•Fracture energy of rocks is measured from size effect experiments.•Semi-circular notched specimens of various sizes are used. They easily are obtained from cores.•A fracture energy of ...86 J/m2−90 J/m2 for shales, and of 8 J/m2 to 70 J/m2 is obtained for various limestones.
An experimental technique based on size effect for the measurement of the fracture energy of rocks from core-based specimen is presented. It requires a simple apparatus and specimens may be easily obtained from cores. In order to obtain the fracture energy of the material, the non-dimensional energy release rate appearing in the size effect law, which is not available in the literature, needs to be computed first. This is done with the help of a continuum damage model. Computations have been performed for 4 different geometrically similar specimens and for 3 different values of the fracture energy. The non-dimensional energy release rate is back calculated from the fits of the size effect law. For the shale rocks tested, we obtain a fracture energy in between 86 J/m2 and 90 J/m2. For the limestones, we obtain a wider range, from 8 J/m2 to 70 J/m2, depending on their porosity.
Glass waste has been widely explored in geopolymer technology as a precursor blended with other pozzolanic materials or as a source for synthesizing alkali activators. In room temperature curing ...(which is desirable in concrete manufacturing), glass waste is not considered reactive. This is because the major silica content of glass is very resistant to dissolution at low temperatures. Therefore, in usual concrete manufacturing practices, glass fine is mainly considered as an aggregate. While it is known that glass particles will eventually take part in the reaction due to their proximity with the alkaline solution in the matrix, there is no clear understanding of the differences between the geopolymer gel with glass aggregates versus sand aggregates. In this study, two geopolymer systems, with a similar mix design, are made with glass and sand as aggregates, and the characteristics of the growing geopolymer gels have been monitored and compared. It is shown that the higher extent of alkalinity that glass particles introduce to the system at the very beginning has an important impact on the hydration stage of the geopolymer reaction. The kinetics of the reaction is different between the glass and sand system, and the resulting geopolymer gels have different chemical bonding structures. While the gel development and organization take longer in the glass group, the final binder is shown to be denser and stronger.
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•Glass fines is used as a replacement of fine sand aggregates in geopolymers.•The glass particles enhance the hydration stage of geopolymerization reaction.•The geopolymer gel development and organization is delayed in the glass system.•The final geopolymer binder is denser and stronger in the glass system.
This paper shows the potential benefits of using recycled glass fines as a sustainable and low-cost alternative compared to fly ash, silica fume and fine sand in concrete foam. One of the main ...concerns of using recycled glass in concrete is durability and particularly alkali-silica reaction (ASR). SEM-EDS analysis of the mortar with glass showed the formation of ASR gel with a higher silicon to calcium ratio around the glass particles which justified the large ASR expansion. On the other hand, microtomography analysis showed that the porous structure of concrete foam can accommodate the expansion of ASR gel inside the pores with considerably less expansion and no cracking. The majority of the pores filled with ASR gel are at the outer edge of the sample which was in contact with the alkaline solution. The smaller pores are more likely to be filled with ASR gel compared to the larger pores. Replacement of 10 wt% of Portland cement with milled glass resulted in higher strength compared to the other cementitious materials because of better compaction within the binder and pore strengthening effect due to rearrangement of pore sizes, as well as reduction of the unit weight. Recycled glass slightly decreased water absorption but substantially reduced the drying shrinkage of concrete foam.
•Recycled glass fine can be used as an alternative to fine sand in geopolymer foams.•The paste with glass aggregate is 100 kg/m3 lighter than that with sand aggregate.•Less foaming is needed to ...target low densities for the paste with glass aggregates.•Geopolymer foams with glass aggregates are 77% stronger at 600 kg/m3 density.•Thermal performance of the foams with glass aggregate is better at 600 kg/m3 density.
The ever increasing demand for high-quality river sand in construction, which poses economic and environmental concerns, highlights the necessity for finding suitable alternatives. Waste glass has a very low impact tolerance, which makes it very easy to grind and use as a fine aggregate in the construction of lightweight building elements. In this study, glass fines are used as a replacement for fine sand in manufacturing geopolymer foams. The thermal and mechanical properties of the two systems with different densities are investigated and compared with a control sample of geopolymer foam with no aggregates. The geopolymer paste with sand aggregates has a density that was approximately 100 kg/m3 higher than the paste with glass aggregates. The heavier samples with sand aggregates required a higher degree of foaming to drop the density to a similar range, which negatively affects their strength. For a density of 1000 kg/m3, the geopolymer foams with glass aggregates are 25% stronger than the foams with sand aggregates. The strength improves further by 31% and 77% as the density drops down to 800 kg/m3 and 600 kg/m3, respectively. The shape of the bubbles in geopolymer foams with glass aggregates is more regular with less interconnectivity between pores, especially at lower densities. This pore characteristic enhances the insulation capacity of lightweight foams with glass aggregates where thermal conductivity of 0.15 W/mK was achieved in the sample with 600 kg/m3 density.
