtures and SCMs, 3DPC can achieve or exceed the freeze–thaw durability of cast concrete, making it a reliable option for cold-region applications.

The results of the F/T test are normally determined by two factors: the change in mechanical strength and mass. All the papers with F/T tests that were reviewed included a compressive strength test [31,32,51,54,56]. Three papers looked at flexural strength before and after F/T tests [31,32,51]. The other two did not include flexural strength testing, with GivKashi et al. [56] focusing on compressive strength change and Dong et al. [54] including the mass loss as a measurement of the F/T resistance, as was also performed by Du et al. [31]. Equation (1) below shows how the F/T resistance is measured with the property varying from compressive strength to mass.

$$\Delta \left(Property\right), \%={\displaystyle \frac{Property of mix not exposed to\frac{\mathrm{F}}{\mathrm{T}}-Propert of mix after given\frac{\mathrm{F}}{\mathrm{T}} Cycle}{Propert of control mix not exposed to\frac{\mathrm{F}}{\mathrm{T}}}}\times 100$$

(1)

Sikora et al. [32] focused on the durability of cast vs. printed concrete specimens, using three different samples, cast, 1 layer of printed concrete (1 L), and 3 layers of printed concrete (3 L), ll of which used the mix shown in Table 3. The paper found cast concrete had higher initial strength properties than 3DPC, as seen across all the papers that compared cast and printed samples. They also found that the 3DPC behaved anisotropically [31,32,54]. However, Sikora et al. [32] and Dong et al. [54] found that printed samples had a slightly larger decrease in compressive strength and mass, whereas Du et al. [31] found the printed sample to have a smaller decrease in mass. The printed sample performs better with larger pore size and better pore connectivity [31].

Assaad et al. [51] and GivKashi et al. [56] compared 3D-printed concrete (3DPC) with and without air-entraining additives. Although the initial compressive strength of air-entrained mixes was lower than the control, the strength loss under the freeze–thaw cycle or acid attack was significantly smaller. This improvement is attributed to the modified pore structure, where uniformly distributed microbubbles help relieve internal pressure, reduce microcracking, and lower permeability. As a result, air-entrained 3DPC matched or even exceeded the durability performance typically expected from cast concrete with similar air-void systems. To optimize this balance, a controlled AEA dosage (≤0.10%) and the incorporation of pozzolanic materials are recommended to mitigate strength loss while maintaining durability [51,56]. Du et al. [31] show evidence that a mix with silica fume performs better than one with limestone. This is seen by the initial strength characteristics and those after F/T cycles [31]. More research must be conducted to find if adding silica fume will improve the freeze–thaw resistance of 3DPC compared to traditional concrete mixes.

2.4. Chemical Resistance

For 3DPC to be considered a viable alternative to cast concrete, it must be ensured that it is effective in harsh environments. The harsh environments include acidic environments, sewage systems, maritime, and underground [40]. The chemical resistance of concrete is often challenged. Gu et al. [57] wrote that sewage systems create an environment with one of the most rapid rates of concrete degradation. There has been significantly less 3DPC used in these environments compared to traditional cast concrete. This lack of information requires extensive research into the chemical resistance of 3DPC to ensure that it is a feasible alternative to cast concrete.

2.4.1. Sulfuric Acid

Many papers state that Portland cement has little resistance to acid attacks due to its relatively high alkalinity [57,58]. This allows the degradation of concrete to occur due to many different processes. Sulfuric acid (H₂SO₄) is considered to be among the most harmful acids when it comes to the degradation of concrete. This is due to the combined effects of the sulfate and acid attacks [58], the result of which is shown in Figure 14. The sulfate attack is caused by the reaction between the sulfate and calcium hydroxide (CH) in the hydrated cement paste. The reaction produces gypsum. The gypsum causes a volume increase in the concrete. Further reactions cause larger volume increases. The large change in volume leads to internal pressure and, eventually, expansion. The expansion causes cracking and spalling, as well as a loss in mechanical strength and, eventually, complete failure [56,58]. GivKashi et al. [56] state that sulfuric acid causes damage to concrete through the following chemical reactions.

$$\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}$$

(2)

Equation (2) shows the first reaction, which occurs between the sulfuric acid (H₂SO₄) and the calcium hydroxide (CH), producing calcium sulfate dihydrate, commonly known as gypsum (CaSO₄⋅2H₂O). Gypsum can increase its volume by a factor of 2.2 [56]. The soluble gypsum then attempts to leech out of the concrete. In cases where that is not possible, the gypsum increases the internal pressure, leading to cracking and spalling.

$$\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}{\mathrm{O}}_2\cdot 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4+\mathrm{S}\mathrm{i}{\left(\mathrm{O}\mathrm{H}\right)}_4+{\mathrm{H}}_2\mathrm{O}$$

(3)

Equation (3) shows a second reaction. The sulfuric acid (H₂SO₄) reacts with the hydrated calcium silicate (CaSiO₂⋅2H₂O) to produce gypsum and silica hydroxide [56]. This reaction greatly impacts the mechanical properties of the concrete. In particular, it significantly reduces the compressive capacity. The fine network of the structure is broken down, causing the loss of cohesion and adhesion of the concrete. This, combined with the additional gypsum from the reaction, increases the degradation of the concrete.

$$3\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\cdot 12{\mathrm{H}}_2\mathrm{O}+3\left(\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}\right)+14{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\cdot 3\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 32{\mathrm{H}}_2\mathrm{O}$$

(4)

Equation (4) shows the third reaction: the gypsum (CaSO₄⋅2H₂O) produced from the first two reactions now reacts with calcium aluminate hydrate (3CaO⋅Al₂O₃⋅12H₂O), producing ettringite [56]. Ettringite has an increased volume by a factor of 7 in relation to the original compound [58]. This large expansion, combined with the structure’s network breakdown, causes the concrete’s failure.

