Production of lightweight aggregates from mining residues, heavy metal sludge, and incinerator fly ash

In this study, artificial lightweight aggregate (LWA) manufactured from recycled resources was investigated. Residues from mining, fly ash from an incinerator and heavy metal sludge from an electronic waste water plant were mixed into raw aggregate pellets and fed into a tunnel kiln to be sintered and finally cooled rapidly. Various feeding and sintering temperatures were employed to examine their impact on the extent of vitrification on the aggregate surface. Microstructural analysis and toxicity characteristic leaching procedure (TCLP) were also performed. The results show that the optimum condition of LWA fabrication is sintering at 1150 degrees C for 15 min with raw aggregate pellets fed at 750 degrees C. The rapidly vitrified surface envelops the gas produced with the increase in internal temperature and cooling by spraying water prevents the aggregates from binding together, thus forming LWA with specific gravity of 0.6. LWA produced by sintering in tunnel kiln shows good vitrified surface, low water absorption rate below 5%, and low cylindrical compressive strength of 4.3 MPa. In addition, only trace amounts of heavy metals were detected, making the LWA non-hazardous for construction use.     

Effects of distribution of lightweight aggregates on internal curing of concrete

This study investigates the effects of spatial distribution of lightweight aggregates (LWAs) on internal curing of concrete. As replacements for normal aggregates, different sizes and amounts of natural pumice LWAs were used as water reservoirs to provide internal curing in mitigating autogenous deformation. Water in the pre-soaked LWAs flows into cement paste during hydration and provides internal curing to counteract the RH loss due to self-desiccation of binding paste. The results show that variations in the autogenous strain of concrete can be evaluated in terms of LWA–LWA proximity. The protected paste volume approach, previously used for air-entrained concrete, is applied to calculate the internally-cured volume of paste. The results show that the experimental rate of mitigation of autogenous strain for different series of concrete specimens, with respect to the reference concrete, gave the best-fitted values at water flow distance of 1 mm. The results indicate that the protected paste volume in internal curing can be determined by calculating the water-entrained volume using image analysis.     

Effect of natural coarse aggregate type on the physical and mechanical properties of recycled coarse aggregates

Many environmental problems caused by the large volumes of construction and demolition waste (C&DW), the lack of adequate deposition sites and the shortage of natural resources have led to the use of C&DW as replacement of natural aggregates in the production of new concrete. As in the case of natural aggregates, when recycled aggregates are used to manufacture structural concrete, the assessment of their physical, mechanical and durable characteristics is a key issue. The different physical and mechanical properties of the recycled coarse aggregate (RCA) are evaluated. RCA was obtained by crushing conventional concretes with different strength levels (different w/c ratios) containing four different types of natural coarse aggregates (three crushed stones and a siliceous gravel), which differ in shape, composition and surface texture. There is a significant influence of the natural coarse aggregate (NCA) on the properties of RCA, which in many cases is greater than that of the w/c ratio of the source concrete.     

Internal curing of concrete using lightweight aggregates

Internal cured concrete (ICC) has been recently used in the local and international construction markets. ICC contains surplus amount of water to compensate the shrinkage of the mix and the volumetric changes which result in early-age cracking of concrete. Concrete cracking is a direct result of the shrinkage of the water–cement paste during early stages of the hydration process and continues for a significant amount of time during the life span of the concrete section. Early-stage shrinkage, prior to the concrete hardening, is associated with volumetric changes, until final setting is achieved. Afterward, the reduction in cement paste particle size results in increased voids within the concrete structure. These voids result in increased permeability, additional sulfate and chloride attacks on steel reinforcement, and internal tensile stresses in concrete, which result in significant cracking. ICC uses the additional water added to the mix in counteracting the reduced volume of the concrete. Several techniques are used for internal curing (IC). In this research, water-saturated lightweight aggregates (LWAs) are used in partial replacement of normal weight aggregate as a source of additional water. LWA is submerged in water prior to concrete mixing to absorb a significant amount of water, which is stored within the LWA particles. Once added to the mix, the water is gradually desorbed and compensates the water losses during hydration. Hence, it counteracts the shrinkage induced. Different ICC mixes are developed in this research using two different sizes of LWA, and supplementary binding materials are used to improve compressive strength. ICC compressive strength and reduced shrinkage attained are presented. ICC mixes developed in this research can be successfully used in pouring highway segments and bridge decks with lower cracks and reduced life cycle cost due to reduced maintenance.     

