Stability of three-dimensional printable foam concrete as function of surfactant characteristics

Abstract

Extrudability is one of the most critical factors when designing three-dimensional printable foam concrete. The extrusion process likely affects the foam stability which necessitates the investigation into surfactant properties particularly for concrete mixes with high foam contents. Although many studies have been conducted on traditional foam concrete in this context, studies on three-dimensional printed foam concrete are scarce. To address this research gap, the effects of surfactant characteristics on the stability, extrudability, and buildability of three-dimensional printed foam concrete mixes with two design densities (1000 and 1300 kg/m³) using two different surfactants and stabilizers (synthetic-based sodium lauryl sulfate stabilized with carboxymethyl cellulose sodium salt, and natural-based hingot surfactant stabilized with xanthan gum) were investigated in this study. Fresh density tests were conducted before and after the extrusion to determine stability of the foam concrete. The results were then correlated with surfactant qualities, such as viscosity and surface tension, to understand the importance of key parameters in three-dimensional printing of foam concrete. Based on the experimental results, surfactant solu1tion with viscosity exceeding 5 mPa·s and surface tension lower than 31 mN/m was recommended to yield stable three-dimensional printable foam concrete mixes. Nevertheless, the volume of foam in the mix significantly affected the printability characteristics. Unlike traditional foam concrete, the variation in the stabilizer concentration and density of concrete were found to have insignificant effect on the fresh-state-characteristics (slump, slump flow, and static yield stress) and air void microstructure of the stable mixes.

Graphical abstract

Keywords

foam concrete / 3D printable concrete / stability / rheology / air void microstructure

1 Introduction

The pace of digital construction is advancing owing to the active participation of researchers and entrepreneurs. Extrusion-based three-dimensional (3D) concrete printing, recognized as the most widely used type for digital construction, has been proven to meet demand sustainably by minimizing waste generation, eliminating the need for formwork, and utilizing sustainable materials [1–4]. Despite the lack of standardization and reinforcement choices, trial projects are being constructed worldwide to investigate 3D printed concrete structures in practice [5,6]. Similarly, 3D printing of lightweight foam concrete has attracted significant attention [7–14], although the current status of studies conducted in this area is limited.

Foam concrete is cellular concrete produced with cement, sand, and foam generated using surfactants and water as ingredients. Adequate control of foam dosage can yield a wide range of densities, from 300 to 1800 kg/m³ [15–17]. Surfactants (or foaming agents) play a vital role in foam generation and stabilization by reducing the surface tension of the water. Surfactants are generally classified into two types based on their origin: natural and synthetic. Synthetic surfactants can be subdivided into ionic (anionic and cationic) and nonionic surfactants [18,19]. The efficacy of surfactants used in foam formation significantly varies because they originate from diverse sources. Consequently, surfactant selection and foam properties likely impact the foam concrete microstructure [20]. Among various foam properties, the foam density is key in determining the amount of foam required to achieve the target concrete density. ASTM C796 [21] prescribes a foam density range of 30–65 kg/m³ for preformed foam concrete.

However, stability of the formed foam is critical in maintaining the desired concrete density. Hence, type of surfactant (which influences the bubble microstructure) and rheology of the mortar enclosing the bubbles play a crucial role in achieving a stable 3D printed foam concrete (3DP-FC) mix with an actual density within 50 kg/m³ of the design density. Cho et al. [8] investigated the influence of paste rheology on foam degradation during the pumping of 3DP-FC. Their experimental results showed that mixes with a higher foam content exhibited better stability owing to the cushioning effect of bubbles, regardless stiffness of the base mix. In contrast, Markin et al. [10] observed that when density of a concrete mixture decreased, the foam stability also decreased significantly. They found that although an increase in surfactant concentration improved the stability, the enhancement was trivial. Cho et al. [9] used calcium sulfoaluminate cement (CSA) to reduce the setting time and subsequently enhance the thixotropy of 3DP-FC at an early age. A similar approach was applied by Jones et al. [22] for traditional foam concrete mixes to solve the foam instability issue. They observed that the buoyancy forces of foam bubbles and bubbles with thin lamellae added to the foam instability, particularly in mixes with low density. If the buoyancy force on the bubble is not counteracted by surface forces (such as the paste surrounding the bubble), the bubbles will cluster and cause the separation of solid–gas phases. This coalescence (merging of bubbles) effect can be minimized by using rapid-hardening CSA cement, which helps maintain the stability of foam bubbles.

