Search

The urgent need to tackle the effects of global warming has led to a worldwide compromise and ever-more demanding regulations. In this respect, as an important greenhouse gas emitter, the cement industry has to implement major changes in its production processes to achieve future goals. In this perspective, low-carbon eco-efficient cement, such as the thermoactivated recycled cement from concrete waste (RCC), seem to be a promising alternative to current carbon-intensive binders, such as ordinary Portland cement (OPC). This study aimed to demonstrate the potential contribution of RCC to the reduction in the environmental impacts of the cement industry, by means of a comparative life cycle assessment of three production methods of this binder (wet (WM), dry (DM) and air clean (ACM) methods) and OPC. Overall, RCC WM did not turn out to be a good alternative to OPC, essentially owing to the amount of fuel and electricity required for washing and drying the particles before the magnetic separation. On the other hand, RCC DM and RCC ACM proved to be promising alternatives to RCC WM and OPC, with a relevant reduction in all impact categories.

Various thermal conductivity models for multi-phase materials have been suggested in the literature. However, none have been specifically developed or adapted for structural lightweight aggregate concrete (SLWAC). In this context, the main purpose of this study was to develop a new simple model to predict the thermal conductivity of SLWAC, taking into account the volume fraction and thermal characteristics of each phase and an empirical factor that considers the uneven heat flux distribution through them. The study was based on an extensive experimental campaign carried out for different water to cement ratios and types and volume fractions of aggregate. Moreover, given that there are no standards for determining the thermal conductivity of lightweight aggregates (LWAs), a new technique was devised. The proposed model is easily implemented in practice, contributing to expeditious estimation of the thermal conductivity of the most common SLWAC regardless of the type of LWA and concrete composition.

The urge to reduce greenhouse gas emissions, in particular carbon dioxide, is a global problem, not only in spatial terms but also in terms of the scope of activities and sectors involved. Nevertheless, some sectors/industries are more critical due to their overall contribution to the problem, which is the case of the Portland cement industry. The present research estimates the energy consumption and carbon emissions associated with a novel process for producing cement by recycling used concrete and mortars. The novel process assessed resorts to the magnetic separation of the cement paste from the aggregates, followed by the thermal reactivation of the cement paste. Comparing the recycled cement production with the clinker production, higher energy consumption (over 9000 MJ/t compared with roughly 4000 MJ/t for Portland cement) and lower carbon dioxide emissions (average 730 kg CO

Chloride-induced corrosion has been one of the main causes of reinforced concrete deterioration. One of the most used methods in assessing the chloride penetration resistance of concrete is the rapid chloride migration test (RCMT). This is an expeditious and simple method but may not be representative of the chloride transport behaviour of concrete in real environment. Other methods, like immersion (IT) and wetting–drying tests (WDT), allow for a more accurate approach to reality, but are laborious and very time-consuming. This paper aims to analyse the capacity of RCMT in assessing the chloride penetration resistance of common concrete produced with different types of aggregate (normal and lightweight) and paste composition (variable type of binder and water/binder ratio). To this end, the RCMT results were compared with those obtained from the same concretes under long-term IT and WDT. A reasonable correlation between the RCMT and diffusion tests was found, when slow-reactive supplementary materials or porous lightweight aggregates surrounded by weak pastes were not considered. A poorer correlation was found when concrete was exposed under wetting–drying conditions. Nevertheless, the RCMT was able to sort concretes in different classes of chloride penetration resistance under distinct exposure conditions, regardless of the type of aggregate and water/binder ratio.

This paper intends to contribute to a better knowledge of the production and rehydration of thermoactivated recycled cement and its incorporation in cement-based materials. To this end, the influence of the treatment temperature on the properties of recycled cements and recycled cement pastes was assessed by means of a wide array of tests. Anhydrous recycled cement as well as the resulting pastes were characterized through density and particle size, water demand and setting time, thermogravimetry, X-ray diffraction, field emission gun scanning electron microscopy, isothermal calorimetry, 29Si nuclear magnetic resonance spectroscopy, flowability, mechanical strength, mercury intrusion porosimetry and scanning electron microscopy. The treatment temperature had a significant influence on the dehydration and hydration of recycled cement, essentially resulting in the formation of C2S polymorphs of varying reactivity, which led to pastes of different fresh and hardened behaviors. The high water demand and the pre-hydration of recycled cement resulted in high setting times and low compressive strengths. The highest mechanical strength was obtained for a treatment temperature of 650 &deg, C.

