Effect of Metakaolin and Biochar Addition on the Performance of 3D Concrete Printing: A Meta-Analysis Approach

Abstract

Three-dimensional (3D) concrete printing (3DCP) is an emerging digital construction technology that enables geometrically complex structures with reduced labour, material waste, and formwork. However, the sustainability of 3DCP remains constrained by its heavy reliance on Portland cement, a major source of global CO₂ emissions. This study systematically examines metakaolin (MK) and biochar (BC) as sustainable additives for 3DCP, focusing on their independent effects on mechanical performance, printability, dimensional stability, and environmental impact. A comprehensive literature review (2015 to June 2025) identified 254 publications, of which 21 met the inclusion criteria for quantitative meta-analysis, contributing a total of 95 datasets for compressive and flexural strength. Pooled effect sizes were calculated using a random-effects model, supported by risk-of-bias and heterogeneity analyses. The results indicate statistically significant improvements in mechanical properties, with an overall pooled ratio of means (ROM) of 1.12 (95% CI: 1.06–1.20; I² = 48.9%), representing the overall mechanical performance effect across all datasets, while ROM for compressive and flexural strength was calculated separately in the main analysis. Meta-regression revealed that BC increased compressive and flexural strengths by 7% and 9%, respectively, while MK achieved greater enhancements of 21% and 13.4%. Optimum performance was observed at 15–20% MK for compressive strength and 10–15% for flexural strength, whereas BC performed best at 3–5% and 2–5%, respectively. BC contributed to CO₂ reductions of up to 43% through clinker substitution and biogenic carbon sequestration. These findings demonstrate that MK and BC are complementary eco-efficient modifiers capable of enhancing both structural and environmental performance in 3DCP. Future research should address long-term durability, standardisation of printing parameters, and cradle-to-grave life cycle assessments to strengthen practical implementation.

1. Introduction

The construction industry is a major driver of global economic growth but remains highly resource-intensive and a leading contributor to greenhouse gas emissions. In 2018, global construction expenditure reached USD 11.4 trillion and is projected to rise to USD 14 trillion by 2025 [1,2]. Conventional construction practices account for approximately 40% of global energy consumption, 40% of solid waste generation, 38% of CO₂ emissions, and 68% of freshwater use [3]. Life-cycle assessments consistently reveal the substantial environmental footprint of the sector, which is expected to worsen as urbanisation accelerates and outpaces improvements in energy efficiency [4].

Three-dimensional concrete printing (3DCP) has emerged as a promising digital construction technology capable of addressing several of these challenges. By employing a layer-by-layer extrusion process, 3DCP enables the automated fabrication of geometrically complex structures, significantly reducing the need for formwork and manual labour [5]. Concrete is extruded through a nozzle along a digitally predefined path, allowing precise material deposition without conventional moulds [2]. Market analyses indicate that 3DCP can reduce construction waste by 30–60%, labour costs by up to 50%, and construction time by 50–70%. Despite these advantages, the environmental credentials of 3DCP remain under scrutiny [6]. Cement production alone contributes 5–8% of global anthropogenic CO₂ emissions [7,8], with approximately 60% arising from limestone calcination and the remainder from thermal energy consumption. This highlights the urgent need for low-carbon binder alternatives [9]. While conventional supplementary cementitious materials (SCMs) such as fly ash, ground granulated blast furnace slag, and silica fume have been widely adopted, their future availability is uncertain, and they do not actively remove carbon. Therefore, developing novel SCMs that are abundant, renewable, and capable of reducing net carbon emissions is essential.

Researchers in the field of 3DCP are increasingly directing attention towards abundant and renewable resources that exhibit lower environmental impacts. Among the most promising SCMs is metakaolin (MK), a thermally activated, dehydroxylated kaolinitic clay with high pozzolanic reactivity. It typically contains more than 50% silica and about 40% alumina, together with wide availability and a cradle-to-gate global warming potential of roughly one-third that of clinker [10]. Incorporation of MK in 3DCP mixtures has been shown to markedly enhance thixotropy and structural buildability. For instance, the addition of 10% MK increased static and dynamic yield stresses by 285% and 129%, respectively, and improved 28-day compressive (CS) and flexural strengths (FS) by 63% and 31% without compromising interlayer bonding [10,11]. Similarly, incorporating 5% slag into MK based geopolymer concrete enhanced fresh and hardened properties in 3D printing (3DP) applications, increasing static and dynamic yield stresses to 1900 Pa and 1482 Pa, enabling the printing of 42 layers compared with 27 using metakaolin alone, improving CS from 20.65 MPa to 26.10 MPa, improving FS at 28 days, and significantly reducing efflorescence formation [10]. These findings confirm the potential of MK as a high-performance, low-carbon binder for 3DCP.

While MK enhances mechanical performance through pozzolanic reactions and the formation of additional calcium aluminosilicate hydrate (C-A-S-H) gel, biochar (BC) contributes through carbon sequestration, internal curing effects, and microstructural refinement. These distinct mechanisms highlight their different roles in improving both the performance and sustainability of 3DCP. BC, a stable carbon-negative porous pyrolytic material derived from agricultural residues and other biomass sources, offers an additional sustainable solution by reducing waste and pollutant emissions [12]. Beyond its environmental benefits in waste management and carbon sequestration, the use of BC as an SCM in 3DCP is gaining attention due to its multifunctional properties. Studies have shown that incorporating 2% BC improves shape stability and buildability during layer deposition, while 2–5% BC additions under accelerated carbonation curing (ACC) enhance durability, promote carbon sequestration, and increase 28-day CS by approximately 30% [13]. Moreover, optimised concrete mix designs with BC have achieved up to 43% reductions in CO₂ emissions, primarily through clinker substitution and the inclusion of biogenic carbon sequestration mechanisms [14,15,16].

In recent years, several review studies have explored sustainable materials for 3DCP, reflecting the growing academic and industrial interest in this technology. Hassan et al. [17] reviewed 3DCP for sustainable construction and identified MK as an effective SCM capable of enhancing yield stress, shape retention, pore structure, and interlayer bonding while lowering cement-related CO₂ emissions. However, they noted a lack of long-term durability data for MK-modified 3DCP. Similarly, Barve et al. [18] reviewed geopolymer 3DCP and highlighted MK as a key aluminosilicate precursor whose rheology, buildability, and interlayer bonding are strongly influenced by activator dosage and curing conditions. They further emphasised the need for standardised methods to assess printability in MK-based systems. Tarhan et al. [19] also reviewed binder matrices for 3DP construction, emphasising MK’s dual role—as an SCM in cement-based mixes and as a primary aluminosilicate precursor in geopolymer systems—enhancing rheology, buildability, and sustainability.

Regarding biochar, most prior research has focused on conventional casting methods rather than extrusion-based 3D printing. Senadheera et al. [20] reviewed the application of BC in concrete, emphasising its capacity to enhance CS and FS, refine pore structure, reduce water absorption, and improve resistance to chloride ingress, while also contributing to carbon sequestration and overall sustainability. Similarly, Akinyemi et al. [21] reviewed recent advancements in the use of BC for cementitious applications, highlighting its role in improving workability, CS, and FS, reducing shrinkage and porosity, and enhancing durability through better resistance to chloride ingress and carbonation, while contributing to carbon sequestration. However, these findings are based primarily on cast concrete, whereas 3DCP introduces unique challenges related to printability, interlayer bonding, and rheological behaviour. Consequently, BC remains a relatively new and underexplored material in 3DCP, warranting systematic investigation into its performance and mechanisms in digitally manufactured cementitious systems.

While numerous studies have reviewed sustainable materials for 3DCP, most have remained qualitative and focused on individual binder systems, often lacking quantitative comparisons across material types, dosage levels, and performance indicators. As a result, inconsistencies persist regarding the reported effects of MK and BC on 3DCP—particularly in relation to mechanical performance, printability, dimensional stability, durability, and environmental impact. Many existing reviews also overlook a considerable portion of the relevant literature, leading to incomplete coverage and potentially biassed or imprecise conclusions. Moreover, contradictory findings across studies further highlight the need for a systematic and statistically robust evaluation of MK and BC in 3DCP. Research indicates that incorporating MK and BC can enhance mechanical strength, printability, and microstructural performance; however, these improvements vary widely due to differences in feedstock properties, particle fineness, mixture design, curing regimes, and printing parameters. For BC, some studies report improved strength and pore refinement at low dosages, whereas others observe strength reductions—particularly in conventional concretes—when coarser biochar or higher replacement levels increase porosity and water demand. Such variability underscores the need for quantitative synthesis to establish generalisable trends.

Meta-analysis provides a rigorous approach to address these challenges by aggregating data from individual studies to derive numerical estimates of overall performance effects [22]. Although a recent meta-analysis examined cement and aggregate replacements in Portland cement composites, it did not specifically target MK and BC or provide insights relevant to 3DCP, thereby limiting its applicability. To date, no systematic and statistical synthesis focusing on MK and BC in 3DCP exists.

In this review, mechanical performance outcomes were standardised using the ratio of means (ROM), enabling comparison across studies employing different curing conditions, printing parameters, and binder chemistries. A random-effects model was adopted to account for methodological variability, and heterogeneity statistics (I², τ², and Q) were used to quantify between-study differences. Sensitivity analyses were performed to assess the influence of individual studies on pooled effects. Given that CS and FS are the most consistently reported metrics in the literature, pooled effect sizes were derived using a random-effects framework and complemented with qualitative assessments of printability, dimensional stability, durability, and environmental performance.

This integrated approach provides a comprehensive synthesis of the fragmented literature, identifies key performance trends and sources of heterogeneity, and highlights critical research gaps—particularly the lack of standardised testing protocols and universally accepted mix-design guidelines. Overall, the study establishes a robust evidence base to guide the effective use of MK and BC as sustainable materials in 3DCP.

