Adhered Mortar Research Articles

To Analysis the Feasibility of Utilization of Modified Recycled Aggregate for Recycled Cement Bound Mixtures (A Review)

Construction and Demolition (C&D) wastes are generated through the demolition of concrete structures, and they are often discarded into the open area causing environmental problems. Various components such as bricks, concrete, steel, wood, etc., are generated through the demolition of structures. Perhaps the necessity of construction materials, especially aggregates, increases rapidly due to urbanization and industrialization. Construction activities mainly depend on natural resources as a source of raw materials, leading to ecological imbalance. So, the construction industries tend to depend on alternative materials as the source of aggregates in concrete production. Recycling C&D wastes and using them as aggregates will be a viable option to counteract the problems mentioned above. The recycled aggregates are broken fractions of C&D wastes with smeared cement particles adhered to on the surface of natural aggregates. In this study, recycled aggregates were collected from a 20-year- old demolished building at the university and recycled into fractions of 1.18 mm~2.36 mm and 10 mm~20 mm and replaced for both fine aggregates and coarse aggregates. However, the poor quality of recycled aggregates resulting from the presence of adhered mortar affects the properties of the concrete. Thus, this paper aims to improve the property of recycled aggregates through surface treatments. Though various treatments were developed either to remove or coat the adhered mortar surface on the recycled aggregates, this study uses microbes in conjunction with advanced mixing approach techniques to improve the quality of recycled aggregates and properties of the recycled aggregate concrete (RAC). This paper finds Feasibility of Utilization of Modified Recycled Aggregate in various construction work by study the different Literature review and articles

Overcoming the brittleness of sustainable compression-cast recycled aggregate concrete through steel spiral confinement

The loose and porous adhered mortar of recycled aggregates produces concrete with low strength and poor durability. The newly developed compression casting technology applies pressure to fresh concrete to squeeze out excess water and air in the recycled aggregate concrete (RAC) and force the cement paste into the old adhered mortar to strengthen it, thereby effectively solving the aforementioned problems. However, the significant increase in the compactness and compressive strength of the RAC is accompanied by a pronounced increase in compression brittleness. Hence, this study attempts to overcome the brittleness of sustainable compression-cast RAC through proper design of transverse reinforcement. To achieve this objective, the axial compression behaviour of steel spiral-confined RAC specimens was experimentally investigated. It is found that the compression casting approach almost doubled the compressive strength of the unconfined RAC and improved the ultimate axial stress of the steel spiral-confined RAC by up to 69.9 %. The steel spiral-confined normal-cast RAC exhibited typical compression failure accompanied by a stress–strain curve characterised by strain hardening, whereas the steel spiral-confined compression-cast RAC exhibited localised shear failure accompanied by a stress–strain curve characterised by strain softening. The strain-softening behaviour of steel spiral-confined compression-cast RAC can be significantly improved by reducing the spiral pitch. Based on the test results obtained in this study and in the literature, stress–strain models were proposed separately for the steel spiral-confined compression-cast RAC and normal-cast RAC. The stress–strain models exhibited a significantly higher prediction accuracy than the selected models in the literature.

Structural implementation of recycled lump prepared from waste concrete after elevated temperatures: Mechanical and environmental performances

To find a sustainable treatment for construction and demolition waste, the utilization of recycled lump (RL) prepared from waste concrete after elevated temperatures as coarse aggregate in concrete was investigated. The mechanical and environmental properties of recycled lump concrete (RLC) and reinforced recycled lump concrete (RRLC) were discussed with three RL replacement ratios and five treatment temperatures. The results show that increasing the RL replacement ratio would reduce the slump, and axial compressive performance of RLC, resulting from the presence of old adhered mortar on RL. The treatment of elevated temperature for RLC would improve the adverse effect of increasing the RL replacement ratio, resulting from the decreasing amount of old adhered mortar and interfacial transition zone on RL. The prediction formulas for the mechanical model of RLC were proposed while the design method for the axial compressive strength for RRLC was modified against the existing standard to conduct the full-range parametric analysis. The environment assessment indicates that the sustainable utilization of RL as coarse aggregate could reduce the environmental impact and global warming from concrete production whilst the RL prepared from waste concrete after 400 °C treatment is an appreciated option for the production of RLC and RRLC products from a mechanical and environmental perspective. In addition, the increase in external diameter would remarkably dwindle the environmental impact caused by the utilization of RL in structural members.