A study of concrete properties using phyllite as coarse aggregates

Nowadays, industrial activities generate a huge amount of waste. One such activity is underground mining which generates phyllite wastes that are recycled as coarse aggregates for use in concrete production. Aggregate use in concrete is dependent on availability. This paper reports of an experimental study on some of the physical and mechanical properties of phyllite aggregate concrete as compared to granite (conventional) aggregate concrete. The obtained physical and mechanical properties of both aggregates for specific gravity, water absorption (%), dry density, aggregate impact value (%), aggregate crushing value (%), 10% fines, elongation index (%), flakiness index (%) and Los Angeles abrasion values satisfied minimum requirements for aggregates suitable for concrete production. Five mixes of concrete mix proportions designated M1, M2, M3, M4 and M5 were cast using phyllite and granite aggregates. A total of 400 concrete cubes and 210 modulus of rupture beams were cast and cured by total submerging in water for ages 3, 7, 14, 28, 56, 90, 180 and 360 days before compression and bending tests were performed. The results show that the trends in the development of compressive and bending strengths of plain phyllite concrete were similar to those in granite (conventional) aggregate concrete. However the compressive and bending strengths of phyllite concrete mixes were on the average 15–20% lower than those of the corresponding granite concrete mixes at all ages. The same concrete mix proportions gave lower concrete classes for phyllite compared to granite with the exception of the lowest grade. This was probably because the flakiness and elongation properties coupled with reactive materials in phyllite aggregates affect the absorption and bond characteristics of its concrete.     

Use of paraffin impregnated lightweight aggregates to improve thermal properties of concrete panels

In this study, the effect of aggregates impregnated with phase change material (paraffin type) on properties of concrete is investigated. The experimental series consists of two stages. The first stage is to investigate the techniques used to impregnate phase changed material (paraffin type) into lightweight aggregates and the properties of aggregates with paraffin inside (PLA). Two impregnation techniques are introduced, (1) heat only and (2) heat and pressure (autoclaving). Using the obtained results, the aggregate with the highest level of impregnation in the shortest time is selected to use in the concrete production process of the next stage. In the second stage, the properties of concrete mixed with non-paraffin and paraffin impregnated lightweight aggregates (PLA) at different proportions are investigated. The experimental series include density and absorption, compressive strength, thermal storage (and insulation) and sound transmission loss. Results in aggregate level show the increase in specific gravity and the decrease in absorption with paraffin inserted into aggregates. In concrete form, the density, compressive strength and sound insulation are found to increase with the PLA replacement ratio. The sound transmission loss, on the other hand, becomes less efficient with increasing PLA replacement ratio.     

Quantification of the residual mortar content in recycled concrete aggregates by image analysis

As the supply of suitable fresh aggregates in some locations is rapidly dwindling and such aggregates need to be transported from distant locations, there will be more and more economic and environmental reasons to use recycled concrete aggregates in making new concrete. The nature and properties of recycled concrete aggregates have a definite impact on the performance of recycled aggregate concrete and the effects can vary considerably. Existing literature and studies conducted by the authors have shown that the amount and properties of the residual mortar in the recycled concrete aggregates significantly affect the mechanical and durability properties of the new concrete. Consequently, a quick laboratory method was developed to determine the residual mortar content of recycled concrete aggregates, to serve as a quality control tool for such aggregates. In order to validate the results obtained by that laboratory test procedure, image analysis was used to quantify the residual mortar content in the different size fractions of the recycled concrete aggregates tested. The results confirmed that the quick laboratory test provides an accurate measurement of the residual mortar content in recycled concrete aggregates.     

Properties of concrete prepared with PVA-impregnated recycled concrete aggregates

This paper reports an experimental study to improve the properties of recycled concrete aggregates (RCA) by their impregnation with polyvinyl alcohol (PVA). The effects of PVA on the development of strength and durability properties of the recycled aggregate concrete were evaluated. The experimental investigation was conducted in two parts. Firstly, the optimal concentration of PVA solution required to improve the recycled aggregates was determined. The RCA was soaked in 6%, 8%, 10%, 12% PVA solutions, and impregnation was conducted under a controlled laboratory environment. Density, crushing value (10% fines value), and water absorption of the PVA impregnated RCA (PI-RCA) were determined. Secondly, the slump, slump loss, compressive and tensile splitting strength, dimensional change (shrinkage) and chloride penetrability of the concretes prepared with the RCA that had been impregnated with the optimal (10%) PVA concentration were determined. It was found that the 10% fines value of the PI-RCA was higher, and the water absorption of the PI-RCA were lower when compared to the untreated RCA. The results show that there was not only an improvement in the mechanical properties of the concrete made with PI-RCA, but also the shrinkage of PI-RCA decreased while the resistance to chloride-ion penetration of the concrete produced increased.     

