Investigation of Waste Steel Fiber Usage Rate and Length Change on Some Fresh State Properties of 3D Printable Concrete Mixtures

Abstract

In this study, the effects of waste steel fiber and high volume blast furnace slag (BFS) substitution on rheological properties, thixotropic behavior and carbon emission were investigated in order to increase the sustainability of three-dimensional (3D) printable concrete (3DPC). Cement was replaced with BFS at 0%, 25%, 50% and 75% by volume, while waste steel fibers were added to the mixtures at three different lengths (5, 10, 15 mm) and volumetric ratios (0.5% and 1.0%). A total of 39 mixtures were optimized with respect to extrudability, buildability and shape stability criteria, and their rheological and thixotropic properties were characterized by a modified rheometer procedure. Results showed that 50% BFS substitution reduced dynamic yield stress and viscosity by 69% and 52%, respectively, and eliminated the need for a water-reducing admixture. 75% BFS substitution improved structural build-up (A_thix) but required 6% silica fume. The fiber effect interacted with length and BFS content, with short fibers increasing rheological resistance, while the effect of long fibers decreased in mixtures with high BFS. The carbon emission assessment revealed that 75% BFS substitution provided an outstanding CO₂ reduction of up to 71% compared to the control mix. These findings prove that high-volume BFS and waste fibers are an effective strategy to optimize rheological performance and environmental impact for sustainable 3D concrete printing.

1. Introduction

Recently, the construction industry has shown increasing interest in three-dimensional (3D) concrete printing technologies due to quests to increase production efficiency, reduce labor costs, and expand design flexibility [1,2,3]. Working on the principle of additive manufacturing, 3D printable concrete (3DPC) technology not only minimizes material waste by eliminating the need for molds but also enables the production of structural elements with complex geometric shapes [4,5].

Existing literature reveals that the performance criteria that 3DPC mixtures must meet in their fresh state differ significantly from those of conventional concretes [6,7]. These criteria include extrudability, which refers to the ability of the mixture to be extruded continuously and homogeneously in layers [8]; buildability, which is defined as the structural strength of each deposited layer to support the load of subsequent layers [9]; and form stability, which is defined as the ability of the printed element to maintain its design geometry without deterioration [10]. Meeting these fundamental requirements is directly related to the rheological properties and thixotropic behavior of the mixture. Rheological parameters play a decisive role throughout the printing process, from the mixtures flow behavior during pumping to its interlayer load-bearing capacity and shape stability [11]. Therefore, optimizing rheological and thixotropic properties is crucial for a successful 3D printing process [12,13].

Avoiding the use of coarse aggregate in the production of 3DPC mixtures is a fundamental requirement for achieving these fresh-state properties [14]. However, the lack of an aggregate skeleton necessitates the use of high amounts of binder, which leads to increased cement content [15]. High cement usage poses a significant environmental sustainability problem due to increased production costs, high energy consumption, and associated CO₂ emissions. Industrial-scale adoption of 3DPC technology requires mitigating these negative environmental impacts. Therefore, reducing cement consumption and utilizing industrial waste materials is crucial for improving the economic and ecological sustainability of 3DPC mixtures [16]. Cement production is known to be a major source of global greenhouse gas emissions due to its high energy requirements and fossil fuel consumption [17]. Current literature indicates that global cement production accounts for approximately 8% of total CO₂ emissions [18]. In this context, the use of industrial by-products and waste materials such as blast furnace slag (BFS) and waste steel fiber instead of cement holds significant potential for reducing the carbon footprint and supporting circular economy principles in the construction sector [19,20]. The use of these substitute materials in 3DPC mixtures can make positive contributions not only to reducing production-related CO₂ emissions but also to improving mechanical performance and economic feasibility [21].

A byproduct of the iron and steel industry, BFS is a mineral additive widely used as a cement replacement due to its high calcium oxide (CaO) content [22]. Research shows that BFS can be effectively used as a replacement for Portland cement even at high substitution rates of up to 80–90% by mass [23,24]. Using BFS instead of cement significantly contributes to environmental sustainability by directly reducing carbon emissions from cement production [25].

Xu et al. [26] reported that BFS can be used in 3D-printable systems at up to 40% by weight of the binder, and that substituting 20% BFS increases the fluidity of the mixture, but that fluidity decreases at higher substitution rates. This behavior is explained by the fact that at low rates, the relatively smooth surface morphology of BFS particles reduces friction and reduces water requirements. At higher rates, the water demand increases due to the increased specific surface area, negatively affecting fluidity. In a study conducted by Yu et al. [27], it was reported that up to 10% by weight BFS substitution increased the static yield stress, but a significant decrease was observed in this value when the substitution rate was increased to 40%. The same study also emphasized that BFS substitution negatively affected the thixotropic behavior of the mixture, regardless of the usage rate. Similarly, in a study conducted by Bayat and Kashani [28], it was determined that 10% BFS substitution increased the static yield stress, but this value decreased when the substitution rate was increased to 20%. In a study conducted by Panda et al. [29], it was reported that the static yield stress increased with the replacement of cement with slag. In a study conducted by Arularasi et al. [30], in ternary blend paste mixtures (OPC + fly ash + BFS), cohesion improved with increasing BFS content, while cohesion decreased with increasing fly ash content.

