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Blends of ordinary Portland cement (OPC) and calcium sulfoaluminate (CSA) cement can be used to adjust the properties of cement for specific applications. In this study, CSA cement was used as a shrinkage-compensating admixture to improve the hydration behavior and performance (compressive strength and drying shrinkage) of OPC; the expansion behavior of the blended cement mortar was evaluate based on the saturation index of ettringite. The experimental results showed that incorporating CSA cement resulted in a delayed effect on the hydration of C3S, shortened the induction periods of the blended cement and decreased the setting time. The CSA cement also improved the early compressive strength and drying shrinkage of the OPC due to its compact microstructure. The drying shrinkage of the OPC mortar decreased by 27.8% when 6% CSA cement was used, but the formation of microcracks due to expansion could negatively impact its late compressive strength development and associated pore structures of the blends when the replacement content of CSA cement exceeded 6 wt.%. The results relevant to the expansion behavior of the CSA cements could induce crystallization stress, enhancing its resistance against shrinkage cracking.

This study aimed to find an effective, inexpensive, and safe washing treatment for municipal solid waste incineration bottom ash (MSWIBA) in order to reduce its potential harmful effects in disposal and recycling. The washing solutions, namely tap water (TW), saturated lime water (SLW), and wastewater from concrete batching plant (WW) were used to wash MSWIBA at different liquid-solid (L/S) ratios and for different durations. Leaching behavior of some heavy metals, chloride, and sulfate from MSWIBA was tested and evaluated. From the TCLP leaching test, when the L/S ratio was above 5, WW was the most effective solution in reducing As, Cd, Se, and Sb emissions from MSWIBA. The calcium and iron ions present in the WW were essential for controlling the leaching of As, Cd, and Sb from MSWIBA due to the formation of stable crystalline pharmacosiderite, cadmium hydroxide sulfate, and hydromeite during the washing process. Using WW showed the best effect in removing sulfate from MSWIBA. At a L/S ratio of 10, about 83% of the sulfate could be removed from MSWIBA after 20 min of washing. The L/S ratio was most influential in removing chloride from MSWIBA. The three washing treatments chosen were effective in reducing the chloride level in MSWIBA to below the level of hazardous waste. Nevertheless, there were still substantial amounts of chloride remaining in the treated MSWIBA. Under the Dutch Building Materials Decree, the treated MSWIBA may be used as a building material in parts which allow isolation, control, and monitoring (ICM).

In this study, modification methods of recycled coarse aggregate (RCA), namely spraying colloidal nano silica (NS) and silica fume (SF) on the surface of RCA, were adopted to enhance the performance of recycled aggregate concrete (RAC). In the RCA modification methods, colloidal NS with different particle sizes (12 nm, 54 nm and 106 nm) and colloidal SF with a particle size of 420 nm were considered. To understand the effectiveness of silica with different particle sizes, mechanical properties and durability of RAC prepared with the modified RCA were evaluated. Microhardness and scanning electron microscopy (SEM) tests were conducted to investigate the microstructural properties. Test results showed that the mechanical properties and durability of RAC were enhanced remarkably after spraying colloidal NS and SF on the surface of RCA, whilst the performance enhancement was larger at a larger particle size of NS. Moreover, it was found that spraying colloidal SF on RCA enhanced both the new mortar and the new interfacial transition zone (ITZ) in RAC, while spraying colloidal NS only enhanced the new ITZ, as evidenced by microhardness investigations. This study deepens the understanding of enhancing the performance of RAC to approach that of natural aggregate concrete.

Many studies have focused on the feasibility of using nanoparticles including nanoalumina (NA), in cement composites and reported some positive effects on the properties of concrete. However, there are still uncertainties about the effects of NA addition on cement hydration. In this study, γ-NA particles with a size of about 30 nm was incorporated into Portland cement pastes and the physicochemical and microstructural properties of the NA-modified cement pastes were analyzed. Conduction calorimetry results showed that NA accelerated the silicate and aluminate phases reactions in OPC. Compared to the plain cement paste, the NA-modified cement pastes exhibited improved compressive strength at all ages due to the reduced macropores and the accelerating effect on cement hydration induced by NA, but the strength enhancement at 28 days was less obvious. Moreover, back scattering electron imaging and energy-dispersive spectroscopy (BSE-EDS) analysis indicated that the Al-enriched outer C-S-H formed in the NA modified paste; and it was mainly due to the NA particles acting as seeding for the growth of the outer C-S-H at the early age, rather than the Al released by the aluminate phases. This also provided a possible explanation for the substantial increase in CH and chemical bounded water contents in the NA-modified pastes at 12 h, but almost no change at 28 days of hydration.