This study aims to characterize the chloride ingress into structural lightweight aggregate concrete (SLWAC) in real marine environment. For this, common normal weight concrete (NWC) and SLWAC produced with different water/binder ratios, types of aggregate and types and contents of binder were exposed to submerged, tidal and atmospheric conditions. Chloride profiles were obtained at 1 and 3 years of exposure and their chloride diffusion coefficients, Dcl, and surface chloride contents, Cs, were analysed. At least for water/cement ratios up to 0.55, Dcl was little affected by the type of aggregate, regardless of the exposure conditions. However, Cs and hence the chloride penetration at a given depth were higher in SLWAC with lightweight aggregates (LWA) of higher porosity or without a protective denser outer shell. SLWAC with dense LWA were able to achieve equivalent chloride penetration resistance to NWC with the same composition.

This paper characterises the durability of concrete produced with thermoactivated recycled cement (RC) obtained from waste cement paste or the cementitious fraction of waste concrete. The durability of concrete produced with 5–100% RC and water/binder ratios of 0.35–0.65 was assessed through capillary absorption, oxygen permeability, rapid chloride migration and carbonation resistance tests. Overall, RC thermoactivated at 650 °C showed high rehydration capacity and effectively improved the durability properties of concrete compared to the same non-treated waste materials. Moreover, concretes with up to 100% RC reached comparable durability to that of reference concretes of equal w/b.

This paper characterizes the oxygen permeability of structural lightweight aggregate concrete (SLWAC). The influence of different parameters is analysed, namely water/binder (w/b) ratio, type of binder and aggregate, relative humidity, water content and concrete age. The correlation between this and other properties is investigated. The oxygen permeability varied over a broad range. Nevertheless, the oxygen permeability of SLWAC with denser aggregates could be as low as that of normal weight concrete. The influence of the type of aggregate tended to increase with w/b ratio, aggregate porosity and percentage of additions. Small differences between mixtures were masked by the high variability of the test.

Separation is a fundamental step for concrete recycling, enabling not only the achievement of reliable and cleaner recycled aggregates but also the better re-use of the remaining concrete constituents. Recently, a new hydraulic binder produced from the hydrated cement paste contained in waste concrete, which is associated with lower processing temperatures and carbon dioxide emissions, has gained momentum in the history of concrete recycling. However, a decisive turning point for its application lies in the successful individualisation of the cement paste and aggregate initially contained in concrete debris. This paper describes a new patented method involving a mechanical comminution stage and a high-intensity magnetic separation process to obtain the cementitious fraction in concrete and retrieve cleaner fine aggregate. The efficiency of the proposed method was assessed through image analysis, thermogravimetry, X-ray diffraction and acid attack. In addition, cement pastes produced with the obtained recycled cement after thermal activation were characterised in terms of mechanical strength. Findings suggest that the proposed method effectively separated the main concrete constituents, especially when the waste material was reduced to the size range 150–500 μm. It was possible to obtain a recycled binder at the current laboratory scale with up to about 80% cement content by weight (nearly 90% in volume). Furthermore, a cleaner recycled fine aggregate, with minimal cement paste contamination, as low as less than 3 wt%, was also obtained. The mechanical strength of pastes with recycled cement retrieved from waste concrete was about 70% of that of reference pastes of equal water/binder produced with recycled cement from non-contaminated waste cement pastes.

The present paper characterises the chloride penetration into a large array of structural lightweight aggregate concrete (LWAC), after a 5-year exposure period to aggressive marine conditions. The chloride profiles, chloride diffusion coefficients and surface chloride contents were determined and the influence of the main composition parameters and exposure conditions was assessed. Current service life models were evaluated through the estimation of chloride profiles. The results suggest that LWAC can present the same durability as normal weight concrete (NWAC). However, despite having the same chloride diffusion coefficients, the surface chloride content of LWAC tended to be greater than that of NWAC, leading to higher chloride penetration at a given depth. In general, the estimated chloride profiles from existing models did not fit the real results.

This paper addresses the prediction of the carbonation-induced corrosion service life of structural lightweight aggregate concrete (SLWAC) by means of a performance-based approach. Abacuses for the prediction of the service life of SLWAC as a function of the w/c ratio are presented. Limiting values of concrete composition for different carbonation exposure classes are prescribed for an intended service life of at least 50 years. The results reveal that even when pastes of low-to-moderate quality are considered, the carbonation-induced corrosion should not be the mechanism determining the service life of SLWAC structures, as long as appropriate thicknesses of concrete cover are adopted.