The above review shows that stability of 3DP-FC can be altered by the paste stiffness encircling the bubble or surfactant characteristics. As discussed previously, the influence of paste rheology on foam stability has been demonstrated by Cho et al. [8]. Therefore, this study intends to investigate the influence of surfactant characteristics on the stability of 3DP-FC. Furthermore, although sufficient reports on the effects of surfactant characteristics on conventional foam concrete are available [19,20,23–27], information on 3DP-FC is scanty. Hence, the main objective of this study is to investigate the stability of 3DP-FC as a function of surfactant characteristics. The effects of two surfactants (stabilized with different stabilizers) on the extrudability and buildability of foam concrete of different design densities (1000 and 1300 kg/m³) were examined. The stability of the 3DP-FC mixes was evaluated based on an assessment of the fresh density before and after extrusion. Finally, the above findings were correlated with the surfactant properties and air void distribution of the printed mixes to clarify the relationship between these parameters.

2 Materials and methods

2.1 Materials and mix design

In this study, experiments were conducted for sand-to-binder ratio of 2 (the sand was sieved through a 300 μm sieve) and water-to-solid ratio of 0.20. The binder for all mixtures contains 70% cement and 30% silica fume (SF) to increase buildability of the mixtures. Hydroxypropyl methylcellulose (HPMC) was used as a viscosity-enhancing agent at a binder content of 0.1%. Two different surfactants were used to examine the effect of surfactant type on 3DP-FC extrudability. The first was an anionic synthetic surfactant (sodium lauryl sulfate (SLS) stabilized with carboxymethyl cellulose sodium salt (CMC)), whereas the second was a saponin-based natural surfactant (hingot (H) stabilized with xanthan gum (XG)) [24,28,29]. The density-based mix design method as specified in ASTM C796 [21] was used. The mix proportions used in this study are listed in Tab.1.

2.2 Foam production parameters

The surfactant solutions were prepared by diluting the surfactant powder in water to a specific concentration. Dilution of the surfactant solution depended on foam stability. The surfactant solution concentrations used in this study were adopted from the literature. For the synthetic surfactant, 5% concentration was adopted; for the natural surfactant, the concentration was kept at 6% [23,29]. CMC and XG (the stabilizers) were added at varying dosages in combination with surfactants SLS and H, respectively. The upper limits of the CMC and XG were fixed at 0.4% and 0.2% of the volume of the prepared solution, respectively, based on preliminary studies.

The experimental setup for the foam generator comprised an air compressor with an outlet valve. A pressure regulator valve in the compressor facilitated the required pressure control. The foam generation pressure was maintained at 4 kg·f/cm² (392.3 kPa) for synthetic surfactants and 5.9 kg·f/cm² (578.6 kPa) for natural surfactants. Pumping pressurized air into the solution caused the foam to expand while producing thick foams.

2.3 Preparation of foam concrete

The ‘mixing process’ and ‘time’ are key to obtain homogeneous foam concrete mixture. The first step of the mixing process involved thoroughly mixing of dry materials for 1 min at 107 r·min⁻¹ using Hobart planetary mixer. Approximately 70%–80% water blended with HPMC was then added and mixed at 198 r·min⁻¹ for 1 min. Subsequently, the sample was scraped for 30 s, and the remaining water was added and mixed at 361 r·min⁻¹ for 30 s. The resulting mixture was inspected for lumps that might affect homogeneity of the mixture. The preformed foam was added and mixed at 107 r·min⁻¹ for 30 s, followed by scraping for 30 s. Finally, another round of mixing was performed for 30 s at 107 r·min⁻¹ to ensure homogeneity.

2.4 3D concrete printer setup

A custom-made extrusion-based 3D concrete printer was used (Fig.1) in this study with maximum nozzle size of 20 mm and nozzle travel speed of 200 mm/s. A digital computer-aided design model of the structure was first sliced using simplified 3D software and this information was transmitted to the controller unit, which instructed the 3D printer to precisely deposit the fresh concrete material. The material was extruded through the nozzle orifice using an Archimedes screw to maintain continuity in the material flow [30,31].

2.4.1 Printing parameters

In this study, 3DP-FC was extruded using a screw-type extruder (auger extruder). Beams of 700 mm length and 50 mm height were printed using 10 mm filament height to investigate 3DP-FC extrudability. A print speed of 80 mm/s was maintained in all the printing trails. These beams were cured for further analysis.

Following the extrudability tests, buildability test was conducted by printing the maximum number of layers without discontinuity and structural collapse. For this purpose, a cylinder with a diameter of 200 mm was printed (10 mm filament height) with 80 mm/s printing speed.

