# Rheological characterization of temperature-sensitive biopolymer-bound 3D printing concrete

Julian Christ, Arnaud Perrot, Lisbeth Ottosen, Holger Koss

► To cite this version:

Julian Christ, Arnaud Perrot, Lisbeth Ottosen, Holger Koss. Rheological characterization of temperature-sensitive biopolymer-bound 3D printing concrete. Construction and Building Materials, 2024, 411, pp.134337. 10.1016/j.conbuildmat.2023.134337. hal-05324697

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Submitted on 31 Oct 2025

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<figure><img src="image_1.png" /></figure>
# Rheological characterization of temperature-sensitive biopolymer-bound 3D printing concrete

Julian Christ<sup>a, *</sup>, Arnaud Perrot<sup>b</sup>, Lisbeth M. Ottosen<sup>c</sup>, Holger Koss<sup>a</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

### Keywords:

Gelatin

κ-carrageenan

3D printing

Concrete

Biopolymer

Yield stress

Modulus of elasticity

Printability

## ABSTRACT

3D concrete printing materials with advanced rheological properties are being developed to realize more structurally optimized and sustainable structures. However, traditional mixtures use large shares of cementitious materials presenting reduced sustainability. In this paper, thermo-reversible mammal gelatin and κ-carrageenan are explored as alternative binders for temperature-controlled concrete printing. Rheological properties were found suitable at solution concentrations of 80–120%-w/v mammal gelatin in a 40%-w/w biopolymer-aggregate composite and 3%-w/v κ-carrageenan in a 50%-w/w composite at temperatures of, respectively, 50 °C and 65 °C. The corresponding yield stress increases from 0.1 to 107kPa under cooling to 20 °C demonstrated good buildability.

## 1. Introduction

3D concrete printing (3DCP) shows the potential to drastically reduce environmental impact and increase the economic efficiency of the construction sector [40,41,52]. The process lowers geometrical constraints by making formwork redundant [30] and placing material freely in a three-dimensional space. Sophisticated optimization algorithms can design building components with optimum material usage, which 3DCP can then realize. The degree of geometrical complexity that can be built with 3DCP is the limiting optimization factor which defines its environmental performance [52]. Therefore, the urge to reach higher degrees of freedom can be identified [16,60]. Broadly seen, 3DCP is limited to a vertical build-up since the setting characteristics of cementitious composites are relatively slow and difficult to control, limiting, in theory, possible high degrees of optimization. Conventionally, the process either relies on (1.) the thixotropic behavior and structural build-up of the material [26,28,35,39,50] or (2.) an additional two-component system that injects accelerators into the nozzle of the extruder [16,61]. Since aggregate size is limited by the scale of the printer's nozzle, both processes require higher cement content than conventional concrete composites [21], presenting a drawback on environmental performance even if the use of structural optimization can save material.

Other printing materials like geopolymers [29,39], clay and earth-based materials [18,24,42], polymers [57] and bio-composites [54] are being explored for use in 3D printing for construction to improve the environmental compatibility. While bio-composites with biofiber aggregates for 3D printing are being explored [25,31,54], composites from bio-based binders and mineral aggregates with this application are rather rare [42,55]. Biobased hydrogels are conventionally used to stabilized earth [22,49]. Also the use of hydrogel in concrete composites as additive for rheology modifying purposes is known, e.g. [32]. Some of these hydrogels have temperature-sensitive properties, i.e., thermo-reversibility, which can ease the setting control of the material during the print via an increase of the temperature during the pumping and extrusion steps. The novelty of this research is the application of hydrogel bound concrete composites with mineral aggregates to 3D printing of construction with a heated extruder. The eased setting control through temperature change could enable both higher build freedom and better environmental compatibility.

This research focuses thereby on the investigation of printability and connected rheological properties of hydrogel-concrete-composites made from the hydrogels mammal gelatin (MG) or κ-carrageenan (kC). It identifies the potential benefits compared to cementitious printing materials.

Hydrogel-bound composites can obtain compression strengths in the

<footnote><sup>a</sup> Department of Civil and Mechanical Engineering, Technical University of Denmark, Brovej 118, 2800 Lyngby, Denmark</footnote>

<footnote><sup>b</sup> Univ. Bretagne Sud, UMR CNRS 6027, F-56100 Lorient, France</footnote>

<footnote><sup>c</sup> Department of Environmental and Resource Engineering, Technical University of Denmark, Brovej 118, 2800 Lyngby, Denmark</footnote>

<footnote>* Corresponding author.
E-mail address: julch@dtu.dk (J. Christ).</footnote>
order of magnitude as conventional concretes [8]. Resourced from butchery waste [15] and seaweed [58], the production of hydrogel binders is potentially more environmentally friendly through lower production temperatures of around 100 °C [15,47], compared to cement (~1450 °C). Similar composites for 3D printing in constructions were investigated by [3,55]. However, the use of kC and MG in a heated extruder for construction applications is still unexplored.

Besides these construction applications, the hydrogel binders MG and kC are known as thickeners in food [4,38] and the biomedical industry [13,36]. Another application is in the field of 3D printing of human organs and food [2,14].

This research presents a printability assessment of hydrogel-based concrete with different binder concentrations, binder-to-sand ratios, and printing temperatures. Concretes with printable properties were evaluated and related to rheological properties such as complex modulus, loss factor, and yield stress under varying temperatures and shearing strains in oscillatory and rotational rheometry.

## 2. Materials

### 2.1. Aggregates

A silica sand was used for all experiments. As conventionally chosen for extrusion-based printing processes [5], small grain sizes were selected to enable a smooth print process. Additionally, the exclusion of larger particles ensures homogenous behavior in rheometer tests with a plate-on-plate setup without excessive gap size (>8 times the maximum grain size) [46]. The grading curve was determined using EN 933 [9] and is shown in Fig. 1. The sand shows its greatest mass within the grain sizes of 90–250 µm. A low fine particle content (below 0.063 mm) of 0.78%-wt. and a moisture content of 0.03%-wt. were recorded.

