Green Strength Research Articles

A novel dataset for green bamboo compressive strength analysis

This paper presents a newly assembled dataset tailored for the analysis of mechanical and physical properties of four species of bamboo, Bambusa vulgaris Schrad. ex J.C.Wendl., Melocanna baccifera (Roxb.) Kurz, Bambusa cacharensis R.B.Majumdar and Thyrsostachys oliveri Gamble. Bamboo has huge potential as a sustainable building material, and compressive strength is a key mechanical property. Data on bamboo compressive strength (BCS) along with moisture content (MC) and density (ρ) were acquired through tests conducted in accordance with the IS 6874:2008 protocol. Each bamboo culm was cut down in a manner where the length of the bamboo specimen equaled its outer diameter. The mechanical and physical characteristics of the four bamboo species were compared to analogous species documented in the literature. These data provide insights into the utilization of naturally occurring structural materials harvested in Arunachal Pradesh (India), offering potential applications across various engineering disciplines, notably in the realm of sustainable construction.

Influence of chemical admixtures on buildability and deformation of concrete for additive manufacturing

In this study, the buildability of three-dimensionally (3D) printed (3DP) concrete that uses coarse aggregate was evaluated in the fresh state according to the contents of three chemical admixtures (high-range water-reducing agent (HRWRA), viscosity-modifying agent (VMA), and setting-time modifying agent), and the correlation between fluidity changes and buildability was analyzed. Polycarboxylate HRWRA and VMA contents were varied to control the flowability and static yield stress, which affect the workability. In addition, the SMA content was varied, and slump tests and table flow tests were performed at 30-minute intervals up to 6.5 h after mixing to measure flowability. Simultaneously, static yield stress was measured to analyze rheological changes, and penetration resistance and green strength tests were performed to analyze the main factors influencing buildability over time. The highest buildability was observed when the VMA content was 1.5 %. The static yield stress of fresh concrete could be ≤3000 Pa to guarantee adequate 3DP concrete transport performance while that at 1 h after mixing could be ≥2000 Pa to secure buildability. When the penetration resistance of fresh concrete is ≥80 kPa, the deflection of concrete may be <10 %. Through this, buildability can be approximately judged based on penetration resistance.

Use of aqueous polyvinyl alcohol in binder jetting of Inconel 718

AbstractBinders used in binder jetting often pose health and environmental risks during processing and post processing operations. The print-heads which are used to deposit binder selectively on the feedstock are prone to clogging, despite the trend of print-heads being highly customised to suit different kinds of binders. These factors often hide the advantages of binder jetting as an additive manufacturing process, especially its scalability and its faster printing rates in comparison to powder bed fusion methods. The work presented here takes a step back and focuses on the development of an aqueous, polyvinyl alcohol (PVA)-based liquid binder that is easy to manufacture and store, safe to handle, and can be reliably jetted to print parts. The feedstock considered was Inconel 718, a nickel-based super alloy which can be effectively processed by binder jetting without niobium segregation. PVA was added to the Inconel 718 powder in dry, granular form to manufacture a modified feedstock. The study also investigated the role of molecular weight of the PVA used, sintering environments and post-processing methods like hot isostatic pressing (HIP) on process responses like part densification, tensile strength, and hardness. Three different types of PVA were chosen which had molecular weights 10,000 g/mol (low molecular weight or LMW), 26,000 g/mol (medium molecular weight or MMW), and 84,000 g/mol (high molecular weight or HMW). The compatibility of the liquid, aqueous PVA-based binders with virgin Inconel 718 was examined by measuring the contact angle. The liquid, aqueous binder having MMW PVA reported better wetting with the Inconel 718 powder with a wetting angle of 26.6 which was lower than the wetting angle of 42.4°, seen in case of a commercial resin-based binder. The green strength reported by the MMW PVA liquid binder was 220 kPa which was higher than the other two PVA-based liquid binders. Green parts, upon successful printing, were sintered at 1260 °C. It was observed that a part printed using MMW PVA had a densification of 96.16% when sintered in 99.98% by volume argon gas, which increased to 98.96% after undergoing HIP. The same part reported a densification of 88.69% when sintered in a 95% by volume N2 and 5% by volume H2 gaseous environment, which was later attributed to the uptake of nitrogen by the chromium present in Inconel 718, which prevented necking between particles. Tensile specimens printed using MMW PVA, sintered in a 99.98% argon environment, showed the highest ultimate tensile strength of 220 MPa, which increased to 1010 MPa after the HIP process, which can be compared to commercially available Inconel 718.

