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A nondestructive testing method is developed to monitor the hydration process and predict the strength development of cement-based materials by applying the ground penetrating radar (GPR) and a non-contacting electrical resistivity device. These two instruments allow the measurements of the relative dielectric constant (RDC), the relative amplitude reflected electromagnetic (EM) wave and the electrical conductivity. The RDC and relative amplitude can be measured by calculating the propagation time and the intensity of electromagnetic impulses in material, while the electrical conductivity can be obtained directly by using the non-contacting electrical resistivity device. The results show that both of the relative amplitude and RDC are ideal indicators of the hydration process. The electrical resistivity shifts are well related to the dielectric properties. Linear relationships between the dielectric properties and compressive strength are observed, and a theoretical model is proposed to describe this relation based on the measurements of the dielectric constant and the relative amplitude at early age. This study provides additional theoretical and technical foundations for investigating the hydration progress and estimating the strength, which further helps to have a better understanding of the hydration process and promotes the quality control of cement-based materials. [ABSTRACT FROM AUTHOR]

The highly dense structure which causes the risk of explosive spalling is one of the major limitations of using ultra high-performance concrete (UHPC). This study developed a lightweight UHPC (L-UHPC) and proposed a strategy to improve its material efficiency and lower the risk of explosive spalling. High-strength hollow glass microspheres and fine lightweight aggregates were jointly used in the UHPC system to achieve light weight and ultra high strength. The L-UHPC was cured at different curing regimes to maximize its material efficiency and prevent its spalling after exposure to high temperature. The experimental results showed that the combination of lightweight materials and dry heat curing was effective in reducing the density (<1800 kg/m3) and enhancing the compressive strength (>150 MPa) of the L-UHPC. By means of minimizing the amount of internal free water, the heat curing approach contributed to avoiding the thermal spalling of the L-UHPC exposed to fire conditions. Microstructural results indicated that the elevated temperature curing promoted the pozzolanic reaction and increased the microhardness of the paste matrix by increasing silicate polymerization of the C-S-H structure. The created voids of the collapsed glass microspheres might accommodate some internal vapor pressure and mitigate the degradation caused by the high temperature exposure. The synergetic effect of the porous materials and the dry heat curing lowered the thermal conductivity and significantly improved the material efficiency of the L-UHPC. • A new lightweight ultra high-performance concrete (L-UHPC) was developed. • A strategy was proposed to lower the risk of explosive spalling of L-UHPC. • The combination of lightweight materials and dry heat curing could prevent the spalling of L-UHPC. • The developed L-UHPC had low density (<1800 kg/m3) and high compressive strength (> 150 MPa). • The L-UHPC had low thermal conductivity and high material efficiency. [ABSTRACT FROM AUTHOR]

This study presents the use of IBA for the production of eco-pervious concrete, aiming at enhancing the recycling rate of IBA in construction materials. The IBA aggregates are used to replace the natural aggregates (NA) in various percentages (0%, 25%, 50%,75% and 100% by volume). The effect of IBA replacement ratios and casting methods on the mechanical properties, water permeability, porosity, density and thermal conductivity of the pervious concrete were determined. The volume stability of the pervious concrete incorporating IBA was evaluated. The results indicated that the appropriate content of IBA could benefit the mechanical properties due to the internal curing effect of IBA. However, the excessive content of IBA in the pervious concrete led to reduced water permeability, connected porosity and mechanical properties. Even so, the pervious concrete containing 100% IBA could achieve the required permeability and compressive strength for permeable pedestrian pavers (JIS A 5371). In addition, the incorporated IBA in pervious concrete causes little volume expansion from the reaction between alkalis and components consisting of metallic aluminum and glass in IBA owing to its high connected porosity. Furthermore, the pervious concrete incorporated IBA had a low density and thermal conductivity, showing the potential to be applied in thermal insulation areas. [Display omitted] [ABSTRACT FROM AUTHOR]