The light microscopy pictures in Fig. 2 present sand grains with sub-rounded to sub-angular geometrical properties. The mineralogical composition was identified with an X-ray diffractometer as merely consisting of SiO₂.

### 2.2. Hydrogel-binder

This study investigated two hydrogel binders, one protein binder and one polysaccharide binder. The protein binder is MG, and the polysaccharide binder is a κ-type carrageenan. Both binders are hydrogels chosen due to their thermoplastic phase change character [1,6]. MG was purchased as granular material through the tool manufacturer DICTUM [12] (Art. Nr.: 450141). kC was used in powder form and obtained from Sigma-Aldrich (Mat. Nr.: 22048–100 G-F).

Generally, both binding materials were prepared as hydrogels by mixing with water and heating the solution over the material's specific gel point (for MG >40 °C and for kC >60 °C). To produce a concrete composite material, heated sand at the same temperatures is added to the binder. Under mixing, a homogenous composite material can be made that solidifies when cooled. The maximum strength of the

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Grading curve of used silica sand specified with 0.09–0.25 mm, quantified after EN 933 [9].</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Light microscope recordings from sieving analysis of used sand with 10x objective, showing retained grains on sieves (from top left to bottom right): Pan, 0.063 mm, 0.08 mm, 0.125 mm, 0.250 mm.</figcaption></figure>

hydrogel-bound composite materials is reached by fully or partially removing the water through drying or conditioning. Previously, mean compression strengths of over 20 MPa could be recorded for MG composite [8].

#### 2.2.1. Mammal gelatin

MG is a biopolymer produced by partial hydrolysis of collagen through decocting, alkaline or acidic treatments of animal skin, connective tissue, or bones [38]. The extracted biopolymer is a protein made of long-chained polymer strands of various amino acids. An amino acid analysis could show that the used MG is primarily made up of the amino acids Glycine (32.6 mol-%), Proline (12.23 mol-%), and Alanine (11.2 mol-%). The gelatin consists of randomly placed polymer coils when heated as a solution. Under cooling, these random coils partially regain the collagen structure, an approximately 300 nm long triple-helix structure that is cross-linked through hydrogen bonds. Relevant for the formation of these, for the gelation responsible helix structures, are primarily Proline and Hydroxyproline [51]. The content of these amino acids could be identified in the used MG as 12.34 mol-% and 9.26 mol-%, respectively.

Gelatin is a thermo-reversible material, i.e., heating and cooling cycles can be repeated and lead to similar phase change characters with gelling points around 15–30 °C [34]. However, an irreversible denaturation process of the gelatin polymer network occurs when heated to temperatures that approach boiling temperature or above [59].

#### 2.2.2. κ-carrageenan

kC is a hydrogel that is extracted from red seaweeds (Rhodophyceae) [4] with alkaline treatment (NaOH or Ca(OH)₂) under a temperature of ~100 °C [30]. The polymer consists of repeating D-galactose and 3, 6-anhydrogalactose units. The definition κ-kappa originates thereby from the number and placement of ester-sulfate groups. Correspondingly, the types 1 iota and λ lambda also exist but exhibit different solubility and temperature-dependent properties [4]. Like the MG gelling behavior, the kC hydrogel undergoes a coil to helix transition by cooling, i.e. gelling. However, its gelling temperature is around 55 °C [27] and thereby significantly higher than MG. The helix arrangement is evoked by the κ-unit residue [45].

## 3. Methods

Three performance criteria were used to assess the materials' rheological properties suitable for application on 3D concrete printing: 1. Evaluation of printability by extruding four layers of material through a modified caulking gun to assess buildability and extrudability as key parameters of printability, 2. Oscillatory rheometry with a plate-on-plate geometry to assess rigidity under the influence of varying
temperature and shear strain, and 3. destructive rotational rheometry with shear-vane in cup geometry to measure yield stress development over temperature and time.

In (1.), several concentrations were evaluated as printable. With minor adjustments, these printable compositions were then tested in (2.) and (3.): To be able to draw more detailed conclusions, it was intended to keep most of the parameters identical for (2.), even though they did not show optimal results in (1.). For (3.), merely the highest printable concentration was tested.

For all testing protocols, especially for high temperature testing, drying was identified as the most significant challenge, since the material's rheology was significantly affected. It was avoided by keeping the vapor pressure at an equilibrium state by adding moisture and water around the sample, increasing the relative humidity of the samples' surroundings, and shielding the samples from non-saturated air. The methodologies presented are the result of trials without any measures that could prevent drying, with different oils and greases as cover fluids for rheometer testing and continuous addition of moisture without any coverage. These measurements were discarded and improved. The presented methodologies showed the least significant errors.

## 3.1. Printability assessment

### 3.1.1. Extrusion system

Similar to [56] and [53], a caulking gun was used in the printability assessment. An automatic caulking gun DEWALT DCE581E with a 600 ml capacity was utilized as an extruder basis. The caulking gun is an off-shelf ram extrusion system that can apply pressure (1.05 MPa according to the user manual) on its 5.5 cm diameter piston [11] (see Fig. 3).

The cartridge that held the hydrogel concrete was made from aluminum and had a length of 37 cm. The speed of the piston was kept constant for all tests at 40 mm/min, accepting minor variations due to changing resistance under progressing extrusion state. A relation between applied force and total friction, dependent on filling state of the

<figure><img src="image_5.png" /><figcaption>Fig. 3. The heated ram extruder system, which was mounted on a movable-sled. Tubular films were used to store and refill material.</figcaption></figure>

cartridge, could be identified in [43]. The caulking gun was connected to a cylindrical nozzle with an inner diameter of 13 mm.

The caulking gun was equipped with temperature-adjustable heating features: Around the aluminum tube, a heating coil was applied and insulated with foam rubber. By adding a temperature switch connected with a thermocouple mounted between the foam rubber and the aluminum, the temperature outside the cartridge but inside the insulating layer could be controlled. A drawing of the heated extruder system can be seen in Fig. 3. Since aluminum has a high thermal conductivity, an insignificant temperature difference between the outside and inside of the cartridge was assumed. Thus, the adjusted value on the temperature switch assumed the temperature conditions on the inside of the cartridge.