In recent years, Energy Performance Building Directives have been published and adopted by all EU member states in order to promote the improvement of the energy performance of buildings within the EU, and thus coping with the growing comfort needs and consequent increase in energy consumption for space heating and cooling. Structural lightweight aggregate concrete (SLWAC), due to its thermal properties, presents itself as an alternative to normal weight concrete (NWC) to reduce the thermal bridging effects as well as the building energy needs to maintain thermal comfort levels in buildings. In this paper, the potential of SLWAC to improve the energy performance of buildings was assessed. An experimental study was carried out in order to determine the thermal properties of five different concrete mixtures, four SLWAC and a reference NWC for comparison purposes. These thermal properties were then used in the two-dimensional heat transfer program Therm and in the whole-building energy simulation program EnergyPlus to assess the impact of SLWAC on the thermal bridge heat losses and building energy needs of a case study. Results showed that SLWAC can improve the energy efficiency of buildings and thus be an attractive alternative to the use of the traditional NWC.

This study aims to characterise the carbonation behaviour of structural lightweight aggregate concrete (SLWAC) produced with different types of aggregate, w/c ratios and types and content of mineral admixtures (silica fume, fly ash, lime filler) by means of accelerated carbonation tests. A new biphasic model to describe the carbonation behaviour of SLWAC is suggested. Based on this model, the carbonation coefficient of SLWAC is estimated as a function of the w/c ratio, for different density classes of lightweight aggregates. It is concluded that the carbonation resistance of SLWAC cannot be properly predicted by its compressive strength and that the carbonation rate is little affected by the type of binder, being appropriate to neglect the contribution of mineral admixtures in carbonation resistance of SLWAC. It is also concluded that even with pastes of moderate quality, the carbonation mechanism should not be relevant to the durability of SLWAC.

This paper characterises the thermal conductivity of structural lightweight aggregate concrete with strength classes ranging from LC12/13 to LC55/60 and density classes from D1·6 to D2·0. The thermal conductivity of concrete is assessed by a transient test method, taking into account different types of lightweight aggregate and binder, as well as different water/binder ratios. The influence of the concrete constituents on its thermal conductivity is analysed and relations between this thermal property and both the density and water content of concrete are established. It is shown that the thermal conductivity of structural lightweight aggregate concrete can be half that of normal weight concrete of similar composition. A high exponential correlation between thermal conductivity and density of concrete is achieved. On average, thermal conductivity decreases about 0·6% per each 1% increment in aggregate porosity, 8·3% per each 50 l/m3 increment in coarse aggregate volume and 3–9% per each reduction of 1% by weight in water content.

This paper aims to contribute to a better knowledge of the production and rehydration of thermo activated recycled cement (TRC) and its incorporation in cement based materials. To this end, the influence of different factors in the fresh and hardened properties of mortars with TRC was analysed, such as the water/binder ratio, the treatment temperature, incorporation percentage and particle fineness of TRC. Thermogravimetry, X-ray diffraction, isothermal calorimetry, scanning electron microscopy and 29Si nuclear magnetic resonance spectroscopy were carried out to better understand the hydration behaviour and microstructure development of TRC mortars. Optimal rehydration properties and strength development were obtained for recycled cement thermo activated at 600–800 °C. Binders with CEM I 42.5R and 20, 50 and 100% TRC could be framed in the European normative strength classes 52.5, 42.5 and 32.5, respectively. The main obstacles that hinder the application of TRC are its low setting time and high water demand. However, the water demand and setting time were not significantly affected for up to 50% TRC incorporation. Moreover, the mechanical strength was little affected for up to 20% TRC replacement and was only 12% reduced for 50%TRC incorporation. Therefore, TRC proved to be a very promising supplementary cementitious material for cement replacement.

This paper characterizes the chloride penetration resistance of structural lightweight aggregate concrete (SLWAC) produced with different types of aggregates and binders, for usual compressive strengths of about 20–65 MPa. Concrete is characterized by means of capillary absorption, electrical resistivity and accelerated chloride migration tests. In general, it was found that the chloride penetration resistance can be substantially affected by the type of binder. Besides the microstructural refinement effect, the mineral admixtures should also affect the ionic composition of the pore solution. The type of aggregate had a little influence on the chloride penetration resistance. Chloride penetration resistance should not be related to the compression strength of concrete. However, chloride diffusion is exponentially related to the w/c ratio, regardless the type of aggregate. The capillarity and resistivity results did not correlate well with those from the chloride migration test. It is concluded that SLWAC with less porous aggregate can be at least as resistant as normal weight concrete.

