The effect of expansive agent and cooling rate in the performance of expanded glass lightweight aggregate as an internal curing agent
Introduction
The concrete industry is responsible for the emissions from the production of cement, as it is one of the main components. Cement is the source of about 8% of the world’s CO2 emissions [1]. The other main component of concrete, natural aggregates, are also impacting the environment through direct damage in the lithosphere; approximately 60% of all the natural resources extracted from the lithosphere are used in building and public infrastructure construction [2]. Additionally, according to the USGS report of 2019, limestone and other cement raw materials are geologically abundant and spread all over the world; therefore, overall shortages are unlikely in the future [3] facilitating the situation to remain the same.
Different ways to mitigate the environmental impacts of concrete production have been considered. Some involve a replacement of constituent materials in concrete. The substitutes are generally recycled materials from waste products from several industries or recycled concrete. Natural pozzolanic materials, (i.e., volcanic tuffs and diatomaceous earth), and industrial by-products that have cementitious properties (i.e., fly ash, silica fume and slags) are usually used as cement replacement [4]. Additionally, artificial aggregates originated from industrial by-products (i.e., mining residues, sewage sludge, palm shell, fly ash, marine clay), [5], has been used to decrease the total amount of natural aggregates used in concrete.
Another sustainable approach involves promoting an effective curing generating two main benefits: i) an increase in the degree of hydration of cement, therefore more calcium silicate hydrates and strength are produced for a given amount of cement, and ii) a decrease in shrinkage and cracking which increase the durability of concrete. Consequently, with better curing, the total amount of cement can be reduced without sacrificing strength and the life span of the structure can be increased [6]. An effective curing reduces cracking and the risk of penetration of water and other aggressive chemicals, leading to corrosion of reinforcement bars, deterioration of cement paste and structural failure [7].
In general, curing influences the hydration of concrete, its compressive strength and permeability, playing an important role in minimizing shrinkage and early cement paste desiccation [8]. Curing methods help to increase the durability of concrete by maintaining a high relative humidity inside concrete to proceed with the hydration and by avoiding autogenous and drying shrinkage, and consequently, the development of cracks [9], [10], [11]. There are cases where, even providing proper curing, the traditional curing methods fail to promote hydration. This is the case of low water-cement ratio and low-permeability concretes [12].
Internal Curing (IC) is a method that can increase the degree of hydration in concrete providing curing water initially stored within concrete [13]. There are different approaches to perform internal curing, the most commonly used consists of replacing conventional aggregates for high-absorption lightweight aggregates (LWA). The prewetted LWAs release water through time to improve hydration of concrete [14]. By using IC, the overall hydration of cement increases, also increasing the strength and durability [15]. When added to the mixture, LWAs reduce shrinkage, increase the meso-pore concentration and the degree of hydration and produce a denser microstructure compared to conventional concrete [16]. Drying shrinkage may also be reduced with the extra water left from prewetted lightweight aggregates [13], increasing the durability of concrete. Sustainability of concrete elements can be increased by the use of IC, due to the potential improvement of durability and reduction of life-cycle costs [13], [17].
Most of the research on IC with LWAs has been carried out using expanded clays, shales and slates [13], which are the most commonly used LWA for producing lightweight concrete; although, there are other LWAs that can be formulated and engineered to vary their properties widely as the case of expanded glass lightweight aggregate (EG-LWA) [18]. A recent study [19] found that EG-LWA also generate the beneficial effect of internal curing. EG-LWA are produced by combining glass powder with an expansive agent and applying a thermal treatment, which is responsible for the expansion than ranges from 2 to 10 times the original volume of the constituents [18], [20]. EG-LWA production starts with the milling of glass cullet into a fine powder typically below 100 μm [21]; then, the powder is mixed with an expansive agent, such as silicon carbide (SiC), calcium sulfate (CaSO4) or manganese dioxide (MnO2), and then a heating cycle is applied to produce the expansion and bonding of the glass powder [20], [22]. The heating cycle typically considers a heating ramp between 5 and 10 °C/min, until it reaches an optimal maximum temperature depending on the expansive agent utilized [23], [24], which can vary between 750 °C and 1200 °C [25], [26] depending on the manufacturing process selected for the production of EG-LWA .
The heating cycle of EG-LWAs is performed in either a rotary kiln or a continuous kiln; When using a rotary kiln the production process can yield different particle size distributions [27]. When using a continuous kiln, the relatively large boards of expanded glass have to be reduced in size to produce particles of LWAs, this fragmentation is typically performed by either crushing or quenching which induces microcracks that can reduce the EG-LWA intrinsic strength [23], [28], [29].
For conventional concrete, the interfacial transition zone (ITZ) is responsible for the cracking and failing of concrete; this is not the case of concrete containing LWAs; where the weakest phase are the LWAs themselves [31]. In fact, the concept of strength ceiling has been proposed [32] to explain the strength limitation of concrete imposed by the porous LWA [26].
The strength ceiling of the LWA limits the strength gains from the enhanced cement hydration with IC [8], [19]. Thus, the EG-LWA fragmentation approach (i.e., crushing, quenching), might have a relevant effect on the overall performance of EG-LWA ad IC agents.
The aim of this investigation is to assess the effect of the expansive agent content and cooling rates (i.e., quenching), after the expansion process of the EG-LWA, on the water transport and mechanical properties of EG-LWA as IC agent. These results will help to maximize the capacity of EG-LWA as IC agent maximizing hydration, minimizing self-dissection and minimizing strength losses.
Section snippets
Raw materials
EG-LWAs were produced using glass cullet and silicon carbide (SiC) as the expansive agent. The glass cullet was 100% recycled glass and it was milled down to sizes below 75 μm, which was in accordance with the size used in previous studies [18], [21], [22]. The recycled glass was mainly comprised of 73% of SiO2, 14% of Na2O and 10% of CaO. MgO, Al2O3, and SO3 were all below 1%.
The SiC used as expansive agent had a maximum particle size of 14 μm and was added in proportions between 2.5 and 6.1%
Results and analysis
The main physical, water transport and mechanical properties of the EG-LWAs produced for the experimental program are summarized in Table 1. The name of each case starts with the SiC content, expressed as a % by mass, and then a letter representing the cooling rate (E: extra slow, S: slow, M: medium, R: rapid). Total porosity, connectivity density and average pore diameter were estimated from the CT-Scan results, specific surface was calculated from the sieve analysis assuming spherical
Conclusions
EG-LWAs were produced from waste glass and SiC was utilized as an expansive agent (1.8% to 6.1%) in a continuous kiln with a maximum process temperature of 850 °C. Four different cooling rates were applied after the expansion process: Extra slow, Slow, Medium and Rapid cooling which had cooling times between 72 h and approximately 2 min.
The SiC content and cooling rates changed the microstructure and properties of the EG-LWAs importantly. Porosity, density, pore size distribution, pore
CRediT authorship contribution statement
Karla Cuevas: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Mauricio Lopez: Conceptualization, Methodology, Formal analysis, Resources, Visualization, Writing - original draft, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work included in this article is supported by the National Agency of Research and Development (ANID) under grants Fondecyt No. 1150251, Fondef ID17I10215 and ID19I10031, and by CEDEUS, FONDAP No. 15110020. The testing performed in this study was conducted mainly at the Construction Materials Laboratories at the Pontificia Universidad Catolica de Chile and DICTUC Laboratories in Chile. CT-Scan analysis was performed under the FONDEQUIP project EQM 130028. The authors acknowledge the
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