High strength and light weight are two recent opposite development trends of concrete. This study proposed a design concept of an ultra high-performance lightweight cementitious composite (UHP-LCC), which had a compressive strength of higher than 120 MPa and an air-dried density down to around 1800 kg/m3. The UHP-LCCs were innovatively developed by incorporating micro-sized hollow particles with a high strength shell (hollow glass microspheres, HGM) into an ultra-high performance cementitious composite (UHPC). The roles of HGM in the UHP-LCCs were investigated by evaluating the reactivity of the HGM and the mechanisms on achieving the excellent mechanical properties, low density and superior durability were revealed. The Chapelle test results showed that the HGM exhibited some pozzolanic reactivity, which facilitated the reaction between the shell of HGM and the alkali hydration products of the paste matrix. This chemical reaction was conducive to improving the HGM-paste interface and enhancing the mechanical properties. With the use of microspheres with a high stiff shell, the fundamental properties of the UHP-LCCs including thermal insulation, sound absorption, resistance to water ingress and electrical resistivity were improved significantly. The strategies for preparing the UHP-LCCs with high structural efficiency and great performance were proposed. The results of this study provide a new approach for designing and producing a lightweight UHPC, which would be a promising material for long-span structures. • An ultra high-performance lightweight cementitious composite (UHP-LCC) was designed. • The UHP-LCCs had ultra high strength (>120 MPa) and very low density (<1850 kg/m3). • The UHP-LCCs had high structural efficiency, good functional properties and superior durability. • The strategies and mechanisms on achieving UHP-LCCs with excellent performance were proposed. [ABSTRACT FROM AUTHOR]

To enhance the properties of recycled concrete aggregates (RCA), a liquid-solid carbonation process was developed and compared with the conventional gas-solid pressurized carbonation method. The performance of RCA and the recycled aggregate concrete (RAC) prepared with the carbonated RCA by the two aforementioned carbonation methods was evaluated. The carbonation products assemblage, microstructure and porosity evolution were first characterized. The results indicated that after subjecting to a 10-min liquid-solid carbonation process, a reduction of 12.2% in RCA water absorption and an increase of 2.6% in density were achieved, which were more efficient when compared to the 24-h gas-solid pressurized carbonation. More importantly, the compressive strength of RAC prepared with the 6-h liquid-solid carbonated RCA was significantly improved. In light of microstructural analysis, the enhanced performance of RAC was attributed to more additional mono-carbonate (Mc) and C–S–H gel formed at the interface between the liquid-solid carbonated RCA and the new cement pastes. [ABSTRACT FROM AUTHOR]

• 1. The proposed design concepts was applicable to produce high strength pervious concrete. • 2. The ultra-high performance paste bonded the aggregates tightly. • 3. The improved homogeneity contributed to the enhancement of strength. • 4. The bonding area between aggregates was optimized by adjusting membrane-forming ability. This study proposes a novel design concept of high strength pervious concrete, aiming at improving its compressive strength with adequate permeability. An ultra-high performance paste was developed and used as the binder of the high strength pervious concrete. The effects of aggregate characteristics and membrane-forming ability of the paste on the performance were investigated and their compatibility was evaluated. The experimental results showed that the use of ultra-high performance paste, improvement of homogeneity and compatibility between the paste and the aggregates by elimination of coarse aggregates could improve the mechanical properties. The combined use of aggregates with two-size ranges also resulted in an increase of compressive strength due to the increase of homogeneity and interlocking effect between the aggregates. The enhancement of the membrane-forming ability could optimize the bonding area and thickness of the cement paste between the aggregates. Therefore, the application of the proposed design concepts was able to produce a high strength pervious concrete with a compressive strength of 60.93 MPa and a water permeability of 0.37 mm/s, which would potentially broaden the scope of application of pervious concrete. [ABSTRACT FROM AUTHOR]

In this study, municipal solid waste incineration bottom ash (IBA) was reused as fine aggregates to prepare ultra-high performance concrete (UHPC). The effects of IBA on the mechanical properties, workability, hydration, volume stability and microstructure of UHPC were investigated. The experimental results showed that the incorporation of an appropriate amount of IBA could improve the compressive strength of UHPC due to the internal curing effect. While the addition of excessive amount of IBA would reduce the strength significantly. From the microstructure analysis, the hydration of UHPC was promoted by adding IBA, leading to a higher hydration degree and denser microstructure of the paste compared to reference sample. The use of IBA in UHPC could effectively restrict the expansion due to the presence of metallic Al, glass, gypsum in the IBA as the UHPC matrix acted as a barrier in the vicinity of the IBA particles, which prevented the formation and propagation of cracks. Also, the highly impermeable UHPC cement paste, the use of supplementary cementitious materials and the high self-desiccation of UHPC further prevent the expansive chemical reactions from occurring. Thus, it was practicable to apply IBA in UHPC without considering the expansive risks and with easy pre-treatment, and it was demonstrated that the utilization of IBA in UHPC is a feasible solution for recycling IBA. [ABSTRACT FROM AUTHOR]