The effects of a sulfuric acid attack are measured using a Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution (ASTM C109/C109M) [59]. The test measures the sulfate resistance of mortars using Portland cement, Portland cement with pozzolans or slags, and blended hydraulic types of cement. This makes the test suitable for comparing traditional cement mixes to those with nano silica admixtures. The standard solution for the test contains 352 moles of Na₂SO₄ per meter cubed (50 g/L). The test consists of 6 bars used for the acid attack test and up to 21 cubes used for mechanical property testing. The dimensions of the mortar bars used are 25 × 25 × 285 mm. Once molded, the bars must be placed in an oven at 35 ± 3 °C for 23½ h ± 30 min and removed from the molds. The samples are then tested in a curing tank of saturated lime water at 23 ± 2 °C. Once removed from the curing tank, the initial length is measured. The test should include 625–800 mL of solution per mortar bar. Once the samples are placed in the sulfate solution, such as Figure 15, the change in length is measured at 1, 2, 3, 4, 8, 13, and 15 weeks. The samples can be left in the solution, and further length changes are measured at 4, 6, 9, 12, 15, and 18 months.

Some of the studies [28,60,61] referred to a version of the ASTM C109/C109M test standard [59]. Baz et al. [28] and El Inaty et al. [61] both used a volume of solution four times the volume of the samples as recommended by ASTM C109/C109M. However, the studies used different concentrations of sulfuric acid. Baz et al. [28] had solution baths with 1 and 3% concentration compared to El Inaty et al.’s [61] 0.5% concentration. Three papers used a 5% solution [23,24,60]; the final paper did not state the concentration but had a pH of 2 [56]. Two papers used dry–wet cycles designed per GB/T 50082-2009 [23,60,62]. Rui et al. [23] used 1d cycles, with 15 h wet and 9 h dry cycles. Guo et al. [60] used 3, 7, 14, and 21 d cycles using 3:1 dry–wet ratio. The solution was replaced for tests that lasted longer than 30 d, with some replacement taking place every 30 d [56,60]. Others replaced the solution when mass or length measurement was taking place at 3, 7, 14, 21, 28, 42, 56, 70, 84, 98, 112, 126, and 140 days [28,61]. The sulfuric acid resistance of various 3DPC mixes, as shown in

Table 4. Analysis of sulfuric acid in 3DPC literature: mix design parameters and testing.

Ref.	Mix	Binder Description	Mix Description	Test Method	Procedure	Cast f′c Loss (%)	3DP f′c Loss (%)	Cast Mass Loss (%)	3DP Mass Loss (%)
[28]	A	100% OPC	VMA%/C 0.4, HRWR%/C 0.81	ASTM C1012/C1012M	56 d @ 1% Sulfuric Acid	-	-	10.9	10.9
					56 d @ 3% Sulfuric Acid	-	-	54.5	47.3
	B	100% OPC	VMA%/C 0.47, HRWR%/C 0.95	ASTM C1012/C1012M	56 d @ 1% Sulfuric Acid	-	-	10.7	9.1
					56 d @ 3% Sulfuric Acid	-	-	53.6	46.4
	C	100% OPC	VMA%/C 0.4, HRWR%/C 1.52	ASTM C1012/C1012M	56 d @ 1% Sulfuric Acid	-	-	19.1	14.5
					56 d @ 3% Sulfuric Acid	-	-	60.9	49.1
[56]	C	100%OPC	SP%/B 1.1	ASTM C349–14	90 d @ pH2 Sulfuric Acid	-	−5.64	-	-
	A0.08	100%OPC	SP%/B 1.1, AEA%/B 0.08	ASTM C349–14	90 d @ pH2 Sulfuric Acid	-	−6.53	-	-
	A0.10	100%OPC	SP%/B 1.1, AEA%/B 0.1	ASTM C349–14	90 d @ pH2 Sulfuric Acid	-	−10.61	-	-
	A0.12	100%OPC	SP%/B 1.1, AEA%/B 0.12	ASTM C349–14	90 d @ pH2 Sulfuric Acid	-	−13.44	-	-
[61]	A	90% OPC 10% SF	SP%/B 0.3	ASTM C1012/C1012M	140 d @ 0.5% Sulfuric Acid	7.5	8.75	3.7	3.5
	B	90% OPC 10% SF	SP%/B 0.6	ASTM C1012/C1012M	140 d @ 0.5% Sulfuric Acid	8.75	15	4	4.2
	C	90% OPC 10% SF	SP%/B 0.6	ASTM C1012/C1012M	140 d @ 0.5% Sulfuric Acid	4.75	5	3.2	2.7
[23]	Cast	83% OPC 17% SF	S/B 1.78 W/B 0.47	GB/T 50082-2009	150 d—15 h @ 5% Sulfuric Acid, 7 h drying	−42	-	−1.4	-
	3DP-6-Y	83% OPC 17% SF	S/B 1.78 W/B 0.47	GB/T 50082-2009	150 d—15 h @ 5% Sulfuric Acid, 7 h drying	-	−54.8	-	−1.8
	3DP-12-Y	83% OPC 17% SF	S/B 1.78 W/B 0.47	GB/T 50082-2009	150 d—15 h @ 5% Sulfuric Acid, 7 h drying	-	−43.5	-	−1.6
[24]	OPC	100% OPC	S/B 3.45 W/B 0.45	ASTM C267	30 d @ 5% Sulfuric Acid	34	-	1	-
	FA1GPC	70% FA1 30% SB	S/B 2.38 W/B 0.45	ASTM C267	30 d @ 5% Sulfuric Acid	32	-	−0.28	-
	FA1NSGPC	68% FA1 30% SB 2% NS	S/B 2.38 W/B 0.45	ASTM C267	30 d @ 5% Sulfuric Acid	19	-	−0.77	-
	FA2GPC	70% FA2 30% SB	S/B 2.38 W/B 0.45	ASTM C267	30 d @ 5% Sulfuric Acid	28	-	−0.39	-
	FA2NSGPC	68% FA2 30% SB 2% NS	S/B 2.38 W/B 0.45	ASTM C267	30 d @ 5% Sulfuric Acid	17	-	−0.74	-
[60]	M	80% OPC 20% FA	S/B 1.8 W/B 0.54	GB/T 50082-2009	231 d, 54 h @ 5% Sulfuric Acid, 18 h drying	-	-	4.03	-
					231 d, 126 h @ 5% Sulfuric Acid, 42 h drying	-	-	5.02	-
					231 d, 252 h @ 5% Sulfuric Acid, 84 h drying	-	-	3.28	-
					231 d, 378 h @ 5% Sulfuric Acid, 126 h drying	-	-	2.98	-