A comprehensive experimental study on the concrete–steel bond behaviour of structural expanded clay lightweight aggregate concrete (LWC) was carried out using different compositions, types and initial wetting conditions of lightweight aggregates, anchorage lengths, reinforcement arrangements, strength classes and failure mechanisms. For splitting failure, the bond strength of LWC was about 70% that of normal weight concrete (NWC). But when the confinement is sufficient and pull-out of the reinforcement occurs, the relative performance of LWC and NWC may vary widely. The reduction of the bond strength with LWC was roughly proportional to ρd/2200, where ρd is the concrete dry density. The addition of silica fume had little effect on the LWC bond strength. The bond was impaired when the LWA was pre-soaked. LWC seems to be less affected by the position and orientation of the reinforcement. It is shown that the water/cemet (w/c) ratio has a greater influence on the bond strength than on compressive strength. The bond strength was higher in the lower w/c concrete, regardless of the type of aggregate. Moreover, the bond strength of LWC was more than twice that of NWC of equal compressive strength. It is shown that the application of reduction factors to the LWC bond strength, as suggested in the main standards, may be too unrealistic. A new approach for estimating the LWC bond strength is suggested.

The present study aims to characterize the capillary absorption of structural lightweight aggregate concrete (LWAC), taking into account different compositions with lightweight aggregates (LWA) of very distinct porosity. The influence of the following parameters is analysed: the volume and initial water content of LWA; the cement content and its replacement by fly ash or silica fume; the partial replacement of normal weight aggregate by lightweight coarse or fine aggregate; different curing conditions. It is shown that LWAC usually has higher initial and long-term absorption than normal weight concrete (NWC) of the same composition. However, the sorptivity tends to be similar in both concretes, regardless of the type and volume of aggregate and the water/cement of the surrounding paste. Moreover, the sorptivity is lower in LWAC than in NWC of the same strength. It is also shown that the coarser the porosity of the LWA the lower is the capillary action. The capillary absorption is higher in LWAC with pre-soaked LWA and increases as the percentage replacement of cement by fly ash increases and the percentage of silica fume decreases.

This paper presents a comprehensive review on structural lightweight aggregate concrete (SLWAC) durability. The main transport properties and degradation mechanisms of reinforced concrete are addressed, namely, carbonation and chloride attack. The influence of the main composition parameters, such as type of aggregate, type of binder and water/binder ratio, as well as the influence of cracking, are also analysed. Finally, the current knowledge of SLWAC’s service life prediction is assessed. Although the knowledge of SLWAC’s durability behaviour is still limited, investigation works performed either in laboratory or in real environments indicate that SLWAC can have similar to better durability performance than normal weight concrete, especially when the same strength level is considered. The importance of the quality of the paste over the characteristics of the lightweight aggregates is highlighted. Durability standardization regarding SLWAC is still insufficient and is one of the main gaps of current knowledge. The objective of this review is to foster a better understanding on the durability and service life prediction of SLWAC, contributing to a greater confidence in using this type of concrete.

Knowledge of structural lightweight aggregate concrete (SLWAC) durability is still limited, but its potential durability is demonstrated by its good performance in existing structures. This paper focuses on chloride-induced corrosion of the reinforcement in SLWAC, which is one of the most frequent and serious causes of concrete deterioration. Based on a previous comprehensive experimental work, this study aims to predict the chloride-induced corrosion service life of SLWAC produced with different types of lightweight aggregate and types of binder. The service life prediction is based on a performance-based model suggested in the specification E465 (Concrete. Methodology for estimating the concrete performance properties allowing to comply with the design working life of the reinforced or prestressed concrete structures under the environmental exposures XC and XS. LNEC specification, Portugal 2005), considering a safety factor method. New abacuses are defined to predict the service life of uncracked SLWAC as a function of their chloride diffusion coefficient and concrete cover. Finally, limiting values of concrete composition are recommended for the most common uncracked normal weight concrete and SLWAC, for an intended service life of at least 50 years. This deemed to satisfy approach is extended for concrete with fly ash, silica fume and lime filler.