A Review on Utilization of recycled Aggregate in concrete: Examining the effects of incorporating recycled concrete aggregate or other recycled materials as substitute for natural aggregate in concrete mixtures

In developed countries, concrete is mostly observed as a valuable resource. Recycling efforts have highlighted the importance of maintaining the required compressive strength when using old concrete in new construction. Research shows that compressive strength depends on factors like adhered mortar, water absorption, abrasion resistance, aggregate size, original concrete strength, curing time, replacement ratio, transition zone between old and new concrete, moisture level, impurities, and environmental conditions. While some studies have suggested methods for adopting recycled aggregates into concrete mixes, there's a need for a simple and environmentally friendly approach that considers the percentage of adhered mortar. This project aims to review existing literature on using recycled concrete as aggregates, focusing on compressive strength, and propose a method for incorporating recycled concrete aggregate while maintaining strength. Keywords: Construction and demolished concrete, second generation concrete, Adhere, recycling, workability, recycle aggregate, compressive strength

Mechanical properties, microstructure, and environmental assessment of recycled concrete from aggregate after fire

In terms of ever-increasing fire accidents in cities together with upgrades in the amount of construction and demolition (C&D) waste, the rapid modernization and urbanization process have caused an adverse effect on environmental protection. The lack of natural resources in concrete provides a driving force for the reuse of C&D waste caused by fire accidents. Aiming to conserve natural resources and reduce environmental pollution, the feasibility of using aggregate after fire as an alternative material to produce recycled aggregate concrete (RAC) is investigated experimentally. The materials and environmental properties of the RAC are evaluated with the change of four different replacement ratios (25%, 50%, 100%, including a control group of 0%), and five different heating temperatures (200 °C, 400 °C, 600 °C, and 800 °C, including a control group of 20 °C). The results showed that with the increase of the substitution ratio, the strength, elastic modulus and ascending-branch slope for the stress–strain curves decrease gradually, which the brittle failure phenomenon becomes more obvious, which may have a bearing on the presence of old adhered mortar (AM) on recycled coarse aggregate (RCA). Nevertheless, the utilization of waste concrete treated by heating in RAC can mitigate the adverse effect of increasing the replacement ratio, which is attributed to the decreasing content of old AM in RCA. Numerous calculation formulas related to the material properties and constitutive model are proposed by considering different factors. Based on SEM tests and X-ray diffraction techniques, it was identified that the strengthening mechanism in the mechanical properties and uniaxial compressive behavior for RAC prepared with waste concrete after heating is mainly ascribed to the decreasing possibility of penetrating cracks in concrete under compressive loading. The cost and EI assessment shows that RAC with a 25% replacement ratio produced by waste concrete after 400 °C can probably be a suitable selection in concrete materials from an economic and environmental perspective.

Assessment of fly ash and ceramic powder incorporated concrete with steam-treated recycled concrete aggregates prioritising nano-silica

Present research involves determining the effects of a proposed novel nano-silica prioritized-steam-treated recycled concrete aggregate (RCA) on microstructural, mechanical, and durability aspects of concrete incorporated with waste ceramic powder (WCP). The study on novel nano-silica prioritized-steam-treated recycled concrete aggregate revealed that 3% nano-silica induction with 3-h steam treatment for 50% adhered mortar bonded RCA performed optimally. The physical characterization of treated RCA showed improvement compared to untreated RCA, which was confirmed by microstructure study indicating the formation of additional calcium silicate hydrates in the bonded adhered mortar of treated RCA. Furthermore, as WCP has significant contents of alumina and silica, an optimum ternary binder mix was developed with cement, fly ash, and WCP. Later, a study was performed to analyse the performance of treated RCA incorporated in WCP prioritized concrete mix. The mechanical performance of WCP prioritized concrete with treated RCA was investigated through compressive strength, flexural strength, split tensile strength, and modulus of elasticity. The quality was ensured through ultrasonic pulse velocity, water absorption, and density characterization. The durability of concrete was studied with 5% concentrated hydrochloric acid attack and sea water (pH = 8.3 to 8.7) exposure conditions for a duration of 148 days (including 28 days of portable water curing period). Overall, 30% of the ternary mixture based on WCP prioritization, 50% adhere mortar-based RCA, and 3% of nano-silica prioritization steam treatment (3 h) demonstrated the best performance in terms of both mechanical and durability aspects. The study concluded that due to its improved performance, the innovative nano-silica priority steam treatment approach could replace 100% of RCA in concrete. Furthermore, treated RCA being advantageous because of easy adoptable technique for real-time practices as well as maintaining consistency regards RCA characteristics throughout concrete mixture be the challenge.