A coupled CO2-water curing regime was employed on Ordinary Portland cement (OPC) paste samples immediately after casting, which allowed carbonation and hydration of OPC to proceed simultaneously. The strength development and microstructural evolution was evaluated by using multiple-techniques. The results indicated that, compared to the normal hydrated counterpart, a lower porosity, higher amorphous phase content and overall reaction degree can be achieved in the coupled CO2-water cured OPC sample. By combining with the morphological observations, a new mechanism was proposed for the rapid hardening of OPC. It is shown that the carbonation reactions led to the formation of calcite particles, which provided more nucleating sites for C S H gel growth; and thus, an increase in the overall reaction degree of the cement paste can be achieved within the first 24 h compared to the conventional water curing process.

Optimization of pretreatment and various gas-solid interaction conditions is necessary for effective carbonation of recycled coarse aggregates derived from concrete wastes (RCA). This study demonstrated the use of linear weighted sum method to simultaneously optimize the pretreatment and carbonation test conditions. The optimization of the test conditions produced simplified, and eco-friendly pretreatments and drastically reduced the carbonation duration of RCA from 72 h to 3 weeks in previous studies to 18 h in this study. In addition, the optimum experimental conditions were validated by casting concrete using the carbonated RCAs. The optimized pretreatment and carbonation conditions were spraying Ca2+-rich wastewater at a level of 60% of the 24 h water absorption of RCA, 12 h air drying and 6 h carbonation at 60 °C. This study also demonstrated that wastewater derived from ready-mix concrete batching plants can be utilized for spraying the RCAs to improve carbonation efficiency. Also, the mechanical and durability properties of concrete prepared with RCAs produced from the optimized conventional carbonation and pressurized carbonation were similar. Therefore, optimized conventional carbonation can be utilized as an alternative to pressurized carbonation. The improved mechanical and durability properties of concrete prepared with carbonated RCAs were attributed to the improved physical properties of carbonated RCAs and improved microhardness of the interfacial transition zone of the new mortar. The simplified, faster, ecofriendly, optimized pretreatment and carbonation conditions opens a new vista for industrial carbonation applications and optimization of gas-solid interactions.

Five recycled coarse aggregate (RCA) treatment techniques including flow-through carbonation, pressurized carbonation, wet carbonation, nano silica (NS) pre-spraying and combined pressurized carbonation with NS pre-spraying, were utilized to improve the performance of recycled aggregate concrete (RAC). The characteristics of the stress–strain curves of RACs including peak stress, peak strain, elastic modulus, ultimate strain and toughness were evaluated after using the above RCA treatment techniques. A theoretical model for natural aggregate concrete was used to analyse the stress–strain curve of RAC. Additionally, the carbonation resistance of RAC after using different RCA treatment techniques were investigated. The results showed that the calculated stress–strain curve of RAC based on the theoretical model matched well with the experimental results. Among the three types of carbonation techniques, pressurized carbonation caused the highest improvement in peak stress and elastic modulus of RAC, followed by flow-through carbonation, the last was wet carbonation. The NS pre-spraying method contributed to even higher improvement in peak stress and elastic modulus of RAC than the pressurized carbonation method. The combined pressurized carbonation with NS pre-spraying exhibited the highest enhancement of RAC because both the RCA and the new interface transition zone (ITZ) were improved. The carbonation resistance of RAC was improved after using all the studied RCA treatment techniques.

Urban solid waste disposal has become a global concern due to the huge waste volume and its adverse environmental impacts. Construction and demolition waste (CDW) and municipal solid waste incinerator bottom ash (IBA) are two main urban solid waste streams that have attracted global research interest for their recycling and reutilization. It is acknowledged that cement treatment is an effective method to facilitate the wastes to be used in road subbase. In this chapter, cement-treated recycled concrete aggregates (RCAs) derived from CDW and IBA generated from MSW incinerators were evaluated and the feasibility of using the cement-treated semirigid pavement base course was explored. Additionally the environmental impact was investigated in terms of assessing the effect of the cement treatment of the blended RCA-IBA mixture on stabilizing heavy metals and reducing their leaching to the environment.