### 3.1.2. Moving traverse

The extruder was mounted on a traversing sled to obtain a linear print and constant layer height (see Fig. 4), restraining all movements except the traversing direction. The sled rolled along two rails with a total length of ~2 m and a distance of 40 cm. The extruder head was mounted vertically in the center of the sled, still allowing for vertical adjustment of printing height. The layer height was kept constant at 10 mm. After a printed layer, the caulking gun was adjusted manually to the next layer height.

### 3.1.3. Material preparation

Both hydrogels investigated show phase change properties and are sensitive to temperature [1,6]. The same accounted for the fresh composites investigated. Due to a possible dependency of temperature on handling history, a strict protocol was followed for the tests, keeping shearing and temperature history similar:

1) Soaking of MG {kC} in water, 2) Heating of solution in an oven at 70 °C {90 °C} (1 h), 3) addition of preheated sand at 70 °C {90 °C}, 4) mixing with a spatula under heating from a heating plate for 3 min, homogeneity was assumed thereafter.

Subsequently, all material mixtures were filled into tubular films, made from 50 µm {100 µm} thick low-density polyethylene with a width of 80 mm (see Fig. 3) to avoid drying. The volume of each sample was 200–300 ml. The material tubes were stored for a short time under cooled conditions until used in the printability assessment.

### 3.1.4. Mixtures

The material's rheological properties vary with temperature, binder to sand ratio, and the binder solution concentration. Therefore, an experimental matrix was designed to indicate the influence of all three parameters mentioned. The corresponding nomenclature was defined by MGxxByy (xx%-MG-granulate-wt./H₂O-vol.) (yy% MGgelxx-wt./sand-wt.).

**MG:** The concentration of MG150 presents one of the largest mixable concentrations without loss of workability. The solution concentrations investigated were 60%, 80%, 100%, 120%, 150%-w/v of MG. The ratio w/v describes weight (w) of MG granulate to volume (v) of water. The binder to sand ratios were chosen with 0.3, 0.35, 0.4, 0.45, and 0.5, and temperatures of 30 °C, 35 °C, 40 °C, 50 °C, and 70 °C.

**kC:** Similarly, the kC compositions of 2%, 3%, 4%-w/v were selected with the corresponding binder to sand ratios by weight of 0.4, 0.5, and 0.6 with the printing temperatures of 45 °C, 65 °C, and 85 °C.

### 3.1.5. Experimental procedure

The cooled tubular films were heated in an electric oven to the specific testing temperatures, specified in Section 3.1.4. The extruder was heated to the same temperature. After one hour of heating, the tubular film bag was placed inside the extruder and cut open, closest to the nozzle. After the nozzle was put back on, and the extruder was set in the moving traverse, the setup was ready for the experiment.

The nozzle extruded the material with a maximum velocity of 10 mm/s during the tests. Depending on the material consistency and
<figure><img src="image_6.png" /><figcaption>Fig. 4. Moving travers with extrusion system, mount, temperature switch and four layers of extruded sample.</figcaption></figure>

the resistance to the piston, the extrusion rate varied slightly. Hence, the printing speed was accordingly adjusted to avoid tearing of the material. The print speed was 5–10 mm/s. The layer height and the nozzle’s distance to the preceding layer were kept constant at 10 mm. The layer width was 13 mm. The produced samples had a length of 15 cm. After each layer, the extruder was stopped to keep a layer time of 90 s. If the material did not show sufficient yield strength to stay inside the cartridge, the material flow was stopped during the waiting period by clogging the nozzle.

Given that the material could not pass the nozzle under pressure, the material was defined as not extrudable. If it could be extruded, but the four layers (each 10 mm height) were deviating more than 5 mm in total (total height <35 mm), the material was defined as not buildable.

## 3.2. Rheological characterization

The rheological characterization of 3D printing composites requires the determination of mainly three parameters: the apparent elastic modulus to prevent buckling [62], the yield stress to prevent the structure from collapsing at the basis [44] and the description of the viscosity for pumping and extrusion [35,46,50]. To ensure the stability of the printed shape, the initial elastic modulus of printable cementitious materials ranges between 10 and 100 kPa and the initial yield stress between few hundreds Pa to a couple of kPa. This roots in the requirement for great rigidity and static yield stress after extrusion to enable the buildability of the material and a liquid state material during extrusion to avoid segregation and clogging. The complex modulus G*, loss factor tan(δ), and yield stress τc have been measured and computed for the rheological tests. Since the materials have both solid and liquid-like behaviors for most tests under temperature change, the complex modulus in conjunction with its loss factor was used to describe and quantify the visco-elastic behavior.

The yield stress build-up and consistency change were expected to be mainly due to the temperature change and the connected polymer folding to helix-like structures [48]. Additionally, the thixotropic build-up due to particle flocculation can contribute to the yield stress development over time. The rheological tests aimed to mimic the shearing and temperature process of the actual printing procedure: 1. Initial state – heated material in extruder cartridge at printing temperature, 2. Extrusion – shearing of material, 3. Deposition – material at rest and initial cooling, 4. Temperature set - cooling of material to ambient temperature. The influence of aggregate particles in the hydrogel composite was studied and compared with the rheological properties of the raw binding material. All tests were made in at least triplicates.

### 3.2.1. Testing setup

**Oscillatory tests:** All rheological tests were performed with an Anton Paar MCR 702 Rheometer. A plate-on-plate testing geometry was used with diameters of 25 mm (PP25) and 50 mm (PP50), respectively, for the composite materials and the pure hydrogels. The larger plate geometry was chosen to decrease the torque error measured under low friction for the pure binders. A heated lower plate was used for temperature control of the sample. With a gap size of 1 mm for pure hydrogels and 2.5 mm for suspensions, the temperature gradient inside the sample was assumed to be negligible.

