Effect of recycled glass fines on mechanical and durability properties of concrete foam in comparison with traditional cementitious fines

Abstract

This paper shows the potential benefits of using recycled glass fines as a sustainable and low-cost alternative compared to fly ash, silica fume and fine sand in concrete foam. One of the main concerns of using recycled glass in concrete is durability and particularly alkali-silica reaction (ASR). SEM-EDS analysis of the mortar with glass showed the formation of ASR gel with a higher silicon to calcium ratio around the glass particles which justified the large ASR expansion. On the other hand, microtomography analysis showed that the porous structure of concrete foam can accommodate the expansion of ASR gel inside the pores with considerably less expansion and no cracking. The majority of the pores filled with ASR gel are at the outer edge of the sample which was in contact with the alkaline solution. The smaller pores are more likely to be filled with ASR gel compared to the larger pores. Replacement of 10 wt% of Portland cement with milled glass resulted in higher strength compared to the other cementitious materials because of better compaction within the binder and pore strengthening effect due to rearrangement of pore sizes, as well as reduction of the unit weight. Recycled glass slightly decreased water absorption but substantially reduced the drying shrinkage of concrete foam.

Introduction

Municipal waste generation is fast-growing around the world because of a significant increase in urban population. Therefore, recycling end-of-life materials for sustainable and cleaner production is becoming a major initiative worldwide. About 1.5 million tons of municipal waste glass is generated annually in Australia, in which only about two-thirds was recycled [1]. The rest, which are mainly glass fines (i.e. glass which was crushed to very fine particles during the recycling process), are unsuitable for use in glass manufacturing. This is attributed to the fineness of the glass particles which makes different types of recovered glass indistinguishable based on their colour for recycling into glass containers [2]. Despite the limited application of glass fines e.g. in pipe bedding, waste glass fines can be used in construction components as a sustainable alternative to fine cementitious materials, and aggregates in mortar and concrete.

Different applications for waste glass in cement and concrete, mainly as a replacement for traditional aggregates or as a raw material replacing Portland cement, have been reviewed by Jani and Hogland [3], Shi and Zheng [4], Rashad [5], Mohajerani, Vajna, Cheung, Kurmus, Arulrajah and Horpibulsuk [6]. Recycled glass has been investigated as a source of silica to replace clay in the manufacturing of Portland cement [7]. However, the high content of alkali (sodium) in glass leads to flash setting and poor strength development. Pozzolanic activity of glass increases with its fineness [[8], [9], [10]], which can benefit mechanical properties by the transformation of calcium hydroxide Ca(OH)₂ to calcium silicate hydrate (C−S−H) when used as raw material. However, this can cause durability issues due to alkali-silica reaction (ASR) expansion if not treated well. ASR occurs over time in concrete between the highly alkaline mortar and the reactive non-crystalline (amorphous) silica found in many common aggregates, if sufficient moisture is present. ASR forms an expanding gel around the aggregates which could lead to excess pressure inside concrete, thereby leading to internal cracking. ASR is a main durability concern in formulating concrete due to the addition of aggregates and cementitious materials with reactive silica. Recycled glass has a high content of potentially reactive silica. Therefore, ASR could be a long-term durability issue that can limit the application of glass into concrete. The expansion of ASR gel around glass in concrete can cause micro-cracks and this also depends on the degree of glass reactivity. Different colours of waste glass have different chemical compositions which result in various ASR reactivity [11,12]. Despite this, a few researchers concluded that using fine glass (<1 mm) limits ASR [13,14]. Regardless of ASR, de Castro and de Brito [15] studied the durability of concrete with crushed glass aggregates and found that other durability-related properties are less affected when the crushed glass is used in concrete.

There is significant potential in using recycled glass fines in concrete but the uptake by industry has been limited due to restrictions in standards and durability (mainly ASR) concerns. For instance, the Australian standard for general purpose and blended Portland cement (AS 3972- 2010) only recognises fly ash and ground granulated blast furnace slag as supplementary cementitious materials for use with Portland cement. The main problem can be attributed to concerns regarding mechanical and durability performance, and also the lack of long-term performance data for blended Portland cement with recycled glass. This barrier has limited the uptake of glass in traditional concrete despite a number of research studies focusing on this area. On the other hand, the application of recycled glass fines in lightweight or concrete foam can be a viable potential alternative with less restrictions. Lightweight concrete is formed by a process of aeration of concrete without autoclaving. This provides an efficient system, which offers a high strength-to-weight ratio, fire-resistance, and thermal and sound insulation properties [[16], [17], [18], [19]]. However, research on the effects of recycled glass on the mechanical and durability properties of concrete foam is very limited. For instance, Al-Sibahy and Edwards [20] used a mixture of recycled glass, metakaolin and expanded clay, and showed that recycled glass can help to lower density and improve thermal performance. Therefore, the benefits of using recycled glass fines in non-load bearing lightweight concrete foam as a low-cost and sustainable solution were studied here.

In this study, the performance of recycled glass was compared to that of traditional cementitious fines (i.e. fly ash and silica fume) and fine sand in concrete foam. The main concerns arising from the addition of recycled glass in concrete is ASR. The research aims to investigate the extent of ASR in concrete foam compared to traditional concrete. This is the first time, to the knowledge of the authors, that ASR was thoroughly investigated in concrete foam. Micro-tomography images, before and after ASR, of the same sample were used to monitor ASR gel expansion in concrete foam. In addition, the compressive strength, water absorption and drying shrinkage were investigated amongst samples with different cementitious fines compared to recycled glass fines. The size effect of glass particles on the properties of concrete foam was also studied. The research aims to show the potential benefits of using recycled glass fines in concrete foam as a sustainable and low-cost alternative to traditional cementitious fines.