In this study, municipal solid waste incineration bottom ash (MSWIBA) and waste glass were recycled and reused to synthesize aerated or cellular alkali-activated concrete (AAAC) in order to minimize their disposal at landfills and to produce a value-added construction material. The experimental results showed that incorporating 20% waste glass powder (WGP) in the alkali-activated MSWIBA-WGP products would not cause a decrease in strength, and WGP can be considered as a precursor as well as a waste-derived activator to provide the reactive silica for geopolymerization. The MSWIBA as the main precursor contributed to the structure forming of AAAC as well as acting as a source of foaming agent due to the presence of metallic aluminium. The prepared AAAC incorporating 20% WGP exhibited low thermal conductivities ranging from 0.14 to 0.38 W/m·K with compressive strengths ranging from 0.9 to 10.4 MPa, and density values from 494 to 1295 kg/m3. The results obtained by optical photography and Scanning Electron Microscopy-backscattered electrons imaging methods showed that the prepared AAAC had less spherical air voids and wider air-void size distribution from 0.02 to 3.0 mm when compared to traditional aerated cement concrete, and the reactive silica was able to form C(N)-A-S-H which provided the mechanical strength development of AAAC.

Municipal solid waste (MSW) is being considered as a renewable resource for producing energy in Waste-to-Energy (WtE) incineration plants. However, a considerable amount of MSW incineration bottom ash (MSWIBA) is generated in the process of MSW incineration (MSWI). Disposal of MSWIBA in landfills poses a threat to the environment due to leaching of pollutants (e.g. heavy metals and soluble salts). Over the last decade, with increasing awareness of sustainable development, many researchers carried out studies on pre-treatment techniques to upgrade the quality of MSWIBA as construction materials. Moreover, its utilization as an alternative recycled construction resource to replace natural materials has been promoted, but various limitations for its engineering applications still exist due to the potential environmental risks as well as its relatively inferior properties compared to natural aggregates (e.g. high porosity and alkali-silica reaction). This paper presents an overview of the environmental and engineering issues facing the utilization of MSWIBA to replace natural aggregates in different applications, as well as the current pre-treatment techniques for upgrading the quality of MSWIBA. In the literature, most of the past research studies focused on the leaching of pollutants from MSWIBA. However, techniques for improving the properties of MSWIBA as aggregates for engineering applications have been relatively less studied. Moreover, the pre-treatment methods had been reported in individual case studies, but there was no comprehensive review for large-scale operation. To provide scientific and technical ways for utilization of MSWIBA, it is suggested to develop integrated pre-treatment processes of MSWIBA combining with two or more techniques in association with each targeted application, which not only reduces the environmental risk of MSWIBA, but also improves its engineering properties in applications.

Increasing attention is now paid to sustainable waste management from construction due to the increasing disposal cost as well as the growing public appreciation on environmental impacts. Since 1990′s, recycling and reuse of hardened concrete from construction and demolition waste has received considerable attention, while the principles, methods of treatment and associated technologies developed are generally not applicable to unhardened or early hardened concrete wastes generated during concrete production processes. This review focuses on the source, classification and management of the processing wastes from ready-mixed concrete (RMC) plants and their potential reutilization. In recent years, washing-out systems have been adopted at many concrete batching plants. Depending on reclaiming systems such as aggregate and water reclaimers, the wastes generated are classified as reclaimed aggregates, concrete slurry waste, wastewater and reclaimed water. Although the potential to reuse these wastes has attracted lots of interest, the respective principles and methods of treatments have only been reported in discretely, and sometimes inconclusive manners. The management challenges such as low re-usage rate, poor product performance, increasing cost and strict local regulations obviously limit their sustainable utilization. In order to minimize the dispose of the processing wastes from the RMC plants, the installation of both mechanical aggregate and water reclaiming systems are found to be necessary. All kinds of processing wastes can then be reused in producing different low carbon footprint products in order to improve the sustainable development of the RMC industry.

The modeled recycled aggregate concrete (MRAC) which is an idealized model for the real recycled aggregate concrete (RAC) was used in this study. The MRCAs prepared with two types of old mortars were modified by an accelerated carbonation process. The effects of carbonation of MRCA on the micro-hardness of MRCA and the mechanical properties of MRAC were investigated. The results indicated that the micro-hardness of the old interfacial transition zone (ITZ) and the old mortar in the carbonated MRCAs was higher than that in the uncarbonated MRCAs, and the enhancement of the old ITZ was more significant than that of the old mortar. The compressive strength and modulus of MRACs increased when the carbonated MRCAs were utilized, and the improvement was more significant for MRAC prepared with a higher w/c. In addition, a numerical study was carried out and it showed that the improvement in strength by carbonation treatment was less obvious when the difference between the new and old mortar was larger.