Like in [46], the gap size between the plates of the rheometer was more than eight times the maximum grain size of the mortar mix to ensure a homogeneous behavior of the suspension. To prevent drying, a hood from foamed plastic was made with an inner diameter of 9 cm and an outer diameter of 13 cm, covering the sample space. As shown in

<figure><img src="image_7.png" /><figcaption>Fig. 5. Setup to measure rheological parameters by oscillatory shear rheometry (left) through a plate-on-plate setup and determination of yield strength (right) in rotational fashion with four blade vane-cup setup. Both samples were protected against drying.</figcaption></figure>
Fig. 5, the inner surfaces were covered with a bentonite-water mixture with sufficient yield stress to stay at the inner surfaces of the foamed plastic, to keep the testing environment moisturized. The bentonite was mixed with a water-to-bentonite ratio of 2.35. Additionally, the foamed chamber was moisturized throughout all tests with sprayed water through the recess on the foamed hood's top.

**Rotational destructive testing of yield stress:** The same Anton Paar MCR 702 Rheometer was used for rotational tests. Similar to [33], a four-blade vane-cup setup was chosen, as presented in Fig. 5. The material was placed to a height of 70 mm in the cup with a diameter of 29 mm. The 4-blade vane probe with a height of 8.8 mm and a diameter of 10 mm was then approx. 2 cm fully submerged into the sample. The exposed sample surface was moisturized by water spray and covered by a thin plastic film. The film was not to any time in direct contact with the torque-inducing parts of the rheometer.

### 3.2.2. Data analysis

**Oscillatory tests:** G* denotes both the modulus of elasticity of viscoelastic (VE) fluids and VE solids and can be defined as the vector sum of the real component, or storage modulus G' and the imaginary component, or loss modulus G'':

$$G^* = G' + iG'' \quad (1)$$

The tangent of the phase angle δ in

$$G^* = |G^*|\cos(\delta) + i|G^*|\sin(\delta) \quad (2)$$

can be identified as the loss factor tan(δ), denoting the relative magnitude between G' and G''. Being that G' describes the properties of a viscoelastic solid and G'' describes a VE fluid, tan(δ) > 1 is defining a fluid. In contrast, tan(δ) < 1 represents a VE solid.

The gel point T<sub>G</sub> is defined as the temperature for this physical state transition.

I.e.:

$$T_G = T(\tan\delta = 1) \quad (3)$$

**Yield stress:** The yield stress is the maximum shear stress applied on a sample at yielding failure with T as the maximum torque at this point. A cylindrical shearing surface was assumed with the dimensions of the 4-blade vane (see Fig. 5) geometry of D= 10 mm (R=5 mm) and H= 8.8 mm. The corresponding yield stress τ<sub>c</sub> could be calculated by:

$$\tau_c = \frac{T}{2\pi R^2 (H + \frac{2R}{3})} \quad (4)$$

### 3.2.3. Mixtures and preparation of samples

In the printability assessment described in Section 3.1., the concentrations of MG60B35, MG80B40, MG100B40, MG120B40, MG150B45, kC03B50 and kC04B50 were evaluated as well printable (see Section 4.1). These printable compositions were further investigated with minor adjustments: To draw more detailed conclusions, the binder-to-sand-ratio and temperature were kept constant. Just the solution concentration was varied. The compositions of MG60-150B40 and kC03-04B50 were selected to be measured within rheometer tests at the corresponding printing temperatures of 50 °C (MG), 65 °C (kC03), and 85 °C (kC04). In similar fashion, the pure binder compositions of MG60-150 and kC03-04 were investigated. Yield stress measurements were merely conducted using the highest concentration of kC04B50 and MG150B40.

Like Section 3.1.3, the materials were prepared by 1. soaking, 2. heating and mixing, 3. addition of heated sand. To prevent the mixtures from drying and repeated heating and cooling cycles, the batches were split into several samples of ~10 ml and filled into Ziplock bags. The bags were stored in cooled conditions until testing. The application procedure for MG {kC} is as follows:

1) A ziplock bag with the material was emerged in 70 °C {95 °C} water for 30 s {180 s}; 2) the PP25 or PP50 probe is lowered 90 s

{240 s} past the start of heating; 3) the excess material is removed to reach material application states as shown in Fig. 5; 4) the sample space is moisturized with water spray (plates and bentonite in the hood); 5) hood is closed; 6) the test is started 210 s {360 s} past the start of heating.

### 3.2.4. The testing protocol for amplitude strain sweep of hydrogel solution

An oscillatory amplitude strain sweep of the binder solutions was conducted by recording 30 data points with a maximum recording time of 90 s each. The strain sweep was done logarithmic, starting from 0.0005% and reaching 100%. After the material application, the instrument was defined with a resting time of 2 min. The tests were conducted at the respective testing and printing temperatures of 50 °C (MG60-MG150), 65 °C (kC03), and 85 °C (kC04). A generally low frequency of 3 Hz was chosen. A large diameter PP50 setup was used to increase the measurements' torque and reduce errors. A gap size of 1 mm was defined.

### 3.2.5. The testing protocol for the temperature sweep of binder solution

An oscillatory temperature sweep was conducted by recording 46 data points with a recording length of 15 s each. The temperature was varied with a magnitude of 2.5 °C/min during the recording. This gradient was chosen by approximating a constant cooling rate for a printed layer. Because the cooling rate varies significantly for different locations in a printed sample for different printing temperatures, a constant cooling rate enabled better comparison of the samples and allowed for better conclusions. The sample was left to rest for 2 min before testing and after the application of the sample. After the temperature sweep, the data was accumulated for ten more data points to record possible time dependencies. It was tested with a frequency of 3 Hz and a shear strain of 0.1%. A gap of 1 mm was defined with a PP25 setup. The binders were cooled from the printing temperatures of 50 °C (MG60-MG150), 65 °C (kC03), and 85 °C (kC04) to 20 °C.

### 3.2.6. The testing protocol for the amplitude strain sweep of composite material

Like the testing protocol for the amplitude strain sweep for the pure binder solutions in Section 3.2.4, an amplitude strain sweep for the composite material was conducted under the same parameters. However, a PP25 setup with a gap size of 2.5 mm was used instead.

