Up-Cycling Waste Glass to Minimal Water Adsorption/Absorption Lightweight Aggregate by Rapid Low Temperature Sintering: Optimization by Dual Process-Mixture Response Surface Methodology

Mixed color waste glass extracted from municipal solid waste is either not recycled, in which case it is an environmental and financial liability, or it is used in relatively low value applications such as normal weight aggregate. Here, we report on converting it into a novel glass-ceramic lightweight aggregate (LWA), potentially suitable for high added value applications in structural concrete (upcycling). The artificial LWA particles were formed by rapidly sintering (<10 min) waste glass powder with clay mixes using sodium silicate as binder and borate salt as flux. Composition and processing were optimized using response surface methodology (RSM) modeling, and specifically (i) a combined process-mixture dual RSM, and (ii) multiobjective optimization functions. The optimization considered raw materials and energy costs. Mineralogical and physical transformations occur during sintering and a cellular vesicular glass-ceramic composite microstructure is formed, with strong correlations existing between bloating/shrinkage during sintering, density and water adsorption/absorption. The diametrical expansion could be effectively modeled via the RSM and controlled to meet a wide range of specifications; here we optimized for LWA structural concrete. The optimally designed LWA is sintered in comparatively low temperatures (825-835 °C), thus potentially saving costs and lowering emissions; it had exceptionally low water adsorption/absorption (6.1-7.2% w/wd; optimization target: 1.5-7.5% w/wd); while remaining substantially lightweight (density: 1.24-1.28 g.cm-3; target: 0.9-1.3 g.cm-3). This is a considerable advancement for designing effective environmentally friendly lightweight concrete constructions, and boosting resource efficiency of waste glass flows

On the reliability of experimental data in the geomechanical characterization of dimension stones

In the last 40 years different procedures to estimate standard properties of dimension stones have been proposed by Italian and European technical committees as well as by international associations. The introduction of numerous and different standards, based on different sampling and testing conditions, has produced many practical consequences over the years, such as the doubtful reliability of results from the tests. This paper deals with this issue, analysing the differences between uniaxial compressive strength values of a Miocene limestone from the Central–Southern Apennines, Italy, calculated with four different standard procedures and evaluating the effects of loading rate, environmental conditions, specimen size and shape on the measured values. An additional finding of this study is a specific correlation between uniaxial compressive strength data and the size-corrected point load strength index. Although there are several studies asserting that point load test is an attractive alternative for indirect uniaxial compressive strength evaluation, there are many variables affecting this correlation, as shown by the high variability of results reported in the literature