Note: Compressive Strength (f’c), Flexural Strength (fr), Ordinary Portland Cement (OPC), Viscosity-Modifying Agent (VMA), High-Ranger Water Reducer (HRWR), Superplasticizer (SP), Air-Entraining Agent (AEA), Silica Fume (SF), Sand/Binder (S/B), Water/Binder (W/B), Fly Ash Type 1 (FA1), Fly Ash Type 2 (FA2), Sodium Bisulfate (SB), Nano-Silica (NS), Fly Ash (FA).

, reveals critical insights into durability under aggressive chemical environments. Mass loss in printed samples exposed to 3% sulfuric acid ranged up to 49.1%, though this was consistently lower than in cast samples in similar conditions. In one case, a 100% OPC printed mix lost 46.4% mass, while the cast equivalent lost 53.6%. The presence of silica fume dramatically improved acid resistance; a mix with 10% SF showed only 2.7% mass loss after 140 days. Compressive strength losses also varied, with printed samples sometimes outperforming cast ones despite being exposed to the same acid concentrations. These outputs underline the effectiveness of SCMs such as SF and the importance of microstructural control in resisting acid degradation in 3DPC.

The change in mass is the most common measurement recorded across the papers, followed by porosity, water porosity, and mechanical properties. Five of the six papers measured the loss or, in some cases, gain in mass [23,24,28,60,61]. Baz et al. [28] examined the microstructural analysis of 3DPC exposed to a sulfuric acid attack. The concrete mix used consisted of Portland cement, crushed limestone, and admixtures consisting of a water reducer and viscosity modifier. The results saw a decrease in mass across all samples over 53 days. El Inaty et al. [61] look at the long-term durability of 3DPC with concrete mixes consisting of Portland cement, sand, silica fume, and a superplasticizer. The results over 140 days showed that the mix with the largest silica fume content had the smallest loss in mass, implying that it had the greatest resistance to sulfuric acid. Rui et al. [23] look at how the anisotropy impacts the effects of sulfate attack on 3DPC, finding mass loss was the smallest when attacked in the x-direction, then z-direction, and the largest in the y direction. Loading directions are shown in Figure 16. It was also noted that the printed sample performed worse than the cast when the period between print layers was 6 min and better than the cast sample when the period between print layers was 12 min [23]. Two papers reported an increase in mass either throughout the testing [24,60]. This is likely due to the reactions that occur due to the sulfuric acid, cases where gypsum has not leached out of the samples, causing a reduction in mass.

In addition to mass change, sulfuric acid exposure also causes chemical degradation through gypsum and ettringite formation, which induce expansive stress and microcracking, particularly concentrated in near-surface zones. Microstructural investigations, such as those by Baz et al. [28] and Gu et al. [57], indicate the gradual decomposition of calcium hydroxide and C-S-H phases, leading to increased porosity and reduced cohesion of the cement matrix, as shown in the SEM images (Figure 17). XRD results [63] further reveal ettringite formation gradients that intensify with increasing acid concentration, confirming progressive sulfate ingress through the matrix (Figure 18). While such degradation mechanisms are well documented in conventional concrete, studies on 3DPC remain limited, particularly regarding the influence of anisotropic pore networks and interlayer interfaces on acid transport and mineral phase transformation.