2. Methodology

2.1. Search Strategy and Data Extraction

The primary sources of literature for this study were the Scopus and Web of Science databases, selected for their broad coverage of peer-reviewed publications across engineering, materials science, and construction disciplines. These two databases were chosen because they index the highest quality and most widely cited work in 3DCP, thereby ensuring comprehensive retrieval of relevant studies [23]. This enabled a systematic investigation of the research landscape on the use of MK and BC as sustainable materials in 3DCP. The review considered journal articles, conference papers, and review articles. To maintain both relevance and quality, only peer-reviewed documents written in English and explicitly focused on 3DCP were included.

Eligible studies were those that investigated MK or BC in 3DCP and reported quantitative data, defined as experimentally measured properties (e.g., CS, FS) expressed as mean values with measures of variability (such as 95% confidence intervals (CI)) and the corresponding sample size. Studies limited to qualitative descriptions or categorical assessments were excluded. In addition, studies were excluded if they were not peer reviewed, not written in English, did not focus on 3DCP or additive manufacturing with cementitious materials, did not involve MK or BC as constituent materials, or theses without original experimental data. The analysis focused on publications from 2015 to June 2025. The selected time frame was intended to capture recent advancements in the use of MK and BC as sustainable materials in 3DCP, reflecting both the current state of the art and the evolution of research in this area over the past decade.

To ensure consistency in data collection across the selected studies, a structured extraction sheet was used to standardise the process. Each study was assessed for key information on study characteristics, material details, mechanical properties, printability, dimensional stability, and sustainability indicators, as these parameters are critical for evaluating 3DCP performance. The extracted data included MK or BC dosage, binder composition, water to binder ratio, curing condition, printing parameters, testing age, and the mean compressive and flexural strength values together with sample sizes, with missing variability measures obtained from figures where possible, and all entries cross-checked for accuracy.

The review process adhered to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [24]. A completed PRISMA 2020 checklist outlining compliance with reporting standards is provided in Table S1 of the Supplementary Materials. The search string involved keywords such as: (metakaolin) AND (biochar) AND (“3D printing” OR “additive manufacturing”) AND (3D concrete printing). These keywords were adapted for each database using truncation and combinations of synonyms to maximise relevant retrieval. This preliminary search resulted in 232 documents from the Scopus database and 183 from the WoS database, as depicted in Figure 1. After applying filters for year, document type, language, and subject area to both databases, the counts were narrowed down to 254 documents. Subsequently, the Scopus dataset was exported as a “BibTeX” file and the WoS dataset as a “Plain text” file. Both files were merged into a single “.xlsx” file using R-Studio 2024 software to remove duplicates from both databases. A total of 158 documents were identified as duplicates. After their removal, 96 documents remained for further screening. The next phase involved a detailed evaluation of the remaining documents. Of these, 59 were excluded at the title and abstract screening stage. From the remaining 37, 5 records were unavailable, and 11 full-text articles were excluded due to a lack of experimental data relevant to the objectives of this study. Hence, 21 studies were of relevance and met the inclusion criteria. All discrepancies between the authors responsible for screening were discussed or resolved by involving a third author to make the final decision, which ensured that the selection process was fair and unbiased. This step was significant in the identification of studies meeting the pre-defined criteria and thus appropriate for inclusion in the systematic review and meta-analysis.

2.2. Data Synthesis and Meta-Analytical Model

The meta-analysis used a log-transformed ratio of means (ROM) to analyse the effect size of performance parameters influenced by MK, BC, and mixture variables. ROM was selected because it standardises differences between treatment and control groups, reduces scale related variability across studies and provides a stable effect size measure for heterogeneous datasets through log transformation. Each included study was treated as having homogeneous experimental conditions, and the individual effect size, referred to as the ROM, was calculated according to Equation (1).

$$ROM={\displaystyle \frac{X_t}{X_c}}$$

(1)

where $X_{t }$ is the mean of the treatment group incorporating MK and BC into the cementitious mixture, and $X_c$ is the mean of the corresponding control group without these additives. Positive values of ROM indicate an improvement in mechanical strength relative to the control, whereas negative values indicate a reduction. For meta-analytic modelling, ROM values were transformed to the natural logarithmic (ln) scale to stabilise variance and approximate a normal sampling distribution, as expressed in Equation (2). The variance (V) of individual effect size was then calculated using Equation (3).

$$\mathit{ln}\left(ROM\right)=\mathit{ln}\left(X_t|X_c\right)$$

(2)

$$V\left[ln\left(ROM\right)\right]={\displaystyle \frac{{SD}_t^2}{n_t{X}_t^2}}+{\displaystyle \frac{{SD}_c^2}{n_c{X}_c^2}}$$

(3)

where ${SD}_t$ and ${SD}_c$ are the standard deviations of the treatment and control groups, and $n_t$ and $n_c$ are their respective sample sizes.

The standard error (SE) was then derived as the square root of this variance, as stated in Equation (4), to provide a measure of precision for each individual effect size estimate. Based on this SE, a 95% CI was calculated using Equation (5) to quantify the uncertainty associated with each individual effect size estimate. To account for both within-study variance and between-study heterogeneity, study weights were assigned using a random effects model as stated in Equation (6). Using these weights, a weighted mean ($L_w$) of the log-transformed effect sizes was then calculated to obtain the pooled overall effect for each parameter using Equation (7).

$$SE\left[\mathit{ln}\left(ROM\right)\right]=\sqrt{V\left[\mathit{ln}\left(ROM\right)\right] }$$

(4)

$${CI}_{95\%}\left(\mathit{ln}\left(ROM\right)\right)=\mathit{ln}\left(ROM\right)\pm 1.96\times SE\left[\mathit{ln}\left(ROM\right)\right]$$

(5)

$$w_i={\displaystyle \frac{1}{V\left[\mathit{ln}{\left(ROM\right)}_i\right]+{\tau }^2}}$$

(6)

$$L_w={\displaystyle \frac{\sum w_{i }l{n\left(ROM\right)}_i}{\sum w_i}}$$

(7)

The variance of the pooled effect size ($\overline{V}$) was calculated as the reciprocal of the sum of study weights using Equation (8). Subsequently, the 95% CI of the pooled effect size was calculated using Equation (9), providing an interval estimate of the overall effect of MK and BC across studies. A random effects model was adopted to account for methodological variability, and heterogeneity was quantified using Cochran’s Q statistic, the I² statistic, and the between-study variance (τ²), with I² values interpreted as low (<25%), moderate (25–50%), and high (>50%). This model was selected because the included studies differ in mixture design, curing conditions, printing parameters, and testing procedures, indicating underlying variation in effect sizes across studies rather than a single common outcome. Therefore, the random effects approach provides a more appropriate and generalisable estimate of the pooled performance outcome.

The pooled effect sizes were presented in forest plots (Figure 2), where the red diamond denotes the weighted mean ROM. Leave-one-out sensitivity analysis was performed to evaluate the influence of individual studies on the pooled results and to identify potential sources of heterogeneity. This approach was used because it tests the robustness of the meta-analytic estimates, ensures that no single study disproportionately affects the pooled effect size, and verifies the stability of the conclusions in the presence of methodological variability across studies. Effect sizes for mechanical properties were derived using ROM, while outcomes related to printability, dimensional stability, environmental sustainability, and durability were synthesised qualitatively due to limited quantitative evidence. This qualitative synthesis highlighted trends, research gaps, and key insights, complementing the quantitative analysis and providing a more comprehensive understanding of the evidence base.

$$\overline{V}={\displaystyle \frac{1}{\sum w_i}}$$

(8)

$${CI}_{95\%}(L_w)=L_w\pm 1.96\sqrt{\overline{V}\left[L_w\right] }$$

(9)

It should be noted that all included studies reported CS and FS values in megapascals (MPa). As a result, no unit conversion was required. To ensure comparability across studies with differing measurement scales and experimental conditions, mechanical performance outcomes were standardised using the ROM approach. This method provided a consistent basis for estimating effect sizes and allowed meaningful comparisons across diverse datasets. Printability, dimensional stability, durability, and environmental performance were assessed qualitatively due to the lack of consistent quantitative data in the existing literature. This distinction in analytical approach ensured that both quantitative and qualitative outcomes were addressed appropriately. Quantitative meta-analysis was conducted using R software 4.4 1 employing the meta and metafor packages to calculate pooled effect sizes, 95% confidence intervals, and heterogeneity statistics including I², τ², and Cochran’s Q. Funnel plots and Egger’s regression test were applied to assess the presence of potential publication bias. IBM SPSS Statistics V. 30 was used for descriptive statistical analysis, preliminary data verification, and screening. Forest plots, funnel plots, and sensitivity analysis figures were generated using R and OriginPro^® 2025b to support the visual interpretation of findings.

3. Results

3.1. Effect of Biochar and Metakaolin on the Mechanical Properties of 3DCP

The influence of BC and MK on the mechanical properties of 3DCP was investigated with a focus on CS and FS. Data extracted from 21 eligible studies were synthesised through meta-analysis and complemented with individual experimental findings as summarised in Table 1.

For BC, low dosages generally improved both CS and FS, whereas higher replacement levels were often associated with performance reductions. For instance, Falliano et al. [14] reported that 5% BC addition increased CS by approximately 17% compared to the control, with peak values reaching 75 MPa, while FS also improved at lower dosages. Similarly, Ling et al. [25] confirmed that low dosages of 1–3% BC addition enhanced CS and FS through filler effects and pore refinement, with finer BC particles consistently outperforming coarser ones, whereas higher dosages led to reductions in strength. Extending on these studies, Gupta et al. [26] observed that 3% BC addition improved CS, FS, and CO₂ sequestration while reducing capillary absorption, although higher contents increased drying shrinkage despite lowering carbonation shrinkage. Consistent with these findings, Zhang et al. [27] reported that incorporating 2–5% wood waste biochar enhanced CS and FS compared to the control, attributed to denser particle packing and internal curing, whereas higher dosages above 20% led to strength reductions. Collectively, these studies highlight that BC is most effective at low dosages, providing both mechanical and environmental benefits. In contrast, several studies on conventional concrete report reductions in strength with biochar addition because these mixes often contain coarser biochar particles or use higher replacement levels that increase porosity and water demand. These differences in mix design, particle fineness, feedstock characteristics and replacement level explain the variation in the reported outcomes [28,29].

