High strength/density ratio in a syntactic foam made from one-part mix geopolymer and cenospheres

Abstract

By designing a composite of one-part mix geopolymer and hollow cenospheres, a commercially viable and environmentally-friendly foam was synthesised with a high strength/density ratio. The composite is made of a dry mix powder of geopolymer source materials, sodium silicate alkali activator and cenospheres, which starts to react when mixed with water. As the geopolymer reacts and gains strength over time, the surface of the cenospheres takes part in the reaction and forms a strong bond with the binding matrix. Synchrotron-based Fourier transform infrared microspectroscopy revealed, for the first time, the chemical bonding interaction of the amorphous interfacial layer between the geopolymer and cenospheres. The resulting foam composite gained a strength of 17.5 MPa at a density of 978 kg/m³, which is noticeably higher than that of existing environmentally-friendly lightweight foams made under ambient conditions. The thermal conductivity of the foam was measured to be around 0.28 kW/mK, which is similar to that of foam concrete. This foam produced in this study is found to be lightweight, strong and possess a desirable insulating capacity, while the preparation process of the one-part mix composite is maintained simply by adding water and curing the mixture at an ambient temperature.

Introduction

Lightweight prefabricated panels have been widely used for construction and refurbishment [1]. The application of these building elements in construction has many advantages. Their light weight simplifies the handling of the panels and reduces the dead load of buildings. By specialising the design of these panels for rapid assembly and insulation, they remarkably reduce the construction time, and improve the acoustic and thermal performance of buildings [2]. Also, by reducing the quantity of required materials and increasing the potential for recycled waste, lightweight building elements can reduce the embodied energy and carbon footprint of buildings.

Many studies have been conducted on developing lightweight concrete and composites for non-structural and structural building components [3,4]. Polymeric foams have been explored as a core of lightweight sandwich panels, and in fact expanded polystyrene (EPS) is widely used in Australia. However, the problem with polymeric foams is their vulnerability to high temperatures and fire. There is growing interest in improving the properties of these panels while maintaining their low cost in order to develop environmentally friendly options [5]. Lightweight concrete has been researched for decades as a potential fire-resistant building component; with many advantages such as high durability, long service life and low cost, concrete seems to be an ideal material for modular construction if it can be lighter [6]. Many techniques have been applied for manufacturing concretes with a lower density, and the most popular technique is the application of lightweight aggregates [7]. The low fire resistance of polymeric foams can be compensated by embedding them in a fire-resistant cementitious matrix, and the resulting composite would thereby have the advantages of both components to some extent.

In addition to using the lightweight aggregates, the other popular technique available for reducing the weight of concrete is by inserting air voids into the cement matrix. This technique, which is known as foaming, can be conducted by mechanically mixing a pre-made foam with cement paste or adding a chemical foaming agent (such as hydrogen peroxide) that releases gas as a result of its reaction in an alkaline environment [4]. Compared with the use of lightweight aggregates, the foaming technique is generally more efficient in reducing overall density. Therefore, ultra-lightweight components with densities as low as 300 kg/m³ can be developed. However, the major problem associated with foaming is that controlling the density is not straightforward [8]. The stability of pre-made foam, the setting time of the cement, and the simultaneous reaction of the binder and chemical foaming agent, bring complications to the design and manufacturing of the foam concrete. Some variations in the quality and density of the foamed concrete seem unavoidable during the manufacturing process. Above all, the main problem is that foam concrete is in general not strong enough for applications as a structural component. Lightweight and foam concrete with a density of 1000 kg/m³ (or lower) have been broadly researched for non-structural applications. The typical lightweight concrete in this density range can reach a strength of about 2–7 MPa [9]. However, for structural lightweight systems such as floor systems, a strength in the range of 10–14 MPa is required [10].

This study explores the possibility of developing high-strength lightweight composites using one-part mix geopolymers and cenospheres. Geopolymers are known as environmentally-friendly construction materials that can convert landfill wastes such as fly ash and blast furnace slag into useful cementitious binders [[11], [12], [13]]. The conventional geopolymer reaction process involves the alkali activation of aluminosilicate powders, often sourced as waste materials, by using alkaline solutions such as water glass and sodium hydroxide. The key silica and alumina elements of the powder precursors are released into the alkaline aqueous environment and undergo speciation, reorganisation, gelation and polymerisation stages until they form a three-dimensional aluminosilicate network. One of the problems with this form of geopolymer synthesis is the difficult handling of the alkali solutions; nevertheless, one-part geopolymers have been explored to resolve this problem [14].

A cenosphere is an aluminosilicate hollow sphere which is filled with air or inert gas, and is generated as a by-product of coal combustion in thermal power plants, readily separated from the bulk of the combustion ash by density separation [15]. The rigidity, lightweight, small size and spherical shape of cenospheres have made them very useful for manufacturing syntactic foams [16]. Syntactic foams are lightweight composites that are made from hollow spheres and a binding matrix. The matrix material can be any metal, ceramic, polymer or resin that can hold the lightweight filler together and give it the desired shape. Lightweight cementitious composites made from cenospheres are gaining much attention recently because of their attractive structural and thermal performance.