The effect of the alkali activation reaction on the characteristics of slag-based foams made by the mechanical foaming technique is investigated. The mechanical foaming method is selected in order to ...control the density and quantity of the pre-made foam and minimise the impact of alkali reaction on the amount of initial foaming. Despite the similarity of the pre-made foams mixed with the altered binder formulae, a change in the alkali reaction is shown to affect the pore size distribution, pore homogeneity, density and properties of the inorganic foams. The water content of the binders is shown to be the key factor affecting the bubble breakage and foam sorting during the mechanical mixing stage. Later in the reaction, the reaction rate of the binders and the size of the gel particulates govern the pore size distribution and directional homogeneity of pore distribution within the matrix. Also, the reaction rate and the size of the gel particulates regulate the final density of the inorganic foams. The thermal conductivity is higher in the samples with higher density, but it is largely affected by the directional homogeneity of pore distribution. Apart from the strength of binding matrix and size distribution of pores, the directional homogeneity of porosity is shown to be the vital factor for high strength development.
Same amount of premade foam with the same density is added to three different mix formulations of alkali activated slag binders, and the impact of alkali reaction on foam characteristics is investigated. It is shown that characteristics of the inorganic foams don't have to be controlled by the quality and quantity of pre-made foam blended with them. It is possible to control the density and properties of inorganic binders by manipulating their mix formulation. Display omitted
•Alkali reaction can be altered to control the characteristics of slag-based foams.•The water content is the key factor in bubble arrangement during mixing stage.•Thermal conductivity is largely affected by homogeneity of pore distribution.•Homogeneity of porosity is the vital factor for high strength development.
In geopolymers foamed by H2O2, it is important to consider the impact of mix design on both geopolymerisation reaction and the H2O2 foaming reaction to control the porosity of the geopolymer foam ...matrix. In this paper, the effect of mix design on both reactions is discussed and correlated with the properties of the resulting geopolymer foams. It is found that when the mix design provides a chemically stable environment for the foaming reaction and simultaneously facilitates rapid setting of the binders, it results in homogeneously distributed fine pores within the matrix. The homogeneity of pore distribution is determined by a non-destructive test and correlated with the thermal conductivity results. It is suggested that for better thermal insulating properties, geopolymer foams are preferably poured and cured parallel to the heat flow direction. The size distribution of pores is shown to be a critical parameter in determining the strength of foams, and the mesoscale-based finite element analysis is performed to create a predictive tool for correlating the pore size distribution of geopolymer foams with their mechanical properties.
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•Higher percentage of sodium silicate in geopolymer mix solution results in more stable H2O2 decomposition (foaming process).•The percentage of open porosity decreases from 58 to 22 vol% as the foaming process becomes more stabilised.•Homogeneity of pore distribution increases from 82% to 98% by stabilising the decomposition of H2O2.•Insulation capacity of metakaolin foams increases by 16% as a result of improved foam homogeneity.•The size of the pores has a greater effect on compressive strength compared to the homogeneity of pore distribution.
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•Five surface treatments of tyre crumbs yielded strength improvements of 27–56%.•Silica fume coating improves the adhesion between tyre crumbs and concrete foam.•A denser gel with ...lower Ca/Si was observed around crumbs for silica fume treatment.•Chemical treatments reduced water contact angle on rubber (increased hydrophilicity).•Sulfuric acid etched the crumbs and formed zinc sulphate crystals on the surface.
The uptake of recycled tyre crumb (RTC) in concrete can result in a significant loss of strength because of weak adhesion between cement and rubber. Five RTC surface treatment methods (cement coating, silica fume coating, sodium hydroxide, potassium permanganate and sulphuric acid soaking) were used to mitigate strength loss in concrete composite foam with RTC. All five methods yielded strength improvements between 27% and 56%, with sulphuric acid and silica fume coating having the largest improvement. Silica fume coating, a treatment method used for the first time, improves strength because of the better adhesion between RTC and mortar. In addition, the formation of low Ca/Si gels was observed at the rubber-mortar interface based on SEM-EDS analysis. A reduced water contact angle on rubber showed that the three soaking methods were able to successfully reduce the hydrophobicity of RTC. Microscopic analysis of sulphuric acid treated RTC revealed etching of the surface and possible formation of polar zinc sulphate crystals. Silica fume coating provides an economically feasible solution with less safety and environmental risks associated compared to the chemical treatments.