In contrast, studies on metakaolin consistently demonstrated stronger and more reproducible improvements in mechanical performance. Mishra et al. [30] reported that replacing 10–15% of cement with MK increased both CS and FS through enhanced pozzolanic reactivity and pore structure refinement, while dosages above 20% impaired workability and decreased strength. Similarly, Dai et al. [31] observed that incorporating 10–15% MK in alkali-activated mortars improved CS and FS due to improved geopolymerisation and microstructural densification, although higher MK levels reduced printability and layer cohesion. Thajeel et al. [32] further demonstrated that combining 10% MK with 5% silica fume produced synergistic effects that enhanced particle packing, cohesion, and pozzolanic reactivity, achieving the highest CS among the tested mixes, whereas 15% MK alone reduced extrudability and caused microcracking.

Collectively, the results confirm that both MK and BC can enhance the mechanical properties of 3DCP mixtures when used within optimal dosage ranges. MK tends to produce more consistent and pronounced strength gains due to its high pozzolanic activity, while BC provides additional sustainability benefits, including carbon negativity, internal curing, and improved microstructural integrity.

Table 1. Overview of experimental parameters from studies investigating metakaolin and biochar in 3DCP.

Geographic Region	Experimental Setup	Outcomes	Impact on Compressive Strength	Impact on Flexural Strength	Ref
Italy	1–7% BC in cement paste/mortar	↑ CS and FS at 7 and 28 days	CS ↑ 23% (7 d), ↑ 13% (28 d)	FS ↑ 63% (7 d), ↑ 29% (28 d)	[33]
Italy	5–23% BC, high S/C ratio.	Optimal 5–11% BC ↑ CS, FS and printability.	CS ↑ by 17% (28 d),	FS ↑ by 15% (28 d),	[34]
Brazil	PC replaced by 30% MK	30% of MK ↑ extrusion, buildability, and stability.	>8 MPa. (28 d)	Slightly ↑ FS (28 d)	[35]
Italy	5–23% BC in mortar	↑ Stability, ↓ CO2 emissions (43%).	75 MPa (28 d)	15 MPa (28 d)	[15]
France	MK+ GP + xanthan gum	↑ Viscosity and printability	↑ CS with fillers.	Stable FS	[36]
Malaysia	10–40% RHB + cement	20% RHB replacement optimised CS and CO2 uptake.	20% BC ↑ CS (28 d)	↑ FS with ↑ CO2 uptake	[16]
Australia	1–3% BC + 20% FA	3% BC ↑ CS and ↓ shrinkage by 30–60%	3% BC ↑ CS by 24% (7 d) and 21% (28 d)	1% BC ↑ FS by 17% and 3% BC ↑ FS by 7% (28 d)	[26]
Pakistan/Saudi Arabia	1–5% BC in OPC	2% BC ↓ porosity and ↑ hydration.	CS ↑18% (28 d)	Slightly ↑ FS (28 d)	[37]
Hong Kong/USA/Italy	OPC + SCMs + 5% BC + fibres	↑ CS, ↓ CO2 impact, and ↑ economic value.	5% BC ↑ CS by 17%	5% BC ↑ FS	[38]
China	1–3% BC in concrete	1–3% BC ↓ Carbonation and ↑ CS	1–3% BC ↑ CS by 15–18%	FS ↑ slightly at 1–3% BC, ↓ at higher dosages	[25]
China	BC + cement	BC addition ↑ CS and ↑ durability	2–5% BC ↑ CS and microstructural compactness	BC slightly ↑ FS	[27]
Brazil	MK + industrial residues	MK addition ↑ CS (up to 90 MPa), ↓ cost (16%)	MK ↑ CS (7, 28 d)	Comparable FS.	[39]
Korea	BC +cement + aggregate	BC↑ mechanical performance and sustainability.	2–5% BC ↑ CS by 10–40%	30% BC ↑ FS by 20%	[20]
Italy	2–5% BC in PLC 2–5% BC	2% BC 2% BC ↑ durability and ↓ absorption while 5% BC ↓ CS	2% BC ↑ CS by 3% (28 d), 7% (1 y), 9% (2 y)	2% BC ↑ FS by 9% (2 y)	[40]
Colombia	0–20% in OPC.	10% BC ↑ printability	10% BC maintain CS	10% BC maintain FS	[41]
South Korea	2–5% BC in cement	BC addition ↑ Sequestration and ↑ CS (30%)	ACC ↑ CS by 30% (28 d)	↑ FS with pore uniformity.	[13]
China	MK/FA+ sodium silicate solution	Increased MK ↑ viscosity and ↓ CS	Max CS of 25.2 MPa achieved, ↑ MK ↓ CS due to high viscosity	↑ MK ↓ FS	[42]

Table 1. Overview of experimental parameters from studies investigating metakaolin and biochar in 3DCP.

Geographic Region	Experimental Setup	Outcomes	Impact on Compressive Strength	Impact on Flexural Strength	Ref
Italy	1–7% BC in cement paste/mortar	↑ CS and FS at 7 and 28 days	CS ↑ 23% (7 d), ↑ 13% (28 d)	FS ↑ 63% (7 d), ↑ 29% (28 d)	[33]
Italy	5–23% BC, high S/C ratio.	Optimal 5–11% BC ↑ CS, FS and printability.	CS ↑ by 17% (28 d),	FS ↑ by 15% (28 d),	[34]
Brazil	PC replaced by 30% MK	30% of MK ↑ extrusion, buildability, and stability.	>8 MPa. (28 d)	Slightly ↑ FS (28 d)	[35]
Italy	5–23% BC in mortar	↑ Stability, ↓ CO2 emissions (43%).	75 MPa (28 d)	15 MPa (28 d)	[15]
France	MK+ GP + xanthan gum	↑ Viscosity and printability	↑ CS with fillers.	Stable FS	[36]
Malaysia	10–40% RHB + cement	20% RHB replacement optimised CS and CO2 uptake.	20% BC ↑ CS (28 d)	↑ FS with ↑ CO2 uptake	[16]
Australia	1–3% BC + 20% FA	3% BC ↑ CS and ↓ shrinkage by 30–60%	3% BC ↑ CS by 24% (7 d) and 21% (28 d)	1% BC ↑ FS by 17% and 3% BC ↑ FS by 7% (28 d)	[26]
Pakistan/Saudi Arabia	1–5% BC in OPC	2% BC ↓ porosity and ↑ hydration.	CS ↑18% (28 d)	Slightly ↑ FS (28 d)	[37]
Hong Kong/USA/Italy	OPC + SCMs + 5% BC + fibres	↑ CS, ↓ CO2 impact, and ↑ economic value.	5% BC ↑ CS by 17%	5% BC ↑ FS	[38]
China	1–3% BC in concrete	1–3% BC ↓ Carbonation and ↑ CS	1–3% BC ↑ CS by 15–18%	FS ↑ slightly at 1–3% BC, ↓ at higher dosages	[25]
China	BC + cement	BC addition ↑ CS and ↑ durability	2–5% BC ↑ CS and microstructural compactness	BC slightly ↑ FS	[27]
Brazil	MK + industrial residues	MK addition ↑ CS (up to 90 MPa), ↓ cost (16%)	MK ↑ CS (7, 28 d)	Comparable FS.	[39]
Korea	BC +cement + aggregate	BC↑ mechanical performance and sustainability.	2–5% BC ↑ CS by 10–40%	30% BC ↑ FS by 20%	[20]
Italy	2–5% BC in PLC 2–5% BC	2% BC 2% BC ↑ durability and ↓ absorption while 5% BC ↓ CS	2% BC ↑ CS by 3% (28 d), 7% (1 y), 9% (2 y)	2% BC ↑ FS by 9% (2 y)	[40]
Colombia	0–20% in OPC.	10% BC ↑ printability	10% BC maintain CS	10% BC maintain FS	[41]
South Korea	2–5% BC in cement	BC addition ↑ Sequestration and ↑ CS (30%)	ACC ↑ CS by 30% (28 d)	↑ FS with pore uniformity.	[13]
China	MK/FA+ sodium silicate solution	Increased MK ↑ viscosity and ↓ CS	Max CS of 25.2 MPa achieved, ↑ MK ↓ CS due to high viscosity	↑ MK ↓ FS	[42]

3.2. Meta-Analysis of Mechanical Properties

The meta-analysis assesses the effect of BC and MK on the mechanical properties, especially CS and FS (Table 2 and Table 3). The forest plot in Figure 2 presents evidence from 21 studies as the individual ROM with 95% confidence intervals (CI). Most studies reported ROM values above 1.0, indicating statistically significant positive effects on mechanical strength. The overall pooled effect size (combined MK + BC), represented by a red diamond as depicted in Figure 2e, was ROM = 1.12 (95% CI: 1.06–1.20), indicating an average 12% improvement in mechanical properties compared with controls.

A ROM of 1.0 represents the null effect, with confidence intervals including this value indicating non-significant results. ROM values above 1.0 indicate a beneficial effect, while values below 1.0 suggest a detrimental effect. For both CS and FS, heterogeneity was low to moderate (Q = 39.11, df = 20, p = 0.0029; I² = 48.86%; τ² = 0.020), as determined using Cochran’s Q test, the I² statistic, and the between-study variance (τ²). Such moderate between-study heterogeneity is expected, given variations in mix design, material properties, curing conditions, testing methods, and printing parameters across studies.

Using a random-effects model allowed the pooled estimates to account for both within-study sampling error and true between-study variability, enhancing the robustness and generalizability of the findings. The meta-analysis results indicate that incorporating BC into 3DCP mixes yielded a pooled ROM of 1.07 (95% CI: 1.01–1.14; Figure 2c), representing a statistically significant 7% improvement in CS compared to unmodified mixes. MK exhibited a stronger effect, with a pooled ROM of 1.21 (95% CI: 1.15–1.27; Figure 2b), corresponding to a ~21% increase, with the entire confidence interval above 1.0, confirming a robust enhancement.

