Thermal properties of lightweight dry-mix shotcrete containing expanded perlite aggregate
Introduction
As the demand for minerals increases the world over, it takes mining ever deeper. As a result, one has to contend with increasing temperatures in the working area due to the geothermal heat trapped in the surrounding rocks. One consequence of this is an escalating cost related to ventilation and cooling systems in order to keep the working environment comfortable for the miner [1]. It is here that the application of a thermal insulation, which is also mechanically sound, assumes importance [2], [3], [4].
Shotcrete, also termed as sprayed concrete, refers to a cement-based mixture that is projected pneumatically at high velocity towards the target surface [5]. Further, it is stipulated that it must be compacted by its own momentum [6]. This technique has been used in a wide range of applications in construction and mining industries. The latter has now become a major consumer of shotcrete especially for use in underground rock support [7], so much so that the annual consumption of shotcrete in North America is estimated to be over 200,000 m3 [8].
There are two methods of producing shotcrete, namely, the dry-mix and the wet-mix processes, and both of them are routinely used in tunnels and mines. Specifically, in the dry-mix process, the bone-dry ingredients, including the cementitious binder, the aggregates, and any powder admixture are pneumatically conveyed from the spraying equipment through the delivery hose to the spraying nozzle, at which point a water ring introduces the water under pressure from another hose. The water mixes with the dry ingredients inside the nozzle and the fresh mix is then projected to the target at high velocity. As a result of this distinctive production process, shotcrete differs from the conventionally cast concrete both in its rheology and in its hardened properties [9]. While the rheology of fresh shotcrete influences the shootability and pumpability, using lightweight aggregates has been shown as favourable to producing shotcrete, especially through the dry-mix process [10]. Shotcrete has been extensively used as a support to tunnels and its mechanical performance including resistance to rock burst has been widely documented [7], [11]. However, barring a single report from the USBM [12], no research has yet been undertaken to characterize its thermal properties, especially as resulting from lightweight inclusions.
In this study, expanded perlite aggregate (EPA) was incorporated as a lightweight substitute for sand. Raw perlite occurs in nature as a siliceous volcanic rock, which contains 2–5% water [13]. After heating at over 870 °C, this water vaporizes and causes the volume of raw perlite expanding from 4 to 20 times [14] and thereby it forms the lightweight porous expanded perlite. It is well known that expanded perlite can be blended with normal aggregates in suitable proportions to achieve a variety of benefits including lightweight, superior thermal resistance, acoustic insulation and shrinkage resistance in conventional cement-based systems [15]. The material has been applied in tiles, stucco, brick/block masonry, precast products, roof fill, pipe coating, oil–gas and geothermal wells, etc. While extensive studies have been made on the mechanical and thermal properties of concrete containing expanded perlite [16], [17], [18], [19], limited information is available with regard to its use in the mining industry. To the authors’ knowledge, the only prior study was done by the U.S. Bureau of Mines (USBM) [12] who sprayed an insulation of shotcrete containing expanded perlite, to achieve a thermal conductivity of 0.36 W/(m K), and a 90-day uniaxial compressive strength (UCS) of 20 MPa. Unfortunately, this promising research track was apparently discontinued following the disintegration of USBM in 1995.
The objective of this study is to examine the use of expanded perlite as fine aggregate in dry-mix shotcrete and characterize the thermal properties of the resulting material. Further, this study also highlights the process dependence if any, of thermal properties by comparing those derived for a sprayed mix with those from a conventionally cast counterpart.
Section snippets
Materials and mixtures
The expanded perlite aggregate (EPA), Type GU Portland cement [20] and sand were locally sourced in Edmonton. The EPA was mainly composed of SiO2 (70–75%) and Al2O3 (12–18%). As shown in Fig. 1, it had a porous structure and with a bulk density of 71 kg/m3 in oven-dry conditions, it was rated to absorb water at 100% of its dry mass. The sand was at saturated surface dry (SSD) condition, corresponding to a moisture content of 2.04% by mass. It had a bulk density of 1675 kg/m3 in oven-dry (OD)
Results and discussion
The rebound associated with spraying each mix is listed in Table 1. The pre-mixed powder for the SP0 mix underwent setting overnight due to moist sand. Thus, this mix was ignored in further discussion. Nonetheless, it appears from Table 1 that there was an increase in the rebound with an increase in the amount of EPA in the fine aggregate. An earlier study on the effect of particle density on rebound in dry-mix shotcrete revealed that a lower density was beneficial to the reduction of rebound
Concluding remarks
This project investigated the use of expanded perlite aggregate (EPA) as a lightweight inclusion to achieve a thermally more resistant structural material for underground insulation. Five mixes, with increasing EPA/sand ratio were prepared through the dry-mix shotcreting technique. The thermal constants were evaluated using a device based on the transient plane heat source method. The results show that dry-mix shotcrete with up to 75% of sand substituted with EPA offers superior thermal
Acknowledgements
The authors thank Mr. Chaoshi Hu and Mr. Lang Liu for their assistance with the shotcrete production and sample preparation. Continued financial support from the Natural Sciences and Engineering Research Council (NSERC), Canada, is gratefully acknowledged.
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