Potential applications of phase change materials in concrete technology

In internal curing, pre-wetted lightweight aggregates (LWA) serve as internal reservoirs to supply the extra water needed by the cementitious and pozzolanic components of the concrete during their hydration processes. Due to their porous nature and reasonably high absorption capacity, the LWA can also be filled with other materials, such as phase change materials (PCMs). In this paper, three potential applications of PCM-filled LWA in concrete technology are presented. In addition to the previously explored application of increasing the energy storage capacity of concrete in residential and commercial construction by using a PCM with a transition temperature near room temperature, applications for higher and lower temperature PCMs also exist. In the former case, a PCM can be used to reduce the temperature rise (and subsequent rate of temperature decrease) of a large concrete section during (semi)adiabatic curing, to minimize thermal cracking, etc. In the latter case, a PCM can perhaps reduce the number or intensity of freeze/thaw cycles experienced by a bridge deck or other concrete exposed to a winter environment. In this paper, these latter two applications are preliminarily explored from both experimental and modeling viewpoints.     

Influence of steam curing on the pore structures and mechanical properties of fly-ash high performance concrete prepared with recycled aggregates

In this research work, High Performance Concrete (HPC) was produced employing 30% of fly ash and 70% of Portland cement as binder materials. Three types of coarse recycled concrete aggregates (RCA) sourced from medium to high strength concretes were employed as 100% replacement of natural aggregates for recycled aggregate concrete (RAC) production. The specimens of four types of concretes (natural aggregate concrete (NAC) and three RACs) were subjected to initial steam curing besides the conventional curing process. The use of high quality RCA (>100 MPa) in HPC produced RAC with similar or improved pore structures, compressive and splitting tensile strengths, and modulus of elasticity to those of NAC. It was determined that the mechanical and physical behaviour of HPC decreased with the reduction of RCA quality. Nonetheless steam-cured RACs had greater reductions of porosity up to 90 days than NAC, which led to lower capillary pore volume.     

Influence of internal curing using lightweight aggregates on interfacial transition zone percolation and chloride ingress in mortars

The microstructure of the interfacial transition zone (ITZ) between cement paste and aggregate depends strongly on the nature of the aggregate, specifically its porosity and water absorption. Lightweight aggregates (LWA) with a porous surface layer have been noted to produce a dense ITZ microstructure that is equivalent to that of the bulk cement paste, as opposed to the more porous ITZ regions that typically surround normal weight aggregates. This ITZ microstructure can have a large influence on diffusive transport into a concrete, especially if the individual ITZ regions are percolated (connected) across the three-dimensional microstructure. In this paper, the substitution of LWA sand for a portion of the normal weight sand to provide internal curing (IC) for a mortar is examined with respect to its influence on ITZ percolation and chloride ingress. Experimental measurements of chloride ion penetration depths are combined with computer modeling of the ITZ percolation and random walk diffusion simulations to determine the magnitude of the reduced diffusivity provided in a mortar with IC vs. one with only normal weight sand. In this study, for a mixture of sands that is 31% LWA and 69% normal weight sand by volume, the chloride ion diffusivity is estimated to be reduced by 25% or more, based on the measured penetration depths.     

Influence of recycled aggregates and high contents of fly ash on concrete fresh properties