Synergistic enhancement of magic triangle properties of PC tread stocks modified by amine-capped trans-1,4-poly (butadiene-co-isoprene)

The development of high-performance “green tires” with synergistically improved “magic triangle” properties like lower rolling resistance, higher wet-skid resistance and higher abrasion resistance has always been a hot issue. In this work, an effective strategy for developing high-performance “green tires” with simultaneously improved “magic triangle” properties of solution-polymerized styrene-butadiene rubber (SSBR)/cis-1,4-polybutadiene rubber (BR) nanocomposites modified by amine-capped trans-1,4-poly(butadiene-co-isoprene) copolymers (F-TBIR) was proposed. A series of F-TBIR with 10–60 mol% amine-capped efficiency (CE) and 30-90 × 104 weight-average molecular weight (Mw) were synthesized by using heterogeneous TiCl4/MgCl2–Al(i-Bu)3 Ziegler-Natta catalyst with dicyclohexylamine (DCHA) as chain transfer agent (CTA). With the increase in CE of F-TBIR, the silica-filled SSBR/BR/F-TBIR compounds exhibited improved green strength, modulus at 100 % elongation and bound rubber, and their vulcanizates showed synergistically improved “magic triangle” properties like obviously reduced rolling resistance and abrasion loss, and increased wet-skid resistance. It was found that the incorporation of 10 phr F-TBIR3 with CE of 60 mol% and Mw of 32 × 104 resulted in highly expected properties of the SSBR/BR/F-TBIR3 nanocomposite. The contribution mechanism of F-TBIR3 was discussed based on the improvements of polymer network structures and filler network structures. This work is expected to provide an effective strategy to construct the desired network structures for high-performance rubber composites.

Qualitative analysis of rivers sand sources (Silica sand, Ravi river sand, Indus river sand, and Jhelum river sand) for metal casting: a comparative study

Abstract Recent geological investigations revealed extensive use of Silica sand in Pakistan resulting in depletion of tectonic resources. Hence, it is important to explore cost-effective and alternative sources of sand for its applications. Also, Pakistan has a river network that provides river sand as a potential substitute for Silica sand. However, it is essential to systematically study and assess the potential applicability of river sand as an alternative to Silica sand. In this context, this study examined sand samples collected from various rivers in Pakistan to investigate their suitability for metal-casting molds. For that purpose, random samples were selected from major rivers in Pakistan for characterization by utilizing chemical composition analysis, microscopy, XRD, TGA-DSC (thermogravimetric analysis), clay content analysis, grain size distribution analysis, pH measurement, green and dry compression strength testing, shear strength testing, permeability analysis, shatter index analysis, compactibility testing, and Specific gravity measurement. The chemical composition analysis revealed that all types of sands contained over 90% SiO2 content. The mechanical tests demonstrated that river sand exhibited acceptable mechanical properties, with a moisture content ranging from 4.5-5%. The pH value of all the sands exceeded 7, making them suitable for casting purposes. These investigations suggest that river sand can be effectively utilized in casting applications by implementing pre-processing techniques and incorporating additives.

Comparative analysis of ternary blended cement with clay and engineering brick aggregate for high-performance 3D printing

Utilising recycled brick aggregate in cementitious mixtures can decrease the overdependency on limited natural resources and improve the sustainability of concrete. This paper presents a potential solution to lower the amount of waste being landfilled and increase the sustainability of 3D concrete printing technology by combining low-carbon ternary blended cement with recycled aggregates. Hence, the effect of incorporating two types of recycled brick aggregate - clay brick (CBA) and engineering brick (EBA) on the properties of 3D printable ternary blended cement were investigated. The natural aggregate in a pre-existing 3D printable blend was substituted by up to 50 wt.-% with two varieties of recycled brick aggregates available throughout Europe. The recycled brick aggregates underwent characterisation to determine their properties. The fresh property evaluation using the green strength test was used to assess the effect of aggregate replacements on the mixture's shape stability. The mechanical performance of mixtures containing CBA and EBA, both cast and 3D printable mixes, was evaluated and compared to that of the control sample. The results indicated that incorporating recycled brick aggregate enhances green strength and Young's modulus significantly. Mechanical strength performance showed significant enhancement when incorporating RBA, which reached up to 67% and 55% for both cast and 3D printing methods, respectively. The suitability of the developed mix formulations for 3D printing was assessed by printing cylindrical objects.