•Introducing a simple method to solve HENs synthesis using the GA-MQLP model.•Using GA to find the structural variables.•Using the MQLP, which consists of two inner and outer surfaces, to find the ...optimal values of each HEN's continuous variables.•Finding the local optimal values of variables by combining an LP and a search loop, on the outer surface.•Increasing probability of finding the global optimal of these variables and the OOF on the inner surface by solving a set of linear equations.
In this paper, an efficient as well as reliable approach to deal with heat exchanger networks (HENs) synthesis problems, which is inherently known as a mixed-integer non-linear programming model, is presented. The structural variables as the discrete variables are optimized by a genetic algorithm (GA), whereas continuous variables are handled by a modified quasi-linear programming (MQLP) model. Each HEN is considered as a chromosome consisting of a sequence of genes. Each gene also contains the address of the exchanger(s) in the network. The HENs generated by the GA are sent to the MQLP to calculate their overall objective function (OOF) (i.e. minimum total annual cost (TAC)). The MQLP model includes two inner and outer surfaces. On the outer surface, the local optimal values of the continuous variables are found according to the maximum energy recovery of HEN, while on the inner surface, the globally optimal values of them are found to reach the minimum TAC of HEN. Due to the relatively linear behavior of the proposed method, a comparison of results with references showed that this method can reduce TAC of HENs compared to the studied references by about (0.51% to 2.37%).
•A two-surface hybrid approach was introduced for multi-period HENs synthesis.•It was combined linear programming (LP) + imperialist competitive algorithm (ICA).•The ICA is used to find the ...structural variables on the 1st surface.•The continuous variables are handled with a combined LP and ICA on the 2nd surface.•The relatively linear behavior improved its ability to deal with HENs problems.
In this paper, an efficient hybrid approach to deal with multi-period heat exchanger networks (HENs) synthesis, which is inherently formulated as a mixed-integer non-linear programming (MINLP) model, is proposed. In this novel two surfaces approach, the creation and determination of optimal HENs structures are accomplished by an imperialist competitive algorithm (ICA) on the first surface. In each HEN structure, there is a sequence of stages containing the addresses of the exchanger(s). The HENs generated by the ICA are sent to the second surface, where their minimum total annual cost (TAC) is calculated as an overall objective function. This surface works on two levels. For all periods of each HEN, the local optimal solutions are determined based on the maximum energy recovery at the outer level, including an external search loop and a linear programming (LP) model. Based on the outer level results, the ICA is re-used, at the inner level, to find the final minimum TAC of the network. As a result, the MINLP model is transformed into a relatively linear LP + ICA hybrid model, which is easily solvable. The results demonstrate that this approach can sometimes reduce the network TAC even by over 7.2% compared to the literature.
End-of-life waste tyres are known to have negative economic and environmental impacts due to the difficulty associated with their disposal and recycling. The use of recycled tyre crumb (RTC) as a ...component of construction materials has emerged as a potentially sustainable solution to this environmental issue. There exists extensive research covering the use of RTC as a filler in conventional concrete. However, there are very limited studies focused on insulation properties of RTC in lightweight cellular concrete (LCC) as a sustainable application that can add value to the final product. This study investigates the effects of different RTC contents on compressive strength, porosity, thermal conductivity, sound insulation and water permeability of LCC as an insulator. A rather homogeneous distribution of tyre crumbs within LCC structure was achieved. It has been found that LCC samples containing RTC are suitable as insulators because the sound and thermal insulation are improved as rubber content is increased compared to the sample with similar density but without RTC. The insulation properties of this product also come with the same total porosity and a significant reduction in the rate of water permeability compared to the sample without RTC. 3D restructuring of micro-CT scan images of this composite revealed the effects of tyre crumb on the total and local porosity of this composite. Despite the fact that addition of tyre crumb reduced the strength, surface treatment of crumb rubber with sodium hydroxide solution exhibited significant improvement in the compressive strength of LCC as the result of better physical bonding with hydrated cement proven by scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) analysis.
•LCC-RTC composite has shown excellent water, thermal and sound insulation properties.•LCC-RTC composite offers a sustainable and cleaner production of insulating panels.•3D micro-CT scan images provided significant insights towards porosity calculation.•Surface treatment of RTC with sodium hydroxide (NaOH) resulted in better binding.•NaOH-treated rubber partially compensates the strength loss by the addition of RTC.