2.4.2. Chloride

Chloride attack must be particularly considered for reinforced concrete structures, as it corrodes the reinforcement [40,64]. In comparison to cast concrete, 3DPC can have a higher porosity. This can lead to an increased rate of chloride ingress [25]. The combination of higher porosity with increased cracking rate makes 3DPC significantly more vulnerable to chloride attacks. This makes the resistance to chloride attacks one of the key durability criteria that must be assessed [25].

Two different tests measure chloride attack: the rapid chloride permeability (RCP) test and the chloride penetration depth (CPD) test [39]. ASTM C1202—17—Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration, a commonly used RCP test for evaluating concrete’s resistance to chloride ion penetration, uses disk-shaped samples that are 50 mm thick and 100 mm in diameter [65]. The same dimensions are used for GB/T 50082-2009 (Figure 19) [62]. The electrical resistance of the sample is found by passing 60 V through the sample, which has one-half immersed in chloride solution and the other half in sodium hydroxide solution. The chloride resistance is then calculated using the electrical resistance [65]. The CPD test places a sample in the salt solution for an extended period, giving a closer description of the chloride resistance of the sample [39]. The RCP test can have a duration of 6 h, which leads to many studies opting for it over the lengthy CPD test [65].

This review found that many papers used the RCP test when determining the chloride resistance of 3DPC. The majority of papers [30,31,36,39,66] took guidance from the ASTM C1202—17 [65], or the equivalent Chinese standard GB/T 50082-2009 [62]. Some of the studies used 60 V for the chloride migration inline from ASTM 1012-17 [31,39], while Moelich et al. used 10 V. There were varying concentrations used for the NaCl and NaOH solutions, with the NaCl varying from 3% [39] to 10% [30,45] and 1 M [66], and the NaOH varying between 0.1 M [66] and 0.3 M [30,39,45]. Shafiq et al. [39] ran the test for the shortest length at 6 h, and Du et al. [31] the longest at 96 h. Bran-Anleu et al. [66] and Bekaert et al. [30] tested the sample for 24 h, and the remaining papers did not specify a test duration [36,45]. As seen in

Table 5. Analysis of chloride attack in 3DPC literature: mix design parameters and testing.

Ref.	Mix	Binder Description	Mix Description	Test Method	Procedure	Curing	D (×10−12 m/s2)	PD (mm)
[30]	REF	50% OPC 50% GGBS	S/B 1	RCM—NTBuild492	24 h @ 0.3 M NaOH 10% NaCl	28 d @ RH = 60	7.33	-
						28 d @ RH > 95	4.67	-
	M1	100% OPC	S/B 1.5	RCM—NTBuild492	24 h @ 0.3 M NaOH 10% NaCl	28 d @ RH = 60	10.33	-
						28 d @ RH > 95	8.33	-
	M2	75% OPC 25% GGBS	S/B 1.5	RCM—NTBuild492	24 h @ 0.3 M NaOH 10% NaCl	28 d @ RH = 60	20.33	-
						28 d @ RH > 95	10.33	-
[31]	P-O	44.6% OPC 29.1% FA 26.3% LP	S/B 1 W/B 0.3, Cast	RCM—NTBuild492, CPD	96 h	28 d	56.3	9.45
	P-Z	44.6% OPC 29.1% FA 26.3% LP	S/B 1 W/B 0.2, Printed	RCM—NTBuild492, CPD	96 h	28 d	78.8	12.36
	H-O	61.7%OPC 32.7% FA 5.6% SF	S/B 1 W/B 0.2, Cast	RCM—NTBuild492, CPD	96 h	28 d	21.3	3.82
	H-Z	61.7%OPC 32.7% FA 5.6% SF	S/B 1 W/B 0.2, Printed	RCM—NTBuild492, CPD	96 h	28 d	52.5	8.91
[66]	P	48% OPC 26% MSS 26% FA	2, 13, 1440 min/layer	µXRF	24 h @ 0.1 M Na OH 1 M NaCl	28 d @ 20 °C RH = 65%	-	-
	Q	48% OPC 26% MSS 26% FA	2, 13, 1440 min/layer	µXRF	24 h @ 0.1 M NaOH 1 M NaCl	28 d @ 20 °C RH = 65%, 3 d @ 40 °C	-	-
[45]	T0	100% OPC	100% N	RCM-GB/T50082-2009 [62]	0.3 M NaOH 10% NaCl	28 d	5.8	-
	T15	100% OPC	85% N 15% BT	RCM—GB/T50082-2009	0.3 M NaOH 10% NaCl	28 d	5.2	-
	T25	100% OPC	75% N 25% BT	RCM—GB/T50082-2009	0.3 M NaOH 10% NaCl	28 d	4.33	-
	T35	100% OPC	65% N 35% BT	RCM—GB/T50082-2009	0.3 M NaOH 10% NaCl	28 d	4.2	-
	T45	100% OPC	55% N 45% BT	RCM—GB/T50082-2009	0.3 M NaOH 10% NaCl	28 d	4.6	-
[36,67]	Mold	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	CCI-UCT Manual	5 M NaCl	21 d @ ambient, 7 d @ 50 °C	0.325 mS/cm	-
	Lab	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	CCI-UCT Manual	5 M NaCl	21 d @ ambient, 7 d @ 50 °C	0.4 mS/cm	-
	R-Site	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	CCI-UCT Manual	5 M NaCl	21 d @ ambient, 7 d @ 50 °C	0.296 mS/cm	-
	U-Site	70% OPC 20% FA 10% SF	S/B 1.4 W/B 0.45	CCI-UCT Manual	5 M NaCl	21 d @ ambient, 7 d @ 50 °C	0.346 mS/cm	-
[38]	Cast	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	RCM	-	90 days @ 20 °C	284.5 kΩcm
	BM	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	RCM	-	90 days @ 20 °C	165.5 kΩcm
	OL2-Z	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	RCM	-	90 days @ 20 °C	99.6 kΩcm
	OL4-Z	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	RCM	-	90 days @ 20 °C	113.3 kΩcm
	Z’-Z	55% OPC 30% MK 12% CaCO3 3%G	PVA%/B 0.225 S/B 1.5 W/B 0.4	RCM	-	90 days @ 20 °C	177 kΩcm
[39]	CM	100% OPC	S/B 1.47 W/B 0.35	RCM—ASTM C1202	3% NaCl	28 days	1575 C	11.83
	C-NS-1%	99% OPC 1% NS	S/B 1.47 W/B 0.35	RCM—ASTM C1202	3% NaCl	28 days	1650 C	-
	C-NS-2%	98% OPC 2% NS	S/B 1.47 W/B 0.35	RCM—ASTM C1202	3% NaCl	28 days	2600 C	-
	C-MK-5%	95% OPC 5% NS	S/B 1.47 W/B 0.35	RCM—ASTM C1202	3% NaCl	28 days	850 C	5.8
	C-MK-10%	90% OPC 10% MK	S/B 1.47 W/B 0.35	RCM—ASTM C1202	3% NaCl	28 days	350 C	9
	C-MK-10%-NS-1%	89% OPC 10% MK 1% NS	S/B 1.47 W/B 0.35	RCPT—ASTM C1202, CPD	3% NaCl	28 days	311 C	3.43
	C-MK-10%-NS-2%	88% OPC 10% MK 2%NS	S/B 1.47 W/B 0.35	RCPT—ASTM C1202, CPD	3% NaCl	28 days	318 C	3.08