An Innovative Approach for Evaluating the Quality of Recycled Concrete Aggregate Mixes

The objective of this research is to develop an innovative methodology for evaluating the quality of coarse aggregate mixes that incorporate recycled concrete aggregate for use in structural concrete. The proposed approach consists of three steps: measuring the packing density, quantifying the adhered mortar, and characterizing the mechanical resistance of the aggregate mixes containing both natural and recycled concrete aggregate. Comprehensive practical recommendations for each step are discussed in detail to facilitate the broader future utilization of recycled concrete aggregate in structural concrete. The mechanical resistance of aggregate mixes is significantly influenced by both packing density and the quantity of adhered mortar. Predictive models are used to illustrate the mechanical resistance of aggregate mixes based on packing density and adhered mortar amount, recognizing the direct correlation between these properties.

Effects of pretreated recycled fine aggregates on the mechanical properties and microstructure of alkali-activated mortar

Compared with traditional cement-based binders, alkali-activated binders can be improved upon combination with recycled aggregates to produce a promising and new green construction material. However, during this process, a pretreatment method is needed to alleviate the negative impact of recycled aggregates on the properties of alkali-activated materials. In this study, fly ash (FA), ground granulated blast furnace slag (GGBFS) and recycled powder (RP) were used as precursors, and recycled fine aggregates (RFAs) were used to replace natural fine aggregates (NFAs) to prepare recycled alkali-activated mortar (RAAM). Various RFA replacement ratios (0, 25%, 50% and 100%), pretreatment methods (calcination temperature and carbonation time) and alkaline moduli (0.8, 0.95, 1.1 and 1.25) were investigated. The effects of these parameters on the fresh and hardened properties, chemical composition and microstructure of RAAM were investigated. The results showed that the mechanical properties of RAAM gradually decreased with an increasing RFA replacement ratio due to the poor physical properties of RFA and its poor matrix binding. Two pretreatment methods (optimal calcination at 400 °C and carbonation at 6 h) were effective in improving the properties of RFA and thus the mechanical properties of RAAM. Calcination removed the adhered mortar from RFA and thus improved the mechanical properties of RAAM. Carbonation of RFA enhanced the quality of the attached mortar, which strengthened the RFA and provided better mechanical properties. In addition, microstructural analyses showed that RAAM prepared from pretreated RFA had a dense microstructure and a low critical pore size.

Effect of the strength grade of parent concrete on the performance of recycled aggregate treated by cement-fly ash slurry under prolonged soaking duration

The detrimental effects of quarrying natural resources have contributed to large-scale environmental pollution. Utilizing recycled concrete aggregate (RCA) obtained from construction and demolition waste as a replacement for natural aggregates has become an effective solution for environmental sustainability. However, the inevitably diminished characteristics of the adhered mortar and the existence of an additional interfacial transition zone (ITZ) have negatively suppressed the widespread use of these materials in production lines. The present study aims to comprehensively investigate the effects of a cement-fly ash slurry with a mass ratio of 50% of the total weight of RCA on the microstructure and engineering properties of RCA derived from three different parent concretes with strength grades of 20, 30, and 60 MPa. The RCAs were soaked for 24 h, 48 h, and 72 h to evaluate the long-term effects of the pozzolanic slurry. A sequence of laboratory tests was conducted, including mercury intrusion porosimetry (MIP) test, thermal gravimetric–differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, and measurements of water absorption and crushing value. The MIP results demonstrated a significant increase in medium capillary and gel pores, and a reduction in large capillary pores in the RCA with an increase in soaking time. TG-DTA results indicated an increase in Ca(OH)2 content during the first 24 h and 48 h of treatment. However, after 72 h of treatment, the Ca(OH)2 content of the treated RCA decreased in all cases. XRD analysis revealed the appearance of a few additional ettringite peaks among the treated RCAs in all cases. SEM images demonstrated the presence of additional hydration products after treatment, which filled the pores and improved the surface texture. Furthermore, after treatment, the crushing value of all treated RCA decreased by 6.0–27.1%, whereas the water absorption decreased by 12.0–30.9% compared to the untreated RCA. Based on several experimental results, the formation of hydration products and the improvement mechanism of RCA from three different parent concretes by slurry treatment with different soaking durations were discussed. In conclusion, a prolonged impregnation time of 72 h yielded a dominant performance, effectively intensifying their inferior properties, regardless of the initial strength grade of parent concrete.