Waste steel fibers, mechanically recovered from end-of-life tires, are a promising secondary raw material for developing sustainable building materials. Literature indicates that these fibers can be used as distributed reinforcement in concrete, limiting crack propagation and improving the mechanical performance of composite materials (especially tensile strength and toughness) [31,32]. The bridging effect provided by fibers in cementitious matrices has been reported to increase the material’s tensile capacity and energy absorption capacity, as well as provide crack control [33]. Fiber reinforcement is known to increase interlayer adhesion strength, particularly in 3DPC mixtures. The use of waste steel fibers in these mixtures offers both economic and environmental benefits. However, the literature frequently reports that the addition of steel fibers generally has a limiting effect on the rheological properties of fresh concrete. Arunothayan et al. [34] stated that an increase in fiber volume fraction caused an increase in both static and dynamic yield stresses, as well as an increase in apparent viscosity, which negatively affected the workability of the mixture. Alberti et al. [35] similarly showed that fiber addition reduced the fluidity of self-compacting concrete. These findings indicate that the performance advantages to be obtained from the integration of waste steel fibers into 3DPC mixtures should be evaluated by carefully managing these negative effects on rheological properties. The literature findings summarized above indicate that industrial wastes can be used to make 3DPC mixtures more sustainable, but the number of existing studies on this subject is limited. In the current study, waste steel fibers recovered from tires were used at two different volumetric fractions (0.5% and 1.0%) and three different lengths (5, 10, and 15 mm). Additionally, the effects of replacing cement with BFS at 0, 25, 50, and 75% volume were investigated. Mixing ratios were optimized based on extrudability, buildability, and shape stability criteria. The developed mixtures were comprehensively evaluated for their rheological properties, thixotropic behavior, and carbon emission potential.

2. Materials and Methods

2.1. Materials

CEM I 42.5R type Portland cement (Vezirhan Cement in Bilecik/Türkiye) and blast furnace slag (BFS) were used as binders in the preparation of 3DPC mixtures. The chemical, physical, and mechanical properties of the materials used, as provided by the manufacturer, are shown in

Table 1. Chemical composition, physical and mechanical properties of binder materials.

Oxides (%)		Cement	Blast Furnace Slag
SiO2		18	35.5
Al2O3		4.75	12.4
Fe2O3		3.58	1.5
CaO		63	38.9
MgO		1.4	5.05
Na2O + 0.658 K2O		0.7	1.07
SO3		3.11	1.67
Specific gravity		3.06	2.5
Specific Surface Area (cm2/g)		3441	4950
Compressive Strength (MPa)	7-day	42.8	-
	28-day	51.8	-
Pozzolanic Activity Index (%)	28-day	-	80
	90-day	-	90
Setting Time (min)	İnital	170	-
	Final	240	-

.

Some properties of the water-reducing admixture used to ensure the required workability in 3DPC mixtures, shared by the manufacturer, are shown in

Table 2. Some properties of water-reducing admixture.

Admixture	Density (g/cm3)	Solid Content (%)	pH	Chlorine Content (%)	Alkaline Content, Na2O (%)
Polycarboxylate-ether based high range water reducing	1.060	32	2–5	<0.1	<10

.

River sand with a maximum particle diameter of 2 mm (D_max) was used as the fine aggregate. The specific gravity and water absorption capacity of the aggregate were calculated as 2.54 and 0.4%, respectively, in accordance with the EN 1097-6 Standard.

The study also used waste steel fibers recovered from end-of-life tires in three different lengths (5, 10, and 15 mm) and two different volume fractions (0.5% and 1.0%). Some properties of the fibers are given in

Table 3. Some properties of waste steel fiber.

Fiber Type	Fiber Length (mm)	Tensile Capacity (MPa)	Modulus of Elasticity (MPa)	Specific Gravity
Steel	5, 10, 15	1500	200,000	7.8

. The scissors and cut fibers used to cut the waste fibers to the appropriate length are shown in Figure 1.

2.2. Preparation of Mixtures and Method

3DPC mixtures that met the extrudability, buildability and shape stability criteria were optimized based on the procedure suggested by Şahin et al. [36,37,38,39].

The mixture preparation process was carried out in three stages, as schematically illustrated in Figure 2. In the first stage, Portland cement, BFC, aggregate, and waste steel fibers were dry mixed for 30 s at 62.5 rpm. In the second stage, water and, if necessary, a water-reducing admixture were added, and the mixing process continued for 1 min at 62.5 rpm. In the final stage, the mixture was mixed for 2 min at 125 rpm to achieve a homogeneous structure, and the pressing process was then initiated. The mixtures were printed immediately after preparation. The printing speed is 16 mm/s.

2.2.1. Rheological Measurements

Rheological measurements of the mixtures were taken immediately after production using an MCR52 (Anton Paar) type rotational rheometer (Anton Paar GmbH in Graz, Austria) with an 8 mm ball diameter. To determine the rheological parameters, a modified measurement protocol consisting of 7 periods, whose flow chart is given in Figure 3, was applied. This protocol was developed by modifying the methods proposed by Mardani-Aghabaglou [40] and Yao et al. [41] to better characterize the structural recovery behavior of 3DPC. This rheological measurement method is based on previous experience of the researchers [36,37,38,39].

The rheological measurement protocol consists of the following steps:

Period 1 (Shear History Removal): To remove past shear stress (shear history) generated during mixing in the mixer, measurements were taken for 30 s at a constant strain rate of 5 s⁻¹.