Strengthening the attached old cement mortar of recycled concrete aggregate (RCA) is a common approach to enhance the RCA properties. Accelerated CO2 curing has been regarded as an alternative way to enhance the properties of RA. However, the improvement of the properties of RCA was limited by the shortage of reactive components in the old cement mortar available for the carbonation reactions. In this study, a CO2 curing process associated with a limewater saturation method was performed cyclically on cement mortar samples, aiming to enhance the properties of cement mortars via artificially introducing additional calcium into the pores of the cement mortars. The results indicated that the adopted treatment method promoted the level of carbonation which was demonstrated by higher CO2 uptake by the limewater saturated cement mortar when compared to that without limewater treatment. After 3-cycles of limewater-CO2 treatment, the density of the cement mortar slightly increased by 5.7%, while the water absorption decreased by over a half. For mechanical properties, the compressive and flexural strength were increased by 22.8% and 42.4%, respectively. Compared to the untreated cement mortar samples, the total porosity of cement mortar was reduced by approximately 33% and the densified microstructure therefore resulted in a higher microhardness.

Fine municipal solid waste incineration bottom ash (MSWIBA) contains more contaminants (e.g. metallic aluminium, glass and gypsum) that cannot be effectively removed by current recycling techniques and this thus limits its engineering application in concrete products. Instead of removing these contaminants, the objective of this research was to investigate the influences of using different casting methods and various supplementary cementitious materials (SCMs) on improving the properties of cement mortar prepared with the fine MSWIBA. In this study, the mortar bar specimens incorporating 100% fine MSWIBA as sand were prepared using the conventional wet-mixed method and a dry-mixed method. The SCMs applied were fly ash (FA), ground-granulated blast-furnace slag (GGBS) and waste glass powder (GP), with a replacement ratio of 30 wt% of ordinary cement in the mortar. The curing conditions of the specimens were in the 20 °C water, in the 80 °C water and in an 80 °C NaOH solution for 28 days, respectively. The experimental results showed that the dry-mixed method was an effective method to mitigate the expansion of the mortar prepared with MSWIBA as well as to enhance its mechanical properties after being subjected to the NaOH solution. The order of effectiveness of SCMs on the reduction in the expansion of the mortar prepared with MSWIBA was FA, GGBS and GP. It was indicated that FA was the most effective in mitigating the alkali-silica expansion as well as the alkaline-Al reaction.

In order to reduce the leaching of pollutants and remove the Al and Zn/Al alloy from municipal solid waste incineration bottom ash (MSWIBA), an optimized alkaline pre-treatment procedure was developed in this study. The influences of alkaline conditions on the removal rate of Al and Zn/Al alloy were investigated, including [OH] − concentration, temperature, particle size, liquid/solid ratio and treatment duration. The experimental results showed that the optimized alkaline pre-treatment conditions to efficiently remove the Al and Zn/Al alloy was by using a minimum of 1.0 mol/l [OH] − , at 55 °C and with a minimal liquid/solid ratio of 5. The removal rate of Al and Zn/Al alloy followed an S-shape curve, in which the slow beginning stage was attributed to the protection of the oxidation layer and the quenched product around the Al and Al/Zn alloy. After 3 h of the optimized alkaline pre-treatment, the leaching of Cr, Cu, Pb and Zn of the treated MSWIBA was reduced by more than 90% of that of the original MSWIBA. The alkali-silica reaction test further indicated that the expansion of concrete prepared with the pre-treated MSWIBA was significantly reduced and there was no macro-crack or spalling damage on the surface of the tested specimens.

As the world’s most widely used construction material, the production of concrete has been recognized to lead to major environmental impacts. To promote sustainability in the concrete industry, various kinds of supplementary cementitious materials (SCMs), such as fly ash, blast furnace slag and silica fume have been used to replace cement in concrete production. The nature of these SCMs has therefore been changed from wastes to co-products or by-products. Life cycle assessment (LCA) has been increasingly adopted in the concrete industry to assess environmental sustainability. However, the choice of an appropriate method for impact distribution in the LCA of concrete incorporating SCMs is a research challenge. This study aimed to present a comprehensive review of the impact allocation approaches for assessing the environmental impacts of SCMs-incorporated concrete. Furthermore, the use of the system expansion approach was compared with the conventional mass and economic allocation approaches. A case study of concrete production using SCMs in Hong Kong was conducted using the different approaches. The results were then analyzed and it was concluded that the system expansion approach is appropriate for the assessment of the environmental impacts of SCMs-incorporated concrete.