Notes: Permeability Coefficient (D), Penetration Depth (PD), Ordinary Portland Cement (OPC), Ground Granulated Blast-Furnace Slag (GGBS), Sand/Binder (S/B), Rapid Chloride Migration (RCM), Relative Humidity (RH), Fly Ash (FA), Limestone Powder (LP), Water/Binder (W/B), Chloride Penetration Depth (CPD), Micro Silica Suspension (MSS), Micro-X-ray fluorescence (µXRF), Natural (N) Sand, Bauxite Tailings (BT), Silica Fume (SF), Metakaolin (MK), Chloride Conductivity Index (CCI), University of Cape Town (UCT), Calcium Carbonate (CaCO₃), Gypsum (G), Polyvinyl Alcohol (PVA) Fibers, Nano-Silica (NS), Rapid Chloride Permeability Test (RCPT).

, chloride migration coefficients (D-values) for 3DPC ranged from 311 to 2600 Coulombs (C), indicating a wide performance variation based on mix design and curing. High relative humidity curing (RH > 95%) consistently improved results, with one study showing a drop in permeability from 10.33 to 4.67 × 10⁻¹² m²/s. The integration of nano-silica and metakaolin reduced chloride permeability by up to 80%. The penetration depth was also substantially reduced in optimized mixes, from 12.36 mm to just 3.08 mm. These results demonstrate that 3DPC’s chloride resistance can match or exceed traditional concrete when enhanced with SCMs and controlled curing.

The results of the RCP test are determined using Equation (5), which calculates D, the non-stationary chloride migration coefficient (m²/s) [30,31,45]. U is the voltage passed through the test sample (V). T is the mean of the initial and final temperature of the test solution in (°C). L is the thickness of the sample (mm). X_d is the mean value of the chloride penetration depth (mm). t is the length of time that the sample was tested for (hours) [30,31,45].

$$D={\displaystyle \frac{0.0239\times \left(275+T\right)}{\left(U-2\right)5}}(X_d-0.0238\sqrt{{\displaystyle \frac{\left(273+T\right)L{X}_d}{U-2}}})$$

(5)

Zhou et al. [45] look at how adding bauxite tailings impacts the durability of 3DPC. The study compared printed samples with different concentrations of bauxite tailings. The results showed an optimal content of 35% bauxite tailings with a migration coefficient decrease from 5.8 to 4.2 × 10⁻¹² m²/s [45]. Bran-Anleu et al. [66] studied the impact of printing time intervals and curing conditions on chloride through cold joints using micro-XRF. The study found very little difference between samples that had 2 and 13 min between printing layers. However, the samples with 24 h printing intervals experienced a large increase in chloride ingress as the bond between layers was impacted [66]. Recent studies highlight the microstructural mechanisms by which layer interfaces accelerate chloride ingress. Interfaces formed during layer-by-layer deposition inherently possess increased porosity, interfacial voids, and microcracks, creating preferential pathways for chloride ions [68,69]. This effect is aggravated by drying–wetting cycles that cause dissolution, migration, and recrystallization of reaction products, further widening these pathways and accelerating deterioration processes [66,70,71]. Despite this critical relationship, explicit research into anisotropy-driven chloride penetration in 3DPC remains relatively limited.