2.5 Studies on surfactant characteristics

Previous studies have demonstrated that the essential characteristics of surfactants, such as viscosity and surface tension significantly influence various foam properties [25]. Therefore, we attempted to evaluate these surfactant characteristics in this study. The surfactant viscosity is due to the collision of adjacent particles in a fluid flowing at different speeds. As the number of particles increases, the viscosity increases. In this study, the surfactant viscosity was measured using a rheometer (Anton Paar, MCR 101 model) with a concentric cylinder geometry at a constant rotational speed of 150 r·min⁻¹ and a constant temperature of 25 °C.

Generally, the surface tension in liquids is caused by the discrepancy between the intermolecular cohesive forces of fluid particles at their boundaries. A tensiometer (Kyowa DY300) was used to measure the surface tension of the surfactant solution by applying the Wilhelmy plate method. In this method, a platinum plate (selected because of its chemical inertness and ease of cleaning) was immersed in the liquid, and the force required to hold the plate (that is, restrict the surface tension to pull the plate) was measured.

2.6 Studies on foam characteristics

The term “foam density” describes the unit weight of the foam. The initial foam density (IFD) of the foam was measured immediately after its generation. The free foam drainage test was conducted to assess the foam stability. The foam drainage setup consisted of a drainage pan with a conical base and a nominal volume of 1.612 L. The conical base of the drainage pan was equipped with a polymethyl methacrylate tube with a 12.7 mm internal diameter, 25 mm length, and a 1.6-mm-diameter hole at its bottom end (according to DEF STAN 42-40/2). The drainage pan was filled with foam, and the volume of the solution drained for 30 min was measured to determine the foam stability.

2.7 Studies on fresh properties of 3DP-FC

Fresh density is a critical physical property that helps assess the stability of a mix. In this study, the stability of air bubbles in 3DP-FC was determined by measuring the fresh density of the material before and after extrusion.

The consistency of the foam concrete was measured using slump and slump flow tests. The mix was fed into the slump cone in two layers to determine the slump, with each layer tamped 20 times in compliance with ASTM C230 [32]. A trowel was used to remove the additional material from the top surface. Next, the cone was lifted gently, and the reduced height of the sample was measured and reported as a slump. Flow table tests were performed according to ASTM C1437 [33] to assess the flowability of the mixture. The mean diameter was determined by measuring the diameter of the flow in the two perpendicular directions after 25 blows.

The static yield stress of the material was measured to determine the force required to initiate flow. The yield stress of the 3DP-FC mixture was measured using four-bladed (24 mm height (H) and 12 mm diameter (D)) vane shear equipment, which is often used to determine the shear strength of soft clays. The vane was entirely driven into the 3DP-FC-filled mold until its top was 20 mm below the surface of the material. The rotation was maintained at 0.1°/s until the sample failed. The maximum torque (T) at failure was calculated based on the difference between the initial and final readings. The yield stress ({\tau }_{\mathrm{y}}) of the 3DP-FC sample was calculated using Eq. (1) [34]:

(1)\mathit{T}=\frac{\text{π}{\mathit{D}}^3}{2}{\tau }_{\mathrm{y}}\left(\frac{\mathit{H}}{\mathit{D}}+\frac{1}{3}\right).

2.8 Air void characterization

In this study, the air void parameters of concrete were analyzed using optical microscopy with image analysis. The surface treatment quality of a specimen is crucial for generating reliable data to successfully perform image analysis. From the printed beam, four 50 mm cubes for each density were cut and moist-cured for 28 d (Fig.2(a) and 2(b)). All the specimens were split into nine samples, considering the interlayer porosity (Fig.2(c) and 2(d)). The surface treatment was performed according to ASTM C457 [35]. Fig.2(e) shows the specimens after surface treatment. We have used Image J software to analyze the macropores. Air voids with sizes exceeding 50 µm were considered for the analysis to examine the entrained macropores [36]. Four microstructural images of each sample were captured using an optical microscope at 0.7× magnification. Subsequently, these images were digitized and converted into a binary form for further analysis.

3 Results and discussion

3.1 Effect of stabilizer on foam properties

The experimental results indicated that adding 0.1% and 0.2% dosages of XG improved the foam density by 16% and 184%, respectively (Fig.3(a)). Despite significant reduction in the foam drainage at 0.2% dosage of XG, the upper limit of XG was fixed at 0.1% as the foam density did not satisfy the prescribed density range in ASTM C796 [21]. Nevertheless, foam drainage at the 0.1% dosage of XG resulted in 24% decrease in foam drainage. Similarly, in the case of the synthetic surfactant, adding CMC up to a 0.4% dosage increased the foam density by 17% and decreased the foam drainage from 99% to 63% (Fig.3(b)). The improvement in the foam properties can be attributed to the enhanced surfactant viscosity and lamella thickness in the bubble microstructure, as established in previous studies [23,27]. These facts are experimentally verified and discussed in the following sections. A comparative evaluation of the performance of XG and CMC indicated that XG significantly improved the IFD and decreased the foam drainage. This could be attributed to the thickening property of XG and viscosity of the surfactant solution compared to CMC.