Period 2 (Rise Curve): To establish the rise portion of the yield curve, the deformation rate was linearly increased from 0 to 30 s⁻¹ over 150 s, and data were recorded at 5 s intervals.

Period 3 (Descent Curve): To establish the fall portion of the yield curve, the deformation rate was linearly decreased from 30 to 0 s⁻¹ over 150 s, and data were recorded at 5 s intervals. The dynamic yield stress and apparent viscosity values of the 3DPC mixtures were calculated by fitting the raw data obtained from this period to the Herschel-Bulkley model shown in Equation (1).

Period 4 (Resting-Structural Recovery): The sample was held for 30 s without any deformation to allow for structural recovery before static measurement.

Period 5 (Initial Static Measurement): To measure the initial static yield stress of the material, measurements were taken at a constant, low strain rate of 0.02 s⁻¹ for 30 s (at 2-s intervals, a total of 15 measurements).

Period 6 (Structural Recovery): To measure the thixotropic structural recovery rate of the mixtures, the sample was held for 480 s without any deformation.

Period 7 (Final Static Measurement): To measure the static yield stress at the end of the holding period, measurements were taken again at a constant strain rate of 0.02 s⁻¹ for 30 s (at 2-s intervals, a total of 15 measurements).

The obtained yield curve data were analyzed using the Herschel-Bulkley model Equation (1) and the dynamic yield stress and viscosity values were calculated.

τ = τ₀ + b(γ˙)^p

(1)

where, τ = Shear stress (Pa), τ₀ = yield stress (Pa), b = Herschel-Bulkley consistency coefficient, γ˙ = Deformation rate (s⁻¹) and p = Herschel-Bulkley index.

2.2.2. Thixotropic Measurements

The thixotropic behavior of the mixtures was quantitatively evaluated using three different methods to reflect both dynamic and static properties:

• Dynamic Structural Build-up (D-SBU): This approach, proposed by Zhang et al. [42], was calculated using data obtained from dynamic measurements Equation (2). This method represents the structural state of the material after the dynamic shear history.

  $$D-SBU={\displaystyle \frac{{\tau }_{3.p}}{{\tau }_{2.p}}}$$

  (2)
  Here, τ_3.p represents the dynamic yield stress (Pa) obtained from the 3rd period (descent curve) and τ_2.p from the 2nd period (ascent curve).
• Structural Build-Up Development (A_thix): Calculated with data obtained from static measurements to characterize the rate of structural strength increase observed during a given rest period Equation (3).

  $$A_{thix}={\displaystyle \frac{{\tau }_{s,f}-{\tau }_{s,i}}{t_d}}$$

  (3)
  Here, A_thix represents the structural build-up development (Pa/s), τ_(s,f) represents the static yield stress value (Pa) obtained from the 7th period, τ_(s,i) represents the static yield stress value (Pa) obtained from the 5th period, and t_d represents the waiting time (s).
• Thixotropic Index (I_thix): The method proposed by Qian and Kawashima [43] was used to characterize the time-dependent behavior of shear stress under a constant deformation rate Equation (4). This index represents the ratio of the maximum shear stress required to initiate flow to the shear stress in steady-state flow.

  $$I_{thix}={\displaystyle \frac{{\tau }_i}{{\tau }_e}}$$

  (4)
  Here, I_thix represents the thixotropic index, τ_i represents the shear stress (Pa) required to initiate flow, and τ_e represents the steady-state flow shear stress (Pa).

2.2.3. Carbon Emission Assessment

The carbon footprint of the mixtures produced within the scope of the study was calculated by considering the embodied carbon values per unit weight (kgCO₂/kg) of each component. A sensitivity analysis was conducted using a range of the lowest and highest values reported in the literature for carbon emission calculations. The embodied carbon value ranges used and the relevant references are provided in

Table 4. Embodied CO₂ emission and embodied energy factors used within the scope of the study.

Mixing Component	Embodied Carbon (kgCO2/kg)
	Minimum Value	Maximum Value
Cement	0.804 [44]	0.94 [45]
BFS	0.00 [46]	0.07 [47]
Silica Fume	0.014 [48]	0.024 [49]
Fine Aggregate	0.0026 [49]	0.017 [44]
Water	0 [47]	0.001 [50]
Steel Fiber	0.43 [47]	1.59 [51]
Water Reducing Admixture	0.0000052 [52]	2.388 [44]

. Total carbon emissions per 1 m³ of concrete were calculated using wet mix ratios and Equation (5).

$$c{o}_2={\Sigma }_{i=1}^n\left(W_{i}xc{o}_{2i-\epsilon }\right)$$

(5)

Here, CO₂ is the total embodied CO₂ value of 1 m³ of concrete. Its unit is kgCO₂/m³. n is the total raw material in the mix. W_i is the total amount of material (in kg) required to produce 1 m³ of concrete. CO_2i−e is the equivalent CO₂ value of material i (in kg CO₂/kg).

3. Results and Discussion

3.1. Mixing Ratios

Within the scope of the study, optimum 3DPC mixtures were developed that met printability criteria (extrudability, buildability, and shape stability) for each substitution level using 0, 25, 50, and 75% of the cement volume. The water/binder ratio (by volume) was kept constant at 0.40 in all mixes, and a total of 39 different mixes were formulated by varying the sand/binder ratio, aggregate amount, fiber parameters (length and volume ratio), and superplasticizer dosage.

Table 5. Amounts of materials used in the production of 1 m³ 3DPC mixture.

