Effect of microwave curing on the hydration properties of cement-based material containing glass powder
Introduction
In China, municipal solid waste in the large and middle cities is about 168.161 million tons in 2014 [1]. According to the statistics by Wang et al. [2], the glass waste is about 4.2% of municipal solid waste generated in northeast China. Utilization of waste glass in concrete undoubtedly is a potential and incentive way to provide a sustainable solution for waste glass storage. In addition, addition of glass powder is beneficial to aesthetic function [3] and durability [4].
Waste glass consists of mainly amorphous SiO2 (about 70%). In addition, a certain amount of CaO, Al2O3 and Na2O exist [5], [6]. Glass can be used in the concrete as fine aggregate. However, this is easy to come up alkali-silica reaction (ASR) by the delayed Na+ dissolution [7], which leads to a remarkable strength regression and excessive expansion [4]. This has limited the use of waste glass as fine aggregate replacement. Waste glass can also be used as a kind of supplementary cementitious material [8], [9]. The composite binder containing glass powder shows an enhancement in hydration at later ages. This is attributed to the hydration improvement due to a fine particle size and the pozzolanic reaction of glass powder [10]. Increasing the specific surface area [11] or curing temperature [12] can improve the activation of glass powder at early ages. Mirzahosseini et al. also found that glass composition was seen to have a large impact on reactivity. Green glass showed higher reactivity than clear glass [9]. Glass powders can be well encapsulated into dense and mature gel [13], increasing the long term compressive strength, flexural strength, resistance to ASR and sulphate attack [14], [15]. Glass powder is also beneficial to reducing ASR expansions in a mortar prepared with glass waste aggregate [16], [17], [18], [19]. Studies have found that the glass powder does not release lots of alkalis into the pore solution to trigger deleterious reaction [20], [21], but the Na+ dissolved from the glass powder is considered as an activator, which is conducive to the early-age hydration of cement [22], [23], Na+ has adverse effects to the later-age compressive strength [22], [24].
Microwave curing is characterized by energy saving and rapid stripping for precast concrete [25], [26] or concrete repair [27], which is effective in accelerating the hydration of cement [28], [29], [30] or improving the pozzolanic reaction of SCMs i.e. metakaolin [31], coal gangue [32], rice husk ash [33], lithium slag [34], fly ash [35] and silica fume [36]. Microwave also plays an important role in nucleation, crystallization and ion transfer during curing. According to the pieces of research conducted on alkali activated fly ash, microwave can increase the dissolution of the amorphous phase to form crystal phase and increase the degree of polymerization for N–A–S–H gel [37], [38]. Stubby AFt and small sized CH is formed under microwave curing [30]. According to the research of glass fiber reinforced concrete by Pera et al., microwave curing impedes K+ transfer to glass fiber when compared with normal curing [39]. This may be caused by the adsorption of K+ by C–S–H [30]. Because K+ has strong similarity with Na+, therefore, microwave curing may also affect the transformation of Na+ in the cement-glass powder-based material.
The utilization of glass powder under microwave curing has not yet been reported. In order to investigate the effects of glass powder on the hydration properties of cement-based materials under microwave curing, coarse waste glass powder with specific surface area of 230 m2/kg was used. The cement-coarse glass powder-based material under microwave curing was compared against that cured using (a) normal curing at 20 ± 1 °C, >90% RH, (b) steam curing at 40 °C for 10 h and (c) steam curing at 80 °C for 4 h by performing compressive strength, X-ray diffraction (XRD), Thermogravimetry-differential scanning calorimetry (TG-DSC), Scanning electron microscope-Energy disperse spectroscopy (SEM-EDS) and Mercury intrusion porosimetry (MIP) analysis.
Section snippets
Raw materials
The chemical composition of P.I 42.5 cement and the glass powder are listed in Table 1. The China ISO standard sand complied with GB/T 17671-2005 was used. The specimens with the diameter of 4 cm and the height of 8 cm were prepared by cylindrical molds processed by Nylon with wall thickness of 4 mm. The complex binder containing glass powder of 30 wt, % was used to prepare the mortars and pastes. Mortars with water to binder ratio of 0.45 and with the sand to binder ratio of 3.00 were
Compressive strength
The compressive strength of mortar is shown in Fig. 6, Fig. 7, Fig. 8, Fig. 9. In order to analysis the effect of replacement of cement by glass powder, the compressive strength of mortar prepared with pure cement is cited and shown in Fig. 6 [30].
The most competitive advantage of microwave curing is that the curing period is shortened significantly [44]. It can be seen from Fig. 6, Fig. 7, Fig. 8, Fig. 9 that, the compressive strength of the mortar prepared with composite binder containing 15%
Conclusions
Utilization of coarse glass powder in concrete is economical and ecological for precast concrete production. The effect of microwave curing on the hydration properties of cement-based materials containing coarse glass powder with specific surface area of 230 m2/kg was investigated. The conclusions are as follows:
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Microwave is a kind of clean energy, which can reduce the energy consumption for precast concrete production. The advantage of shortened curing period by microwave curing was maintained
Acknowledgments
This work was supported by the National Key Technology R&D Program during the Twelfth Five-year Plan Period (2012BA20B02) and the National Natural Science Foundation of China (51208391).
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