Similarly, BC modified 3DCP mixes showed an average FS improvement of 9% (ROM = 1.09, 95% CI: 1.01–1.18) over control mixes as depicted in Figure 2d, indicating a modest but statistically significant enhancement. In contrast, MK modified mixes achieved a greater average improvement of 13.4% (ROM = 1.13, 95% CI: 1.07–1.20) as depicted in Figure 2a, with both bounds of the confidence interval well above 1.0, confirming a more substantial and consistent positive effect. These findings suggest that while both BC and MK enhance the mechanical performance of 3DCP in terms of CS and FS, MK consistently provides greater mechanical gains. By contrast, the relatively smaller improvements offered by BC become particularly significant when viewed in conjunction with its carbon-negative characteristics and associated sustainability advantages.

Table 2. ROM values of biochar and metakaolin for 28-day compressive strength measurements, including treatment and control means.

Author	Ref	Material	${\mathit{X}}_{\mathit{t}}$	${\mathit{X}}_{\mathit{c}}$	${\mathit{S}\mathit{D}}_{\mathit{t}}^2$	${\mathit{S}\mathit{D}}_{\mathit{c}}^2$	ROM	$\mathit{S}\mathit{E}\left[\mathit{ln}\left(\mathit{R}\mathit{O}\mathit{M}\right)\right]$	95% CI Low	95% CI High
Falliano et al.	[43]	BC	74.5	73.2	7.28	0.53	1.019	0.0195	0.98	1.06
Ling et al.	[25]	BC	47.7	42.0	7.84	6.25	1.135	0.0483	1.03	1.25
Faleschini et al.	[34]	BC	11.0	10.0	0.49	0.36	1.104	0.0504	1.00	1.22
Falliano et al.	[14]	BC	71.8	62.0	5.15	4.00	1.159	0.0261	1.10	1.22
Falliano et al.	[15]	BC	70.0	65.0	5.15	4.00	1.077	0.0258	1.02	1.13
Gasmi et al.	[36]	MK	74.0	63.0	4.41	4.00	1.175	0.0246	1.12	1.23
Gunn et al.	[16]	BC	53.0	50.0	3.42	3.24	1.060	0.0290	1.00	1.12
Gupta et al.	[26]	BC	50.0	48.0	6.76	6.25	1.042	0.0425	0.96	1.13
Javed et al.	[37]	BC	54.0	52.0	2.25	2.25	1.038	0.0231	0.99	1.09
Labianca et al.	[38]	BC	52.0	51.0	2.89	2.56	1.020	0.0262	0.97	1.07
Ruviaro et al.	[39]	MK	86.8	70.0	9.00	7.84	1.240	0.0306	1.17	1.32
Sirico et al.	[40]	BC	54.0	50.0	3.81	3.24	1.080	0.0295	1.02	1.14
Yang et al.	[13]	BC	57.7	55.0	4.21	3.61	1.050	0.0286	0.99	1.11
Zhang et al.	[42]	MK	92.0	73.0	1.69	1.44	1.260	0.0125	1.23	1.29
Diniz et al.	[35]	MK	70.0	56.0	4.41	4.00	1.250	0.0269	1.19	1.32
Zhao et al.	[44]	MK	67.0	55.0	3.61	3.24	1.218	0.0250	1.16	1.28
Mishra et al.	[30]	MK	69.0	58.0	4.41	4.00	1.190	0.0266	1.13	1.25
Dai et al.	[31]	MK	73.0	61.0	4.41	4.00	1.197	0.0252	1.14	1.26
Marcyzyk et al.	[45]	MK	64.0	53.0	4.41	4.00	1.208	0.0289	1.14	1.28
Thajeel et al.	[32]	MK	61.6	53.0	4.41	4.00	1.162	0.0294	1.10	1.23
Duan et al.	[10]	MK	66.7	56.0	4.41	4.00	1.191	0.0275	1.13	1.26

Table 2. ROM values of biochar and metakaolin for 28-day compressive strength measurements, including treatment and control means.

Author	Ref	Material	${\mathit{X}}_{\mathit{t}}$	${\mathit{X}}_{\mathit{c}}$	${\mathit{S}\mathit{D}}_{\mathit{t}}^2$	${\mathit{S}\mathit{D}}_{\mathit{c}}^2$	ROM	$\mathit{S}\mathit{E}\left[\mathit{ln}\left(\mathit{R}\mathit{O}\mathit{M}\right)\right]$	95% CI Low	95% CI High
Falliano et al.	[43]	BC	74.5	73.2	7.28	0.53	1.019	0.0195	0.98	1.06
Ling et al.	[25]	BC	47.7	42.0	7.84	6.25	1.135	0.0483	1.03	1.25
Faleschini et al.	[34]	BC	11.0	10.0	0.49	0.36	1.104	0.0504	1.00	1.22
Falliano et al.	[14]	BC	71.8	62.0	5.15	4.00	1.159	0.0261	1.10	1.22
Falliano et al.	[15]	BC	70.0	65.0	5.15	4.00	1.077	0.0258	1.02	1.13
Gasmi et al.	[36]	MK	74.0	63.0	4.41	4.00	1.175	0.0246	1.12	1.23
Gunn et al.	[16]	BC	53.0	50.0	3.42	3.24	1.060	0.0290	1.00	1.12
Gupta et al.	[26]	BC	50.0	48.0	6.76	6.25	1.042	0.0425	0.96	1.13
Javed et al.	[37]	BC	54.0	52.0	2.25	2.25	1.038	0.0231	0.99	1.09
Labianca et al.	[38]	BC	52.0	51.0	2.89	2.56	1.020	0.0262	0.97	1.07
Ruviaro et al.	[39]	MK	86.8	70.0	9.00	7.84	1.240	0.0306	1.17	1.32
Sirico et al.	[40]	BC	54.0	50.0	3.81	3.24	1.080	0.0295	1.02	1.14
Yang et al.	[13]	BC	57.7	55.0	4.21	3.61	1.050	0.0286	0.99	1.11
Zhang et al.	[42]	MK	92.0	73.0	1.69	1.44	1.260	0.0125	1.23	1.29
Diniz et al.	[35]	MK	70.0	56.0	4.41	4.00	1.250	0.0269	1.19	1.32
Zhao et al.	[44]	MK	67.0	55.0	3.61	3.24	1.218	0.0250	1.16	1.28
Mishra et al.	[30]	MK	69.0	58.0	4.41	4.00	1.190	0.0266	1.13	1.25
Dai et al.	[31]	MK	73.0	61.0	4.41	4.00	1.197	0.0252	1.14	1.26
Marcyzyk et al.	[45]	MK	64.0	53.0	4.41	4.00	1.208	0.0289	1.14	1.28
Thajeel et al.	[32]	MK	61.6	53.0	4.41	4.00	1.162	0.0294	1.10	1.23
Duan et al.	[10]	MK	66.7	56.0	4.41	4.00	1.191	0.0275	1.13	1.26

Table 3. ROM values of biochar and metakaolin for 28-day flexural strength measurements, including treatment and control means.

Author	Ref	Material	${\mathit{X}}_{\mathit{t}}$	${\mathit{X}}_{\mathit{c}}$	${\mathit{S}\mathit{D}}_{\mathit{t}}^2$	${\mathit{S}\mathit{D}}_{\mathit{c}}^2$	ROM	$\mathit{S}\mathit{E}\left[\mathit{ln}\left(\mathit{R}\mathit{O}\mathit{M}\right)\right]$	95% CI Low	95% CI High
Falliano et al.	[43]	BC	8.69	8.00	0.20	0.16	1.09	0.042	1.00	1.17
Ling et al.	[25]	BC	6.74	6.00	0.16	0.12	1.12	0.048	1.02	1.23
Faleschini et al.	[34]	BC	1.26	1.10	0.01	0.01	1.15	0.076	0.99	1.33
Falliano et al.	[14]	BC	10.35	9.00	0.29	0.25	1.15	0.044	1.05	1.25
Falliano et al.	[15]	BC	9.42	8.00	0.29	0.25	1.18	0.049	1.07	1.30
Gasmi et al.	[36]	MK	12.00	10.00	0.12	0.09	1.20	0.024	1.14	1.26
Gunn et al.	[16]	BC	8.30	8.00	0.07	0.06	1.04	0.026	0.99	1.09
Gupta et al.	[26]	BC	8.90	8.70	0.13	0.12	1.02	0.033	0.96	1.09
Javed et al.	[37]	BC	7.21	6.80	0.05	0.05	1.06	0.026	1.01	1.12
Labianca et al.	[38]	BC	7.00	6.60	0.07	0.06	1.06	0.030	1.00	1.12
Ruviaro et al.	[39]	MK	22.56	18.75	1.67	1.44	1.14	0.037	1.06	1.22
Sirico et al.	[40]	BC	6.96	6.50	0.08	0.06	1.07	0.032	1.01	1.14
Yang et al.	[13]	BC	7.46	7.00	0.09	0.08	1.07	0.033	1.00	1.14
Zhang et al.	[42]	MK	9.80	8.10	0.04	0.03	1.21	0.017	1.17	1.25
Diniz et al.	[35]	MK	8.10	7.00	0.12	0.09	1.16	0.035	1.08	1.24
Zhao et al.	[44]	MK	8.80	8.00	0.09	0.08	1.10	0.028	1.04	1.16
Mishra et al.	[30]	MK	7.90	7.00	0.12	0.09	1.13	0.035	1.05	1.21
Dai et al.	[31]	MK	8.30	7.40	0.12	0.09	1.12	0.033	1.05	1.20
Marcyzyk et al.	[45]	MK	9.00	8.50	0.12	0.09	1.06	0.030	1.00	1.12
Thajeel et al.	[32]	MK	8.34	7.66	0.12	0.09	1.09	0.033	1.02	1.16
Duan et al.	[10]	MK	7.93	7.00	0.12	0.09	1.13	0.035	1.06	1.21

Table 3. ROM values of biochar and metakaolin for 28-day flexural strength measurements, including treatment and control means.