A study by Bekaert et al. [30] investigated the service life of 3D-printed concrete (3DPC) by comparing a mix with ground granulated blast-furnace slag (GGBS) to one without it. The results showed that the mix containing GGBS had a lower chloride migration coefficient, indicating better resistance to chloride ingress. One of the most notable findings of the study was the effect of relative humidity on chloride permeability. Samples cured at 60 percent relative humidity exhibited a significant increase in chloride permeability, rising from 23.3% to 98.9%, compared to those cured at relative humidity above 95% [30]. Du et al. [31] examined the influence of pore structure on the durability of 3DPC, while Shafiq et al. [36] focused on the role of metakaolin and nano-silica. Both studies found that the addition of nano-silica improved the resistance of concrete to chloride attack, identifying it as a promising admixture for enhancing the durability of 3DPC [31,39]. Moelich et al. [36], in a study that included reference to the work of Du et al. [31], investigated the effect of early-age restraint on the durability of 3DPC. Their findings showed that printed samples had a lower chloride migration coefficient than cast specimens. However, Du et al. [31] reported that the chloride migration coefficient of printed samples was either similar to or slightly higher than that of cast concrete. Currently, there is a lack of comprehensive research comparing the durability of 3DPC and conventional cast concrete. Further studies are needed to better understand the durability characteristics of 3DPC and to support its development as a reliable and long-lasting construction material.

2.5. Carbonation

The carbonation process occurs due to air infiltration through the pores and cracks of concrete. The carbon dioxide in the air reacts with the calcium hydroxide and produces calcium carbonate [40]. Carbonation can cause the corrosion of steel reinforcements. Thus, the resistance to carbonation is paramount for the service life of a structure. This makes carbonation testing a key durability criterion to be tested [72]. The industry’s understanding of the carbonation resistance of 3DPC is significantly lower than that of cast concrete. De la Flor Juncal et al. [38] stated that the durability of 3DPC structures should be further investigated and that they should be exposed to accelerated carbonation tests to obtain their resistance.

A standard test for accelerated carbonation is ISO-1920-12-2015—Determination of the carbonation resistance of concrete—Accelerated carbonation method [72]. The test exposes the concrete sample’s vertical side to an increased carbon dioxide level. The storage room had a carbon dioxide concentration of 3 ± 0.5%, a temperature of 22 ± 2 °C, and a relative humidity of 65%. The test is used to compare the carbonation resistance of different concrete samples. Two different sizes are used for the concrete samples: a 100 mm cube for carbonation depth tested after one exposure period and a 400 mm long prism with a 100 × 100 mm face for a sample tested at multiple exposure periods. As previously mentioned, the samples’ tops, bottoms, and ends are covered in paraffin wax, exposing the vertical faces. For samples tested at different lengths of exposures, 50 mm is removed from the end of the 400 mm sample. The sample is then resealed with paraffin wax. The carbonation depth is determined by coloration after 56, 63, and 70 d. The total time is 112 d, 28 d curing, 14 d conditioning, and 70 d testing [72].

Bekaert et al. [30] used 400 × 100 × 100 mm for cast concrete samples and printed a hollow wall 400 mm × 100 mm × layer width; they measured the carbonation depth at 0, 7, 14, 28, and 56 d exposure, and a high carbonation rate was observed. The other papers in the review used cubes [45,73]. Zhou et al. [45] used 100 mm cubes, and Sanchez et al. [73] used 25 mm printed hollow cubes. Sanchez [73] measured carbonation after 2 months. Zhou et al. [45] measured carbonation penetration in the z-direction (Figure 20) when the sample had reached the carbonation time in line with the Chinese standard GB/T 50082-2009.

Table 6. Analysis of carbonation in 3DPC literature: mix design parameters and testing.

Ref.	Mix	Binder Description	Mix Description	Test Method	Procedure	Curing	PD (mm)
[30]	REF	50% OPC 50% GGBS	S/B 1	AC	60 d @ RH = 70% 50–90% CO2	28 d @ RH = 60	4.8/√day
						28 d @ RH > 95	2.4/√day
	M1	100% OPC	S/B 1.5	AC	60 d @ RH = 70% 50–90% CO2	28 d @ RH = 60	7.4/√day
						28 d @ RH > 95	1/√day
	M2	75% OPC 25% GGBS	S/B 1.5	AC	60 d @ RH = 70% 50–90% CO2	28 d @ RH = 60	12/√day
						28 d @ RH > 95	0.8/√day
[73]	C	100% OPC	W/B 0.6	-	60 d @ RH = 70% 50–90% CO2	60 d	0.5
	3DP	77% OPC 15% FL 8% SF	S/B 1.9 W/B 0.30	-	60 d @ RH = 70% 50–90% CO2	70 d	1
[45]	T0	100% OPC	100% N	GB/T 50082-2009	-	28 d	8.25
	T15	100% OPC	85% N 15% BT	GB/T 50082-2009	-	28 d	7.75
	T25	100% OPC	75% N 25% BT	GB/T 50082-2009	-	28 d	6.5
	T35	100% OPC	65% N 35% BT	GB/T 50082-2009	-	28 d	5.25
	T45	100% OPC	55% N 45% BT	GB/T 50082-2009	-	28 d	5.13

Note: Penetration Depth (PD), Ordinary Portland Cement (OPC), Ground Granulated Blast-Furnace Slag (GGBS), Sand/Binder (S/B), Accelerated Carbonation (AC), Relative Humidity (RH), Water/Binder (W/B), Filler Limestone (FL), Silica Fume (SF), Natural (N) Sand, Bauxite Tailings (BT).

evaluates the carbonation resistance of 3DPC using accelerated testing methods. Results show that carbonation depth ranged from 2.4 to 4.8 mm/√day, strongly influenced by curing conditions. Increasing curing humidity to above 95% halved the depth of carbonation compared to samples cured at 60% RH. Mixes containing 50% GGBS also showed superior carbonation resistance compared to mixes with higher OPC content. Although 3DPC tends to have higher carbonation rates than cast concrete due to its porous interfacial zones, these results affirm that carbonation resistance can be significantly improved through SCM inclusion and proper curing.