Mixture Name	Mixture No	Cement (kg/m3)	BFS (kg/m3)	Aggregate			Water Reducing Admixture (kg/m3)	Fiber Length (cm)	Fiber Usage Amount (kg/m3)	Silica Fume (kg/m3)	w/b	Extrudability	Buildability	Shape Stability
				Dmax (mm)	Specific Gravity	Dosage (kg/m3)
K	1	700	0	2	2540	1216.1	5	0	0	0	1.74	* 0	0	0
	2	700	0	2	2540	1209	3	0	0	0	1.73	** 1	1	0
	3	700	0	2	2540	1211.3	2	0	0	0	1.73	1	1	1
K-0.5-0.5	4	700	0	2	2540	1198.6	2	0.5	39	0	1.71	1	1	1
K-1-0.5	5	700	0	2	2540	1185.8	2	0.5	78	0	1.69	1	1	1
K-0.5-1	6	700	0	2	2540	1198.6	2	1	39	0	1.71	1	1	1
K-1-1	7	700	0	2	2540	1185.8	2	1	78	0	1.69	1	1	1
K-0.5-1.5	8	700	0	2	2540	1198.6	2	1.5	39	0	1.71	1	1	1
K-1-1.5	9	700	0	2	2540	1185.8	2	1.5	78	0	1.69	1	1	1
25	10	525	143	2	2540	1211.3	2	0	0	0	1.81	1	0	0
	11	525	143	2	2540	1211.8	1.8	0	0	0	1.81	1	0	0
	12	525	143	2	2540	1212	1.65	0	0	0	1.81	1	1	0
	13	525	143	2	2540	1212.5	1.5	0	0	0	1.82	1	1	1
25-0.5-0.5	14	525	143	2	2540	1199.8	1.5	0.5	39	0	1.80	1	1	1
25-1-0.5	15	525	143	2	2540	1187	1.5	0.5	78	0	1.78	1	1	1
25-0.5-1	16	525	143	2	2540	1199.8	1.5	1	39	0	1.80	1	1	1
25-1-1	17	525	143	2	2540	1187	1.5	1	78	0	1.78	1	1	1
25-0.5-1.5	18	525	143	2	2540	1199.8	1.5	1.5	39	0	1.80	1	1	1
25-1-1.5	19	525	143	2	2540	1187	1.5	1.5	78	0	1.78	1	1	1
50	20	350	285.9	2	2540	1212.5	1.5	0	0	0	1.91	1	0	0
	21	350	285.9	2	2540	1213.8	1	0	0	0	1.91	1	1	0
	22	350	285.9	2	2540	1215	0.5	0	0	0	1.91	1	1	0
	23	350	285.9	2	2540	1216.1	0	0	0	0	1.91	1	1	1
50-0.5-0.5	24	350	285.9	2	2540	1203.4	0	0.5	39	0	1.89	1	1	1
50-1-0.5	25	350	285.9	2	2540	1190.6	0	0.5	78	0	1.87	1	1	1
50-0.5-1	26	350	285.9	2	2540	1203.4	0	1	39	0	1.89	1	1	1
50-1-1	27	350	285.9	2	2540	1190.6	0	1	78	0	1.87	1	1	1
50-0.5-1.5	28	350	285.9	2	2540	1203.4	0	1.5	39	0	1.89	1	1	1
50-1-1.5	29	350	285.9	2	2540	1190.6	0	1.5	78	0	1.87	1	1	1
75	30	175.2	429.4	2	2540	1215.5	0	0	0	0	2.01	1	0	0
	31	175.2	377.9	2	2540	1215.5	0	0	0	43.3	2.04	0	0	0
	32	175.2	412.2	2	2540	1215.5	0	0	0	14.4	2.02	1	1	0
	33	175.2	395	2	2540	1215.5	0	0	0	28.9	2.03	1	1	1
75-0.5-0.5	34	175.2	395	2	2540	1202.8	0	0.5	39	28.9	2.01	1	1	1
75-1-0.5	35	175.2	395	2	2540	1190	0	0.5	78	28.9	1.99	1	1	1
75-0.5-1	36	175.2	395	2	2540	1202.8	0	1	39	28.9	2.01	1	1	1
75-1-1	37	175.2	395	2	2540	1190	0	1	78	28.9	1.99	1	1	1
75-0.5-1.5	38	175.2	395	2	2540	1202.8	0	1.5	39	28.9	2.01	1	1	1
75-1-1.5	39	175.2	395	2	2540	1190	0	1.5	78	28.9	1.99	1	1	1

* if suitable. ** if not suitable.

summarizes the material quantities used for 1 m³ of mix and the printability performance of each mix.

Two requirements for print quality have been proposed: extrudability and buildability. All of these requirements must be met by a mixture with acceptable print quality. After the print quality assessment, the shape stability of the accepted mixture should be evaluated using Equation (6). It has been reported that a 3DPC mixture is acceptable in terms of shape stability if this value is greater than 95% [37].

$$SS\left(\%\right)={\displaystyle \frac{b_{layer}}{b_{nozzle}}}·100$$

(6)

According to Table 5, printability criteria are shown as 1 if suitable and 0 if not suitable.

The naming of the mixtures was based on the BFS substitution rate, fiber volume fraction, and fiber length. For example, the blend without BFS (Control), with 0.5% fiber content and 5 mm fiber length was named K-0.5-0.5, while the blend with 25% BFS, 1% fiber content, and 15 mm fiber length was named 25-1-1.5.