Enhancing concrete performance: Surface modification of recycled coarse aggregates for sustainable construction

In this study, the researchers introduce an innovative method to enhance the utilization of recycled coarse aggregates (RCA) derived from construction and demolition waste (C&D) in sustainable concrete production. The presence of old cements mortar on the surface of these aggregates poses a challenge to their mechanical properties. To tackle this issue, the researchers propose a unique approach that combines chemical treatment and abrasion techniques. Firstly, RCA is immersed in a 0.5 mol hydrochloric acid (HCl) solution, weakening the adhering mortar. Subsequently, abrasion treatment removes the weakened mortar from the RCA surface. The study explores the incorporation of these surface-modified RCAs at varying levels, from 0% to 100% in 25% increments. Various concrete properties, including workability, compressive strength, flexural strength, split tensile strength, and microstructural characteristics (XRD, SEM, and EDAX), along with modulus of elasticity, are assessed. Results indicate that the combination of these modification techniques significantly enhances RCA properties. At the 100% replacement level, surface-modified RCA results in a reduction of compressive, flexural, and split tensile strength by 08.31%, 10.61%, and 07.90%, respectively, in comparison to the reference concrete. Likewise, the modulus of elasticity decreases to 30.82, and porosity decreases to 27.82% at the same replacement level. However, SEM analysis illustrates improved bonding between RCA and cement matrix due to surface modifications, leading to reduced porosity and the formation of dense Interfacial Transition Zones (ITZs). Cost analysis reveals that recycled aggregate concrete (RAC) mixtures can be more economical than those utilizing natural aggregates (NA). In summary, this novel surface modification technique presents a promising avenue for optimizing RCA utilization in sustainable concrete production, promoting environmental conservation, and efficient waste management in the construction sector.

Bacterial Treatment of Recycled Concrete Aggregate

Microbial treatment of recycled concrete aggregate (RCA) may improve the quality of the aggregate, and enhance its use in the production of structural concrete and promote the recycling of concrete waste. The mortar phase of the RCA is responsible for the poor performance of the aggregate. Treating the old adhered mortar or removing it from the natural aggregate (NA) is an option to make RCA beneficial for the production of quality recycled aggregate concrete (RAC). Removing the adhered mortar from recycled concrete aggregate using silicate-solubilizing bacteria was investigated. The bacteria could synthesize the silicates in the calcium silicate hydrate phase of the cement paste leading to the breakdown of the old adhered mortar. Four SSB strains were tested for survivability and activity in an alkaline medium to simulate the concrete environment. The Serratia marcescens bacterial strain, which survived the environment, was inoculated into screw-cap glass vials containing recycled concrete aggregate fragments and glucose-enhanced nutrient broth and then incubated for 14 days. Partial removal of the old adhered mortar was observed based on the weight lost from the RCA. The S. marcescens bacterial strain could survive the alkaline concrete environment and solubilize the silicates present in cement paste resulting in the removal of the old adhered mortar.

Unraveling the Interplay of Physical-Chemical Factors Impacting the Carbonation Performance of Recycled Aggregate Concrete.