### 3.2.7. The testing protocol for the structural build-up of composite material at printing temperature

The composite materials were exposed to oscillatory shear for 30 s with a strain amplitude of 0.5%. As can be seen in Section 4.2.1, this value is in the material's non-linear region, which causes a shearing thinning behavior. After, the structural build-up was measured with a strain amplitude of 0.002%, which is in the linear elastic regime of all materials, 30 measurements were taken every 20 s. A PP25 geometry was used with a gap size of 2.5 mm. The materials were tested at the corresponding printing temperatures. As for other oscillatory experiments, the tests were conducted at a frequency of 3 Hz.

### 3.2.8. The testing protocol for the temperature sweep of composite material

Similar to the temperature sweep of the pure binding materials in Section 3.2.5, the composite materials were cooled down from printing temperatures with 2.5 °C/min to 20 °C. Measurements were collected every 15 s with a shear strain of 0.002% and 3 Hz. The measurements were proceeded for 200 s after reaching 20 °C to record potential time-dependent effects. Like the testing procedure in Section 3.2.7, the materials were sheared at a strain amplitude of 0.5% before the temperature sweep to see the joint effects of thixotropic built up and temperature change. A PP25 setup was used with a gap size of 2.5 mm.

### 3.2.9. Testing protocol of composite's yield stress

The four-vane-cup setup was used to test the yield stress of the
material under the temperatures of 85 °C, 65 °C, 45 °C, 20 °C and 50 °C, 40 °C, 30 °C, 20 °C, respectively for kC and MG. As in [33], a rotational test with a constant shear rate of 0.1 s⁻¹ measured the torque at failure. All samples were prepared at printing temperature, cooled down to testing temperature, and rested for 10 min before testing to allow a homogeneous temperature distribution over the sample's cross-section.

# 4. Results and discussion

## 4.1. Printability assessment

Table 1 and Fig. 6 show the results of the printability assessment.

Table 1 states the buildable, extrudable, and printable concentrations. Fig. 6 shows respectively for kC and MG representative printing samples. Generally, the MG composites showed better printability results. MG generally exhibited no to little segregation, contrary to the low concentrations of kC. The MG also presented good robustness in the printing process, allowing for homogeneous extrusion. A fast phase change at higher temperatures for the kC composites impeded the printing process through high sensitivity to temperature changes, causing non-homogeneous behavior during printing, clogging the nozzle.

Generally, 'not buildable' samples increased their layer width significantly after extrusion, while the layer height was simultaneously decreased – a plastic failure. For 'printable' samples a slight change in layer height was observed. For 'good printable' samples, there was hardly any change in layer geometry after extrusion.

### 4.1.1. The mammal gelatin composites

The MG composites with a binder concentration MG80–120 showed the best printability and the greatest printable concentration span (see Table 1). The MG150 binders tended towards high viscosity and slow extrudability, while MG60 was less buildable. A tendency towards greater buildability at lower printing temperatures and better extrudability for higher printing temperatures was observed for all specimens especially those with high binder concentrations.

### 4.1.2. kC composites

The general tendencies observed in Section 4.1.1 are also applicable for the kC composite materials: Higher temperatures lead to good extrudability. Low temperatures appear to have better buildability. The best printable binder concentration is kC03. A higher binder content of B50 + was needed to produce a printable material composition.

<figure><img src="image_8.png" /><figcaption>Fig. 6. The classification of printability tests for the kC- and MG composites. The MG composites show the best printability. For legend, see Table 1.</figcaption></figure>

## 4.2. The rheological characterizations of printable material

### 4.2.1. The strain amplitude sweep

#### 4.2.1.1. The hydrogel solution.
The mammal gelatin showed Newtonian behavior over the strain from 0.005% to 100% with VE liquid properties for all concentrations at 50 °C. Fig. 7 shows the magnitude of the complex modulus and loss factor over the hydrogel concentrations measured as the mean value over repetitions and strain. An exponential increase of the shear modulus with an increase in concentration was observed while shifting the loss factor towards solid-like material behavior. This is in consistency with reported findings in [63], suggesting non-Newtonian behavior for lower concentrations.

The kC solutions at 65 °C and 85 °C, respectively for the concentrations of 3%-w/v and 4%-w/v, showed similar behavior for shear strains lower than 10% (see Fig. 8). Above this threshold, shear-thinning behavior was observed.

At printing temperatures, all tested hydrogels show VE liquid-like behavior. The material's complex modulus increases with increasing concentrations. Both the 3%-w/v kC concentration at 65 °C and the 4%-w/v at 85 °C showed a similar modulus order-of-magnitude compared to

**Table 1**

Printability assessment of MG- and kC- composites for various concentrations and temperatures. MG (kC)xx shows the dried biopolymer to water ratio (w[kg]/v[I] %). Bxx defines the wet hydrogel to sand ratio (w/w %).

<table><thead><tr><th rowspan="2">T[°C]</th><th colspan="5">MG60</th><th colspan="5">MG80</th><th colspan="5">MG100</th><th colspan="5">MG120</th><th colspan="5">MG150</th></tr><tr><th>30</th><th>35</th><th>40</th><th>50</th><th>70</th><th>30</th><th>35</th><th>40</th><th>50</th><th>70</th><th>30</th><th>35</th><th>40</th><th>50</th><th>70</th><th>30</th><th>35</th><th>40</th><th>50</th><th>70</th><th>30</th><th>35</th><th>40</th><th>50</th><th>70</th></tr></thead><tbody><tr><td>B50</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>B45</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>B40</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>B35</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>B30</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>T[°C]</td><td colspan="5">kC02</td><td colspan="5">kC03</td><td colspan="5">kC04</td><td colspan="5">•</td><td colspan="5">Not buildable</td></tr><tr><td>B40</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>B50</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>B60</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr></tbody></table>
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Complex modulus G* and loss factor tan(δ) of MG solution measured over increasing strain sweep (0.0005–100%) at 3 Hz and 50 °C. All strain sweeps showed linear behavior. Presented are mean values and standard deviations computed from at least three test repetitions and over the scope of strain.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Complex modulus G* and loss factor tan(δ) of kC solution measured over increasing strain sweep (0.0005–100%) at 3 Hz and 65 °C, 85 °C respectively for the concentrations of 3%-w/v and 4%-w/v. Standard deviation of G* shown as dashed line.</figcaption></figure>

the 80–100%-w/v concentration of MG. By comparing this result with the results in Section 4.1., a region of G* between 10 and 50 Pa of the hydrogel proved to relate to good printability.