Blanco et al. [17] manufactured lightweight concretes using cenospheres. They used powder packing theory to optimize the properties of the concrete, but no microstructural enhancements and high curing temperatures were applied. The optimum strength achieved for low densities (around 1000 kg/m³) was 5 MPa [17]. Huang et al. [18] used cenospheres and industrial wastes to produce environmentally friendly lightweight composites. They achieved mixtures as light as 1649 kg/m³ with the compressive strength of 25 MPa, whereby the samples were cured under ambient conditions [18]. Nematollahi et al. [19] studied lightweight geopolymer composites using cenospheres, expanded perlite and expanded glass aggregates, achieving densities of 1586–1833 kg/m³ and reporting strengths of 43.4–56.8 MPa [19]. Gao et al. [20] used aerogels to reduce the density of cement composites to 1000 kg/m³ and gained a strength of about 8.3 MPa [20]. Topҫu et al. [21] fabricated lightweight cement composites using diatomite and pumice lightweight aggregates. The density of the samples with pumice was dropped to around 1500 kg/m³, and with diatomite as low as 900 kg/m³. For the lightweight diatomite samples, the maximum strength was reported to be around 6 MPa [21]. Ng et al. [22] reported making lightweight composites from cement and aerogels. They reported that the optimum amount of aerogel addition is about 50 vol%, yielding samples of density 1400 kg/m³ with a strength of 20 MPa at 28th day; the strength of the samples sharply dropped with higher amounts of aerogel (i.e. lower densities) [22].

Some researchers also applied microstructural enhancement techniques to achieve higher strength to density ratios. Hanif et al. [23] produced lightweight composites using cenospheres and aerogel, and achieved high strength to density ratio. They enhanced the binder performance by the addition of silica fume, PVA fibre and superplasticizer. Their oven dried samples could reach 1003 kg/m³ density and 18.63 MPa strength, but the oven temperature and the duration of drying was not reported [23]. Senthamarai et al. [24] also investigated replacing cement with cenospheres, and adding silica fume to compensate for the strength reduction caused by the cenospheres. The density of the samples was not reported, but 12% silica fume replacement helped in enhancing the microstructure of the binder and maintaining the strength of matrix [24]. Liu et al. [25] made high strength lightweight cement composites adding cenospheres to a cement binder. The strength of the lightest samples (1300 kg/m³) was reported to be about 58 MPa, with the use of silica fume, PVA fibre and superplasticizer, curing for 28 days in high humidity (>95%) [25]. Wang et al. also made lightweight samples using cement and cenospheres, with a density of about 1040 kg/m³ and compressive strength of 25 MPa. Silica fume, shrinkage reducing admixtures and superplasticizer were used, and the curing condition of the samples was not reported [26]. Wang et al. have also reported making high performance lightweight composites using metakaolin-based geopolymer and cenospheres, reaching 36.5 MPa strength for samples (small cylindrical specimens of ø20 × 20 mm) with 820 kg/m³ density. In their study, metakaolin was calcined at 800 °C for 4 h, and the geopolymer composite was cured at 80 °C for 6 days [27]. Shao et al. [28] have reported making high strength to density geopolymer composites using ultra-fine fly ash with a mean particle size of 4.6 μm, and hollow glass spheres as lightweight fillers. They reported achieving 22 MPa in compression, for the samples as light as 782 kg/m³ [28]. Wu et al. made lightweight cement composites using hollow cenospheres, at a density of 1154 kg/m³ after one day, with a strength of 33 MPa at 28 days. They used silica fume, superplasticizer, viscosity modifier, shrinkage reducing admixture, and polyethylene fibres, curing in a fog room at 28–30 °C until the testing age. They mentioned a further drop of the density to 1042 kg/m³ in the oven-dried samples, but the temperature and duration of the drying process were not reported [29].

From this summary of some of the available literature, it is evident that the application of ultra-fine fly ash, silica fume, and other admixtures can improve the microstructure and enhance the performance of lightweight cementitious composites. Also, high humidity, high temperature curing, calcination of the source materials at high temperatures, or extending the curing duration of composites can all enhance the performance of composites. However, costly source materials and energy-intensive processes negatively impact the environmental sustainability and commercial viability of the products for construction applications.

In this study, a syntactic foam of cenospheres with a matrix of one-part mix geopolymers has been explored for the first time to synthesise an environmentally-friendly and commercially viable lightweight composite. One-part mix geopolymers are user-friendly binders that improve the commercial viability of geopolymers by eliminating the difficulties associated with handling corrosive alkali solutions. They facilitate manufacturing of a dry component that can be activated simply by just adding water (similar to cement) [14]. Samples are manufactured and cured at ambient conditions, and it was targeted to achieve high strength to density ratios at very low densities (below 1000 kg/m³).

The importance of interfacial microstructure on strength development of composites is well known [30]. Since cenospheres have a similar chemical composition to the other fractions of the bulk fly ash, there is a possibility that, if exposed to an alkaline environment, the surface of the filler can take part in the reaction. The bonding of a filler with its surrounding matrix can improve the mechanical performance of the composite in the long term and facilitate the development of lightweight structural foams [31]. Wang et al. [27] mapped the elemental distribution at the interface of geopolymers and cenospheres, and reported that an interfacial layer is forming due to the elemental diffusion. Li et al. [32] studied the interface of phosphate geopolymers with cenospheres. They reported that the formation of the amorphous layer at the interface indicates chemical reaction between cenospheres and geopolymer, although their electron diffraction results could not distinguish the nature of the chemical bonding due to the amorphous structure of interface region [32].

This summary of the literature highlights that there is a large research gap in understanding how cenospheres interact within geopolymer composites. The superior characteristics of high photon flux density and diffraction-limited spatial resolution with enhanced spectral quality achieved by synchrotron Fourier transform infrared (SR-FTIR) microspectroscopy was demonstrated to be the key requirement allowing for spatially resolved chemical mapping measurement of amorphous materials at a micron-scale spatial resolution [33]. In this research, we utilized the SR-FTIR technique to reveal the spatial distribution of chemical bonding interaction at the interface of the geopolymer composites.