3D Printed Concrete: Fresh and Hardened Properties

3D concrete printing (3D CP) is an advanced form of additive manufacturing (AM), that allows for the creation of complex geometrical structures using concrete. This technology has the potential to reduce construction time and labour costs while minimizing material waste. However, there are also challenges related to material properties, structural integrity, and standardization of construction practices. The primary challenge in developing 3D printable cementitious materials lies in engineering specific fresh properties that enable extrusion-based printing. Unlike cast concrete, where formwork provides dimensional stability during the printing process, the absence of formwork in 3D printing necessitates fresh mixtures with low-slump and high-thixotropy to be suitable for concrete 3D printing. Additionally, the orthotropic nature of the resulting 3D printed object due to the extrusion process impacts its mechanical properties. Therefore, this research aims to study the effect of water content and loading directions on 3D printed concrete. A pre-mix material from Sika was used in this study. Three mixes were prepared by using three different water to dry material (w/d) ratios, 0.140 l/kg, 0.145 l/kg and 0.150 l/kg. The experiments focused on analysing flowability, green strength, mechanical properties (compressive strength, flexural strength, and modulus of elasticity), water absorption, porosity, and chemical transformations at high temperatures through thermoanalytical measurement. The results showed that the mixture containing 0.145 l/kg (w/d) exhibited satisfactory 3D printability, optimal mechanical performance, lower porosities compared with mixtures 0.140 and 0.150, and same total porosity compared with cast specimen.

Castable refractory materials from magnesium oxychloride cement-bonded cordierite-mullite

Cordierite-mullite ceramics are well-suited for use as kiln furniture due to their superior mechanical strength and thermal shock resistance. This study explores the development of cordierite-mullite refractory castables bonded with magnesium oxychloride (MOC) cement. These castables were created using calcined kaolin aggregates and a matrix comprised light-burned magnesia, calcined alumina, milled sand, and magnesium chloride hexahydrate. Interaction between light-burned magnesia and magnesium chloride hexahydrate produced a needle-like MOC cement phase, significantly enhancing the green strength of the castables. After firing at 1350 °C for 4 h, the MOC crystals decomposed, forming robust cordierite and mullite phases. The resulting sintered castables demonstrated a modulus of rupture of 14.55 MPa, a bulk density of 1.95 g/cm3, a porosity of 24.84 %, and a thermal expansion coefficient of 2.89x10−6°C−1. These findings indicate the effectiveness of MOC cement bonding for manufacturing high-strength cordierite-mullite refractory castables, making them ideal for kiln furniture applications.

Corrosion Evaluation of Metal Binder-Jetted Stainless Steel Parts: Influence of Printing Parameters on Performance