#### 4.2.1.2. Composite.
Fig. 9 shows the results for a strain sweep of the five tested mammal gelatin composites. A non-Newtonian, shear-softening behavior was observed for large strains. A linear VE region is visible for more minor strains, proposing that, similarly to cementitious composites [10], a yield stress fluid can be identified. The concentration-dependent quantitative order of G* of the raw hydrogel solutions (Fig. 7) was resembled for the high strain measurements. Low binder moduli lead to low composite moduli and high binder values to high composite values. These tendencies cannot be observed for the settled materials at low shear. Here, the MG60B40, MG80B40 showed an increased complex modulus and, at the same time, a decrease in its loss factor, i.e., the material illustrated solid-like behavior - likely because of an elevated number of particle-particle interactions through a low viscosity solvent leading to insufficient lubrication between the particles, also justifying the printability difficulties of the MG60B40 composite material.

The results of the strain sweep of the kC composite material are shown in Fig. 10. Despite more significant deviations, a similar behavior to the MG composites can be stated for strains below 10%, describing a

linear region at low shearing and shear softening behavior for higher shear strains.

With its complex modulus magnitudes of ~10⁴ Pa, the printable MG composites have significantly higher values for lower shear strains than the kC composites with ~10³ Pa; leading to similar values around the order of magnitude of 10⁻¹ kPa when sheared with a large strain. The value obtained for MG composites align with the values obtained with printable cementitious materials [19].

### 4.2.2. Temperature sweep

#### 4.2.2.1. Hydrogel.
Fig. 11 shows MG's complex modulus and loss factor in time and temperature dependency. For temperatures close to 50 °C, similar values were observed in Fig. 7.

The general tendencies of low concentrations relating to lower modulus and the opposite remain. A general increase in G* and decrease of tan(δ) over temperature decreases could be presented. The most drastic increase in rigidity of the gels could be observed between the temperatures of 40–20 °C. The exact temperature is dependent on the hydrogel's concentration: Low concentrations lead to a later (lower temperatures) drastic increase of G*. The gel transition point emphasizes this dependency as plotted in Fig. 12. The gel point varies between 23.1 °C for the lowest concentration and 28.4 °C for the highest concentration.

From about 730 s onwards, the temperature was kept constant at 20 °C (compare Fig. 11). The results suggest with a continuously increasing value of the complex modulus that the rheological behavior of the gels is not only dependent on the temperature change: A time-dependent behavior related to its temperature history could be recorded. This inertial effect decreases over time, faster for low concentration gelatins. The inertial effect could also be verified by [17], relating it with inefficient cross-linking through hydrogen bonds, with increasing effect in higher concentration gelatins.

The kC hydrogel exhibited a different temperature span for an increase in rigidity as shown in Fig. 13. The most rapid rise in G* was recorded for temperatures between 35 °C and 50 °C. Fig. 12 shows the significant higher gelling temperatures of 42.3 °C and 49.4 °C for the two concentrations. A shift towards higher modulus values with higher concentrations can be observed for the kC gel, similar to the MG. No significant time-dependent effect was observed past the termination of cooling.

#### 4.2.2.2. Composite.
A general increase of G* with a temperature decrease can be shown for the MG composite material in Fig. 14 similar to the pure hydrogel. Adding sand to the hydrogels did not substantially alter any general temperature-dependent behavior. Similar changes in lower concentration composites were observed for the gels (Fig. 11). The non-printable concentration BG60B40 shows the highest complex modulus at printing temperature and solid-like behavior. Again, a strong particle-particle interaction without sufficient lubrication is expected to evoke this result. At lower temperatures, past gelling point, the rigidity of the concentrations follows expected principles from Fig. 11, where a qualitative relation between concentration and complex modulus was observed. Likewise, the gelling point for the MG composite materials shown in Fig. 12 follows the tendencies of the raw hydrogels. A general increase in gel point temperature can be seen, expecting that, similarly to cementitious composite materials [7], the mineral aggregates will absorb some of the water, decreasing the water content in the binder. Hence, altering the binder at a higher concentration.

Fig. 15 shows the results of the temperature sweep of the kC composite. Like the MG60B40 concentration, a higher value of G* at printing temperature for the lower concentration of kC03B50 was observed. The elevation of the gel point (see Fig. 12) for the concentration rises accordingly, raising it to a higher value than the kC04B50 material. Even if a high-temperature gelling point was observed for the low
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Complex modulus G* and loss factor tan(δ) of MG composite measured over increasing strain sweep (0.0005–100%) after resting the material. Recorded at 3 Hz and 50 °C. The standard deviations are shown as dashed lines.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Complex modulus G* and loss factor tan(δ) of kC composite measured as increasing strain sweep (0.0005–100%) at 3 Hz and at temperatures of 65 °C and 85 °C for the concentrations shown. The measurements are given as mean value of at least three measurements with a coefficient of variation of 13.5% and 45% respectively for tan(δ) and G*.</figcaption></figure>

concentration, the corresponding complex modulus exhibits a lower temperature for the maximum increase in rigidity. The difference in rheological change is largest for the kC04B50 concentration. Hence, it exhibits more temperature-sensitive properties.

