Preparation and Characterization of Ultra-Lightweight Foamed Concrete Incorporating Lightweight Aggregates

1Building Materials and Construction Chemistry, Technische Universität Berlin, Germany
2Structural Engineering Department, Mansoura University, Elgomhouria St., Mansoura, Egypt
3Department of Civil and Environmental Engineering, Sejong University, Korea
4Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology, Poland

Increasing interest is nowadays being paid to improving the thermal insulation of buildings in order to save energy and reduce ecological problems. Foamed concrete has unique characteristics and considerable potential as a promising material in construction applications. It is produced with a wide range of dry densities, between 600 and 1600 kg/m3. However, at a low density below 500 kg/m3, it tends to be unstable in its fresh state while exhibiting high drying shrinkage in its hardened state. In this study, lightweight aggregate-foamed concrete mixtures were prepared by the addition of preformed foam to a cement paste and aggregate. The focus of the research is the influence of fly ash, as well as fine lightweight aggregate addition, on the properties of foamed concrete with a density lower than 500 kg/m3. Concrete properties, including stability and consistency in the fresh state as well as thermal conductivity and mechanical properties in the hardened state, were evaluated in this study. Scanning electron microscopy (SEM) was used to study the microstructure of the foamed concrete. Several mixes with the same density were prepared and tested. The experimental results showed that under the same bulk density, incorporation of fine lightweight aggregate has a significant role on compressive strength development, depending on the characteristics of the lightweight aggregate. However, thermal conductivity is primarily related to the dry density of foamed concrete and only secondarily related to the aggregate content. In addition, the use of fine lightweight aggregate significantly reduces the drying shrinkage of foamed concrete. The results achieved in this work indicate the important role of lightweight aggregate on the stability of low-density foamed concrete, in both fresh and hardened states.

Introduction

Foamed concrete can be considered to be a cellular material containing many pores, with their distribution, shape, and size being significant factors controlling the material characteristics of the concrete [1]. It has high flowability and high thermal insulation properties [2,3]. Foamed concrete is used as a promising construction material in several fields, due to its reduced energy consumption, low cost, high thermal insulation, and energy saving characteristics [4,5]. It has low self-weight, a low cement content, and low aggregate usage [6]. Foamed concrete can be produced with different densities, ranging from 400 to 1600 kg/m3 [1,7]. Due to its numerous advantages, foamed concrete can be applied in many civil engineering areas, such as filling insulation, lightweight blocks, thermal and acoustic insulation, trench reinstatement, and soil stabilization [6,8]. Pore size distribution and pore volume and shape strongly affect the microstructure and characteristics of foamed concrete [9,10]. Chung et al. [11,12] examined the influence of pore size, distribution, and shape on the properties of foamed concrete with different densities with the help of X-ray micro-computed tomography (micro-CT) imaging. They reported that as the density increases, the size of the pores becomes smaller. Moreover, the density of the solid part of the foamed concrete influences pore size and shape because of the ability of the solid structure to hold the pores inside the concrete specimen. With increasing the foamed concrete density, the microstructure becomes more compact and denser [12]. As such, the physical and mechanical properties of foamed concrete can be adjusted by controlling its density.

Foamed concrete contains many pores, which enhances thermal insulation. However, this also has a negative effect on compressive strength and stiffness, which are both significantly decreased [13–15]. In addition to pore shape and size, foam stability has a considerable effect on the properties of foamed concrete. Several parameters influence the stability of foamed concrete, including the amount of foam added and the water content in the cement paste [16]. The basic materials for foamed concrete are cement, water, and foam. Sometimes fine sand, fly ash, superplasticizer, fibers, or silica fume are used as well [17]. Falliano et al. used directional composite grid reinforcement to improve the mechanical properties of foamed concrete [18]. For a given dry density, foamed concrete prepared using fly ash as a filler has better mechanical properties than the equivalent sand-based foamed concrete [19]. Foaming agents are commonly synthetic, with their content and type having a considerable influence on the properties of both fresh and hardened foamed concrete [20]. The quality of the foam is of great importance in the production of foamed concrete, because it influences the stability of the foamed concrete in the fresh state and significantly affects its stiffness and strength in the hardened state [21]. In foamed concrete mixtures, strength is strongly influenced by the type and dosage of foaming agent, as well as by the water: solid ratio [6]. Falliano et al. demonstrated that there is a strong relationship between the water: solid ratio and the type of foaming agent used, with both factors influencing the final characteristics of foamed concretes [22]. The stability of the concrete mix depends mainly on the water: solid ratio, as well as on the foam quantity and quality. These parameters also have considerable influence on the consistency of foamed concrete [23].

One of the main disadvantages of foamed concrete is the high drying shrinkage that takes place at early ages [24,25]. Due to the absence of coarse aggregates in the foamed concrete, its drying shrinkage is about 4–10 times higher than that of traditional concrete [6]. The incorporation of supplementary materials can reduce both drying shrinkage and hydration heat [26]. Several factors significantly affect the drying shrinkage of foamed concrete, including the water: binder ratio, foam volume, and foam agent type [16,23]. The addition of lightweight aggregates is considered to be an efficient method to reduce drying shrinkage [27]. Nambiar et al. [16] examined the effects of different filler materials on the shrinkage behavior of foamed concrete. They concluded that, as the filler: cement ratio increases, the shrinkage drops significantly due to the restraining influence of the increased aggregate content. Foam volume and cement paste characteristics are important factors influencing the drying shrinkage of foamed concrete [23]. The addition of fibers improves the mechanical properties of foamed concrete and reduces its drying shrinkage [28,29]. Moreover, the incorporation of mineral admixtures also affects concrete shrinkage significantly [30]. The drying shrinkage mainly depends on the type of fine material used; it increases with slag content and can be reduced with the use of fly ash [31,32]. Silica fume mixes suffer from cracking, which can be reduced significantly through the incorporation of polypropylene fibers.

Foamed concretes with a density lower than 600 kg/m3 are being developed and applied in various applications. However, at a low density below 500 kg/m3, they tend to be unstable in the fresh state and exhibit high drying shrinkage in the hardened state. Lightweight concrete with a density of 400 kg/m3 has been produced through a combination of expanded polystyrene and foam [33]. Zhihua et al. [34] developed foamed concrete with a very low density (<300 kg/m3). Huang et al. [35] reported that the addition of chemical stabilizing admixtures improves concrete stability and reduces the collapsing of foam bubbles. Foamed concrete with a density lower than 500 kg/m3 has been developed, but more investigation is needed to optimize its characteristics. The target of this research is therefore to study the properties of low-density foamed concrete. As such, several foamed concretes were prepared and tested to investigate the influence of various parameters on the properties of foamed concrete, including the incorporation of fine lightweight aggregates and the addition of fly ash as a partial cement replacement material.