Recycled aggregate concrete (RAC) includes recycled concrete aggregates (coarse and/or fine) as substitutes for natural aggregates as an approach to achieving a circular economy. Some concerns remain about its performance, including the carbonation resistance. The higher porosity of recycled concrete aggregates is logically a disadvantage, but the analysis must address many other factors. This paper provides an in-depth examination of recent advances in the carbonation performance of RAC. The emphasis is on factors that influence CO2 diffusion and the carbonation rate, e.g., the replacement ratio, source concrete quality, interfacial transition zone features, residual portlandite content, and porosity. The influences of previous treatments, combined action with supplementary cementitious materials, and loading conditions are also discussed. The replacement ratio has a significant impact on the carbonation performance of concrete, but it is also dependent on other factors. During carbonation, the physical effects of the porosity of the aggregate and the physical-chemical effects of the portlandite content in the adhered mortar are particularly important. The residual portlandite is especially significant because it is the primary hydration product responsible for the alkaline reserve for carbonation and the potential pozzolanic reaction, which are per se competing factors that determine the carbonation rate.

Sustainable development of recycled concrete aggregate through optimized acid-mechanical treatment: A simplified approach

Present research develops an optimized acid-mechanical treatment for recycled concrete aggregate (RCA). RCA prepared in the laboratory jaw crusher is further soaked for 24 h in a mild acetic acid solution. Acid-soaked recycled aggregates (SR) are mechanically treated at a series of charges and drum revolutions using Los Angeles machines. A performance-based quality identification approach is used to compare acid-mechanically treated recycled concrete aggregates (SRij). The VIKOR indices (QI) for each SRij is calculated by applying a multi-criteria decision-making approach. Based on performance, three SRij with the lowest QI are selected for an experimental study. The present research achieves 71.27 MPa compressive strength for concrete composed of a SRij with the lowest QI. This strength is around 16.06% higher than concrete composed of parent aggregate. Flexural strength, split tensile strength, fracture energy, ultrasonic pulse velocity (UPV) values, rapid-chloride penetration test (RCPT) values, water absorption, and abrasion resistance are in the range of what is expected for parent aggregate concrete. The scanning electron microscope (SEM) image analysis shows that the well compacted and uniform surface morphology of aggregate strengthens the newly developed interfacial transition zone (NITZ) and old interfacial transition zone (OITZ). Acid treatment reduces microcrack lengths and widths and mechanical treatment reduces microcracks and pores by reducing adherent mortar optimally. X-ray diffraction (XRD) analysis shows the lower calcium hydroxide (CH) content in the mortar attached to the SR. A reduced CH content lowers the porosity in the adhered mortar. Thus, improved quality of attached mortar and its reduced porosity strengthen the old interfacial transition zone (OITZ) and lowers the sorptivity of the concrete than reported and almost equal to that of parent aggregate concrete.

Thermo-mechanical treatment as an upcycling strategy for mixed recycled aggregate

The study investigates thermo-mechanical treatment for mixed recycled aggregates (MRA). Six treated aggregates were prepared by heating MRA between 300 °C and 800 °C, followed by 100 abrasion cycles. The thermo-mechanical treatment causes loss in the adhered mortar and improves aggregate crushing, impact and abrasion value up to 700 °C. Concrete prepared with MRA treated at 700 °C, as compared to natural aggregates, shows higher workability (166.67%) and comparable mechanical strength, i.e., compressive (93.72%), flexural (99.94%) and split tensile (97.87%). The sustainability assessment shows that treating MRA with renewable energy can have lower economic costs (89.36%) and carbon emissions (58.05%) than manufacturing natural aggregates.

Mechanical properties of recycled aggregate concrete prepared from waste concrete treated at high temperature