Zhou et al. [45] compared the carbonation of samples with different concentrations of bauxite tailings. The results showed that the 35% bauxite tailing concentration sample had the largest carbonation resistance [45]. Bekaert et al. [30] studied the addition of GGBS (ground granulated blast-furnace slag) into the concrete mix, as well as having RH = 60% and RH > 95% (relative humidity). The results found that the 3DPC mix with GGBS had a high level of carbonation depth at RH = 60% and RH > 95%. The paper also noted that the carbonation depth was significantly higher at RH = 60% compared to RH > 95%. Sanchez et al. [73] performed the microstructural examination of carbonated 3DPC. The microstructural examination found zero carbonation penetration in the cast sample. However, significant carbonation was observed through the depth of the printed sample, with the highest level observed at the layer interfaces [73]. Both studies found that 3DPC had a higher level of carbonation penetration when compared to cast samples [30,73]. Sanchez et al. [73] used 15% limestone and 8% nano-silica as binder substitutions for the 3DPC mix. It is impossible to obtain the nano-silica’s impact on the durability as the test compared a cast sample with the traditional mix to the 3DPC nano-silica mix. It is clear from the literature that printed samples have an increased carbonation rate. Further testing must be conducted to determine if adding nano-silica can reduce this rate.

Mechanistically, 3DPC is more prone to carbonation due to its distinct layered architecture, which introduces interfacial transition zones (ITZs) and lubrication layers with higher porosity and poor compaction. These features increase gas permeability, facilitating CO₂ ingress, especially along anisotropic paths parallel to the printing direction. Recent studies [73,74,75] using X-ray CT and SEM also confirm that interlayer voids and directional pore networks, which act as preferential carbonation pathways, are not present in cast concrete, as shown in Figure 21. Despite the growing body of data on carbonation resistance in traditional concrete, systematic evaluations of 3DPC under long-term carbonation exposure remain limited, particularly regarding the influence of print parameters on microstructure evolution.

3. Discussion and Future Work

The durability performance of 3DPC is governed by a complex interplay between material properties, printing process parameters, and environmental exposure conditions. This review identified key challenges, including elevated porosity at interlayer regions, increased susceptibility to shrinkage, and accelerated degradation under aggressive conditions such as freeze–thaw cycles, acid exposure, and chloride ingress. While the integration of SCMs and the refinement of printing parameters have shown considerable promise in improving durability, several research gaps persist.

Quantitative data summarized in six tables throughout this review underscore the multifaceted nature of 3DPC durability. Porosity in printed mixes ranged from as low as 0.09% to over 40%, with substantial reductions achieved through the use of SCMs such as silica fume and nano-silica. Shrinkage data revealed that plastic and drying shrinkage are more pronounced in 3DPC compared to cast concrete, but the use of fly ash and slag significantly mitigated cracking risk and improved dimensional stability, extending the crack initiation time by up to 57%.

Freeze–thaw resistance results indicated that the addition of air-entraining agents and styrene–butadiene rubber (SBR) emulsions led to marked improvements, with strength losses reduced by up to 79% compared to control mixes. In some cases, printed samples even outperformed their cast counterparts in terms of mass retention. Under sulfuric acid attack, 3DPC mixes containing silica fume showed enhanced chemical resistance, with mass losses reduced to as low as 2.7% after prolonged exposure, outperforming cast concrete in certain scenarios.

Chloride ingress resistance varied considerably, but the use of nano-silica and optimized curing methods (e.g., high relative humidity) reduced diffusion coefficients by up to 80%. Although 3DPC typically exhibited greater carbonation depth than cast concrete, performance improved significantly when GGBS was included in the binder and curing was conducted under RH > 95%, reducing carbonation depth by nearly 50%.

In addition to OPC-based systems, recent studies have demonstrated that geopolymer-based 3DPC systems can also benefit from similar durability strategies. The use of aluminosilicate precursors combined with optimized curing and SCM additions has shown potential in reducing porosity and enhancing chemical resistance. However, despite the presence of studies investigating the durability performance of geopolymer 3DPC in aspects such as acid resistance, carbonation, and chloride penetration, these systems are typically explored in isolation rather than through direct comparison with OPC-based 3DPC. Moreover, current literature lacks a systematic investigation of the differences and mechanisms between geopolymer and OPC-based 3DPC under various deterioration processes, including pore structure evolution, interlayer bonding performance, and degradation under environmental or chemical exposure.