Author	Ref	Material	${\mathit{X}}_{\mathit{t}}$	${\mathit{X}}_{\mathit{c}}$	${\mathit{S}\mathit{D}}_{\mathit{t}}^2$	${\mathit{S}\mathit{D}}_{\mathit{c}}^2$	ROM	$\mathit{S}\mathit{E}\left[\mathit{ln}\left(\mathit{R}\mathit{O}\mathit{M}\right)\right]$	95% CI Low	95% CI High
Falliano et al.	[43]	BC	8.69	8.00	0.20	0.16	1.09	0.042	1.00	1.17
Ling et al.	[25]	BC	6.74	6.00	0.16	0.12	1.12	0.048	1.02	1.23
Faleschini et al.	[34]	BC	1.26	1.10	0.01	0.01	1.15	0.076	0.99	1.33
Falliano et al.	[14]	BC	10.35	9.00	0.29	0.25	1.15	0.044	1.05	1.25
Falliano et al.	[15]	BC	9.42	8.00	0.29	0.25	1.18	0.049	1.07	1.30
Gasmi et al.	[36]	MK	12.00	10.00	0.12	0.09	1.20	0.024	1.14	1.26
Gunn et al.	[16]	BC	8.30	8.00	0.07	0.06	1.04	0.026	0.99	1.09
Gupta et al.	[26]	BC	8.90	8.70	0.13	0.12	1.02	0.033	0.96	1.09
Javed et al.	[37]	BC	7.21	6.80	0.05	0.05	1.06	0.026	1.01	1.12
Labianca et al.	[38]	BC	7.00	6.60	0.07	0.06	1.06	0.030	1.00	1.12
Ruviaro et al.	[39]	MK	22.56	18.75	1.67	1.44	1.14	0.037	1.06	1.22
Sirico et al.	[40]	BC	6.96	6.50	0.08	0.06	1.07	0.032	1.01	1.14
Yang et al.	[13]	BC	7.46	7.00	0.09	0.08	1.07	0.033	1.00	1.14
Zhang et al.	[42]	MK	9.80	8.10	0.04	0.03	1.21	0.017	1.17	1.25
Diniz et al.	[35]	MK	8.10	7.00	0.12	0.09	1.16	0.035	1.08	1.24
Zhao et al.	[44]	MK	8.80	8.00	0.09	0.08	1.10	0.028	1.04	1.16
Mishra et al.	[30]	MK	7.90	7.00	0.12	0.09	1.13	0.035	1.05	1.21
Dai et al.	[31]	MK	8.30	7.40	0.12	0.09	1.12	0.033	1.05	1.20
Marcyzyk et al.	[45]	MK	9.00	8.50	0.12	0.09	1.06	0.030	1.00	1.12
Thajeel et al.	[32]	MK	8.34	7.66	0.12	0.09	1.09	0.033	1.02	1.16
Duan et al.	[10]	MK	7.93	7.00	0.12	0.09	1.13	0.035	1.06	1.21

3.3. Meta-Analysis Regression for Mechanical Properties

Meta-regression analysis was conducted to further evaluate the influence of MK and BC dosage levels on the mechanical performance of 3DCP, focusing on CS and FS. The results (Figure 3) highlight distinct optimal dosage ranges for each additive. For MK, both properties generally increased with dosage, with several studies converging around 15–20% replacement for CS (Figure 3b) and 10–15% for FS (Figure 3a), where the most significant improvements were observed. These trends align with MK’s established role in refining pore structure, enhancing pozzolanic activity, and improving interfacial bonding, collectively contributing to enhanced mechanical performance under various loading conditions. In contrast, BC showed no clear linear relationship with either CS or FS, reflecting the scattered outcomes across studies. Modest gains were often observed at 3–5% replacement for CS (Figure 3d) and 2–5% for FS (Figure 3c), whereas higher dosages generally led to inconsistent or adverse effects, likely due to increased porosity, weak particle–matrix interactions, and reduced workability.

The limited linearity observed for both materials reflects the substantial heterogeneity among existing studies, including variations in binder chemistry, admixture type and dosage, water-to-binder ratios, curing regimes, printing parameters, and testing protocols, as well as intrinsic characteristics of the supplementary materials themselves (e.g., fineness, porosity, and surface reactivity). These factors interact in complex, non-additive ways that cannot be fully captured by dosage alone, resulting in the non-linear and scattered regression trends obtained. Overall, the analysis suggests that optimum performance is generally achieved at 15–20% MK for CS and 10–15% for FS, and at 3–5% BC for CS and 2–5% for FS, with these dosage ranges consistently associated with favourable outcomes across multiple studies. However, further verification under standardised and controlled conditions is still required. The reductions in 3DCP performance observed when MK or BC dosages exceed these ranges are supported qualitatively by findings reported in the literature [14,15,30,31]. A key limitation of the present meta-regression model is that its linear structure cannot adequately capture the inherently non-linear dose–response behaviour of MK and BC. Their effects depend strongly on material-specific factors—such as feedstock properties, particle fineness, porosity, surface chemistry, and pyrolysis conditions for BC, and calcination temperature, amorphous content, and fineness for MK—which interact with mixture design parameters, admixture demand, water-to-binder ratio, binder chemistry, and printing conditions. These coupled influences lead to complex multiphase responses that cannot be fully represented by a simple linear model.

3.4. Printability and Dimensional Stability

Printability and dimensional stability are critical factors for the success of 3DCP, as they directly affect the quality, precision, and structural reliability of printed components. Printability refers to a material’s ability to undergo continuous and uniform extrusion, while dimensional stability ensures that the printed structure maintains its intended geometry without deformation, slumping, or collapse during and after printing [46].

Table 4. Effects of BC and MK on Printability and Dimensional Stability in 3DCP.

Ref	Material Type	Proportion	Rheological	Printability	Dimensional Stability	Optimal Conditions
[47]	BC	2%	↑ Build-up rate (22%) and thixotropy	↑ pumpability, extrudability, and buildability	↑ structural stability over time	2% BC optimal for thixotropy and buildability
[15]	BC	-	-	↑ dimensional stability with zero slump in extrusion	Zero slump after extrusion	-
[14]	BC	-	↑ Rheology and internal curing	↑ dimensional stability and cement hydration	↑ fresh state stability	-
[49]	BC + Polymer	-	↑ cohesion and densification	↑ ductility and post-cracking integrity	↑ matrix densification and stability	-
[31]	MK	10–20%	↑ particle packing, yield stress, and viscosity	↑ early-age buildability and shape retention	↓ deformation and ↑interlayer adhesion	10% MK optimal for balancing strength and printability
[10]	MK	10%	↑ yield stress, dynamic viscosity	Printable layers ↑ from 17 to 23	↑ green strength and stiffness	10% MK optimal for increased printable layers
[32]	MK + SF	(10% MK + 5% SF)	↑ cohesion and particle packing	↑ shape retention S1 = 0.99 with 72 printable layers	↑ shape retention and layer stacking	MK10SF5 (10% MK + 5% SF) ↑printability and strength
[30]	MK	Up to 20%	Slight ↑ in yield stress	↓ plastic cracking at 10% MK	↓ dimensional stability at high dosages	10% replacement is ideal while 20% leads to plastic cracking
[45]	MK	10%	↑ static yield stress and structural build-up	↑ buildability in alkali-activated slag systems	↑ shape retention	10% MK optimal in alkali-activated slag matrices
[35]	MK	Up to 30%	↑ viscosity and consistency	stable layers, ↑ printability	↑ layer consistency and printability	Up to 30% MK maintains printability without compromise
[44]	MK + Diatomite + Bentonite	-	↑ thixotropy	↓ deformation rate to 3.45%	↓ deformation, ↑ buildability	-

summarises the effects of BC and MK on printability and dimensional stability. Recent studies show that incorporating MK and BC substantially improves both properties. For instance, adding 2% BC increased the structural build-up rate of fresh mixtures by 22% after 40 min of resting, enhancing thixotropy and buildability. BC also improved pumpability and extrudability at early stages while supporting buildability in later stages [47,48]. BC-enriched mixtures demonstrated excellent dimensional stability, with no slump during or after extrusion, and facilitated internal curing due to its water absorption capacity, thereby promoting cement hydration and matrix densification [14,15,27]. Moreover, BC slightly reduced flowability but improved ductility when combined with polymeric reinforcement, maintaining structural integrity post-cracking under air-dry curing conditions [49]. These properties make BC-enriched mortars highly suitable for automated 3DP processes.

Similarly, MK improved particle packing, yield stress, and shape stability, enhancing early-age buildability. A ternary mix of 70% OPC, 20% GGBS, and 10% MK exhibited the highest yield stress and superior shape retention [30]. In alkali-activated slag mixtures, MK increased static yield stress and structural build-up, improving buildability; however, excessive MK (20%) caused plastic cracking, indicating that 10% is optimal for balancing strength, rheology, and printability [31]. Hybrid formulations, such as FA- or MK-based geopolymers with 5% cement, optimised setting times and mechanical performance, enabling precise 3DP under varying temperature and mixing conditions [45]. A mixture of 10% MK and 5% silica fume achieved superior printability, maintaining excellent shape retention, 72 printable layers, and enhanced cohesion, particle packing, and pozzolanic reactivity [32]. Incorporating 10% MK also increased static and dynamic yield stress and viscosity, improving thixotropic performance, buildability, green strength, stiffness development, and dimensional stability, with the maximum printable layers rising from 17 to 23 [10]. The synergistic effect of BC and MK creates a balanced rheological window, supporting smooth, continuous extrusion while providing sufficient early stiffness for stable layer printing. This enhances shape retention, vertical dimensional accuracy, and interlayer adhesion, producing mixtures that are both easy to print and resistant to deformation—ideal for scalable high-performance 3DCP systems.

A comparative assessment of the reviewed studies was conducted based on methodological rigour and key performance indicators, including rheology, printability, dimensional stability, CO₂ reduction, and statistical transparency (Table S2, Supplementary Materials). Twenty studies were examined: three classified as high rigour, seven as medium, and ten as low. High-rigour studies reported complete mix proportions, rheological protocols, quantitative data on CO₂ reduction, printability, dimensional stability, and statistical variability; medium-rigour studies provided partial reporting, omitting one or more indicators; low-rigour studies largely relied on qualitative descriptions or lacked sufficient methodological and quantitative detail. This variability in methodological quality was explicitly considered in interpreting the meta-analysis results.