This study presents the fresh properties of concrete with supplementary cementitious materials (SCM) and recycled concrete aggregates (RCA), with emphasis on the feasibility of using high volumes of fly ash (FA) in RCA concrete. For this purpose, two mix families (0% coarse RCA and 100% coarse RCA) were produced, both with and without superplasticizers (SP). The coarse natural aggregates (NA) were replaced with coarse RCA at 0% and 100%, respectively. For each of the mentioned families, three incorporation levels (0%, 50% and 100%) of fine RCA were used with 0%, 30% and 60% of FA, resulting in 28 compositions. Each mix was tested in the fresh state by means of slump, density and air content. The results of this study show that RCA decreased the slump of concrete mixes, but the required water content can be minimized by incorporation FA. Regardless of the water absorption of the aggregates, for a given fine RCA incorporation ratio and the same ratio of FA, no increase in water content is required to obtain the same target slump as in the reference concrete. On the other hand, for a given coarse RCA incorporation ratio, a five times lower FA ratio is enough to obtain the same target slump as in the reference concrete. Air voids in concrete mixes were more affected by the shape of the aggregates than by their water absorption. The air content of concrete mixes increased as the incorporation levels of FA and RCA increased. However, in comparison with the individual effects, the air content decreased by combining the incorporation of both FA and RCA. Moreover, the rate of reduction in fresh density by increasing the incorporation of RCA and FA was similar in concrete mixes with and without SP.     

Utilization of lignite power generation residues for the production of lightweight aggregates

A novel process is proposed for the utilization of lignite combustion solid residues in the production of inflammable lightweight aggregates (LWA). The process consists of two stages, pelletization and sintering, and carbon contained in BA was used as the process fuel. The main residues bottom ash (BA) and fly ash (FA) from Megalopolis power plant were characterized, mixed in different proportions and treated through pelletization and sintering process. Sintering benefits from combustion of BA carbon content and the product is a hardened porous cake. The energy required for achievement of high temperatures, in the range of 1250 °C, was offered by carbon combustion and CO₂ evolution is responsible for porous structure formation. Selected physical properties of sintered material relevant to use as lightweight aggregates were determined, including bulk density, porosity and water absorption. Bulk density varies from 0.83 to 0.91 g/cm³, porosity varies from 60% to 64% and water absorption varies from 66% to 80%. LWA formed is used for the production of lightweight aggregate concrete (LWAC). Thermal conductivity coefficient varies from 0.25 to 0.37 W/mK (lower than maximum limit 0.43 W/mK) and compressive strength varies from 19 to 23 MPa (higher than minimum limit 17 MPa). The results indicate that sintering of lignite combustion residues is an efficient method of utilization of carbon containing BA and production of LWA for structural and insulating purposes. Carbon content of BA is a key factor in LWA production. Finally, this research work comprises the first proposed application for utilization of BA in Greece.     

Release of internal curing water from lightweight aggregates in cement paste investigated by neutron and X-ray tomography

A sealed sample of cement paste containing a pre-wetted and a dry lightweight aggregate (LWA) particle was investigated in the period between 0.5 and 20.3 h after mixing. Changes in the local water distribution in the sample during hydration were evaluated using the subtraction of 3D images obtained by subsequent neutron tomographies (NT). As both water retention in the LWA and its release to the cement paste are influenced by the pore structure of the aggregate, a high-resolution image of the sample was subsequently captured by X-ray tomography. The internal curing water released from the LWA traveled at least 3 mm from the LWA into the cement paste in the first day. Hardly any gradient in the water content of the cement paste against the distance from the LWA was observed. This suggests that the release of water for internal curing (IC) is relatively fast and the water is distributed fairly homogeneously from the LWA for at least 3 mm within the hydrating cement paste.     

Interfacial transition zone of cement composites with steel furnace slag aggregates

As previous studies of mortar and concrete with steel furnace slag (SFS) aggregates have shown increases or decreases in the bulk mechanical properties, this study investigated the microstructural cause of these opposing trends through characterization of the interfacial transition zone (ITZ) with quantitative image analysis of backscatter electron micrographs. Three SFS types – basic oxygen furnace (BOF), electric arc furnace (EAF), EAF/ladle metallurgy furnace (EAF/LMF) – were examined as aggregates in a portland cement mortar. The ITZ size for all SFS mortar mixtures was similar, with the ITZ of BOF and EAF/LMF being slightly more porous than mortar mixtures with EAF or dolomite. Microstructural examinations of the SFS particle revealed that BOF and EAF/LMF aggregates have different outer and interior compositions, with the outer composition consisting of a porous layer, which likely contributes to the reduced strength relative to EAF. The imaging results demonstrated that the type of SFS and its spatial composition greatly influences the bulk properties of mortar and concrete, mainly as a function of porosity content in the ITZ and the outer layer and interior porosity of the SFS aggregate.     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Concrete with recycled aggregates: the Portuguese experimental research