Quantitatively, the MG and kC materials reach a similar complex modulus of around 1 MPa when cooled, which is a high value in comparison with cement-based materials leaving a printer's nozzle. As an

origin at printing temperature, the kC-composites result in lower complex modulus values than the MG-composites. Therefore, the kC differs more in its rheological behavior between printing temperature and when cooled (3–4orders) of magnitude. However, when including the shear-softening behavior of BG80B40 and BG100B40, the change in G* is like the kC-composite, but generally being one order of magnitude larger than the kC-composites. kC composites are more temperature sensitive. While MG composites exhibit both, significant shear softening behavior and temperature sensitivity.

### 4.2.3. Thixotropic build-up

The time-dependent behavior of kC and MG composites after shearing is shown in Fig. 16. The complex modulus increases over time for all tested materials i.e., they exhibit a thixotropic behavior.

The increase of complex modulus through temperature change in Fig. 14 and Fig. 15 exceeded the thixotropic build-up for all concentrations. Therefore, the temperature is the most significant parameter in relation to buildability and extrudability. In future studies, the thixotropic build-up of the material can be optimized for the printing process by alternating the grain size distribution.

### 4.2.4. Yield stress

Fig. 17 shows the yield stress development measured over the composite's temperature. Similar to the complex modulus values in Fig. 11, the values of the yield strength increase towards lower temperatures. MG shows the most significant yield stress increase, stretching over three orders of magnitude (0.07kPa-106kPa), while the kC composite extends over two orders of magnitude (0.05kPa-15.1kPa). Directly relating to the composite materials' gelling point in Fig. 12, the most
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Complex modulus G\* and loss factor tan(δ) of mammal gelatin measured with a constant shear strain of 0.001 and a frequency of 3 Hz. The temperature profile is shown in red. Measurements are given as mean value. Standard deviations for the complex modulus are shown as dashed lines.</figcaption></figure>

significant increase in yield stress is between 45 and 65 °C (kC) and 20–40 °C (MG).

It is important to note that these yield stresses for MG and kC composites align with a fast printing of cementitious materials. According to [44], the material is theoretically able to withstand the weight of around 50 cm for kC and more than 3 m for MG composites which makes MG a very efficient and promising binder for 3D printing applications.

### 4.3. Rheological properties in relation to the composite's printability

The composites' printability originates from the rheological difference between conditions during extrusion (sheared and heated) and after extrusion (at-rest and cooled). Generally, both materials show a shear-softening behavior and thermo-reversibility, i.e., beneficial properties for their application and printability within a heated extruder.

**kC composites** perform best compared to MG composites in the following properties:

- **gelling point:** kC shows the highest gelling point. Exposed to environmental temperatures, the heated material cools faster. Therefore, an elevated gelling point exhibits a faster change in rheological properties after extrusion. This is beneficial for the material's buildability.

- **fast change in complex modulus:** The quickest change in complex modulus of the material (see Fig. 15) is also elevated in temperature, which leads to a fast change in the material's rigidity. The fast rheological change is beneficial for the material's printability, but it requires a well-tuned printing process. Especially for kC04B50, a problem with its temperature sensitivity was observed. Layer-time, extrusion- and printing speed needed to be fine-tuned to allow continuous printability. Similar problems can be described for fast-setting cementitious printing mortars [37].

- **the lowest rigidity when heated:** kC04B50 had a significantly lower complex modulus of ~0.2kPa when heated compared to the MG composites. A good extrudability was also observed, while more force input on the extruder was needed for the MG composites.

- Among the best printable **MG composites** (MG80-100B40), the beneficial properties were:

- **Gelling point:** The lower gelling point of the MG composites resulted in a lower printing temperature. This results in lower energy consumption to heat and keep the material at temperature. Contrary to the kC composites, the MG composites exhibited a robust printing process without strong dependencies on its parameters.

- **yield stress:** Both printable kC and MG composites exhibit a good buildability through its yield stress increase after cooling. However, the yield stress difference between printing- and cooled temperature
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Gel-transition point of MG and kC–hydrogel and –composites. Dashed lines show hydrogels and black lines visualize the corresponding composite materials. The data describes the crossing values for tan(δ)= 1 of Fig. 11, Fig. 13, Fig. 14 and Fig. 15.</figcaption></figure>

is most significant for the MG composite. This could not be observed
in its printability assessment, but might lead to significantly faster
build rates for full-scale building components [20].

- shear softening behavior: The shear-softening behavior could be identified as the largest for the low concentration MG composites. Since MG60B40 did exhibit no extrudability, the shear-softening behavior of BG80B40 showed the best properties of the printable composites. The thixotropic build-up after extrusion lead to good printability by not only relying on the rheological change through thermo-reversibility. This was beneficial, corresponding to the low-temperature gelling temperatures since the stiffening of the material is slower.

4.4. Evaluation: best printability

The best printability was found for the MG80–120B40 composites
through:

- The largest span in G* through its shear-softening/thinning behavior under extrusion with 10³-10² Pa in the tested range of < 100% shear strain, decreasing the materials rigidity.

- The low-temperature range for the largest increase in G* (10³-10⁶ Pa) for robustness of printing process through eased temperature control and sufficient buildability without deformation.

- The low gelling temperatures (25–28 °C) for a low temperature and a low energy-consuming printing process by still allowing solidification by cooling through ambient temperatures.

- The greatest yield stress development (10⁻¹·10²kPa) for fast build rate and low plastic deformation during the process.