3.2 Effect of stabilizer on surfactant characteristics

The viscosity and surface tension of the surfactant solution showed contrasting trends with increase in the stabilizer concentration for both natural and synthetic surfactants. Increasing the dosage of XG from 0% to 0.2% increased viscosity of the H surfactant solution by 302% and decreased its surface tension by 6.3% (Fig.4(a)). This can be ascribed to thickening property of the XG, which improved viscosity of the solution and condensed the liquid film around the bubbles [27]. Consistent with most relevant studies, the influence of XG on the viscosity of the surfactant solution was more significant than its effect on surface tension [37]. Similarly, an increase in the CMC concentration from 0% to 0.4% increased the viscosity of the SLS surfactant solution by 472% and decreased the surface tension by 6.5% (Fig.4(b)). This trend can be attributed to an increase in the concentration of the water-soluble polymer, which modified the micellar structure of the molecules, increasing the viscosity [23,38]. This increase in viscosity owing to increased stabilizer concentration might be correlated to the positive impact on foam properties (density and drainage), as discussed in Subsection 3.1.

3.3 Effect of surfactant characteristics on fresh properties of 3DP-FC

The fresh density of the mixture was examined before and after the extrusion. ASTM C869 [39] recommends that the variation between the target and achieved density should be within the range of ±50 kg/m³. SLCM3, SLCM4, and HXG1 mixes achieved the target density (within the tolerance limits) at both measurement stages (before and after printing) for the selected target densities (Fig.5). Although SLCM2 mix could achieve the target density of 1300 kg/m³, it failed to sustain foam stability at 1000 kg/m³. Moreover, SLCM0, SLCM1, and HXG0 (mixes without a foam stabilizer and with a low foam stabilizer dosage) also failed to satisfy the density requirements within the tolerance limits, indicating the instability of air bubbles for the target density mixes. The failure of these mixes to achieve the target density indicates the significance of adding foam stabilizers to achieve stable foam concrete mixes. Furthermore, a detailed analysis of these unstable mixes (SLCM0, SLCM1, and HXG0) indicated that the variation in density before and after the printing increased with decrease in density. For instance, SLCM0 of 1300 kg/m³ exhibited a variation of 15%, whereas, for 1000 kg/m³, it was found to be around 37%. Similarly, for SLCM1 and HXG0, the variation increased from 11.45% to 25.13% and 11.22% to 40.26%, respectively. This trend indicates that the stabilizer effect is more significant in mixes for higher amount of foam. Nevertheless, this variation in density (before and after printing) was insignificant in stable mixes, such as SLCM3, SLCM4, and HXG1 for both target density mixes.

Based on the above experimental results, mixes with higher viscous surfactant solutions appear more stable than mixes with lower viscous surfactant solutions. In addition, studies have shown that an increase in the lamella thickness of foam bubbles owing to stabilizer addition contributes to the improved stability of foam concrete mixes [25,40]. Based on the trends observed in Fig.4(a), Fig.4(b), and Fig.5, the desirable values of viscosity and surface tension of the surfactant required to produce stable 3DP-FC mixes can be deduced. A surfactant solution with viscosity exceeding 5 mPa·s and surface tension lower than 31 mN/m can yield stable foam concrete mixes with variations in the fresh density from the target densities within the ASTM-prescribed tolerance limits.

The SLCM2, SLCM3, SLCM4, and HXG1 mixes found to be stable and were selected for further investigation. Tab.2 presents the test results for various fresh-state-related properties, such as slump, slump flow, and yield stress. The slump and slump flow values were within the ranges of 2–6 and 149–165 mm, respectively. The results are consistent with the desirable requirements for printability [41]. Moreover, the difference in the slump values before and after extrusion was mainly unaffected, indicating foam stability during and after extrusion. The slump of SLCM2 mix (1300 kg/m³) after extrusion increased but was still within the printability range. Unlike conventional foam concrete, the influence of foam stabilizer dosage on fresh properties of 3DP-FC are found to be insignificant. Moreover, the static yield stress measured using the vane shear apparatus ranges from 0.34 to 1.03 kPa, which satisfies the printability criteria mentioned in Ref. [42]. Nevertheless, visual observation tests were performed to validate 3DP-FC printability, as discussed in the next sections.