• Recycled fine aggregate (RFA) and carbonated RFA (CRFA) were used to replace sand. • Influences of RFA and CRFA on the properties of AASGM were studied. • RFA had positive effect on the compressive strength and shrinkage of AASGM. • The compressive strength of AASGM decreased dramatically as CRFA content increased. • Both RFA and CRFA had obvious influence on the polymerization reaction of AASGM. Alkali-activated materials have attracted a lot of research interest due to the advantage of less CO 2 emission than ordinary Portland cement (OPC) systems. The use of recycled aggregate or carbonated recycled aggregate in new concrete is considered a good way to solve the problem of construction and demolition waste. This study aims to investigate the influences of recycled fine aggregate (RFA) and carbonated RFA (CRFA) on the properties of alkali-activated slag and glass powder mortar (AASGM). The macro properties (compressive strength, workability, setting time, and shrinkage) and microstructural properties of AASGMs prepared with RFA/CRFA were evaluated. The hydration evolutions of AASGMs and the reactions between RFA/CRFA and sodium silicate were investigated to explore the reaction mechanisms of AASGM with RFA/CRFA. The results showed that with the increase in RFA content from 0 % to 100 %, the flow value of AASGM decreased from 240 mm to 145 mm, the initial setting time was shortened by 44 %, the 7 days autogenous shrinkage decreased by 82 %, the 25 days drying shrinkage decreased by 31 %, and the 28 days compressive strength firstly increased (RFA content ≤ 50 %) and then decreased. On the contrary, with the increase in CRFA content from 0 % to 100 %, the flow value of AASGM increased from 240 mm to 270 mm, the final setting time increased by 37 %, and the compressive strength decreased by 97 %. The roles of RFA and CRFA in AASGM were different from their roles in OPC-based materials. That was because the calcium hydroxide in RFA could react with sodium silicate (SS) and increase the concentration of alkali, which accelerated the polymerization reaction of AASGM. However, the calcium carbonate and silica gel in CRFA reacted with SS and reduced the concentration of alkali, and thus the polymerization reaction of AASGM was decelerated. [ABSTRACT FROM AUTHOR]

Carbonated recycled concrete fines (RCF) possess the potential to serve as supplementary cementitious materials (SCMs). However, calcium carbonate (C c) formed during traditional carbonation methods generally shows low reactivity. Moreover, the remaining calcium silicate hydrate (C-S-H) with a low Ca/Si ratio exhibits enhanced resistance to decalcification, leading to the formation of lower content of silica gels. These factors limit the reactivity and utilization of carbonated RCF in cementitious systems. In the present work, mechanochemistry was introduced into the RCF carbonation process, a technique named mechanochemical carbonation (MC), to enhance the reactivity of carbonated RCF. This study delved into the influence of MC-treated RCF (MCR) on the early performance of cement composites. The findings reveal that MCR exerted a more pronounced promotional effect on cement hydration, as demonstrated by the earlier appearance of hydration peaks for silicates and aluminates, coupled with increased peak intensity. This then resulted in a notable enhancement in early compressive strength. Specifically, with 10 wt% MCR, the compressive strength increased by approximately 19.9 % and 24.7 % at 1 d and 3 d, respectively. In comparison, samples incorporating wet carbonated RCF (WCR) demonstrated lower enhancements, with increases of only 4.5 % and 12.0 % at 1 d and 3 d, respectively. The enhanced performance of MCR can be attributed to the increased formation of nano-sized amorphous silica gels and metastable C c. These components exhibit elevated reactivity and contribute to seeding effects, thereby promoting the overall reactivity and strength development of the cementitious system. These findings underscore the promising application potential of MC in transforming RCF into highly reactive SCMs. [ABSTRACT FROM AUTHOR]