The main objective of this study is to define expedient procedures to estimate the properties of structural concrete that contains recycled aggregates. Experimental results from Portuguese research, most of which supervised by the first author, were used to establish a relationship between some properties of hardened concrete (compressive strength, splitting and flexural tensile strength, modulus of elasticity, abrasion resistance, shrinkage, water absorption, carbonation penetration and chloride penetration) and the density and water absorption of the aggregates’ mixture and also the compressive strength of concrete at the age of 7 days. The workability and density were also analysed for fresh concrete. The graphic analysis of each property shows the relationship between those for recycled aggregate concrete (RAC) mixes and a reference mix using natural aggregates only (RC). The density and water absorption of all the aggregates in the mixture, for each substitution rate, were calculated in order to represent the exact proportion of each type of aggregate (natural and recycled). This method proved to be viable to estimate the variation of the properties of concrete with recycled aggregates by obtaining results for the three parameters mentioned above. This innovative procedure can contribute to increasing the use of recycled aggregates in the construction sector and make it a sustainable activity.     

Flexural behavior of reinforced recycled aggregate concrete beams under short-term loading

Recycling of waste concrete is one of the sustainable solutions for the growing waste disposal crisis and depletion of natural aggregate sources. As a result, recycled concrete aggregate (RCA) is produced, and so far it has mostly been used in low-value applications such as for the pavement base. But, from the standpoint of promoting resource and energy savings and environmental preservation, it is essential to study whether a concrete made of recycled aggregates—recycled aggregate concrete (RAC) can be effectively used as a structural material. The experimental research presented in this paper is performed in order to investigate the flexural behavior of RAC beams when compared to the behavior of natural aggregate concrete (NAC) beams under short-term loading and consequently the possibility of using RAC in structural concrete elements. Three different percentages of coarse RCA in total mass of coarse aggregate in concrete mixtures (0 %—NAC, 50 %—RAC50, and 100 %—RAC100), and three different reinforcement ratios (0.28, 1.46, and 2.54 %) were the governing parameters in this investigation. Full-scale tests were performed on nine simply supported beams until the failure load had been reached. Comparison of load-deflection behavior, crack patterns, service deflections, failure modes and ultimate flexural capacity of NAC and RAC beams was made based on our own and other researchers’ test results. The results of conducted analysis showed that the flexural behavior of RAC beams is satisfactory comparing to the behavior of NAC beams, for both the service and ultimate loading. It is concluded that, within the limits of this research, the use of RAC in reinforced concrete beams is technically feasible.     

Regression models to predict SCC pressure exerted on formworks containing vertical and transverse reinforcing bars

The use of self-consolidating concrete (SCC) containing recycled concrete aggregate (RCA) considerably increased in sustainable structural applications and civil engineering works. However, current literature and construction practices are not clear regarding the influence of RCA additions and presence of steel reinforcement on formwork pressure exerted by the plastic concrete. This paper reports experimental data obtained from 32 SCC mixtures possessing different stability levels and cast in 1.6-m high formwork containing various combinations of vertical and transverse steel bars. Test results have shown that mixtures incorporating recycled aggregates exhibited reduced initial maximum pressure, given the higher RCA surface roughness that promotes internal friction and material build-up at rest. The decrease in pressure was particularly accentuated in presence of steel bars, suggesting that the reinforcement cage confines the plastic concrete and carries part of its load. The transverse steel was around 1.5-times more influential than vertical steel in reducing the formwork pressure. The rates of pressure drop over time were not altered because of steel, implying that pressure decay is governed by the concrete intrinsic properties such as thixotropy, RCA friction, and cement hydration. Special emphasis was placed to develop regression models and examine suitability of existing ones to predict lateral pressure of RCA-modified SCC cast in formworks containing reinforcing bars.     

Properties of cold bonded quarry dust coarse aggregates and its use in concrete

In this study, artificial coarse aggregates are prepared by a cold bonding technique. The waste materials, namely, fly ash and quarry dust, are used for the preparation of the cold bonded artificial aggregate. Portland cement is used as the binder material. The independent variables considered for the preparation of the artificial aggregate are cement and fly ash contents. The properties of the artificial aggregate are determined and regression models are proposed for predicting these properties. The strength and workability of concrete containing artificial aggregate is determined. The slump loss of concrete containing artificial aggregate is found to be gradual. The concretes with strengths of up to 30 MPa is prepared using artificial aggregates. The study promotes the use of waste material and supports sustainable construction practices.