Control Series (0% BFS) Optimization:

The optimization process began with a reference control mix (Mix No. 1) containing 5 kg/m³ of water-reducing admixture and river sand with a maximum grain diameter of 2 mm. Although this mix met the extrudability criteria, it failed to meet the buildability criteria due to excessive fluidity. Reducing the water-reducing admixture dosage to 3 kg/m³ (Mix No. 2) improved extrudability and buildability, but the shape stability criteria were not met due to the spreading of the substrate during the printing of the second layer. Finally, reducing the dosage to 2 kg/m³ (Mix No. 3) resulted in the optimum control mix that met all printability criteria (Figure 4).

As reported in the literature [53,54,55], it was observed that the addition of fibers (Mix No. 4–9) did not lead to any change in the water-reducing admixture requirement due to the negligible water absorption capacity of waste steel fibers. Indeed, at each BFC substitution level, the mixtures with fiber addition could be successfully printed at the same additive dosage as the optimum mixture without fibers.

Optimization of the 25% BFS Substitution Series:

In this series, the same water-reducing admixture dosage (2 kg/m³, Mix No. 10) as in the optimal control mixture was insufficient to ensure constructability. Reducing the dosage to 1.8 kg/m³ (Mix No. 11) maintained extrudability but still failed to ensure buildability. Reducing the dosage to 1.65 kg/m³ (Mix No. 12) improved buildability, but this time, shape stability was not achieved due to the spreading of the lower layers as the upper layers were compressed (similar behavior to Figure 5). Reducing the dosage to 1.5 kg/m³ (Mix No. 13) achieved the optimal fiber-free formulation at this level. This optimum dosage (1.5 kg/m³) has also been used successfully in all 25% BFS series mixtures containing fiber (Mix No. 14–19) (Figure 6).

Optimization of the 50% BFS Substitution Series:

In this series, water-reducing admixture dosages of 1.5 kg/m³ (Mix No. 20—Figure 7) and 1.0 kg/m³ (Mix No. 21) were insufficient to ensure buildability. Reducing the dosage to 0.5 kg/m³ (Mix No. 22) improved extrudability and buildability, but problems with shape stability were observed. The mixture prepared without water-reducing admixture (0 kg/m³, Mix No. 23) yielded optimal results and successfully met all printability criteria (Figure 8). This optimum dosage (0 kg/m³) was also used in all 50% BFS series mixtures containing fiber (Mix Nos. 24–29).

Optimization of the 75% BFS Substitution Series:

This series presented an additional challenge in the mix design. The mix without any additives (Mix No. 30) failed to achieve buildability. The addition of silica fume (SF) was necessary to improve workability, but the mix containing 9% SF by volume (Mix No. 31) became unextrudable due to its excessive cohesiveness (Figure 9). Reducing the SF ratio to 3% (Mix No. 32) made the mix excessively fluid again, and the buildability parameter was not met. Finally, adjusting the SF ratio to 6% (Mix No. 33) achieved the optimum fiber-free balance for this level, ensuring the mix met all criteria. This optimum mix (175.2 kg/m³ Cement, 395 kg/m³ BFS, 28.9 kg/m³ SF, 0 kg/m³ water reducing admixture) formed the basis of all 75% BFS series mixes (Mix No. 34–39) containing fiber (Figure 10).

The findings indicate that as the BFS replacement ratio increases, the water reducer requirement of the mixture decreases significantly. No admixture was required at the 50% replacement level, while SF was required to control fluidity at the 75% replacement level. This observation is consistent with the findings of Özbay et al. [56], who explained the performance of BFS through three main mechanisms. Compared to Portland cement, its lower hydration reactivity improves workability and open time by reducing early-age heat release and water loss [57]. A more spherical and smooth particle morphology improves fluidity by reducing intra-matrix friction and minimizes the need for a lubricant [58]. Its specific surface area (4950 cm²/g), higher than that of cement (3441 cm²/g), increases the adsorption and dispersion efficiency of polycarboxylate ether-based water-reducing admixtures, enabling effective results with lower dosages [38].

The addition of 6% silica fume (SF) was critical to offset the potential overflow tendency of high substitution rates in mixtures containing 75% BFS and to ensure rapid structural build-up after extrusion. The extremely high surface area of SF (~20,000 m²/kg) is known to enhance particle interactions and thixotropic behavior, while also improving shape stability by precisely modulating the viscosity of the paste phase [38]. This combined effect allowed for high printability with controlled rheological performance.

3.2. Rheological Properties and Thixotropic Behavior of 3DPC Mixtures

3.2.1. Rheological Properties

In this study, the rheological behavior of 3DPC mixtures with different compositions was comprehensively examined in terms of dynamic yield stress and plastic viscosity (

Table 6. Rheological properties of 3DPC mixtures.