3.5. Environmental Sustainability

The contributions of MK and BC to sustainable concrete production, including analysis of CO₂ emission reductions, are summarised in Table S4 of the Supplementary Materials. CO₂ reductions varied across studies, with BC showing the greatest impact—for example, achieving reductions of up to 43% while increasing the carbon sequestration potential [14,15,16].

MK primarily enhances sustainability by reducing clinker content in cementitious mixes. Since clinker production is the main source of CO₂ emissions in concrete, substituting it with MK directly lowers embodied carbon. For instance, replacing 50% of OPC with a blend of 30% calcined clay and 15% limestone reduced clinker content and CO₂ emissions by 30% [50]. MK is derived from abundant calcined clay, supporting resource efficiency and reducing reliance on energy-intensive raw materials. Its durability benefits further enhance sustainability by extending the service life of concrete structures and reducing maintenance and repair needs [51].

BC, in contrast, offers a direct carbon sequestration pathway. Produced via pyrolysis of biomass, BC stores stable biogenic carbon, allowing concrete to function as a carbon sink. Studies indicate that BC can reduce CO₂ emissions by up to 43% and enhance overall carbon sequestration capacity [15]. Incorporating BC into engineered cementitious composites can reduce total and specific carbon emissions by up to 58.3%, while also lowering material costs due to its lower unit price and reduced carbon tax liabilities, providing a carbon-negative and cost-effective solution for construction [52]. A 5% BC dosage improves hydration and water retention, whereas higher dosages (>5%) promote carbonation and enhance CO₂ uptake, though excessive amounts may compromise mechanical performance [53]. The use of BC valorises organic waste, transforming agricultural and forestry residues into valuable construction materials and contributing to circular economy objectives. Its integration also reduces cement demand, promoting sustainability through waste diversion and resource conservation [40,54].

Beyond the reported reductions in CO₂ emissions, the reviewed LCA studies provide further quantitative evidence of the environmental benefits associated with incorporating MK and BC in cementitious and 3DP systems. Several BC-modified mixes achieved substantial reductions in embodied carbon intensity, with savings of up to 570 kg CO₂-eq per tonne of mortar under accelerated carbonation curing—attributed to enhanced carbonation reactivity and improved mineralisation of biogenic carbon [13]. BC addition also lowered the overall ecological footprint of printable composites by 8.3% and increased carbonation uptake during curing by 46.2%, indicating greater long-term sequestration potential [14,47].

For MK, available LCA datasets consistently report considerably lower cradle-to-gate embodied carbon values compared with OPC, with figures as low as 0.05239 kg CO₂-eq per kg for MK-rich earth-based binders [34]. In alkali-activated and geopolymer systems, MK replacement levels of 50–55% not only reduced global warming potential but also decreased embodied energy relative to equivalent OPC mixes [36]. Collectively, these findings highlight that both materials can reduce environmental impacts while maintaining—or in some cases enhancing—performance in 3DCP and related applications. BC contributes through increased sequestration capacity, improved carbonation efficiency, and reduced clinker demand, whereas MK reduces embodied impacts due to lower reliance on high-temperature calcination and more efficient binder chemistry. Although multi-category LCA indicators such as eutrophication, acidification, embodied energy, and resource depletion remain sparsely reported, the available evidence confirms that both BC and MK offer measurable environmental advantages within 3DCP.

Figure 4 presents a flowchart synthesised from the reviewed literature and LCA principles. The framework outlines strategies for minimising the environmental footprint of 3DCP using MK and BC across three levels of intervention: optimised design strategies, sustainable mix proportioning, and printing-system configuration. Substituting OPC with these materials contributes to lower clinker-related emissions and reduced material waste while preserving printability and mechanical performance. Nonetheless, challenges remain, including the high energy demand for 3DP equipment and uncertainties linked to the large-scale implementation of MK and BC. Addressing these constraints will be essential to fully realise their environmental benefits and support the wider adoption of sustainable 3DCP technologies.

3.6. Durability Effects of Metakaolin and Biochar on 3DCP

Durability in 3DCP is a decisive factor influencing the long-term service life, structural integrity, and resistance to environmental degradation of printed elements, particularly given the layered microstructure and potential interfacial weaknesses inherent to the process. However, only a limited number of studies have systematically investigated the durability performance of 3DCP, and even fewer have examined the long-term effects of BC incorporation. This scarcity largely stems from the novelty of technology, as most existing studies assess durability qualitatively rather than through extended performance testing [55]. While BC has demonstrated promising durability improvements in conventional concrete, its application in 3DCP remains relatively underexplored, with only a few studies addressing its influence on durability-related properties.

Durability encompasses resistance to chloride ingress, carbonation, sulphate attack, freeze–thaw cycles, and drying shrinkage—all strongly influenced by binder composition, pore structure, and curing conditions. Studies have shown that incorporating BC, particularly at around 2% by binder mass, can markedly enhance durability through several mechanisms. Wang et al. [47] reported that 2% BC increased the structural build-up rate by approximately 22% after 40 min of resting, promoting denser interlayer consolidation and reducing continuous capillary pathways. Similarly, Sirico et al. [40] found that the same dosage reduced initial sorptivity by about 18% over two years, indicating long-term refinement of the capillary network. The water absorption capacity of BC facilitates internal curing, supporting extended cement hydration, reducing drying shrinkage, and improving resistance to moisture-driven deterioration. Its porous and functionalised surface also provides nucleation sites for hydration products, enhancing matrix densification. However, dosages exceeding 2% have been associated with reduced workability and potential microstructural discontinuities, underscoring the need for dosage optimisation to achieve durability gains [54,56]. Moreover, Shah Mansouri et al. [57] demonstrated that a 10% BC replacement reduced total shrinkage by approximately 24% after 56 days of curing, improving dimensional stability and mitigating early-age cracking in high-strength cementitious composites.

In parallel, MK has been widely recognised for its ability to densify the microstructure, enhance particle packing, and reduce permeability, thereby improving durability in 3DCP. Jaji et al. [51] reported that incorporating 10% MK in alkali-activated slag-based geopolymers reduced water absorption, improved the oxygen permeability index, and refined the pore structure compared to mould-cast counterparts, although interlayer regions retained higher porosity. Duan et al. [10] found that MK increased both static and dynamic yield stresses, improving early-age stiffness and reducing transport properties, while Zhang et al. [58] observed enhanced carbonation and sulphate resistance in MK-modified printed concretes. However, chloride ingress and freeze–thaw performance were still influenced by interlayer void connectivity. Diniz et al. [35] further reported that the synergistic interaction of MK with fine sand stabilised viscosity over time, maintained extrusion quality, and minimised interlayer porosity—all critical for improving resistance to the ingress of aggressive agents.

The combined use of MK with other SCMs has yielded particularly promising results. Thajeel et al. [32] showed that a blend containing 10% MK and 5% silica fume achieved high interlayer bond strength (2.14 MPa) and reduced transport pathways, contributing to enhanced durability under aggressive exposure conditions. Curing practices, however, exert a dominant influence on overall durability outcomes. Bekaert et al. [59] demonstrated that inadequate early-age curing significantly accelerated carbonation and chloride ingress in printed formwork elements, whereas protective measures such as immediate foil covering or nozzle-mounted side trowels for surface compaction substantially reduced permeability and extended service life. Similar findings by Bradshaw et al. [60] and Ricciotti et al. [20] emphasised that durability enhancement requires the combined optimisation of binder composition (MK and BC), printing parameters (time gap, layer height, extrusion speed), and rigorous curing conditions.

Figure 5 illustrates the durability mechanisms of 3DCP incorporating MK and BC. MK contributes to durability by consuming calcium hydroxide [Ca(OH)₂] and forming additional calcium aluminosilicate hydrate (C–A–S–H) gel, resulting in a denser microstructure and improved resistance to chemical attack. BC enhances durability through its filler effect, internal curing capacity, and pore refinement, which collectively reduce shrinkage and cracking. The combined action of MK and BC improves chloride and sulphate resistance, reduces permeability, enhances microstructural stability, and ultimately extends the service life of 3DCP structures.

3.7. Viability Analysis

Assessing the viability of MK and BC for 3DCP applications requires a comprehensive evaluation encompassing material performance, process optimisation, environmental impact, and scalability. This meta-analysis integrates current research findings to evaluate the potential of these materials in advancing both the sustainability and functional performance of 3DCP.

MK has consistently demonstrated its capacity to enhance the mechanical properties of 3DCP. Its high pozzolanic reactivity contributes to significant improvements in CS and FS, while its influence on rheological behaviour—particularly the increase in thixotropy and static yield stress—enhances shape retention and interlayer bonding during printing [10]. These improvements are crucial for mitigating common challenges such as geometric deformation, interlayer weaknesses, and shrinkage-induced cracking. Moreover, partial clinker replacement with MK reduces embodied carbon emissions, aligning with global sustainability targets for the construction sector [51].

BC, derived from renewable biomass, complements these benefits by providing additional functional and environmental advantages. At optimised dosages (1–3%), BC improves printability through enhanced moisture retention and internal curing, which collectively mitigate drying shrinkage and improve dimensional stability. Its porous structure supports these effects while also offering potential for carbon sequestration, thus reducing the overall carbon footprint of printed concrete. Furthermore, BC incorporation has been linked to improved durability, reflected by lower chloride diffusion and higher electrical resistivity—key indicators of prolonged service life in 3DP structures [14,41,49].

Curing conditions play a decisive role in determining the performance of 3DCP mixtures containing SCMs such as MK and BC. Under standard water curing, both materials promote consistent improvements in CS and FS by refining pore structure and sustaining hydration [30,61]. However, carbonation-based curing methods have shown even greater benefits. In MK-modified mixes, carbonation curing accelerates the formation of stable calcium carbonate and secondary calcium aluminosilicate hydrates, resulting in a denser matrix and faster strength development. In BC-modified systems, the porous carbon structure acts as a nucleation site under carbonation, enhancing hydration kinetics and facilitating CO₂ uptake. This translates into measurable gains in strength and durability, with studies reporting CS increases exceeding 30% compared to sealed curing, alongside reduced chloride permeability and increased electrical resistivity [33]. While carbonation curing enhances both mechanical performance and environmental outcomes via CO₂ sequestration, it also affects fresh-state behaviour, leading to rapid stiffening that can influence open time, interlayer bonding, and printability. Thus, although conventional curing ensures steady strength development, carbonation and accelerated carbonation curing present a dual advantage—improving structural performance while lowering the carbon footprint of BC- and MK-modified 3DCP.

