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• A novel alkali-activated insulation mortar is prepared for energy-efficient buildings. • The effect of recycled concrete powder (RCP) on thermal insulation is studied. • Low thermal conductivity and high-volume use of RCP are achieved simultaneously. • A simulation is carried out to reveal the satisfactory energy efficiency of such mortar. The influence of recycled concrete powder (RCP) on the thermal insulation of alkali-activated materials remains unclear. Hence, this study develops an innovative alkali-activated insulation mortar, serving RCP and blast furnace slag as the precursor and expanded polystyrene (EPS) beads as fine aggregate, together with air entrainer. Experimental results show that the mortar had satisfactory workability owing to the RCP substitution, supported by appearance and infrared thermal imaging tests. As the ratio increased, the density, compressive strength, and thermal conductivity of the mortar decreased, due to the increasing volume of porous paste and internal air voids. Subsequently, the material design of the mortar was optimized through the adjusted Box-Behnken design. The obtained mortar achieved a high-volume use (61.7 wt%) of RCP, and exhibited an extremely low conductivity (∼0.030 W/(m∙K)) among novel lightweight building materials. Energy simulation further indicates that the optimized mortar can surpass traditional insulation materials, including EPS, polyurethane, and rock wool panels, in reducing air conditioning energy consumption across all seasons and climate zones. This was particularly evident in colder cities such as Harbin and Lhasa. Given that increasing the thermal resistance of the envelope can effectively reduce building energy consumption, the development of such sustainable alkali-activated insulation mortar would undoubtedly contribute to the implementation of energy-efficient buildings. [ABSTRACT FROM AUTHOR]

• Preparing geopolymer with 100% recycled brick/concrete powder (RBP/RCP) is feasible. • Fresh and mechanical properties of geopolymer made of RBP and/or RCP are reviewed. • Effects of precursor, Na/Si, Si/Al, L/S and curing regime are intensively discussed. • CO 2 emission of geopolymer made of RBP and/or RCP is analyzed and compared with OPC. Geopolymers made of recycled brick powder (RBP) and recycled concrete powder (RCP) have attracted huge attention. However, their properties have not been intensively discussed yet, which may limit further popularization. In this paper, an overview of published articles about geopolymers containing RBP and/or RCP during the last decade (2012 – 2021) is presented. The activation process of RBP and RCP, as well as fundamental properties (e.g., flowability, compressive strength, and durability) of corresponding geopolymers, are mainly discussed. The influence of precursor, Na 2 O/SiO 2 molar ratio, SiO 2 /Al 2 O 3 molar ratio, liquid/solid mass ratio, and curing regime is particularly emphasized. Subsequently, the unit carbon emission of geopolymers with a strength beyond 40 MPa is calculated based on the existing data, confirming the feasibility of using RBP and/or RCP as precursors and the necessity of adding pozzolanic materials or Portland cement in the geopolymer system. After clarifying their environmental benefits, potential proportions for sustainable high-performance geopolymers are further provided. More results and prospects are also put forward. [ABSTRACT FROM AUTHOR]

Owing to the high absorption capacity of recycled aggregate (RA), it is crucial to accurately measure its saturated-surface-dried water absorption (WSSD), which largely affects an effective water-to-binder ratio of recycled aggregate concrete. In this study, existing measurement methods for the WSSD of RA are extensively reviewed, including Wiping, Slumping, Centrifugation, Infrared, Evaporation, Airflow drying, Conductivity, Pycnometer, Hydrostatic balance, and Extrapolation. In particular, the physical principles and operability of these methods are emphasized. It was determined that the accuracy of all test results was not satisfactory. For example, the water in pores with an open-ended direction that was opposite to the centrifugal force could largely be retained. In Airflow drying, the temperature change was significantly delayed. In addition, in Hydrostatic balance, RA would pre-absorb water before determining the initial reading. Therefore, several suggestions for optimizing these methods are presented, such as the combination of Evaporation and Airflow drying, the liquid selection in Hydrostatic balance, and the addition of a tiny mixer in each centrifuge tube. In summary, this review facilitates the development of an accurate and convenient method for measuring the WSSD of RA. [ABSTRACT FROM AUTHOR]

Recycled concrete powder (RCP) is a promising supplementary cementitious material, its application in thermal insulation materials is however very limited. In this study, the influence of RCP substitution on the properties of insulation mortars was investigated. The results showed that as the RCP content increased, the compressive strength and thermal conductivity of insulation mortars decreased by 39.8% and 16.8%, owing to the increasing volumes of porous paste and internal air voids. Note that the mortar containing a high content (50 wt%) of RCP struggled to meet the strength requirements of the specification. Therefore, CO 2 curing was further employed here to alleviate the strength loss and successfully increased the compressive strength by 11%∼38%. It was found that since the CaCO 3 particles formed and densified the microstructure (confirmed by the porosity test), the strength reduction derived from the RCP substitution was compensated for. Consequently, insulation mortars incorporating 50% RCP after CO 2 curing for 7 days exhibited satisfactory compressive strength (∼ 0.22 MPa) and relatively low thermal conductivity (∼ 0.048 W/(m∙K)). The mortars showed improved aesthetic appearance and excellent thermal insulation capacity in both hot and cold environments. This study proposed a successful approach to combining reutilization of RCP and CO 2 curing in insulation mortar generation, which effectively improved the sustainability of materials. • A novel and sustainable EPS insulation mortar is developed for energy-efficient buildings. • The synergistic influence of the RCP substitution and CO 2 curing duration is studied. • Remarkably low thermal conductivity is attained through the incorporation of RCP. • Excellent aesthetic appearance and thermal performance are achieved simultaneously. [ABSTRACT FROM AUTHOR]