Additive manufacturing is a rapidly expanding fabrication process in both research and industry, capable of constructing parts layer by layer from a 3D CAD model. ASTM recognizes seven distinct additive manufacturing processes, with the majority capable of producing metal components. Among these technologies, binder jetting stands out, offering the potential to create a variety of metallic parts for applications in the biomedical and energy sectors at reduced costs and shorter lead times. Achieving acceptable corrosion resistance is crucial for such applications. Binder jetting exhibits advantages such as relatively high part density (~99%) and good surface roughness (~6μm for as-sintered parts), making it an ideal candidate for investigating corrosion behavior, given the general correlation between low surface roughness, high density, and improved corrosion performance. Despite its promise, binder jetting lacks standardized procedures for producing parts with specific properties, necessitating users to determine parameters through trial and error. The corrosion behavior of parts created through binder jetting remains uncertain due to various influencing parameters. This study focuses on two key printing factors: layer thickness and binder saturation, chosen due to their impact on surface roughness, density, and thereby on corrosion. Existing literature suggests that increased layer thickness may reduce powder bed density, potentially affecting green part density and causing more significant shrinkage during sintering. Simultaneously, binder saturation is recognized as a critical factor influencing final part quality. Predicting the combined impact of layer thickness and binder saturation on corrosion performance proves challenging. To unravel this relationship, we conducted experiments using an ExOne Innovate Plus metal binder jetting machine and 316L stainless steel metal powder (mean powder size of 22μm) which is widely recognized as the most common powder in research. A number of test runs were conducted by varying layer thickness and binder saturation, offering insights into their nuanced interplay in the context of corrosion performance. Initially, to address insufficient green strength, printer drying time and curing cycle time were optimized by conducting a series of experiments changing the drying time and curing time. Subsequent printing involved changing binder saturation parallel to the layer thickness variations, resulting in distinct parts with varying properties. Then the predetermined curing cycle was executed, followed by de-powdering, and sintering in a furnace. Sintering is imperative to increase the part density and enhance the overall strength of the final product. However, it's essential to acknowledge that sintering may adversely impact corrosion resistance due to the remaining porosity and surface fissures. The choice of sintering profile becomes crucial, necessitating careful consideration to minimize elemental migration during sintering and mitigate porosity. Scanning electron microscopy and optical microscopy are used to analyze sintered parts and interpret corrosion mechanisms. Electrochemical tests were conducted to identify corrosion rates, corrosion potential, passivation behavior, pitting potential, etc. in contrast with conventionally manufactured 316L in M NaCl solution. This study offers insights and recommendations aimed at enhancing the corrosion resistance of binder jetted stainless steel parts.

Enhanced comprehensive properties of alumina ceramic shells by a pennisetum fiber/AlF3•3H2O powder hybrid system

The strength and permeability to gas of ceramic shells are key factors affecting the quality of castings. This research aims to improve the properties of alumina-based ceramic shells (i.e., Al2O3 and SiO2) using a combination of pennisetum fiber and AlF3•3H2O powder. The pennisetum fiber can improve the green strength of ceramic shells, while AlF3•3H2O powder can improve the high-temperature strength of ceramic shells by inducing the generation of mullite (3Al2O3•2SiO2) whiskers at high temperatures. Meanwhile, the mixture of pennisetum fibers and mullite whiskers can also improve the permeability to gas of the ceramic shell. As a result, the modified ceramic shell shows a clear increase in the above-mentioned properties, including 43 % of the green strength, 100 % of the permeability to gas, and 30 % of the thermal diffusion coefficient. In addition, the high-temperature bending strength and deformation under the weight of the ceramic shells are also obtained.

Evaluating pumice as a sustainable raw material in porcelain tile production: impact on technical properties

AbstractPumice, a porous rock resulting from the rapid cooling of tuff fragments during volcanic activity, exhibits a spongy texture and light color due to its low density. Found predominantly in Central Anatolia and Eastern Anatolia, it has drawn interest for industrial applications. This study delved into utilizing micronized pumice within the porcelain tile manufacturing process. Comparative analyses were conducted between formulations incorporating micronized pumice and the standard ceramic tile recipe. In place of feldspar, micronized pumice was introduced at concentrations of 3%, 5%, and 7%, while clay was substituted with micronized pumice at concentrations of 3%, 5%, 7%, and 10% by weight. The prepared bodies were fired in an industrial kiln at 1210 °C for 54 min, and various physical and mechanical properties were evaluated. These included viscosity, sieve residue, green strength pre-firing, firing shrinkage, water absorption, firing strength, and firing color after-firing. The results indicated that the samples incorporating micronized pumice closely matched the physical and mechanical properties of the standard porcelain tile. Phase and microstructural analyses revealed the presence of mullite and quartz phases. Notably, micronized pumice demonstrated promise as a substitute for clay or feldspar, with the optimal usage rate determined to be 7% in the porcelain tile recipe. This indicates that pumice has the potential to be an alternative raw material in the production of porcelain tiles.

Enhancing the Flexural Strength of AlN with an Additional Cross-Linking Mechanism in the Aqueous Isobam Gelling System.