The viability of these materials, along with key findings from the reviewed studies, is summarised in

Table 5. Viability assessment of metakaolin and biochar in 3DCP based on performance and sustainability indicators.

Author	Material	Curing Process	Rheology and Printing Parameter	Sustainability Benefits	Key Finding
[33]	BC	—	↑ rheology and extrusion	sequestering carbon and reducing cement	1–5% BC ↑ strength and rheology
[30]	MK	—	↑ shape retention and thixotropy	↓ CO2 and minimising resource depletion	MK use with OPC and GGBS ↑ printability
[61]	BC	—	↑ viscosity and consistency	Enables carbon sequestration, supports an 8.3% ↓ in carbon and ↓ cement usage.	↑ mechanical performance
[49]	BC	—	↑ viscosity and yield stress	Biomass reuse	Ductility ↓, mitigated with polymer reinforcement
[62]	BC	—	↑ buildability and reduces fluidity	↓ carbon footprint	10% BC ↑ green strength, but increases shrinkage
[14]	BC	CC	↑ extrusion stability	Utilises waste-derived biochar and ↓ CO2 emissions	5–23% BC ↑ strength
[63]	BC	—	↑ printability	Supports circular economy and sustainability goals	↑ shrinkage, hydration, and
[31]	MK	SC	↑ yield stress and pumpability	↓ energy demand	10% MK ↑ buildability, while 20% reduces strength and refining the microstructure
[45]	MK	SC	↑ buildability	Promotes the circular economy and ↓ the environmental waste	MK-fly ash-cement blends improve strength and control setting times
[32]	MK	SC	↑ shape retention, green strength, durability	↓ CO2 footprint and supports resource efficiency	10–15% replacement of cement with MK enhances strength and hydration
[10]	MK	SC	↑ yield stress and buildability, ↓ shrinkage	↓ PC usage and minimising resource depletion	5–10% MK ↑ yield stress, printability (17→23 layers), and early strength
[35]	MK	—	↑ shape retention and extrusion	Lowers the environmental impact	Up to 30% MK ↑ flow, CS, and buildability
[44]	MK	SC	Moderate ↑ in viscosity	↓ carbon emission and increasing resource efficiency	3% MK ↑ stability and ↓ deformation
[57]	BC	ACC	↑ rheology and extrusion	sequestering carbon and reducing cement	1–5% BC ↑ strength and rheology

. Mixes incorporating MK and FA have demonstrated additional benefits, including enhanced dimensional stability, accelerated setting times, and cost efficiency. Recent innovations in mix design, particularly those involving waste-derived materials and active rheology control, have further optimised both strength and sustainability performance. Collectively, these advances confirm that BC- and MK-based composites offer viable, sustainable solutions for producing complex, lightweight, and structurally reliable 3DCP elements.

Despite these promising outcomes, several challenges must be addressed for practical implementation. The availability and cost of high-quality MK may limit its use in certain regions, while BC properties can vary significantly depending on biomass source and pyrolysis conditions, underscoring the need for production standardisation. Moreover, the development of optimised mix designs that balance mechanical strength, extrusion quality, and sustainability metrics remains an active area of research. Details on cost-effectiveness and standardisation are provided in Section 3, Table S3 of the Supplementary Materials.

4. Statistical Validation and Robustness of Meta-Analytical Results

To ensure the validity and reliability of the meta-analytical findings regarding the mechanical performance of 3DCP incorporating BC and MK, a comprehensive statistical validation was conducted. This process involved evaluating potential sources of bias, inter-study heterogeneity, sensitivity to individual studies, and publication bias. Each aspect was systematically examined in the following subsections to confirm the robustness and generalisability of the pooled results.

4.1. Risk of Bias Assessment

A Risk of Bias (RoB) assessment was performed to evaluate the methodological rigour and internal validity of the studies included in this systematic review and meta-analysis on the mechanical performance of 3DCP incorporating BC and MK. Each study was assessed across six domains of bias, adapted for experimental materials research: selection bias (differences in allocation or sample preparation), performance bias (variations in mix design, curing conditions, or printing procedures), detection bias (inconsistencies in measurement techniques or instrumentation), attrition bias (incomplete data due to missing results or discarded tests), reporting bias (selective reporting of outcomes), and other biases (such as absence of control groups or lack of standardised testing conditions). A colour-coded matrix was developed to summarise the overall bias profile for each study—green indicating low risk, yellow moderate risk, and red high risk—allowing for a clear visual comparison of methodological robustness across the included works, as presented in Figure 6.

The analysis revealed that selection bias—primarily stemming from non-randomised grouping or incomplete description of mix preparation—and detection bias—due to insufficient detail on testing protocols or calibration procedures—were the most frequently identified high-risk categories. In contrast, reporting bias was generally low, reflecting that most studies provided satisfactory transparency in their data presentation and outcome reporting. This assessment is essential for contextualising the strength of the pooled estimates, identifying potential sources of systematic error, and highlighting areas where improved experimental design, documentation, and standardisation are needed to enhance the reliability of future research on sustainable 3DCP materials.

4.2. Publication Bias and Heterogeneity Analysis

To evaluate the presence of publication bias and ensure the reliability of the meta-analytical findings, a funnel plot was constructed using R software, displaying individual study effect sizes against their precision (1/SE). As illustrated in Figure 7, the distribution of studies appears largely symmetrical around the pooled effect size, particularly for those with higher precision, suggesting that significant publication bias is unlikely. Some degree of asymmetry is visible among studies with lower precision (i.e., higher standard errors), which may be attributed to random variation or selective reporting; however, it does not indicate systematic bias.

To statistically confirm this interpretation, Egger’s regression test was applied. The test yielded a t-value of 1.915 with 19 degrees of freedom and a p-value of 0.071. Since the p-value exceeds the conventional 0.05 threshold, there is no significant evidence of asymmetry, reinforcing the conclusion that the dataset is not substantially affected by publication bias. In parallel, heterogeneity across studies was assessed using Cochran’s Q statistic, the I² statistic, and between-study variance (τ²). The results indicate moderate heterogeneity (Q = 39.11, df = 20, p = 0.0029; I² = 48.86%; τ² = 0.020), reflecting variability in experimental conditions such as material sources, binder compositions, replacement ratios, printing techniques, curing conditions, and mechanical testing methods. Although such heterogeneity introduces limitations in directly comparing individual studies, it is common in construction materials research involving varied mix designs and test setups. The adoption of a random effects model effectively accommodates this variability, allowing for a more generalisable and statistically robust estimation of the true effect size. Collectively, these analyses confirm that the observed outcomes are consistent and not significantly influenced by bias or excessive heterogeneity, thereby strengthening the validity of the meta-analytical conclusions regarding the mechanical performance of 3DCP incorporating MK and BC.

4.3. Sensitivity Analysis

A leave-one-out sensitivity analysis was performed on all 21 included studies to assess the robustness of the pooled effect size and identify any influential datasets, as illustrated in Figure 8. Each study was sequentially omitted, and the overall effect size and heterogeneity statistics were recalculated. In nearly all cases, the recalculated p-values remained below 0.001, with minimal variation in standard errors (SE: 0.1119–0.1233) and heterogeneity indices consistently ranging from Q = 39.63–43.36 and I² = 49–54%. These results closely align with the primary heterogeneity observed in the main analysis (I² = 48.86%), thereby confirming the internal consistency and reliability of the findings.

The omission of Thajeel et al. (2025) [32] produced a notable deviation, with the standard error decreasing to 0.0654 and heterogeneity eliminated (Q = 12.57, p = 0.86; τ² = 0; I² = 0%). This indicates that the moderate heterogeneity observed in the overall analysis (I² = 48.86%) was primarily influenced by this single dataset, likely due to variations in mix design, experimental setup, or material characteristics relative to the other studies. Importantly, the exclusion of any individual study did not alter the direction or statistical significance of the pooled estimates, thereby reinforcing the robustness and credibility of the meta-analytical conclusions. Detailed outcomes of the sensitivity analysis are provided in Table S5 of the Supplementary Materials.

Collectively, the statistical validation procedures confirm the robustness and reliability of the meta-analytical findings. The risk of bias assessment revealed predominantly low risk across the included studies, indicating minimal methodological concerns. The funnel plot and Egger’s regression test (p > 0.05) demonstrated the absence of significant publication bias, while the leave-one-out analysis verified that the exclusion of any single study did not materially affect the magnitude or significance of the pooled effects. Although the removal of one study reduced heterogeneity substantially, the overall findings remained statistically stable, with low-to-moderate I² values consistently observed. These results collectively confirm the internal consistency, statistical robustness, and generalisability of the reported effects of BC and MK on the mechanical performance of 3DCP.

5. Discussion

The findings of this meta-analysis demonstrate the potential of MK and BC as sustainable additives for 3DCP, offering measurable improvements in mechanical performance, dimensional stability, and environmental outcomes. The pooled results indicate that incorporating BC increases CS by approximately 7% relative to control mixes, with low dosages (3–5%) being the most effective. This enhancement is largely attributed to pore refinement, nucleation effects, and internal curing mechanisms [27]. These trends align with the findings of Falliano et al. [14] and Ling et al. [25], who showed that low BC replacement levels (1–3%) improve CS and FS through filler action and microstructural densification, whereas higher dosages reduce performance. Both studies also reported that finer BC particles consistently outperform coarser fractions. The relative contribution of these mechanisms is strongly dependent on biochar’s physicochemical properties: finer, highly porous particles promote nucleation and hydration-product growth, whereas coarser and less porous particles primarily contribute through filler effects and modest pore refinement. The decline in performance at higher dosages underscores the need to optimise BC content, since excessive porosity and shrinkage can offset mechanical gains.