3.4 Printability of 3DP-FC

Visual observation tests for extrudability and buildability showed that mixes with a design density of 1300 kg/m³ failed at the 13th layer, whereas those of 1000 kg/m³ failed at the 10th layer (maximum), as shown in Fig.6. The failure of these mixes at the 13th and 10th layers for target densities of 1300 and 1000 kg/m³, respectively, can be attributed to plastic deformation of the bottom layer owing to low static yield stress (0.34–1.03 kPa). Subsequently, 700-mm-long beams were printed without any discontinuity. The visual observations indicate that the foam volume significantly influences printability characteristics. Furthermore, the variation in the stabilizer concentration for the stable mixes did not significantly influence printability, in line with the fresh characteristics discussed in the previous section.

3.5 Effect of surfactant characteristics on air void distribution of 3DP-FC

The air void size was characterized using the Feret diameter to examine spherical and irregularly shaped voids, whereas the void shape was investigated using the circularity index. Tab.3 and Tab.4 list the values of the air void size and shape characteristics of 3DP-FC, along with the standard deviation (SD) and coefficient of variation (CoV). D50 denotes the median air void, whereas D90 denotes the air void size below which 90% of voids are present. Similarly, C50 is the median value of the air void circularity, and C90 is the value for air voids with decreased circularity (irregularly shaped voids). The circularity of the air voids was determined using the equation presented by Sahu et al. [19].

3.5.1 Air void size distribution of 3DP-FC

For the design densities of 1000 and 1300 kg/m³, the D50 and D90 values of all the samples were of a similar order, except for SLCM2 (1000 kg/m³) and HXG1 (1000 and 1300 kg/m³) mixes which exhibited slight lower D50 and higher D90, respectively. The results indicate that the stabilizers are efficient, and rheology of all the stable mixes was appropriate, with a good bubble-maintaining capacity [28]. However, in the case of SLCM2 (1000 kg/m³), the mixture did not attain the design density because of the instability of the foam bubbles owing to the surfactant characteristics. Similarly, for mixes containing the H surfactant, the higher D90 values are attributed to the differences in the surfactant type. Hence, unlike conventional foam concrete, the surfactant characteristics and density minimally influenced the air void distribution of 3DP-FC. Therefore, the rheology of the paste surrounding the air bubbles had more significant effect than the surfactant characteristics. Fig.7 shows the air void size distributions of SLCM2, SLCM3, SLCM4, and HXG1 mixes with target densities of 1000 and 1300 kg/m³.

3.5.2 Circularity of 3DP-FC

Based on air void size distributions results (Tab.4), it is clear that the surfactant characteristics and density of concrete had no significant effect on the circularity index. However, circularity of the H surfactant mixes was relatively lower than that of synthetic SLS surfactants. The difference in the viscosity can explain the slightly irregular air voids present in the H mixes. Fig.8 shows the circularity graphs of SLCM2, SLCM3, SLCM4, and HXG1 mixes with target densities of 1000 and 1300 kg/m³.

4 Conclusions

The aim of this study was to investigate the effects of surfactant characteristics on the extrudability and stability of 3DP-FC. Different tests on fresh properties, printability, and air void characterization were conducted for two surfactants (natural and synthetic-based) and with different foam stabilizers. The conclusions drawn from this study are as follows.

1) An increase in the stabilizer concentration significantly increased the foam density and decreased the foam drainage for the surfactants. This improvement in foam behavior can be attributed to a significant increase in the viscosity of the surfactant solution owing to the increase in the stabilizer dosage.

2) A surfactant solution with viscosity exceeding 5 mPa·s and surface tension lower than 31 mN/m could yield stable foam concrete with variations in the fresh density from the target densities within the ASTM-prescribed tolerance limits.

3) The variation in the stabilizer concentration of the stable mixes did not significantly influence the fresh-state-related characteristics, such as slump, slump flow, and static yield stress. Similarly, the impact on the air void characteristics on 3DP-FC was minimal, unlike traditional foam concrete. The above trends indicate that the adopted stabilizers and their dosages are efficient, resulting in stable 3DP-FC mixes with appropriate rheological characteristics that enhance the bubble-maintaining capacity.

4) The foam volume of the mix significantly influenced the printability characteristics. Visual observation tests for extrudability and buildability showed that mixes with design density of 1300 kg/m³ failed at the 13th layer, whereas failure occurred at 10th layer for the 1000 kg/m³ mix.

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