Considering the enormous amount of resources consumption and associated environmental consequences, the selection of sustainable materials is promoted in the construction industry globally. Concrete is one of the mostly used construction materials, but the production of concrete with ordinary Portland cement (OPC) has caused significant environmental burdens. In this study, five mixes of concrete were designed with commonly used binders and potentially alternatives materials to evaluate the mechanical and environmental performances, and their associated costs. The same volumetric replacement ratio of alternative binders, and the same water and aggregates contents were used for effective comparison. The mixes are normal concrete (NC) produced with only OPC, pulverized fly ash (PFA) concrete (PC), ground granulated blast furnace slag (GGBS) concrete (SC), volcanic ash-limestone concrete (VLC) and volcanic ash concrete (VC). Based on their corresponding performances of the selected indicators, multicriteria analysis was conducted. The results show that the potential alternative binders can reduce the environmental impacts significantly. For example, both VLC and VC reduces 25% greenhouse gas emissions compared to NC. Concrete produced with potential alternative binder systems, i.e. VLC and VC were also associated with 10–15% and 9–14% lower total impact than the commonly used industrial by-products. Compared to NC, about 9%, 11%, 12% and 15% lower costs were associated with VC, SC, VLC and PC, respectively. The results also indicate that VLC, SC, VC, PC and NC was ranked first to fifth consecutively based on the performance score evaluated from the multicriteria analysis. The results and analysis could help decision-makers select sustainable and cost-effective binders for concrete production.

Mortar made from concrete slurry waste (CSW) and municipal solid waste incineration bottom ash (MSWIBA) could be recycled into cold bonded lightweight aggregates (CBLA). However, the mechanism by which the CSW-MSWIBA mortar hardened and developed strength remains unexplored. Furthermore, the investigation in the previous study was limited to using the CBLA for wet precast lightweight concrete (LWC). Understanding the characteristic of CSW-MSWIBA mortar and exploring a new application of CBLA can promote the larger-scale recycling of CSW and MSWIBA. Herein, an investigation was performed on assessing the multi-component characteristics of CSW-MSWIBA mortar. The results showed that using MSWIBA (i.e., as compared to river sand) in CSW was advantageous for lower density and higher compressive strength of the mortar. The strength development of hardening CSW-MSWIBA mortar could largely be ascribed to the continuous cement hydration of CSW and the pozzolanic reaction between MSWIBA and CSW providing a better bonding at the CSW-MSWIBA interface. Following a pelletization process, the CSW-MSWIBA mortar was turned into cold bonded lightweight aggregates (CBLA). Through a trial of industrial-scale production, the CBLA were then used to fabricate semi-dry precast LWC, which met the LWC and masonry unit requirements as per BS EN 206 or JGJ/T 12 and BS 8103, respectively.

This study explored the mechanism of the effect of seawater on tricalcium aluminate (C3A) hydration. The results showed that seawater retarded C3A hydration and reduced the reaction degree of C3A. The co-existence or ion pairing of Ca2+ and SO42− onto the surface of C3A poisoning the reactive sites is the main reason for this retardation. Besides, the precipitation of Mg(OH)2 on the surface of C3A would prolong the induction period for another 30 min and then consequently decrease the dissolution rate of C3A hydration. Trace amounts of MgSO40 and Mg2+ present in the alkaline solution had little retardation effect. It was also found that Cl− would preferentially react with C3A to form Friedel's salt, rather than forming hydroxy-AFm as that in the C3A-deionized water paste. The direct formation of Friedel's salt resulted in an accumulation of Al3+ in solution, which would also hinder the subsequent dissolution of C3A.