Mixture	Dynamic Yield Stress (Pa)	Viscosity (Pa·s)
K	130.52	13.47
25	64.5	10.3
50	39.57	6.43
75	108.59	6.89
K-0.5-0.5	201.81	19.03
K-1-0.5	245.75	19.53
25-0.5-0.5	86.69	14.8
25-1-0.5	77.43	13.81
50-0.5-0.5	118.54	14.07
50-1-0.5	113.56	13.27
75-0.5-0.5	154.24	13.59
75-1-0.5	85.2	8.16
K-0.5-1	88.47	14.53
K-1-1	84.45	11.24
25-0.5-1	70.43	11.69
25-1-1	124.09	14.78
50-0.5-1	123.12	15.05
50-1-1	62.04	11.72
75-0.5-1	88.04	11.85
75-1-1	200.35	13.93
K-0.5-1.5	155.17	15.93
K-1-1.5	104.63	15.61
25-0.5-1.5	55.27	7.42
25-1-1.5	56.2	10.5
50-0.5-1.5	67.75	8.17
50-1-1.5	65.05	7.69
75-0.5-1.5	97.76	8.13
75-1-1.5	99.88	12.31

). In fiber-free mixtures, BFS replacement exhibited a clear moderating effect on rheological properties. As summarized in Figure 11, the addition of BFS led to statistically significant decreases in dynamic yield stress and plastic viscosity. A 25% BFS replacement produced reductions of 50% and 23%, respectively, in these rheological parameters, while at a 50% replacement the reductions reached 69% and 52%. This behavior can be primarily attributed to the lower hydration reactivity and more spherical morphology of BFS particles compared with cement. Lower reactivity reduces hydration heat and the associated water loss, whereas the spherical morphology minimizes interparticle friction, thereby enhancing the mixture’s flow potential (rheological performance) [57]. Johari et al. [59] likewise confirmed that BFS contents up to 60% markedly improve the workability of cementitious systems. This improvement has been ascribed to better particle-size distribution, smooth surface morphology, and the low water-absorption capacity of BFS [56,60]. De Belie et al. [61] also reported that BFS use improves mixture flowability and substantially reduces the dosage of water-reducing admixture required to achieve the target spread. Salleh et al. [62] indicate that, in 3DPC mixtures reported in the literature, a 30–40% BFS replacement is recommended to balance optimal workability and mechanical performance. Park et al. [57] found that BFS contents up to 30% significantly increased flowability, attributing this effect to the spherical particle morphology and low hydration reactivity of BFS. The ability of BFS particles to fill the voids formed by cement particles and to reduce interparticle friction contributes to the high flowability of the cement–BFS system. However, higher replacement ratios (50% and above) can lead to lower hydraulic activity, which may adversely affect workability at high replacement levels, as noted by researchers.

To clearly elucidate the effects of waste steel fiber use and fiber length on the rheological properties of 3DPC mixtures, each BFS series was evaluated separately.

In the control series (0% BFS), examining the influence of fiber addition and length showed that using 5 mm fibers at 0.5% by volume increased dynamic yield stress and viscosity by 54% and 41%, respectively. When the fiber dosage was raised to 1%, these increases reached 88% and 45%, respectively. Using 10 mm fibers at 0.5% and 1% led to 32% and 35% decreases in dynamic yield stress, along with an 8% increase and a 16% decrease in viscosity, respectively. For 15 mm fibers, dosages of 0.5% and 1% produced 18% and 19% reductions in dynamic yield stress, and a 16% increase in viscosity in both cases.

In the 25% BFS series, 5 mm fibers at 0.5% yielded 34% and 43% increases in dynamic yield stress and viscosity, respectively, whereas at 1% these increases fell to 20% and 34%. For 10 mm fibers, 0.5% and 1% dosages produced 9% and 92% increases in yield stress and 13% and 44% increases in viscosity, respectively. With 15 mm fibers at 0.5% and 1%, dynamic yield stress changed by −14% and −12%, while viscosity changed by −27% and +2%, respectively, in 3DPC mixtures.

In the 50% BFS series, 5 mm fibers at 0.5% caused striking increases of 199% in yield stress and 118% in viscosity; at 1% the corresponding increases were 186% and 106%. With 10 mm fibers at 0.5% and 1%, yield stress rose by 211% and 57% and viscosity by 134% and 82%, respectively. For 15 mm fibers at the same dosages, yield stress rose by 71% and 64% and viscosity by 27% and 19%, respectively.

In the 75% BFS series, 5 mm fibers at 0.5% increased yield stress and viscosity by 42% and 97%, respectively; at 1% they led to a 21% decrease in yield stress and an 18% increase in viscosity. For 10 mm fibers, 0.5% and 1% dosages resulted in an 18% decrease and an 84% increase in yield stress, and 72% and 102% increases in viscosity, respectively. With 15 mm fibers, yield stress decreased by 9% and 8%, while viscosity increased by 18% and 79%, for 0.5% and 1% dosages, respectively.

Overall, the influence of fiber dosage on rheological properties depends on fiber length. In mixtures with low BFS contents and short fibers, increasing the fiber fraction generally increased dynamic yield stress and viscosity. This finding aligns with Doğruyol et al. [63], who observed that higher contents of waste steel fibers reduce homogeneity and workability in concrete. Similarly, Mohammad et al. [64] reported that increasing fiber content adversely affects mixture flowability, with restrictions on flow growing as total fiber content rises, thereby reducing workability; Figueiredo et al. [65] reported parallel results. Şahin et al. [38] also confirmed that increasing fiber content up to 0.4% elevates dynamic yield stress and viscosity, attributing this to the increase in total surface area with fiber addition [66,67], and similar statements were made by Wang et al. [68]. Conversely, Figueiredo et al. [65] proposed that fibers can reduce flow performance by constraining the relative mobility of aggregate particles due to dimensional incompatibility between fibers and aggregates. In 3DPC mixtures with 6 mm polypropylene fibers, Ma et al. [69] found that dynamic yield stress and viscosity increased with fiber dosage, explaining this by the tendency of PP fibers to bend and form clumps that impair flow; Wang et al. [68] reported similar outcomes.