4.5. Comparison to reference printing mortars

Cementitious mortars or concretes for 3D printing conventionally rely on a purely thixotropic build-up after extrusion [65]. The shear-thinning behavior of the material enables extrudability. The related yield stress fluid properties can be described by a Herschel-Buckley or a Bingham model [10], presenting a linear behavior for smaller strains and a non-linear region for larger strains. As shown in [23], the linear-elastic region of fresh cementitious concrete composites can be significantly higher compared to the bio-composite materials presented here - with values of 0.4% and 0.008% respectively. The corresponding yield stress values for the fresh state cementitious composites are significantly larger than the values of the bio-based composites at printing temperature and at rest with ~0.2-2 kPa [64] and 0.07kPa respectively. Therefore, the shear softening and thixotropic principles are better suited for printing processes in cementitious concrete composites. As shown in [66], the yield stress of cementitious composites at fresh state is dependent on the content of fines in the composite. Since a grain size distribution with a low fine content of 0.78%-wt. was chosen for this study, low values of the fresh properties could have been expected. The addition of fines in the here investigated bio-based materials is expected to have a similar impact on the material's yield stress and linear elastic regime. The optimization of the

<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Complex modulus G\* and loss factor tan(δ) of kC solution measured with a constant shear strain of 0.001 and a frequency of 3 Hz. The temperature profile is shown in red. The measurements are given as mean values. Mean values for the coefficient of variation could be identified with 28% and 19%, respectively for G\* and tan(δ).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 14.** The temperature sweep of mammal gelatin composite from 50 °C to 20 °C. Complex modulus G* and loss factor tan(δ) measured with a constant shear strain of 0.001 and a frequency of 3 Hz, pre-sheared at 50 °C with a strain of 0.005. The temperature profile is shown in red. The measurements are given as mean values. Mean values for the coefficient of variation could be identified with 26% and 10%, respectively for G* and tan(δ).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 15.** The temperature sweep of the kC composite from 85 °C and 65 °C (for the concentrations shown) to 20 °C. The complex modulus G* and loss factor tan(δ) measured with a constant shear strain of 0.001 and a frequency of 3 Hz, pre-sheared at respective printing temperatures with a strain of 0.005. The temperature profile is shown in red. The measurements are given as mean values. Mean values for the coefficient of variation could be identified with 54% and 17%, respectively for G* and tan(δ).</figcaption></figure>

grain-size distribution on its printing parameters of MG composites remains to be investigated.

When cooled to ambient temperatures, the MG composite in Fig. 17 showed a yield stress development over three orders of magnitude to 106 kPa. This being significantly larger than the yield stresses of fresh cementitious concretes of ~0.2–2 kPa [64], a better buildability and build rate can be expected of the presented bio-composite material, possibly enabling thin-walled and higher degrees of free form in constructions. Other biopolymer bound soil composites from bovine blood proteins, tested for 3D printing, showed yield stresses in the same order of magnitude as cementitious concretes with 3–4kPa [3].

The open time of cementitious printing composites can be 30 min [28] or less. This can lead to clogging and breaking of equipment

connected with the printing process [37]. The presented bio-composite with thermoplastic behavior presents an open-time depending on the temperature to which it is heated or kept. If kept at printing temperature and hydrated, the open time is, in theory, infinite, disrespecting the binder's denaturing behavior.

In the here presented experiments, a ram extruder was used. For large-scale construction applications, this process should be upscaled or replaced by a pumping system. This would enable a continuous printing process. To do so, other material properties, like the dynamic yield stress and viscosity should be investigated and a heated pumping process should be developed to keep the material liquid during pumping. Like the here presented results, the pumpability follows temperature- and mixture design dependencies.
<figure><img src="image_18.png" /><figcaption>**Fig. 16.** The structural build up over time for composite materials. Measured with a PP25 setup, 3 Hz, strain of 0.001 and at respective printing temperatures (MG60–150B40 at 50 °C, kC03B50 at 65 °C and kC04B50 at 85 °C). Pre-sheared with a strain of 0.005. Measurements are given as mean value. A mean coefficient of variation could be identified with 36%.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Fig. 17.** The yield stress values for MG150B40 and kC04B50 composites. A four-blade vane setup was used for the tests with a constant shear rate of 0.1 s⁻¹. The data points displayed show the maximum recorded stress at failure.</figcaption></figure>
Looking at the here presented fresh material properties, an application on large-scale building components would be feasible. Other limiting factors like the long-term durability and fire resistance of the materials, as well as mechanical properties need to be further explored and optimized. An application of the printing material to structural building components, as well as printed formwork would be conceivable.

As a waste product, MG is available relatively cheap. The process of extracting the MG from butchery waste does not use temperatures above 100°C. This is significantly lower compared to cement production. In places with high energy costs or limited material supply, the hydrogel could not only be a more climate friendly alternative but also more economical compared to cementitious printing mortars. Large amounts of secondary materials in food production are available through extensive meat consumption. However, with a high demand in construction, this might let prices rise.

## 5. Conclusion

The two thermo-reversible hydrogels, mammal gelatin and k-carrageenan, originating from mammal butchery side-products and seaweed, were investigated as binding materials in 3D printing filaments for construction. The printing assessment within a heated extruder, cooling under ambient-temperature conditions showed that both filaments had concentration and temperature ranges for which the materials were extrudable and buildable. The tested rheological properties showed that the mammal gelatin composites exhibited the best printability. The rheological conditions between extrusion and building state exceeded the ones known for cementitious composites primarily through temperature-dependent changes. Therefore, a faster build rate and higher form stability can be expected due to quick and easy controllable yield stress development. This result could enable higher degrees of freedom in building components, leading to higher degrees of optimization and minimization of material use. As presented in this paper, the rheological properties fit this application.

## CRediT authorship contribution statement

**Koss Holger:** Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing – review & editing. **Perrot Arnaud:** Methodology, Resources, Supervision, Writing – review & editing. **Ottosen Lisbeth M.:** Funding acquisition, Resources, Supervision, Writing – review & editing. **Christ Julian:** Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Validation, Visualization, Writing – original draft, Writing – review & editing.

## Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Julian Christ reports financial support, administrative support, equipment, drugs, or supplies, and travel were provided by Technical University of Denmark.

## Data Availability

Data will be made available on request.

## Acknowledgments

The experimental work published in this paper occurred at the Technical University of Denmark, and the University of South Brittany in Lorient, France. The research was granted financial support from the Technical University of Denmark and the grant (00023307) from VIL-LUM FONDEN. The authors acknowledge the contributions of Mária

Husarcíková, Jonas Holmer Bigom and Oliver Kragelund Vittrup for carrying out experimental work, as well as lab technicians Ebba Schnell and Emma Nørby Jensen. The amino acid analysis could only be presented thanks to the work of Emil Engelund Thybring and Anne Blicher at Copenhagen University, Department of Geosciences and Natural Resource Management.

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