As considerable amount of waste materials and by-products are generated from the urban and industrial production systems, efforts to utilize such materials resourcefully have received increasing attention in order to increase the use of recycled materials and reduce landfill disposal. In addition, construction products often induce significant environmental burden due to the use of emission-intensive materials. Therefore, continuous scientific efforts have been devoted in resource-efficient design with the incorporation of such secondary materials to help save the primary resources and reduce the environmental impacts. In this study, cement-free partition wall block is developed through a number of strategies, whereby emission-intensive Ordinary Portland cement is completely substituted by waste materials and supplementary cementitious materials , and this together with the use of recycled aggregates can make the blocks produced entirely based on secondary materials. The required mechanical performance of the blocks produced under different strategies is verified through laboratory tests. The carbon reduction potentials for such strategies are evaluated through the life cycle assessment (LCA) technique. The LCA results show that partition wall blocks produced with concrete slurry waste (CSW), fly ash and fine recycled concrete aggregates can reduce up to 82% of the total carbon emissions than other strategies. On the other hand, the reuse of CSW in the production of partition wall blocks can potentially save 80,000 m 3 of landfill space and US$4 million of costs associated with landfill disposal in Hong Kong annually. To maximize the advantages of such secondary resources in partition wall blocks production, it is imperative to establish industrial symbiosis networks with relevant supply chain and to adopt the developed partition wall blocks widely in practice.

This study compared the efficiencies of five recycled aggregate treatment methods (pressurized gas–solid carbonation (PC), flow-through gas–solid carbonation (FC), wet carbonation (WC), nano-silica (NS) spraying and combined PC with NS spraying (PCNS)) in enhancing the performance of recycled aggregate concrete (RAC). Mechanical properties, durability and microhardness of RACs were evaluated. The results showed that PC method recorded higher enhancing efficiency than both FC and WC method. NS spraying method caused higher improvement in mechanical properties of RAC than three carbonation methods. Because of the synergistic effect of enhanced old mortar and new ITZ, PCNS method recorded the highest efficiency.

An alternative CO2 curing method for precast concrete products has been proposed in order to achieve rapid strength development at early age, as well as to capture and store greenhouse gas (CO2). In this paper, an experimental study for the development of a maturity approach is presented to estimate the strength development of carbonated concrete blocks. In order to promote the use of industrial flue gas containing CO2, a flow-through CO2 curing regime at ambient pressure and temperature was employed using different atmospheric conditions, such as various CO2 concentrations, RH values and gas flow rates. The experimental results showed that the compressive strength or maturity of the carbonated concrete blocks was affected by two factors: accelerated cement hydration and carbonation extent. A high CO2 concentration, a fast gas flow rate and a moderate relative humidity were essential for enhancing the maturity and the strength development. The developed model based on the maturity approach may accurately predict the strength development of the carbonated concrete blocks.

Accelerated carbonation for recycled concrete aggregate (RCA) was one of the effective and economic solutions for improving the quality of RCA and its concrete products. Previously, a fast wet carbonation method was developed and proved to be effective for property enhancement of the coarse fraction of RCA by transforming the carbonation reaction from a gas-solid phase to a liquid-solid phase. Fast carbonation is essential for providing a cost effective and practical method of enhancing the properties of recycled aggregates by waste CO2. In order to investigate the feasibility and effectiveness of the wet carbonation for fine RCA (i.e.

The efficiency of a nano-silica (NS) treatment method, namely pre-spraying NS suspension on the surface of recycled aggregate (RA), in enhancing strength of recycled aggregate concrete (RAC) was evaluated and compared with that of a traditional NS treatment method by premixing NS with water. Various dosages and particle sizes of NS were considered. Microhardness and scanning electron microscopy tests were performed to investigate the microstructural properties and mechanism. Tests results showed that the NS pre-spraying method had superior effect over the NS pre-mixing method. The optimal NS suspension dosages of 4% and 6% by mass of aggregate were identified for the NS pre-spraying method and the NS pre-mixing method, respectively, whilst NS particle size had little influence on the performance. Such superior performance of the NS pre-spraying method could be ascribed to the better enhanced interfacial transition zone (ITZ) between RA and new mortar, as evidenced by the distinctive increase in microhardness and the formation of low Ca/Si ratio C-S-H in the ITZ.

In this work, a two-step wet carbonation process was developed to synthesize amorphous nano-silica by extracting the silicon phase from recycled cement fines (RCF), aiming to convert RCF into value-added products and sequester CO2. The phases evolution and their microstructure during carbonation were investigated. The results indicated that the RCF was carbonated rapidly to form a mixture of calcite and amorphous silica gel, followed by the formation of silica bearing gel containing aluminosilicate gel and silica gel after two-step wet carbonation. An amorphous silica with a purity of 98.8% and in the form of aggregated nanoparticles (