In summary, while 3DPC presents inherent durability concerns due to its layered anisotropic structure and interfacial porosity, these challenges can be effectively addressed through comprehensive mix design optimization, targeted use of SCMs, appropriate curing strategies, and the incorporation of performance-enhancing admixtures. When properly engineered, 3DPC has the potential to meet or even exceed the durability standards of conventional cast concrete. To further improve the long-term performance of 3D-printed concrete (3DPC), future research should address the following key areas:

• Reducing porosity, particularly at interlayer regions, is essential for enhancing durability. Future studies should optimize the incorporation of nano-materials (e.g., nano-silica) and supplementary cementitious materials (SCMs) to improve pore structure uniformity and reduce overall permeability.
• Plastic and autogenous shrinkage remain critical durability concerns due to rapid moisture loss and the absence of formwork. Research should explore advanced curing techniques, such as controlled humidity environments and the use of internal curing agents or fiber reinforcements to limit early-age shrinkage cracking.
• While air-entraining agents and silica fume have shown positive effects, the long-term freeze–thaw durability of 3DPC under variable field conditions remains uncertain. Further investigation should focus on optimizing compaction quality, print parameters, and mix designs to ensure stable performance.
• Sulfuric acid poses a major threat to 3DPC durability. The development of acid-resistant binder systems using geopolymer formulations, hybrid cements, or surface treatments is necessary to withstand aggressive chemical environments.
• The anisotropic pore structure of 3DPC increases vulnerability to chloride penetration and steel reinforcement corrosion. Future work should design mixes with enhanced microstructural continuity and investigate admixtures such as corrosion inhibitors.
• Because 3DPC has shown higher susceptibility to carbonation than cast concrete, future experiments should employ accelerated carbonation testing across different mix designs, curing conditions, and print orientations to better understand and control carbonation depth.
• Currently, there is no dedicated durability standard for 3DPC. Adapting and validating existing ASTM, ISO, or EN test methods specifically for 3D-printed structures is crucial for establishing reliable long-term performance benchmarks.
• Most current assessments are based on accelerated laboratory testing. Long-term field monitoring of printed structures under varying environmental conditions and loading is needed to evaluate real-world performance.
• Parameters such as layer height, interlayer time gap, and print speed can significantly affect the formation of interfacial defects and long-term durability. Future studies should develop guidelines that balance durability with print efficiency.
• High-resolution synchrotron X-ray computed tomography (CT) is recommended for mapping CO₂ diffusion pathways in 3DPC, particularly across interlayer interfaces and anisotropic pore networks, enabling accurate correlation between microstructure and carbonation behavior.
• Future work should also focus on developing durability prediction models that account for anisotropic transport behavior, evolving interlayer bonding, and environmental influences over time. These models should incorporate the effects of creep, shrinkage, and temperature–humidity cycles to reflect realistic service conditions.
• Although several studies have reported promising durability results for geopolymer 3DPC, future research should systematically compare geopolymer and OPC-based systems under identical exposure conditions. Investigating their differences in pore structure evolution, interlayer bonding, and degradation mechanisms will help clarify their respective advantages and limitations in long-term durability performance.

4. Conclusions

This review systematically assessed key durability challenges associated with 3D-printed concrete (3DPC), a construction technology offering benefits such as faster construction, reduced labor requirements, and minimal material waste. This review focused on essential mix design parameters and testing methodologies addressing durability factors, including freeze–thaw resistance, chloride ingress, chemical attack, and carbonation. Key insights indicate significant impacts from porosity, shrinkage behaviour, microstructural development, and exposure conditions on the long-term durability of 3DPC. The primary conclusions from this study include:

• The durability of 3DPC significantly depends on optimized mix design, precise printing parameters, effective interlayer bond strength, controlled porosity, and environmental exposure conditions. The anisotropic behaviour and high interlayer porosity necessitate careful material and process optimization to achieve robust performance.
• The high porosity at layer interfaces poses a durability risk, which can be effectively mitigated by incorporating supplementary cementitious materials (SCMs) such as nano-silica, metakaolin, silica fume, and fly ash, significantly improving porosity and overall durability.
• Shrinkage-induced cracking, particularly plastic and autogenous shrinkage due to the absence of formwork, remains a critical issue. Slag and fly ash admixtures are recommended to reduce shrinkage rates and delay cracking.
• Enhanced freeze–thaw resistance is achievable through admixtures like silica fume and air-entraining agents, significantly reducing compressive strength loss and mass loss compared to traditional concrete.
• Sulfuric acid resistance in 3DPC can be notably improved by adding silica fume and nano-silica, minimizing mass loss and structural degradation in aggressive chemical environments.
• Chloride ingress is effectively controlled using optimized mixes containing metakaolin and nano-silica, significantly reducing chloride permeability.
• Carbonation depth tends to be higher in 3DPC; however, improvements through the use of SCMs like GGBS and nano-silica combined with high-humidity curing can substantially enhance resistance.
• In conclusion, while 3DPC holds substantial promise for sustainable and efficient construction, its durability needs targeted improvements. To enhance durability, it is recommended to strategically incorporate SCMs like silica fume, metakaolin, fly ash, and nano-silica, optimize printing parameters, and implement rigorous curing protocols. Standardized testing methods such as ASTM C666 [53] for freeze–thaw cycles, ASTM C1202 [65] for chloride permeability, and ASTM C1581 [42] and C1579 [43] for shrinkage assessments are essential for consistent evaluation and improvement. Future research should focus on standardizing long-term durability testing protocols, developing specialized admixtures tailored to durability needs, and exploring deeper into how print-process parameters affect microstructural evolution.