•Recycled concrete aggregate (RCA) attrition during mixing was studied in two pilot mixers.•Mixing parameters and initial RCA properties influenced the breakage rate.•Both attrition and cleavage ...mechanisms influenced the aggregate degradation.•Angularity of the RCA was reduced by the mixing.
In this work, the recycled concrete aggregate (RCA) friability during mixing was studied in order to better understand the evolution of this material during the mixing process and improve the recycled aggregate concrete mix-design. The influence of some important materials and process parameters was evaluated: initial abrasion resistance and initial moisture of the aggregates, mixer geometry, mixing time and mixing speed. To assess the mixing process effect on the recycled concrete aggregate friability, three different aspects were evaluated; the mass loss (mass of fraction inferior to 2.5mm) the grading and the angularity evolutions with mixing time of an initially 10–14mm aggregate. Tests were carried out in two types of laboratory concrete mixers, a planetary 30l mixer from Skako and an intensive 5l Erich mixer. The results revealed that in normal laboratory setting of the mixers configuration, the mass loss for natural aggregate (NA) is less than 1% of the coarse aggregate. This percentage reach 3% for good quality recycled concrete aggregate (MDE value of 21) and 5% for lower quality recycled concrete aggregate (MDE value of 27). The mass loss directly depends on the mixing parameters and the degradation of the recycled concrete aggregate drastically increased when the mixing speed was raised to 500RPM. By analyzing the grading evolution during mixing, it was shown that both cleavage (creation of intermediate size particles) and attrition (creation of small particles) mechanisms influenced the aggregate degradation. However, the configuration of mixing significantly influenced the proportion of attrition and cleavage mechanisms. To complete this work, the angularity evolution showed that recycled concrete aggregate surface becomes smoother and the edges more rounded after mixing.
•Monitoring tools employed for analysing concretes and mortars mixing evolution.•The influence of water and coarse aggregate on the mixing evolution was analysed.•Difference between mixtures due to ...the wall effect exerted by coarse aggregates.•New mixture parameter defined: the “effective water-to-powder ratio in the mortar”.•Better estimation of the real effective water and of the fresh properties.
A new image analysis technique coupled with the power consumption were employed in this paper and proved to be a relevant method to detect the different characteristic times of the mixing evolution, i.e. the cohesion time, the fluidity time and the half-life time of the mixture evolution after fluidity. It was shown that the “effective water-to-powder ratio in the mortar” which excludes from the effective water the quantity of water needed to fill the less compact packing in the vicinity of the coarse aggregates, governed the mixing behaviour before fluidity and the interval of water content where the concrete mixture behaves as a homogenous and fluid granular suspension. It was also shown that the mixing behaviour was adversely affected by the amount of water into the mixture. The fluidity and cohesive times were faster to be obtained when the water proportion was increased. At opposite, it was more difficult to obtain the final consistency for a more fluid mixture, at given super-plasticizer proportioning. As a general remark, the behaviour seemed controlled by the mixture composition before the fluidity point and by the mixture consistency after the fluidity point. To end, the image analysis technique coupled with the mixing power technique can be employed at the end of the mixing to complete the information obtained with the other methods of analysis in order to monitor the consistency of the mixture. But, the use of the cohesion and fluidity times, easy to detect with the texture analysis, could be more efficient monitoring tools from an industrial point of view.
A stronger commitment towards Green Building and circular economy, in response to environmental concerns and economic trends, is evident in modern industrial cement and concrete production processes. ...The critical demand for an overall reduction in the environmental impact of the construction sector can be met through the consumption of high-grade supplementary raw materials. Advanced solutions are under development in current research activities that will be capable of up-cycling larger quantities of valuable raw materials from the fine fractions of End-of-Life (EoL) concrete waste. New technology, in particular the Heating-Air classification System (HAS), simultaneously applies a combination of heating and separation processes within a fluidized bed-like chamber under controlled temperatures (±600 °C) and treatment times (25–40 s). In that process, moisture and contaminants are removed from the EoL fine concrete aggregates (0–4 mm), yielding improved fine fractions, and ultrafine recycled concrete particles (<0.125 mm), consisting mainly of hydrated cement, thereby adding value to finer EoL concrete fractions. In this study, two types of ultrafine recycled concrete (either siliceous or limestone EoL concrete waste) are treated in a pilot HAS technology for their conversion into Supplementary Cementitious Material (SCM). The physico-chemical effect of the ultrafine recycled concrete particles and their potential use as SCM in new cement-based products is assessed by employing substitutions of up to 10% of the conventional binder. The environmental viability of their use as SCM is then evaluated in a Life Cycle Assessment (LCA). The results demonstrated accelerated hydration kinetics of the mortars that incorporated these SCMs at early ages and higher mechanical strengths at all curing ages. Optimal substitutions were established at 5%. The results suggested that the overall environmental impact could be reduced by up to 5% when employing the ultrafine recycled concrete particles as SCM in circular cement-based products, reducing greenhouse gas emissions by as much as 41 kg CO2 eq./ton of cement (i.e. 80 million tons CO2 eq./year). Finally, the environmental impacts were reduced even further by running the HAS on biofuel rather than fossil fuel.