The ever-increasing generation of construction and demolition (C&D) waste caused by the frequent occurrence of building fires has posed a threat to environmental protection and waste management over the last few decades. The shortage of natural resources provides an impetus for the utilization of C&D waste after fire. Aiming to provide a cleaner option for waste concrete after fire, the viability of using waste concrete after fire as a substitute for coarse aggregate in the production of recycled aggregate concrete (RAC) was investigated and analyzed in detail. The failure mode, stress–strain behavior, mechanical properties and microstructure of the RAC was evaluated with the variation of four different replacement ratios (0%, 25%, 50%, and 100%) and five different treatment temperatures (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C). The experimental results showed despite the failure mode of RAC being similar to that of natural concrete (NC), an increase in the recycled coarse aggregate (RCA) replacement ratio would decrease the slope of curves and strength, and the fcu and fc with 25% replacement ratio for RAC decrease by about 15% and 10% respectively as compared to those for NC. Nevertheless, treatment of elevated temperature for waste concrete can eliminate the damage of replacement ratio to mechanical properties, and the fcu and fc with 25% replacement ratio after 400 °C increased by approximately 12% and 8% respectively as compared with untreated RAC. Against the test result, a series of calculation formulas is proposed to predict the mechanical properties of the RAC whilst a constitutive model is modified with the existing standard to describe the stress–strain behavior of that with the consideration of replacement ratio and treatment temperature. The microstructure analysis indicated that the increase in the RCA replacement ratio would result in the increasing content of adhered mortar characterized by numerous poles and less bond on RCA, whereas treatment of high temperature for waste concrete can decrease the content of adhered mortar on RCA, which makes the microstructure of the RAC more densified.

Physical and mechanical behavior of concrete specimens using recycled aggregate coated using recycled cement paste

Various countries have implemented the use of recycled concrete aggregates (RCA) as an alternative to reduce the use of natural resources. The replacement of RCA for natural aggregates (NA) in concretes is limited to small percentages because of the quality of the old mortar which is still adhered to the RCA. This mortar is more porous and absorbent, weakening the RCA and causing losses in the physical and mechanical properties of new concretes. In addition, the smaller the particles, the greater the amount of adhered mortar, which is why its use in concrete is limited and the fine fraction tends to be discarded. In this study, the use of RCA coated with recycled binder paste (RBP) (smaller than 45μm) from the crushing of the RCA itself is sought to strengthen weak adhered mortar and improve the properties of the RCA through an economic and sustainable treatment by using the waste of the waste. Therefore, the mechanical, physical and chemical behavior and durability of the concretes are assessed by the use of RCA coated with RBP (replacing by 0%, 50% and 100%). The results indicate a reduction in the mechanical and physical properties of the concrete for the highest replacement percentage of RCA. Nevertheless, the coated recycled aggregate concrete (RAC) showed superior behavior to the untreated RAC, obtaining compressive strengths close to the control and a remarkable improvement in the physical assays.

Effect of loading-induced damage on chloride ingress behavior of recycled aggregate concrete: A comprehensive review

The recycling technology of construction and demolition wastes is an inevitable choice towards the sustainable development of the construction industry, aiming at addressing the problems of raw materials shortage and waste disposal. The utilization of recycled coarse aggregate (RCA) in new concrete production has been identified as an effective way to reasonable application of natural resources replacement and environmental preservation. In service, external mechanical loading can easily cause the initiation and interconnectivity of microcracks inside concrete, specifically the complex multiple interface structures of recycled aggregate concrete (RAC) due to the old adhered mortar at the surface of RCAs, which can remarkably accelerate the process of water penetration and aggressive ions penetration into concrete. Thus, knowledge of mass transport properties in the loading-induced damaged RAC contributes to the durability evaluation of RAC and its extensive application in the practical engineering. This paper reviews the previous findings on the mechanical loading effects on the chloride transport properties of RAC. The effects of loading-induced damage on microstructure evolution of RAC as well as the damage characteristics were discussed. The research progress on the chloride ingress behavior of RAC subjected to mechanical loading was highlighted from the viewpoint of testing method, loading devices and numerical calculation, and the modified effects on the transport properties were further analyzed. On this basis, the future research on the durability-related properties of RAC would be finally proposed, which is helpful to further perfect theoretical development and technological innovation of RAC durability in the aggressive environment.

Effect of recycled coarse aggregate quality on the interfacial property and sulfuric acid resistance of geopolymer concrete at different acidity levels