Isobam is widely used for fabricating ceramics through spontaneous gelation and has attracted considerable interest. However, the disadvantage of the Isobam system is the low gelation strength. The effects of suitable additives and the mechanism by which they effectively enhance the green body strength and the rheological behavior of an aluminum nitride (AlN) slurry with 50 vol% solid loading were investigated using polyethyleneimine (PEI), hydantoin epoxy resin, and trimethylolpropane triglycidyl ether (TMPGE). Results showed that the additives acted as both dispersants and cross-linkers in the AlN suspension using the Isobam gelling system. The flexural strength of the AlN green body increased by 42%, 204%, and 268% with the addition of 1 wt% PEI, 1 wt% hydantoin epoxy resin, and 0.5 wt% TMPGE, respectively. After sintering at 1700 °C, the AlN ceramic with 0.5 wt% TMPGE had flexural strength and thermal conductivity of 235 MPa and 166.44 W/(m·K), respectively, showing superior performance to the ceramics without additives.

Countries’ green brands within the context of sustainable development goals

This study aims to fill the gap in estimating the Green Country Brand Strength Index and its convergence among EU countries and Ukraine (as a potential EU candidate) from 2006 to 2020. Drawing upon data from the World Data Bank, Eurostat, and The Food and Agriculture Organization, the study employs various analytical tools including principal component analysis, the global Malmquist‒Luenberger productivity index, the entropy method, and σ- and β-convergence theories. The findings suggest a notable increase in the green brand values of the analyzed countries from 2006 to 2020, with a significant expansion of the measurement scale. Notably, France, Germany, Spain, the Netherlands, and Sweden emerged as leaders in the Green Country Brand Strength Index by 2020. Furthermore, the study uncovers convergent policies among EU countries aimed at fostering green brands aligned with the Sustainable Development Goals, as evidenced by a decline in σ- and β-convergences. This research provides valuable insights into the dynamics of green branding and policy convergence within the EU and prospective candidate countries. It introduces a distinctive approach to assessing and analyzing green brand strength, emphasizing its critical importance to the formulation of sustainable development policies and strategies.

Microstructure and properties of tungsten‐40 wt.–% copper composites fabricated by binder jet 3D printing

AbstractIn this work, the tungsten‐copper composites were fabricated by binder jet 3D printing technology. The effects of different printing layer thicknesses (50 μm to 100 μm) on the green density and green strength were investigated. Then, under the optimal layer thickness, the effects of different sintering temperatures (1300 °C to 1600 °C) on the microstructure and densification of tungsten‐40 wt.–% copper composites were studied. The experimental results showed that when the printing layer thickness was set as a lower value (50 μm), the tungsten‐40 wt.–% copper green part had the best strength and its density reached a maximum value of 60.73 %. After sintering for green parts printed with 50 μm layer thickness, the relative density, thermal conductivity and electrical conductivity of sintered parts first increased and then decreased with increasing sintering temperature. In our experiments, the tungsten‐40 wt.–% copper sample fabricated by binder jet 3D printing with a printing layer thickness of 50 μm under a sintering temperature of 1500 °C displayed the best properties, in which the relative density, thermal conductivity and electrical conductivity were 97.04 %, 228 W⋅m−1⋅K−1 and 46.5 % IACS, respectively.

Potential of Reusing 3D Printed Concrete (3DPC) Fine Recycled Aggregates as a Strategy towards Decreasing Cement Content in 3DPC.

This paper explores the new potential strategy of using fine recycled aggregates (fRA) derived from waste 3D printed concrete (3DPC) as a substitute for cement in additive manufacturing. This study hypothesizes that fRA can optimize mixture design, reduce cement content, and contribute to sustainable construction practices. Experimental programs were conducted to evaluate the fresh and hardened properties, printability window, and buildability of 3DPC mixes containing fRA. Mixes with replacement rates of cement with fRA by 10 vol%, 20 vol%, 30 vol%, 40 vol%, and 50 vol% were produced. A comprehensive experimental protocol consisting of rheological studies (static and dynamic yield stress), dynamic elastic modulus determination (first 24 h of hydration), flexural and compressive strengths (2 d and 28 d), and an open porosity test was performed. The obtained results were verified by printing tests. In addition, an economic and environmental life cycle assessment (LCA) of the mixes was performed. The results indicate that up to 50 vol% cement replacement with fRA is feasible, albeit with some technical drawbacks. While fRA incorporation enhances sustainability by reducing CO2 emissions and material costs, it adversely affects the printability window, green strength, setting time, and mechanical properties, particularly in the initial curing stages. Therefore, with higher replacement rates (above 20 vol%), potential optimization efforts are needed to mitigate drawbacks such as reduced green strength and buildability. Notably, replacement rates of up to 20 vol% can be successfully used without compromising the overall material properties or altering the mixture design. The LCA analysis shows that reducing the cement content and increasing the fRA addition results in a significant reduction in mix cost (up to 24%) and a substantial decrease in equivalent CO2 emissions (up to 48%). In conclusion, this study underscores the potential of fRA as a sustainable alternative to cement in 3D printed concrete.

