Properties of concrete incorporating microwave treated coarse aggregate: An experimental study

Abstract

Using microwave heating technology to assist concrete recycling have great potential for improve the quality of recycled concrete aggregate. Unfortunately, there are few studies explored on the properties of concrete incorporating microwave treated coarse aggregates. This study aims to investigate the performance of coarse aggregate after microwave treatment and to evaluate the properties of the resulting concrete incorporated microwave treatment aggregate. The water absorption and crushing value tests were carried out to study the effect of microwave heating treatment on the physical properties of coarse aggregates. Concrete incorporated aggregate with various microwave heating parameters was fabricated and tested. The results indicated that physical performance of microwave treated aggregate was within the range of specification requirement and water absorption and crushing value of coarse aggregate heated under low microwave power levels (2–3 kW) is very closed to the untreated samples, while coarse aggregates treated with high power levels showed a significant increase. It was also observed that concrete made out of low microwave power levels heated aggregates are able to achieve strength on par with natural coarse aggregate concrete. In addition, in case of similar strength, the elastic modulus of microwave treated aggregate concrete is similar to natural aggregate concrete. These results indicate that using low microwave power levels to assisted aggregates recycling can reduce the damage to coarse aggregates to some extent, which provide a further understanding for the application of microwave-assisted aggregates recycling.

Introduction

To reduce the use of natural resource and occupation of land resource, using recycled concrete aggregates (RCA) obtained from concrete debris of construction and building demolition (C&D) wastes to produce new concrete has been extensively studied in the last decades [1], [2], [3], [4]. Compared with natural aggregates (NA), RCA derived from C&D wastes contains considerable attached mortar. The presence of residual mortar will reduce the properties of interfacial transition zone (ITZ) and thus affect the performance of recycled aggregate concrete. It is widely recognized that concrete with weaker ITZ has tendency of lower strength, workability and durability [5], [6], [7]. In view of this, a great effort has been put into ameliorate the quality of the recycled aggregates. The existing methods such as removal of the attached mortar, carbon curing, sequential mixing approaches and addition of mineral admixtures have been extensively studied to improve the characteristics of RCA or enhance the properties of recycled aggregate concrete [8], [9], [10], [11], [12], [13]. Since the residual mortar attached around the aggregate have been regarded as the main obstacles that adversely affect the performance of recycled concrete aggregate, removal of the adhered mortar is considered to the most direct and effective approach to improve the quality of RCA. Based on this concept, researchers have explored various treatment methods to reduce the unfavorable effects by eliminating the partial or total adhering mortar of RCA for the use in recycled concrete.

In order to maximize the removal of old mortar, several methods such as mechanical treatment, thermal treatment, chemical treatment and the combination of the above methods have been widely explored. These techniques have different performance in the removal of attached mortar, and the properties of recycled aggregate concrete prepared with recycled coarse aggregate vary from these methods. Many researchers have investigated the characteristic of recycled aggregates produced by different methods and the performances of concrete mixed with these aggregates. For instance, Al-Bayati [14] compared the ITZ microstructure properties after different recycled treatment methods. The test results indicated that heating treatment method (at 250 ℃) is highly successful in improving properties of microcracks in the ITZ, and acid treatment better reduced the width and length of microcracks. Revathi [15] studied the strength characteristics of recycled aggregate concrete and concluded that heating treatment could improve the quality of recycled aggregate and hence the property of recycled aggregate concrete by comparing the heating treatment and pre-acid treatment. Hanaa [11] also found that heating treatment method is more effective to enhance the quality of coarse recycled aggregate compared with the acid treatment. Studies indicated the bond strength of RAC treated with acid, mechanical and thermal treatment methods range from 96% to 79% of the natural aggregate concrete (NAC), respectively [16], [17]. Moreover, waste obtained after chemical treatment through acid solution can be used as an admixture in concrete [17].

These existing methods have improved the performance of RCA to varying degrees, but at the same time, the implementation of the methods also brought some problems. For instance, excessive acid treatment may generate waste liquid which is unfavorable to the environment [17], [18]. The heating treatment reported in literature is to place the RCA at high temperature for several hours in furnace which increase the energy consumption and processing time [19], [20]. High temperature will decrease the properties of RCA. Moreover, the mechanical and thermal treatment will increase energy consumption and CO₂ emissions. Both the economic and ecological pressure are calling to find a new approach to enhance the quality of RCA. Recently, using microwave heating technology to deal with RCA has been studied by more and more scholars [21], [22], [23], [24]. Due to the selectively and rapidly heating characteristics of microwave heating, thermal stresses between the aggregates and mortar generated, leading to the reduction of the interface bonding strength between mortar and aggregate, separating the adhering mortar after mechanical treatment. Compared with the high temperature and long-time conventional heating treatment methods, microwave recycle technology is more eco-friendly with less energy and time consumed [25]. Researchers indicated that microwave heating treatment could effectively remove the partial or total adhering mortar and reduce impact on the performance of RCA [13], [26], [33]. Despite the advantages of lower energy consumption and shorter processing time, the microwave assisted recycling technology can have a better removal effect by following a short mechanical treatment [26]. However, although the microwave-assisted beneficiation of recycled concrete aggregates has been studied to some extent, there are few studies on the properties of the RCA after microwave treatment and performances of concrete prepared with microwave treated aggregates. In order to make better understanding of microwave recycling technology, it’s vital to evaluate to characteristic of microwave treatment RCA and the concrete made with these aggregates.