However, in mixtures with longer fibers (10–15 mm), increasing fiber dosage led to decreases in dynamic yield stress and viscosity, an effect that became more pronounced at higher BFS contents. This reduction with higher fiber dosages is thought to stem from alignment of the longer fibers with the flow direction under high fiber contents. A related study by Ma et al. [69] on 3DPC mixtures examined the relationship between the dosage of 6 mm PP fibers and fiber alignment, showing that alignment becomes more dominant as the fiber content increases. By contrast, Gueciouer et al. [70] emphasized that while fiber use affects yield stress, it does not exert a notable effect on viscosity.

3.2.2. Thixotropic Behavior

This study evaluated the thixotropic properties of 3DPC mixtures using three different methods that characterize both dynamic and static behavior (D-SBU, A_thix, and I_thix), with the results summarized in

Table 7. Thixotropic behavior of 3DPC mixtures.

Mixture	D-SBU	Athix (Pa/s)	Ithix
K	0.82	0.89	1.21
25	0.71	0.75	1.31
50	0.58	0.22	1.95
75	2.52	1.04	1.49
K-0.5-0.5	1.17	1.03	1.4
K-1-0.5	1.3	4.78	1.23
25-0.5-0.5	0.82	3.56	1.25
25-1-0.5	0.69	2.36	1.47
50-0.5-0.5	0.57	3.09	1.34
50-1-0.5	0.45	1.94	1.34
75-0.5-0.5	0.98	3.19	1.44
75-1-0.5	1.3	0.72	1.45
K-0.5-1	0.97	2.59	1.2
K-1-1	0.88	1.03	1.47
25-0.5-1	0.64	0.131	1.19
25-1-1	0.76	4.38	1.35
50-0.5-1	0.68	0.08	1.3
50-1-1	0.22	1.3	1.3
75-0.5-1	0.91	5.58	1.8
75-1-1	1.5	−0.09	1.37
K-0.5-1.5	0.83	3.77	1.3
K-1-1.5	0.98	9.32	1.67
25-0.5-1.5	1.02	1.09	1.42
25-1-1.5	0.52	2.4	1.41
50-0.5-1.5	1.08	2.04	1.59
50-1-1.5	0.99	3.71	2.05
75-0.5-1.5	1.1	−2.53	1.41
75-1-1.5	1.16	5.76	1.4

. Independent of fiber parameters, the effect of BFS replacement on thixotropy is shown in Figure 12. In parallel with the rheological properties, compared with the control mix, 25% and 50% BFS replacement generally led to decreases in dynamic structural build-up (D-SBU) and the structural build-up rate (A_thix). This decrease can be explained by the relatively lower hydration reactivity of BFS compared with Portland cement, reduced water loss due to lower heat of hydration, and the smoother morphology of slag particles retaining less water during mixing. In contrast, mixtures containing 75% BFS exhibited a marked improvement in structural build-up. This unexpected increase is attributed to the addition of 6% silica fume by volume in this series to satisfy printability criteria. Bayat and Kashani [28] reported that high specific surface area supports structural build-up by increasing water demand and mixture cohesion. This finding is consistent with results reported by Grist et al. [71] and Jiao et al. [72]. The addition of silica fume contributes to a more cohesive concrete and increases rheological resistance via enhanced internal surface forces [61]. Srinivas et al. [73] confirmed that a 10% silica fume replacement significantly increases static yield stress—and thus buildability—without materially compromising extrudability. The literature further shows that concretes containing silica fume are more viscous and more resistant to segregation even at high flow levels, a property that is critical for layer-by-layer manufacturing. Inozemtcev et al. [74] noted that silica fume not only significantly increases compressive strength and ultimate deformation capacity but also affords superior pumpability. In 3D-printable geopolymers, Chen et al. [75] found that a 10% silica fume content improved structural build-up rates during flocculation and polycondensation. Geng et al. [76] reported that increasing silica fume content (≤12%) enhanced initial yield stress and thixotropic behavior and, provided it did not induce excessively high yield stress, significantly increased interlayer bond strength (from 69.7% to 112.3%). However, when yield stress becomes too high, further increases in silica fume content can reduce bond strength. In the present study, D-SBU and A_thix exhibited similar trends; however, the I_thix method was concluded to be unsuitable for evaluating the thixotropic behavior of 3DPC mixtures.

Examining the effect of waste steel fiber addition on thixotropy shows that, with short fibers (5 mm), increasing the fiber volume fraction generally led to decreases in thixotropic metrics (A_thix). This aligns with Şahin et al. [37], who reported that A_thix declines as fiber content increases. By contrast, for mixtures containing longer fibers (10 and 15 mm), no clear and consistent effect of fiber dosage on thixotropic properties was identified. Comparing different fiber lengths at the same fiber content, the highest D-SBU and A_thix values were generally obtained in mixtures with 5 mm fibers. As an overall trend, structural build-up decreased with increasing fiber length, with negative A_thix values observed—indicating structural degradation—particularly in mixtures 75-1-1 and 75-0.5-1.5. This suggests that in matrices with high BFS and silica fume contents, longer fibers may operate via a mechanism that delays or impedes the re-formation of the rheological network.