• 3DPM with RP had the highest compressive strength under X loading direction. • Splitting tensile and flexural strengths of 3DPM-RP were higher than cast samples. • The addition of RP increased the anisotropy coefficient of 3DPM. • The porosity of 3DPM-RP20 increased 40% (from X-CT) considering the interlayers. • RP had limited effect on pore structure of 3D printed filament without interlayer. This study investigated the hardened mechanical properties of 3D printed mortar (3DPM) with different contents (0–30% by mass of cement) of recycled powder (RP) by anisotropic compressive, splitting tensile and flexural strengths. The pore characteristics tested by X-ray computed tomography and mercury intrusion porosimeter as well as hydration products of 3DPM were analyzed to reveal the influence mechanism of RP on 3DPM. The results showed that the highest compressive strength of the 3D printed specimens was obtained on the loading direction paralleled with the length of printing filament (X direction). When the RP replacement ratio was 20%, the X-direction compressive strength of 3DPM was 24.11% lower than that of cast samples, while the splitting tensile strengths of 3DPM were closed to and the flexural strengths of 3DPM were higher than that of cast samples regardless of the loading directions. This can be attributed to the high fluidity loss rate of 3DPM with RP, leading to the reduction of the surface moisture of interlayer. Besides, more pores at the interlayer and interstrips led to the decrease of mechanical properties. In contrast, the addition of RP had a limited effect on the micro pore structure of the single printed filament without considering the interlayer and interstrip, owing to the secondary hydration and filling effect of RP. [ABSTRACT FROM AUTHOR]

Although recycled aggregate concrete (RAC) has been extensively studied, several critical problems with its fundamental behaviors still urgently need to be figured out. Therefore, this series of two reviews re-thinks fundamental behaviors of RAC based on the latest literature. In the last review (part I), the mechanical and deformation behaviors of RAC were intensively discussed. Here in part II, the relationships between the replacement ratio of recycled aggregates (RA) and the durability of RAC are further presented. It is found that, owing to the increase of total porosity, the resistance of RAC to freeze–thaw cycles, carbonation, and chloride ion penetration is usually decreased with the increasing replacement ratio of RA. As a result, the steel corrosion and corrosion-induced cracking in RAC are more serious than those in natural aggregate concrete (NAC). The prediction models for carbonation and chloride ion penetration of RAC and the corrosion-induced crack patterns are also presented. To address the degradation on the performance of RAC, several enhancement technologies such as the combined/optimisation pre-treatment on RA and fiber reinforcement are put forward. Finally, the prospects of the high-performance/efficiency methodologies and applications of RAC are also shown. [ABSTRACT FROM AUTHOR]

Recycled aggregate concrete (RAC) has received huge amounts of attention in the past two decades. However, there are still many critical problems about fundamental behaviours of RAC, which need to be figured out and which otherwise limit its further sustainable popularisation. Therefore, this series of two overviews rethinks fundamental behaviours of RAC based on the latest literature. In this overview (part I), the strength development and its mechanism, as well as failure patterns of RAC under static and dynamic loading, are explored. Then, the influencing factors and prediction models of elastic modulus, shrinkage and creep of RAC are intensively discussed. It is found that, compared with natural aggregate concrete, the 28 day compressive strength of RAC is lower, whereas the long-term compressive strength of RAC may be higher, which partly depends on the strength of the parent concrete. Furthermore, because the elastic modulus of RAC is often decreased, the drying shrinkage and creep of RAC are always increased. However, the autogenous shrinkage of RAC can be decreased, and also generally develops for above 60 days. Finally, some models have been summarised to better predict the elastic modulus and long-term deformation of RAC. [ABSTRACT FROM AUTHOR]

[Display omitted] • This paper presents a critical review on sustainable 3D printing concrete (3DPC). • The performance of 3DPC based on recycled aggregate and fibre is investigated. • Effect of fibre on mechanical and anisotropic properties of 3DPC is evaluated. • Structural behaviour, environmental impact and cost of 3D printed strucrure is given. This paper presents a critical review of reducing the life-cycle environmental impact of 3D printing concrete (3DPC) systems from the perspectives of material preparation, construction process and structure level. The material requirements of 3DPC are first introduced, then the utilization of low-carbon cementitious materials, recycled aggregates, admixture and fibres in 3DPC is explored, along with their effect on workability and mechanical property. The potential for improving the environmental benefits by applying better design and printing parameters are discussed in the subsequent part. Two main delivery systems and the effect of printing parameters (including printing speed, standoff distance) are presented. Finally, the behaviour of 3D printing components (beam, slab and column) is examined at the structural level. 3D printing technology has a high degree of freedom, thus better understanding of the component behaviour can save materials and improve strength. Finding a balance between component's performance and environmental impact is a crucial work in future. [ABSTRACT FROM AUTHOR]