Interclonal variability in sensitivity to wind breakage: Comparative analysis of the mechanical behaviour of stems of two Hevea clones

The rubber tree (Hevea brasiliensis) is the main source of natural rubber production, with more than 14 million tonnes produced each year. Unfortunately, throughout their economic lifespan on the plantation (between 35 and 40 years), heveas trees are sensitive to wind damage, especially trunk breakage. Over a 30-year period, it is estimated that losses due to the wind breakage of tree stems reach nearly 40%. Industrial hevea plantations are organized in monoclonal plots. Clones are selected based on their productivity, disease resistance, etc., but not for their mechanical behaviour or sensitivity to wind-induced breakage, as breeders lack the appropriate tools to assess these criteria. Field observations highlight variations among clones in their susceptibility to wind breakage, underscoring the increased importance of evaluating mechanical criteria. Among the known causes of these differences in Hevea trees are competition between latex production and growth, crown shape and the presence of tension wood. In this article, we investigated the mechanical behaviour of the stems of two Hevea clones. One, IRCA825, is recognized as very sensitive, while the other, IRCA41, is known for its resistance to wind breakage. We measured biomechanical parameters such as trunk bending stiffness (EI), green wood stiffness (E) and modulus of rupture (MOR). To assess these parameters, we performed bending tests until breakage on standing trees of clonal fields in Ivory Coast, followed by wood characterisation tests in laboratory. A comparative analysis of the bending tests revealed that clones IRC825 and IRCA41 exhibit equivalent trunk bending stiffness. Nevertheless, IRCA41 differs from IRCA825 by having higher green wood stiffness and breaking strength, attributes that mainly explains its higher wind resistance. Comparative analysis of the wood structure indicated that the differences in wood stiffness and breaking strength are mainly attributed to the higher wood density in IRCA41 compared to IRCA825, with no discernible differences in microfibril angle and specific modulus.

Sustainable reuse of waste ceramic tiles powder and waste brick powder as a replacement for cement on green high strength concrete properties

Nowadays, reducing construction waste has grabbed the attention. As bricks and ceramic tiles represent more than 50% of the ceramic waste in many European countries. Thus, the recycling of this waste type is one of the most significant challenges within the paradigm of the circular economy. This paper investigated the impact of substitution levels of cement by waste ceramic powder (WCP) and waste brick powder (WBP) at 0%, 5%, 10% and 15%, on the HSC characteristics. The WBP and WCP materials were characterized in detail by laser granulometry, XRF and XRD measurements, followed by standard mixing, production, and curing of concrete samples. The experiments on dry density, modulus of elasticity, flexural strength, splitting tensile strength, compressive strength, resistance to sulfate attack, water absorption and ultrasonic pulse velocity were conducted to evaluate the hardened properties of concrete. It was demonstrated that the durability and strength of concrete containing WBP and WCP as partial replacements for cement are marginally inferior to those of the control sample. On the other hand, samples containing WBP had a lesser negative effect on HSC properties in comparison with samples containing WCP. However, employing a 5% WBP with 10% WCP mixture enhanced the characteristics of the HSC in comparison to samples containing various percentages of WCP individually. In addition, the microstructure analyses revealed that the addition of 10% WCP and 5%WBP to HSC specimens resulted in higher hydration products and a slightly denser concrete matrix compared to samples containing various percentages of WCP individually. Research findings indicate that a 15% substitution of cement with WCP or WBP illustrated an environmental benefit in concrete production due to a 13.1% reduction in specific energy consumption.