3.3. Carbon Emissions Assessment

The carbon emission values of the mixtures produced in this study were calculated by considering the embodied carbon of each constituent per unit mass and are presented in Figure 13. As expected, the highest carbon emissions (703 kg CO₂/m³) were observed for mixtures K-1-0.5, K-1-1, and K-1-1.5, which contain the highest cement content and the maximum waste steel fiber dosage. This outcome is attributable to cement being the component that contributes most to the concrete’s carbon footprint [77].

To more clearly reveal the dominant role of cement in emissions, the control mix (K) was compared with mixtures incorporating 25%, 50%, and 75% blast furnace slag (BFS) replacement. As is well known, BFS is one of the widely used supplementary cementitious materials (SCMs) in concrete technology [78]. Examining the effect of BFS replacement on carbon emissions (Figure 14) shows that 25%, 50%, and 75% replacement levels produced emission reductions of 23%, 47%, and 71%, respectively. The primary reason is the reduction in cement—which has high embodied carbon—and its substitution by BFS, an industrial by-product requiring minimal processing and possessing a very low embodied carbon (0.00–0.07 kgCO₂/kg). Cement-related emissions mainly arise from limestone calcination and kiln heating [79]. Accordingly, among all mixtures produced in this study, the lowest carbon emission (179 kgCO₂/m³) belongs to the mixture without fibers and with 75% BFS.

Assessing the effect of waste steel fibers on emissions shows that, in the control series, increasing fiber dosage led to a 6–12% rise in emissions. In BFS-containing series, this increase became more pronounced: for mixtures with 25%, 50%, and 75% BFS, increasing fiber dosage raised emissions by 8–16%, 11–23%, and 21–43%, respectively. This rise stems from the relatively high embodied carbon of waste steel fibers (0.43–1.59 kgCO₂/kg). Roshan et al. [80] noted that the global annual production of 0.3 million tons of steel fibers entails substantial fossil-fuel consumption and greenhouse-gas emissions; however, the same study emphasized that using waste-derived steel fibers can be a viable strategy to reduce the carbon footprint without sacrificing mechanical performance. The present results likewise highlight the environmental benefit of fiber recovery while indicating that, particularly at high dosages, there remains an embodied-carbon cost.

The values indicated by squares in Figure 13 represent average measurements. Maximum and minimum values are marked with error bars. Additionally, the Standard Deviation and Coefficient of Variation are given in

Table 8. Standard deviation and coefficient of variation of carbon emission values.

Mixture	Standard Deviation	Coefficient of Variation (%)
K	83.2	13.3
25	72.6	15.2
50	60.4	18.4
75	49.1	27.4
K-0.5-0.5	115.1	17.3
K-1-0.5	146.9	20.9
25-0.5-0.5	104.5	20.3
25-1-0.5	136.4	24.6
50-0.5-0.5	92.2	25.2
50-1-0.5	124.1	30.6
75-0.5-0.5	81.0	37.1
75-1-0.5	112.9	43.8
K-0.5-1	115.1	17.3
K-1-1	146.9	20.9
25-0.5-1	104.5	20.3
25-1-1	136.4	24.6
50-0.5-1	92.2	25.2
50-1-1	124.1	30.6
75-0.5-1	81.0	37.1
75-1-1	112.9	43.8
K-0.5-1.5	115.1	17.3
K-1-1.5	146.9	20.9
25-0.5-1.5	104.5	20.3
25-1-1.5	136.4	24.6
50-0.5-1.5	92.2	25.2
50-1-1.5	124.1	30.6
75-0.5-1.5	81.0	37.1
75-1-1.5	112.9	43.8

.

4. Conclusions

Based on the materials used and the experiments conducted in this study, the following findings were obtained:

• 25% and 50% BFS replacement led to substantial reductions in dynamic yield stress and viscosity relative to the control mix—by 50%/23% and 69%/52%, respectively.
• The effect of waste steel fibers on rheology exhibited a complex interaction with fiber length and BFS content. In mixtures with low BFS contents, short fibers (5 mm) increased rheological resistance as fiber dosage rose, whereas in the presence of longer fibers (10–15 mm) and higher BFS contents, this effect diminished or reversed.
• Thixotropic behavior followed trends similar to rheology. While 25% and 50% BFS reduced structural build-up (D-SBU and A_thix), 75% BFS together with silica fume markedly improved it. The high specific surface area of silica fume is considered to enhance cohesion and structural build-up, underpinning this positive effect.
• BFS replacement noticeably reduced water-reducing admixture demand. Owing to increased flowability up to 50% BFS, the admixture dosage decreased, and the 50% BFS mixture achieved optimal properties without any admixture. At 75% BFS, silica fume addition was required to maintain workability. Waste steel fibers did not change the admixture demand needed to satisfy printability criteria.
• BFS replacement demonstrated exceptional potential for environmental sustainability. 25%, 50%, and 75% BFS provided carbon-emission reductions of 23%, 47%, and 71%, respectively. The lowest footprint in the study (179 kgCO₂/m³) was obtained for the fiber-free mixture with 75% BFS. Although waste steel fibers offer mechanical benefits, their relatively high embodied carbon led to emission increases between 6% and 43% as fiber dosage rose.
• In this study, the effects of using 0, 25, 50 and 75% BFS by volume and different lengths (5, 10 and 15 mm) of waste steel fibers on the performance of 3DPC mixtures were investigated. The findings of this study depend on the material properties and application method used.