Recycled cork aggregate for 3D concrete printing: Rheology, pore-driven anisotropy mitigation, and low-carbon thermal buffering
Curated from sciencedirect.com
Abstract
This study presents a multiscale investigation of 3D-printed concrete incorporating recycled cork aggregates (25–100% sand replacement) as a viable route toward low-carbon and climate-resilient construction. Cork addition markedly modifies rheological behavior—reducing dynamic yield stress from 387 to 55 Pa and plastic viscosity from 10.8 to 4.8 Pa·s. It also delays the development of static yield stress, highlighting the need for admixture optimization to maintain buildability. Mercury intrusion porosimetry reveals a sharp increase in total porosity (from 4.7% to 51.1%) and the formation of large capillary voids, resulting in reduced mechanical strength. Despite this, cork incorporation significantly mitigates mechanical anisotropy: circular statistical analysis of macro-pore orientation shows that cork disrupts the horizontal pore alignment induced by printing, promoting a more isotropic internal structure and uniform stress distribution. Thermal performance is substantially improved, with thermal conductivity reduced by 67% (from 0.91 to 0.30 W/m·K) and specific heat capacity increased from 0.8 to 1.33 J/g·°C, enhancing the material’s thermal inertia. Sustainability assessments further demonstrate the material’s promise: at ≥ 75% cork replacement, the mixture becomes carbon-negative (−40.07 kg CO₂-eq/(m³·MPa)). A building-scale simulation conducted for a cold-climate context (Nome, Alaska) reveals 13.2–35.1% annual energy savings and operational CO₂ reductions of up to 15,203 kg compared to conventional concrete systems. These findings validate recycled cork–based 3D printable concrete as a multifunctional solution integrating rheological tunability, structural reliability, thermal buffering, and net-negative carbon performance.
Introduction
Mitigating carbon emissions and energy demand in the construction industry is essential for achieving global sustainability targets, as buildings account for a substantial share of total energy use and CO₂ emissions. This calls for the development of low-carbon materials and construction methods that address emissions across both embodied and operational phases. 3D printed concrete (3DPC) has gained increasing attention as a potentially more resource-efficient alternative to conventional cast construction, owing to its formwork-free fabrication [1], reduced material waste, and high automation potential [2], [3], [4]. These features may lower material use and construction-stage impacts in suitable applications, while enabling greater design flexibility and construction efficiency. In parallel, the energy required for building operation—particularly for space heating and cooling—remains a major contributor to lifecycle emissions. In the U.S. residential sector, for instance, space conditioning alone accounted for ∼31% of total energy consumption in 2020 [5]. Although operational energy varies with climate and occupant behavior [6], [7], [8], the thermal properties of envelope materials play a critical role in influencing heat transfer and energy use [9]. Lightweight aggregate concrete (LWC) exhibits low thermal conductivity and high heat capacity [10], [11], making it an effective material for improving energy efficiency and reducing operational CO₂ emissions. Moreover, LWC formulations can incorporate recycled or waste-based aggregates, further enhancing environmental benefits through resource recovery and carbon footprint reduction [12]. Combining 3DPC with LWC offers a promising pathway toward climate-resilient and low-carbon construction by simultaneously targeting reductions in both embodied and operational emissions within a unified material system.