Geopolymer recycled concrete (GRC) is a green and low-carbon building material which can promote the early achievement of the goals of carbon peak and carbon neutral globalization. This study aims to clarify the effect of recycled coarse aggregate (RA) quality on the interfacial property and sulfuric acid resistance of geopolymer concrete (GPC) at different acidity levels. Specimens with three qualities of coarse aggregates were prepared, which were immersed in sulfuric acid solution of three acidities (pH = 0.5, 1.0, 1.5) for 98 d to evaluate their acid resistance. The results showed that the adhered mortar attached on the surface of RA affected the inhomogeneous distribution of interfacial transition zones (ITZs) and weakened the bond strength between aggregate and paste, threatening the development of concrete strength. Low-quality coarse aggregate accelerated the deterioration of sulfuric acid resistance of GPC. But at the exposure time of 28 d, GRC showed a lower mass loss rate than that of GPC prepared with natural aggregate (GPCNA) regardless of acid environment levels, indicating the presence of RA resulted in the loose ITZ structure in GRC which could provide more space for gypsum generation to some extent. As exposure time goes on, the adhered mortars of RA provided extra calcium source for GPC to generate gypsum. More adhered mortar content caused overmuch gypsum crystals, generating internal stress, thus resulting in spalling of mortar and worsen acid resistance of concrete. In addition, the fracture of Si-O-Al bonds and disappearance of amorphous N-A-S-H gels caused by the dealuminization was another key reason for failure of the GRC in high acid environment (pH = 0.5 and 1.0). At last, based on the carbon emission calculations of raw materials, transportation and production of concrete, more than 20% of total carbon dioxide (CO2) emissions can be reduced using GRC to replace ordinary Portland cement concrete (OPCC).

The influence of environmental factors and precipitation precursors on enzyme-induced carbonate precipitation (EICP) process and its application on modification of recycled concrete aggregates

Recycled concrete aggregates are greatly limited in applications due to the defects in the adhered mortar. In this study, a novel bio-deposition method, enzyme-induced carbonate precipitation (EICP) technology, was applied to improve the quality of recycled concrete aggregates. Firstly, the influence of environmental factors (temperature and pH) and precipitation precursors (urea and Ca2+) on urease activity and EICP process were investigated. The results showed that the urease activity and deposition rate of CaCO3 were increased with the increase in temperature, and pH of 7–10 had little effect on them. However, when the concentration of CO(NH2)2–Ca(NO3)2 was more than 0.5 M, Ca2+ could not be utilized due to the deactivation of urease at high salt concentration. After obtaining the suitable conditions of EICP process, 0–0.5 M CO(NH2)2–Ca(NO3)2 and self-made urease solution were used to modify recycled concrete aggregates. The mass of recycled concrete aggregates increased by 1%–3.5% after EICP treatment. The saturated water absorption of the modified recycled concrete aggregates was decreased by a maximum of 20.9% after 0.5 M CO(NH2)2–Ca(NO3)2 treatment. After ultrasonic pulse treatment, the loss of the formed precipitation was 36.02% when the initial CO(NH2)2–Ca(NO3)2 was 0.2 M. This study has expanded the application field of EICP technology, and the application value of recycled concrete aggregates can be greatly improved.

Enhancing the Performance of Roller-Compacted Concrete Pavement by Synergetic Improvement of Packing Density, Lubrication, and Moisture State of Recycled Concrete Aggregate

Roller-compacted concrete pavement (RCCP) is considered superior to other pavement types with reference to cost, ease of construction, and performance. However, the aggregate demand is significantly higher in RCCP than in the conventional concrete pavement. It is predicted that natural minable limestone sources would be exhausted in India in the next 30 to 40 years. One way to reduce the natural aggregates (NA) requirements in RCCP is through the integration of recycled concrete aggregates (RCA). However, the physical property of RCA is significantly inferior to that of NA owing to the presence of adhered mortar (AM), which increases the water demand by 2.3 to 4.6 times and affects the compactness and hardened-state behavior of RCCP. This study has tried to enhance the compactness and improve the performance of RCCP containing RCA (coarse, fine, and total RCAs) through different synergetic approaches. This, in turn, enhances the interlocking capacity using the particle packing approach, followed by mitigating the negative effects of AM by altering the moisture states, then improving the compactability and lubricating the matrix with superplasticizers. The results depict that altering the moisture states alone could adversely affect the RCCP performance because of the moisture transfer mechanism from the hydraulic gradient. Moreover, the inclusion of superplasticizers in different moisture states could manifest better aggregate rearrangement and compactability. Further, it could improve the tensile behavior of the RCCP compared with the concrete containing NA. These findings favor the complete replacement of NA by RCA for low-volume rural road construction.