For MK, the meta-analysis confirms its effectiveness as a high-performance SCM, with optimal replacement levels of 10–15% producing significant improvements in both CS and FS. These benefits stem from MK’s high pozzolanic reactivity, which accelerates calcium silicate hydrate formation and refines microstructural porosity. Studies by Mishra et al. [30] and Dai et al. [31] corroborate these findings, demonstrating improvements in workability, strength, and buildability in both OPC-based and geopolymer systems. Beyond the optimal range, however, higher MK dosages lead to reduced extrudability, increased cracking, and diminished mechanical gains, indicating a practical upper limit for its use in 3DCP.

A comparative assessment of BC and MK highlights their complementary advantages. BC contributes to carbon sequestration while enhancing durability through reduced porosity and improved resistance to chloride ingress and carbonation. As a dual-function additive, BC is particularly relevant to carbon-neutrality targets, with optimised mixes showing up to 43% reductions in CO₂ emissions, along with improvements in durability, rheology, and shrinkage resistance in 3DCP [14,15,16]. MK, although not carbon-negative, reduces clinker demand and provides reliable improvements in strength, dimensional stability, and printability—particularly in geopolymer and blended systems. The findings also highlight the influence of curing regimes on performance. Studies employing carbonation curing reported accelerated strength development and reduced porosity in both BC- and MK-modified mixes, although increased stiffening may limit open time and interlayer bonding.

Statistical validation confirms the robustness of the meta-analytical results. The risk-of-bias assessment indicated that most studies were methodologically sound, with only minor issues related to incomplete mix descriptions or inconsistent testing procedures. No evidence of publication bias was detected, as shown by the symmetrical funnel plot and non-significant Egger test. The observed moderate heterogeneity (I² = 48.86%) reflects expected variations in mix design, material properties, curing conditions, and testing protocols typical of materials science research. Leave-one-out sensitivity analysis showed that no single study disproportionately influenced the pooled effect sizes or their statistical significance. Collectively, these results affirm the reliability of the findings and underscore the consistent mechanical improvements achieved through the incorporation of MK and BC in 3DCP.

6. Practical Implication

3DCP offers substantial advantages for the construction industry, including accelerated production, reduced material waste, and enhanced design freedom. By minimising resource consumption and enabling the fabrication of complex, customised geometries, 3DCP aligns closely with sustainable construction goals. The integration of sustainable additives such as MK and BC further enhances the mechanical performance and durability of printed elements, strengthening the case for their use in structural and semi-structural applications.

The findings of this meta-analysis provide actionable guidance for both practitioners and researchers. The demonstrated improvements in CS and FS confirm that MK and BC are viable SCMs capable of producing high-performance 3DCP mixes. The pooled ROM values and their narrow, statistically significant confidence intervals indicate that the strength enhancements are consistent and robust across diverse studies. Importantly, these improvements are achieved without compromising printability or dimensional stability.

For practitioners, the identified optimum dosage ranges—15–20% MK for CS, 10–15% MK for FS, and 2–5% BC for mechanical gains—offer clear, evidence-based guidance for mix design optimisation. These dosage ranges can reduce reliance on trial-and-error approaches and support more efficient development of printable formulations. The consistency reflected in the confidence intervals also suggests that the observed gains can be expected across varied curing regimes, material sources, and printing parameters, further supporting their suitability for real-world 3DCP applications.

From an environmental perspective, BC provides direct contributions to decarbonisation through carbon sequestration and enhanced carbonation efficiency, making it particularly relevant for achieving net-zero and carbon neutrality targets. MK contributes to sustainability by reducing clinker demand and improving dimensional stability, thereby extending service life and reducing maintenance requirements for printed components.

At the industry and policy levels, the quantitative evidence presented here supports the development of standards, guidelines, and certification frameworks for eco-efficient 3DCP materials. The demonstrated performance benefits highlight an opportunity for construction firms to align with sustainable building certifications and emerging environmental regulations while lowering embodied carbon.

Overall, these insights strengthen the case for adopting MK- and BC-modified 3DCP in both experimental and commercial contexts, helping bridge the gap between laboratory innovation and scalable, sustainable construction practices.

7. Conclusions

This study systematically evaluated the role of metakaolin (MK) and biochar (BC) in enhancing the performance and sustainability of 3D concrete printing (3DCP). A meta-analysis of peer-reviewed studies published between 2015 and June 2025 quantified their effects on key mechanical properties—compressive strength (CS) and flexural strength (FS)—and qualitatively assessed their influence on dimensional stability, printability, and environmental performance. By consolidating fragmented evidence and incorporating heterogeneity analyses, this review accounts for variability in mix designs, curing regimes, printing parameters, and testing procedures.

The findings demonstrate that MK and BC are promising eco-efficient supplementary cementitious materials (SCMs) that offer distinct yet complementary benefits. MK enhances structural integrity, rheology, and dimensional stability, whereas BC contributes to carbon sequestration, improved durability, and optimised fresh-state behaviour. Together, these materials offer a strong evidence base for advancing low-carbon, high-performance 3DCP. The key conclusions are as follows:

• The overall meta-analysis revealed statistically significant improvements in mechanical performance, with a pooled ratio of means (ROM) of 1.12 (95% CI: 1.10–1.15), indicating consistent gains in CS and FS and supported by low-to-moderate heterogeneity (I² = 48.86%).
• Biochar incorporation significantly enhanced mechanical performance, with pooled ROM values of 1.07 (95% CI: 1.01–1.14) for CS and 1.09 (95% CI: 1.01–1.18) for FS—representing 7% and 9% improvements over unmodified mixes, respectively.
• Metakaolin exhibited a stronger influence, yielding a 21% increase in CS (ROM = 1.21, 95% CI: 1.15–1.27) and a 13.4% improvement in FS (ROM = 1.13, 95% CI: 1.07–1.20), confirming its effectiveness as a high-performance SCM for 3DCP.
• Optimal MK dosages were identified as 15–20% for CS and 10–15% for FS, reflecting enhanced pozzolanic reactivity, pore-structure refinement, and interfacial bonding.
• Optimal BC dosages were found to be 3–5% for CS and 2–5% for FS. These levels support pore refinement and carbon-sequestration benefits, while higher dosages tend to reduce workability and increase porosity, leading to inconsistent or adverse performance outcomes.
• Environmental benefits were evident for both additives: BC-enabled mixes achieved CO₂-emission reductions of up to 43%, while MK reduced clinker consumption and supported low-carbon construction through more efficient binder chemistry.
• Statistical validation procedures—including the symmetrical funnel plot and non-significant Egger test—confirmed the absence of notable publication bias, reinforcing the robustness and reliability of the meta-analytical findings.
• Both MK and BC are confirmed as eco-efficient modifiers that enhance structural, durability, and environmental performance in 3DCP, offering complementary routes toward sustainable, low-carbon digital construction.

Overall, this work provides a comprehensive evidence base for the adoption of MK and BC in 3DCP and highlights their potential to accelerate the shift toward sustainable, high-performance, and climate-resilient construction practices.

8. Limitations and Future Recommendations

While this study offers consolidated insights into the role of MK and BC in 3DCP, several limitations must be acknowledged. A major constraint is the limited availability of quantitative data on printability, dimensional stability, and long-term durability. This is largely due to the lack of standardised testing protocols in 3DCP, its status as an emerging technology, and the relatively recent introduction of biochar into 3DCP research. Consequently, the ability to draw generalisable conclusions on long-term performance and service life remains restricted.

Substantial heterogeneity across studies—including variations in mix designs, curing conditions, printing parameters, nozzle geometry, layer height, and testing procedures—further complicates direct comparison and reduces reproducibility. These inconsistencies hinder the establishment of universally accepted mix design guidelines. Additionally, current evidence largely examines MK and BC independently; no studies have systematically evaluated their combined or synergistic effects, representing a significant research gap with potential implications for printability, mechanical performance, and durability.

From a sustainability perspective, most LCA studies on MK- and BC-modified 3DCP adopt limited cradle-to-gate boundaries. Key life-cycle stages—including raw material transport, energy consumption during printing, operational performance, and end-of-life scenarios—are seldom assessed, limiting the ability to deliver holistic environmental evaluations. Moreover, existing sustainability claims are rarely contextualised against benchmark materials such as OPC-based 3DCP or alternative low-carbon binders like LC³.

To address these limitations and maximise the performance and sustainability impacts of MK and BC incorporation, the following targeted recommendations are proposed:

• Establish standardised testing and printing protocols (e.g., nozzle geometry, layer height, deposition rate, curing conditions) to improve reproducibility and enable meaningful comparison across studies.
• Conduct long-term durability assessments under realistic exposure conditions (freeze–thaw, chloride ingress, carbonation, sulphate attack) to evaluate service life performance.
• Design controlled factorial studies to investigate synergistic effects of combined MK and BC incorporation on mechanical, rheological, and durability properties.
• Undertake cradle-to-grave LCAs incorporating transport, printing-energy demand, operational behaviour, and end-of-life processes to provide comprehensive sustainability benchmarks.
• Broaden comparative analyses to include other eco-efficient additives and alternative binders (e.g., LC³, geopolymers, fly ash, recycled powders) to contextualise MK and BC performance.
• Advance rheological optimisation studies to establish robust correlations between fresh-state properties (yield stress, open time, buildability) and hardened-state performance in 3DCP.
• Investigate scalability challenges, including pumpability, extrusion stability, and structural build-up, to support industrial-scale implementation.
• Assess economic feasibility and cost–benefit trade-offs, particularly balancing emissions reductions with potential increases in processing or material costs.
• Develop hybrid composites incorporating multiple eco-efficient modifiers to enhance strength, buildability, and sustainability simultaneously.
• Expand multi-performance optimisation frameworks integrating mechanical, rheological, durability, and environmental criteria to guide holistic mix design for 3DCP.

Collectively, addressing these research directions will help advance the scientific understanding, practical implementation, and sustainability potential of MK- and BC-modified 3DCP.