Conventional LWC aggregates include expanded clay [13], plastic [14], shale [15], [16], ceramsite [17], diatomite [18], pumice [19], vermiculite [20], and perlite [21]. Prior studies have explored lightweight-aggregate strategies for 3D printed concrete to reduce density and enhance thermal performance. Clay ceramsite sand has been incorporated using a shell-packing method to maintain extrusion stability, but printability remains sensitive to water availability and rheology evolution [22]. Expanded clay is typically feasible only up to about 30% replacement to retain satisfactory printability, indicating an extrudability threshold for highly absorbent aggregates [23]. Expanded polystyrene mixtures designed via excess paste theory can achieve a wide density range but require rheology control to prevent particle floating and segregation [24]. Expanded thermoplastic microspheres further lower density and can reduce thermal conductivity by about 40%, with strength penalties due to increased void content [25]. Waste-derived cenospheres with expanded glass can reduce thermal conductivity up to about 58% yet increase interlayer porosity and reduce mechanical and bond strengths, underscoring an interface-governed strength–insulation trade-off [26]. Despite these technical advances, many lightweight solutions are associated with nontrivial embodied carbon. Mineral lightweight aggregates often rely on energy-intensive kiln firing above 1000 °C [27], while expanded polystyrene products exhibit cradle-to-gate global warming potentials spanning approximately 14.6–119 kg CO2-eq/m3, depending on product form and recycled content [28]. In contrast, recycled cork aggregate offers a low-carbon alternative. Cork, the outer bark of the cork oak tree, is a natural and renewable lightweight material harvested periodically, every 9–12 years [29], sourced from 22,000 km2 of cork oak forests [29], [30]. While cork is widely used in bottle stoppers, flooring, and aerospace applications [31], over 280,000 tons of cork waste are generated globally each year [29], presenting a valuable resource for sustainable construction [32]. Cork’s cellular structure imparts low density, high specific heat, low thermal conductivity [33], [34], and satisfactory fire resistance [35], [36], making it comparable to commercial lightweight aggregates [37]. Additionally, cork is slowly biodegradable under landfill conditions, making its valorization in concrete a more sustainable end-of-life solution than disposal. Incorporating waste cork granules as recycled aggregate into 3DPC not only supports industrial waste reuse but also enables the development of multifunctional, climate-resilient concrete with improved environmental and thermal performance. Currently, though recycled aggregates have been widely used for 3DPC [38], [39], most studies focus on recycled concrete, bricks, or glass aggregates, Specifically, printability (extrudability and buildability) is often constrained by recycled-aggregate water absorption, which drives rapid rheology drift and reduces process robustness [38], [39], [40], [41]. Moreover, high replacement levels of recycled coarse aggregate tend to reduce directional mechanical performance and amplify anisotropy by promoting printing-induced pore or interlayer defects [39]. Compared with recycled concrete sand, recycled cork aggregate can provide lower thermal conductivity and higher specific heat for improved thermal buffering, yet its use in 3D printed concrete remains limited and mechanistically under-quantified.
In recent decades, growing interest has emerged in incorporating cork granules into cementitious materials to improve sustainability [42], [43], [44], [45]. Cork reduces concrete density and enhances thermal insulation [46]. Cyclic compression tests show improved energy dissipation in cork-layered concrete [47]. These findings highlight the potential of recycled cork to transform construction waste into sustainable 3D-printable building materials. However, its application in 3D-printed concrete remains limited by key knowledge gaps. First, the effects of cork content on dynamic yield stress, plastic viscosity, and the evolution of static yield stress are not quantified, constraining mixture design. Second, while cork enhances thermal insulation, it often reduces mechanical strength—necessitating a clearer understanding of the strength–insulation trade-off. Third, cork-induced changes in rheology and density may alter pore orientation during printing, potentially affecting mechanical anisotropy, yet this relationship remains underexplored. Lastly, despite cork concrete’s improved thermal properties, its impact on full-scale building energy use has rarely been quantified in simulations or real-world scenarios.
To address the need for low-carbon, climate-resilient 3D-printed concrete, this study evaluates granulated cork aggregates as a sustainable alternative to natural sand. Mixtures with 0–100% volumetric replacement were assessed for density, rheology, microstructure, mechanical performance, macro pore orientation–driven anisotropy, thermal behavior, and life cycle impact. A stratified application of cork concrete in a 3DCP Arctic single-family detached house was further analyzed through building energy simulations and sustainability metrics, adhering to Alaska Building Code 2021 requirements. By integrating multi-scale experimental data with building-scale validation, this work advances the development of eco-friendly 3D-printable materials, demonstrating how recycled plant-derived aggregates can reconcile structural feasibility, thermal efficiency, and reduced embodied carbon in modern construction.