•Novel heating-air classification system for the recycling of 3 t/h of concrete waste.•The effect of the recovered SCM was assessed in conventional binders.•The results revealed optimal substitutions of 5% of commercial cement.•Accelerated hydration and gains in mechanical strength at all curing ages.•The LCA revealed potential reductions of 41 kg CO2 eq./ton of cement.
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•The use of clean RCA and RFA in new concrete is a viable solution to assure circularity in the built environment.•RCA and RFA can totally replace natural aggregates and sand, despite ...high water absorption.•Recycled ultrafines are recommended as additives instead of cement replacement.•Addition of RMF improves the tensile strength and modulus of elasticity of recycled concrete.•RMF affect the workability of a concrete and are recommended at lower doses.
This study focuses on formulating the most sustainable concrete by incorporating recycled concrete aggregates and other products retrieved from construction and demolition (C&D) activities. Both recycled coarse aggregates (RCA) and recycled fine aggregates (RFA) are firstly used to fully replace the natural coarse and fine aggregates in the concrete mix design. Later, the cement rich ultrafine particles, recycled glass powder and mineral fibres recovered from construction and demolition wastes (CDW) are further incorporated at a smaller rate either as cement substituent or as supplementary additives. Remarkable properties are noticed when the RCA (4–12 mm) and RFA (0.25–4 mm) are fully used to replace the natural aggregates in a new concrete mix. The addition of recycled cement rich ultrafines (RCU), Recycled glass ultrafines (RGU) and recycled mineral fibres (RMF) into recycled concrete improves the modulus of elasticity. The final concrete, which comprises more than 75% (wt.) of recycled components/materials, is believed to be the most sustainable and green concrete mix. Mechanical properties and durability of this concrete have been studied and found to be within acceptable limits, indicating the potential of recycled aggregates and other CDW components in shaping sustainable and circular construction practices.
•Cementitious potential as SCMs with optimal substitutions of 5–7% of clinker.•Filler effect and pozzolanicity induced by the recycled SCMs.•Neoformation of hydrated phases and C-S-H gel.•Accelerated ...hydration and setting time and maintain of the mechanical strength.•Pilot validation with the manufacture of 184 t of blended cement.
The construction industry and more particularly cement manufacture industry are European Green Deal strategic priorities for the circularity of Europe’s construction and demolition waste (CDW) stream with a view to reducing CO2 emissions. The industry is engaged in a number of strategies to that end, one of which is to manufacture new low-carbon, lower clinker/cement ratio cements by replacing portland clinker with inorganic fractions of CDW featuring hydraulic or pozzolanic properties. Against the backdrop of that global challenge, the present study explores the cementitious potential of the limestone and siliceous concrete fines and shatterproof building glass found in CDW as supplementary cementitious materials (SCMs) in new blended cements. The research was conducted in two stages: generation of new laboratory-scale knowledge; and industrial validation of the viability of using the highest volume waste streams. The laboratory-scale findings revealed that the presence of the filler effect and pozzolanicity in micronised inorganic fractions of concrete and building glass waste induces the neoformation of hydrated phases and C-S-H gel. Those two developments improve the short- and long-term physical and mechanical properties of the new blended cements at optimal replacement ratios of 5–7%. The order of material effectiveness in shortening setting times, increasing the heat of hydration and maintaining mechanical strength was observed to be as follows: limestone concrete > siliceous concrete > glass waste. Laboratory analysis was followed by a pilot study consisting in the manufacture of 184 t of blended cement in which 5% of the clinker was replaced by recycled concrete. Higher product performance than the commercial reference cement confirmed the industrial, technical, economic and environmental viability of the new product, estimated to hold potential for CO2 emissions abatement on the order of 41 kg CO2 eq./t of cement, which could translate into 80 Mt CO2 eq./year worldwide.
The dynamics of blue and green water partitioning under vegetation and climate change, as well as their different interactions during wet and dry periods, are poorly understood in the literature. We ...analyzed the impact of vegetation changes on blue water generation in a central Spanish Pyrenees basin undergoing intense afforestation. We found that vegetation change is a key driver of large decreases in blue water availability. The effect of vegetation increase is amplified during dry years, and mainly during the dry season, with streamflow reductions of more than 50%. This pattern can be attributed primarily to increased plant water consumption. Our findings highlight the importance of vegetation changes in reinforcing the decrease in water resource availability. With aridity expected to rise in southern Europe over the next few decades, interactions between climate and land management practices appear to be amplifying future hydrological drought risk in the region.
Plain Language Summary
Forest recovery, due to human land abandonment, has been observed in several regions worldwide. To improve integrated land and water management, it is crucial to explore how these changes affect resource availability in water‐stressed areas. Forest regeneration has resulted in a large decrease in streamflow in a natural catchment in southwestern Europe, which cannot be explained by climate change. During the dry season, forest regeneration amplifies the impact of drought and water availability, with less impacts in the wet season. Therefore, the effects of vegetation recovery on water resources differ based on water availability, with the most serious implications for water resources occurring during dry periods.
Key Points
Forest secondary succession is the main driver of streamflow trends in mountain Mediterranean areas
The effects of vegetation changes on water availability strongly differ between dry and humid periods
Trends in streamflow in response to vegetation changes are mostly recorded during the dry and warm season
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