# Crushing Performance of Ultra-Lightweight Foam Concrete with Fine Particle Inclusions
## 1 Introduction  
Concrete sustainability has emerged as a key theme across modern concrete studies because of the large environmental impact of cement production [1] and natural aggregate exploitation [2]. All three Rs of environmental protection--reduce, reuse, recycle--can be applied in some ways to concrete infrastructure. Recycling concrete from demolished building structures and pavements as aggregates for new constructions represents a strong potential for concrete recycling. While the use of recycled coarse aggregate is well established [3; 4; 5; 6], the fine fractions in recycled concrete aggregates pose greater challenges for material recycling in new construction [7; 8; 9]. Due to the high heterogeneity of recycled fine aggregate, there are a wide variety of concrete durability concerns, such as water permeability, long-term shrinkage, and alkali-silica reaction, which makes it difficult to recycle the fines in the production of normal concrete for structural purposes. However, good potential has been seen for using recycled fine aggregate for making controlled low strength materials (CLSM). New ideas about using fine inclusions in foam concrete, which is classified as a low-density CLSM [10; 11], open new possibilities to accommodate high-volume recycledfines for its functional material applications, such as excavatable landfill, thermal and acoustic isolators, and fire resistant and impact absorbing materials [12; 13; 14; 15]. Foam concrete is a cellularized cementitious solid made from blending aqueous foam with fresh cement paste. Depending on the volume ratio of the foam, the bulk density of foam concrete typically varies from 0.3 to 1.6 g/cm\({}^{3}\)[16]. Mixtures with density higher than 0.8 g/cm\({}^{3}\) may be applicable for structural applications [12; 17; 18]. At lower densities, the strength of the foam concrete is greatly reduced, and the material exhibits other beneficial properties, being ultra-lightweight, permeable, and crushable. In this study, attention is focused on the functional potential for the ultra-lightweight foam concrete. Although the use of fine aggregates is seen in some foam concrete studies, most of the investigated foam mixtures are denser than 0.8 g/cm\({}^{3}\)[12; 18; 19] and only a few studies focused on the influence of adding sand into low-density foamed paste [20; 21; 21]. Based on the existing literature, the strength of low-density foam concrete mixtures is sensitive to the inclusion of particles and can be affected by the filler type. It has been reported that foam concrete with fine aggregates shows a greater tendency of shrinkage, and ensuring good curing is critical [12; 22]. It is generally understood that greater sand or particle content reduces the foam stability, which usually causes strength reduction in foam concrete [18; 23; 24; 25]. For most cases, a small amount of inclusion of finer particles results in a higher strength of foam concrete. Fly ash is also considered as a filler in some studies [20; 26; 27; 28] and a favorable effect has been seen with respects to workability and strength; however, the physical and chemical nature of fly ash can be quite different to typical particle fillers. Some discussions on using other recycled fine aggregates in foam concrete are also found in literature [2; 29]. As a cellular solid, the performance of foam concrete is largely governed by its foam structure [17; 23; 30; 31; 32]. Before implementing recycled fine aggregates in foam concrete, it is pertinent to understand how the cellular microstructure is altered with the inclusion of particles, and how this alters mechanical performance. Given the promise of using recycled fines in ultra-lightweight foam concrete for low strength applications, low-density foam mixture design and processing deserve greater attention by researchers. More knowledge about efficiently incorporating particles into the foam structure is needed for advanced use of recycled fines in foam concrete production.