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 CO 2. In order to investigate the feasibility and effectiveness of the wet carbonation for fine RCA (i.e. < 5 mm), this study presents the results of an experimental investigation by using both laboratory-prepared well-hydrated cement paste particles (RCP) and a real fine RCA derived from demolished concrete. The test results of RCP revealed that a quick 10-min wet carbonation densified the surface layer of the particles, significantly reduced the volumes of the pores less than 10 nm, and increased the carbonation products by 2.6–3.5%. Moreover, the mortar specimens prepared by the carbonated fine RCA showed a 32.6% increase in compressive strength and a 6.4% reduction in drying shrinkage. Replacing 50% river sand by the carbonated fine RCA in the new mortar would not result in significant property deterioration. [ABSTRACT FROM AUTHOR]

Conventional supplementary cementitious materials would decrease the alkalinity of pore solution, which is not beneficial for the stabilization of passive film on the surface of steel bars in the marine environment. By leverage of the alkali-rich and pozzolanic characteristics, this study first valorized waste glass powder (GP) to enhance the corrosion resistance of recycled aggregate concrete (RAC). The results showed that partially replacing cement with GP could improve the steel corrosion resistance of RAC to a level better than conventional concrete prepared with natural aggregates. Moreover, the presence of 20% GP significantly enhanced the chloride penetration resistance at a later age without sacrificing the compressive strength. The enhanced corrosion resistance of GP-RAC was attributed to two main reasons: i) the refined pore structure by the formation of secondary C-(N)-S-H gels with a lower Ca/Si ratio; ii) the increased alkalinity of pore solution due to the depolymerization of GP and the released Na+ from GP. As a result, the use of GP in low-carbon cement was able to compensate for the inferior durability of RAC. The proposed corrosion-resistant concrete with low ecological and economic costs has a promising application potential to promote the wider use of GP and RAC in the construction sector. [ABSTRACT FROM AUTHOR]

Seawater and sea-sand based ultra-high performance concrete (UHPC) is currently regarded as one of the most innovative and promising construction materials. To provide design guidance for the usage of silica fume (SF) during the production of seawater and sea-sand based UHPC, this work investigated the effect of SF on the later-age macro-performances and micro/nanostructure of seawater ordinary Portland cement (OPC) pastes prepared with a low water-to-binder (W/B) ratio. The results showed that the optimal SF dosage in seawater OPC-SF system was 10 wt% while that for the deionized (DI) water OPC-SF system was 20 wt%. This showed that the usage of SF could be reduced for the production of the seawater based UHPC system. It was found that the later-age compressive strength of seawater OPC-SF system was increasingly degraded with increasing SF dosage when compared with the corresponding DI water OPC-SF system. The difference in the distribution of unhydrated clinkers in the matrix as SF addition increased was the main cause. The seawater OPC-10%SF system exhibited the highest compressive strength and the lowest pore content among all the seawater-OPC systems. Additionally, with the usage of this ultra-low W/B ratio, Friedel's salt was only present in the seawater OPC pastes prepared without SF, but absent in the seawater OPC-SF pastes. [ABSTRACT FROM AUTHOR]

• The carbonation reactivity of CS was enhanced by the rapid cooling process. • The compressive strength of highly reactive CS after carbonation reached a maximum of 92.3 MPa. • Highly reactive CS can be adopted as ultra-low calcium and CO 2 sequestration binder. To enhance the carbonation reactivity of wollastonite (CS), three different calcination processes were adopted to synthesis CS with different crystal structures. The effects of the cooling process on the polymorphs, carbonation reactivity, and CO 2 sequestration capacity of CS were measured and evaluated, and its carbonation hardening properties and microstructure evolution were also explored. The results indicated that the carbonation reactivity of CS was enhanced by the rapid cooling process due to the inhibition of the polymorphic transition of α-CS to β-CS. The enhanced carbonation reactivity was attributed to the distorted CaO x polyhedra and extended Ca-O distance in α-CS, which made the dissolution of α-CS to leach Ca2+ easier. The CO 2 sequestration capacity and compressive strength of highly reactive CS reached 27.2% and 92.3 MPa, respectively, after carbonation for 24 h. Therefore, it was demonstrated that highly reactive CS can be adopted as ultra-low calcium and CO 2 sequestration binder. [ABSTRACT FROM AUTHOR]