Formation of green strength in hydraulically bonded refractory castables at room temperature

A sufficient green strength of refractory castables is necessary to avoid damage during demoulding and heating up of refractory linings and prefabricated parts. However, there is a lack of knowledge on the mechanisms underlying the build-up of green strength in refractory castables. This work focuses on how green strength is formed within the matrix of PCE-dispersed and CA cement bonded refractory castables during their setting.Setting was monitored using several measuring methods, such as sonic velocity, amount of hydrate phases formed from CA cement, ζ-potential (stability resp. coagulation of the matrix suspension), porosity, and specific surface area and was linked to the corresponding increase of green strength of refractory castables during setting.Considered by time after adding water, the first stiffening of the refractory castables is related to the collapse of the rheological system of their matrix suspension (coagulation), resulting in a very low increase in green strength (apparent hardening). This stiffening occurs early if Ca-precipitates are formed together with molecules of the dispersing agent (Ca-PCE, PCE with short side chains) or if the amount of pore water is being reduced below a critical value (rheologically stable matrix due to dispersion of hydrate phases by PCE with long side chains). The major increase in green strength occurs when a significant amount of CA cement reacts to hydrate phases and, correspondingly, water is being bonded. This main increase in green strength may occur directly after stiffening (if CA cement is being hydrated quickly) or several hours after stiffening (if the hydration of CA cement is retarded).Regarding the green strength, the predominant factor influencing the build-up of green strength in refractory castables is the reduction of free water in the pores due to its bond in hydrate phases (reduction of a lubricating, interparticle water film). Minor influences are the reduction in porosity (volume expansion by hydration of CA cement), the increase in the specific surface area (hydrate phases are finer than the initial CA cement particles), and the entanglement of plate-like hydrate phases.

Upscaling earth formworks: 3D printing strategies for material optimised reinforced concrete structures

There is a growing need to understand how locally sourced earthen materials can be processed to build more efficiently and sustainably. Earthen formworks combined with 3D printing technologies present a unique opportunity for the concrete construction sector to address the wastefulness and complexity of custom formworks. The current state-of-the-art projects in academia and industry demonstrate that earthen formwork strategies effectively address this challenge, but remain burdened by upscaling issues such as production speed. This research bridges the gap by exploring strategies for 3D Printed earth formworks to efficiently produce structural elements using custom self-compacting and set-on-demand concrete mixtures. A first base earth mix is developed for reduced shrinkage and later modified via a plasticizer for increased green strength, forming the final mix. Two mix iterations are deployed in two corresponding strategies where concrete is cast into the earth formwork in a dry or plastic state. The methods highlighting the setups for 3D printing and procedures for appropriate material processing such as slump flow, shrinkage and rheology are presented. The results are explored via two column prototypes leading to a final demonstrator for a 2 m high reinforced concrete column. Conclusions are drawn on the implications of the two casting strategies, the current persisting challenges and the crucial next steps for development. Thus, the research provides a foundation for how clay formworks can be upscaled effectively for more sustainable production of complex concrete structures.

Adhesive and alloying properties of dual purpose polyfurfuryl alcohol binder for binder jet additive manufacturing of steel

In-situ alloying in steel can be achieved in the binder jet additive manufacturing process by incorporating alloying elements into the binder. However, using binders with nanoparticle suspensions often leads to challenges with particle dispersion uniformity and nozzle clogging. To circumvent these limitations, we investigated the feasibility of using a particle-free binder based on poly(furfuryl alcohol) (PFA) for binder jet 3D printing of steel. The PFA binder serves dual purposes – (i) imparting structural integrity to the as-printed green part and (ii) providing carbon upon pyrolysis to alloy the printed iron parts into steel. The PFA binder was first dispensed layer-by-layer into a low alloy steel powder bed to produce green parts with a storage modulus of 3360 MPa and a compressive strength of up to 9 MPa. Compared to the control, a commercial carbon binder ink, the present particle-free PFA formulation offered better green part rigidity (+18%) and strength (+18.5%) for the same amount of carbon alloying. The green parts were then subjected to debinding and sintering to consolidate the steel powder particles. During vacuum sintering, the PFA was pyrolyzed and left behind a carbon residue which diffused into the steel part to form a hard and strong ferrite-carbide aggregate phase that significantly enhanced the hardness, yield strength and ultimate tensile strength of the sintered steel parts. Furthermore, by varying the amount of this particle-free binder formulation at different locations, components with site-specific microstructures and mechanical responses, confirmed through hardness tests and digital image correlation, were also demonstrated.