Enhancing the foaming effects and mechanical strength of foam glasses sintered at low temperatures

Highlights

• 3 wt% MnO₂ and 1 wt% CaCO₃ as foaming promoters yield a balanced combination of light weight and high strength.

• Molecular mechanism of the effects of CaCO₃ is elucidated by molecular dynamics simulations.

• 8 wt% foaming intensifier ZnO effectively strengthens the foam glasses.

Abstract

Effectively promoting the foaming effects and precisely controlling the pore structures remain challenging owing to insufficient understanding of the molecular mechanism. Herein, we first investigate the effects of two foaming promoters, MnO₂ and CaCO₃, to the pore structures and physical and mechanical properties of the foam glasses, where an optimal amount of 3 wt% MnO₂ and 1 wt% CaCO₃ are determined, which yield a balanced combination of light weight and high mechanical strength. More importantly, the molecular mechanism of the effects of CaCO₃ is elucidated by molecular simulations. In addition, the effects of foaming intensifier ZnO are explored, where the mechanical strength is generally enhanced by adding ZnO. Aiming at a precise control of the pore structures and desirable material behaviors with certain additions, this work should inspire systematic approaches to controlling the foaming effects by bottom-up material design for diversified engineering needs without the conventional trial-and-error approaches.

Introduction

Manufactured by recycling waste glass [1,2], coal fly ash [3,4], cathode-ray-tube panel [5], and sheet glass cullet [3] from the mass-produced industrial waste, foam glasses have been extensively used in floating materials [6,7], building industry [8,9], and refrigeration/thermal storage [9], by virtue of their superior physical and mechanical properties of light weight [7], high strength [10], thermal insulation [9], and corrosion resistance [10]. The foaming effects, which lead to different pore structures, significantly impact the physical properties and mechenical behaviors, such as bulk density and compressive strength, of the foam glasses. However, in spite of decades of research on improving the mechanical and physical properties of foam glasses [11], precisely controlling and promoting the foaming effects, thereby enhancing the mechanical properties, remain challenging owing to a lack of understanding of the molecular mechanism during the sintering process and the corresponding structure-property-process relationship. Furthermore, precise foaming promotion via effective control of the pore structures can lead to high performance (e.g. high strength and low density) and low cost (e.g. a low sintering temperature) with commercially inexpensive foaming agents and additions, which remains largely unexplored. Therefore, to attain an insight into the pore structure at a molecular level, it is important to reconcile computational and experimental approaches to attaining systematic structure-property relationships at multi-spatiotemporal scales, which may provide novel ideas in precisely and efficiently manufacturing and processing foam glass products used in diversified engineering applications. Despite most works [1,3,12,13] are on recycling solid waste into preparing foam glasses which directly meet the needs of turning waste into useful and environmental protection, using pure chemicals as raw mixtures [6,7,14] can prepare foam glasses with improved physical properties and mechanical performances (e.g. commercial foam glasses), and enable a clearer structure-property-process relationship due to explicit initial chemical compositions.

In this study, we investigate the effects of two foaming promoters, MnO₂ and CaCO₃, to the pore structures and corresponding physical and mechanical properties, and the optimal amount of addition is determined. More importantly, the molecular mechanism of the effects of CaCO₃ addition is elucidated by molecular dynamics (MD) simulations via investigating the pore structure at a molecular level, which corresponds well with the experimental findings. In addition, the effects of foaming intensifier ZnO to the properties of foam glasses are explored, where the mechanical strength is found to be enhanced in general. This work enables a precise control of the pore structures and desirable material behaviors by adding a determined amount of additions with insights at a molecular level. More importantly, we expect this study to inspire systematic approaches to controlling the foaming effects and pore structures for diversified engineering needs by bottom-up material design, which could enable higher performances of the foam glasses without the conventional trial-and-error approaches.