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Natural bentonite as an internal curing agent in the production of eco-friendly ultra-high performance concrete with low autogenous shrinkage
... Utilizing BNC in its split form, along with its sulfate-reactive properties, can effectively fill larger capillary voids in concrete. This results in improved moisture resistance, enhanced compressive strength, and reduced weight loss during sulfate and chloride permeability testing [23][24][25]. The combined physical and chemical characteristics of BNC render it appropriate for partly replacing cement in the production of concrete [26][27][28][29]. ...
... However, the mass loss in Stage II (400 • C-500 • C, associated with the decomposition of Ca(OH)₂) followed a slightly opposite trend. This is likely because the internal curing effect of biochar promotes cement hydration, increasing the overall hydration degree while simultaneously reducing the residual Ca(OH)₂ content-similar to the effect observed with bentonite aggregate [55]. ...
Using biochar as a replacement for sand in concrete provides a sustainable solution to the
ecological degradation and environmental pollution. This study investigates the use of biochar for
internal curing when substituting sand in concrete. Various properties, including slump, me�chanical strength, and hydration products, were assessed. The results show that prewetted bio�char effectively preserves the slump of fresh concrete. The prewetted biochar provides internal
curing by extending the hydration process, resulting in a 56 % increase in the duration between
the first and third hydration peaks at the 100 % replacement level. This extended hydration
process reduced the amount of unhydrated cement clinkers and enhanced the development of
Calcium Silicate Hydrate (C-S-H) and CaCO₃, indicating a more complete degree of hydration of
cement. Furthermore, the addition of biochar effectively reduces the total shrinkage of concrete
by 65 % at a 100 vol% replacement level at 28 days and guarantees later strength development of
concrete
... It can be said that the impact of selfdesiccation to generate denser microstructure through secondary hydration is more reflexive at the w/b ratio of 0.15 compared to when it increases to 0.175 and 0.2 (Fig. 7). Previous studies also confirmed the dense microstructure of the UHPFRC mixtures through microstructural examinations, including SEM and EDS analyses (Chen et al., 2019;Dong et al., 2022;Zhao et al., 2023). The formation of pores is expected to be higher in the concrete with a higher w/b ratio, thereby lowering and increasing its resistivity and conductivity, respectively. ...
The presence of low-quality coarse aggregates and exposure to aggressive conditions are the two major problems with the durability of concrete. Therefore, an alternative concrete with enhanced properties to prevent fluid and ionic mobility compared to conventional concrete is needed. This study investigated the effects of main mix parameters on the transport characteristics and corrosion behavior of ultra-high performance fiber-reinforced concrete (UHPFRC). A set of 27 UHPFRC mixtures with different combinations of w/b ratio, cement, and silica fume contents, based on a 3 ³ -factorial experiment design, were prepared and tested for water permeability, chloride penetrability, electrical resistivity, chloride profile, and corrosion current density. The results showed that UHPFRC mixtures exhibited excellent durability properties characterized by negligible water penetration (< 15 mm), negligible and very low chloride permeability when the w/b ratio was 0.15 (< 100 Coulombs) and up to 0.2 (< 300 Coulombs), respectively, and very low chloride concentrations at the rebar level (0.03–0.18 wt.%). All resistivity values were within the range of 26.7–78.8 kΩ cm (> 20 kΩ cm) and pH values were 12.41–13.01, indicating the implausible likelihood of corrosion in the UHPFRC mixtures. This was confirmed through the corrosion current density measurements of reinforced UHPFRC specimens after 450 days of chloride exposure, which were below the critical limit for the corrosion initiation of reinforcing steel. Finally, the experimental data were statistically analyzed and fitted for all the listed tests, and models were developed for them using the regression analysis such that regression coefficients were within 0.90–0.99.
... Various approaches have been employed to mitigate the shrinkage of cement pastes with SCMs based on restraining mechanisms. These methods include incorporating expansive agents to generate expansive products (Shen et al., 2020), introducing shrinkage-reducing admixtures (SRAs) to reduce the surface tension of pore solution (Soliman and Nehdi, 2014;Yoo et al., 2015), and utilizing the porous aggregates or superabsorbent polymers (SAPs) to replenish water through internal curing (IC) (Bian et al., 2023;Liu et al., 2019;Meng and Khayat, 2017;Zhao et al., 2023). In this context, SRAs are an efficient and uncomplicated method to control shrinkage cracking. ...
The demand for massive cement production requires vast raw materials, consumes high energy, incurs high costs, and contributes to generating 8-10% of carbon dioxide (〖CO〗_2) emissions, which causes enormous ecological and health threats. These issues can be mitigated by adopting sustainable practices, such as using eco-friendly materials in cement production, which is more important than ever. Bentonite (BT), an environmentally friendly alternative, has been widely used as a substitute for cement due to its advantageous properties, including cost reduction, lessened emissions, decreased permeability, and increased chemical resistance. Many researchers have explored the utilization of BT as a substitute for cement in cement-based materials (CBMs) production, however, a comprehensive review substantiating its efficacy in this regard remains to be established. This paper presents a robust literature review and bibliometric analysis conducted on BT, utilizing the VOSviewer tool to scrutinize academic publications. Furthermore, physical and chemical properties, morphological analysis, and the effect of incorporating BT on the fresh, mechanical, and durability properties of CBMs were reviewed. On top of that, BT’s life-cycle assessment (LCA) regarding 〖CO〗_2 emissions and overall costs, challenges, opportunities, and future directions were explored, providing valuable insights for researchers and practitioners in the construction field. The literature review of previous studies concluded that incorporating finely ground BT in optimal amounts (10 to 15%) could effectively replace cement in concrete production without compromising strength, performance, or durability. Utilizing BT aligns with sustainability goals, offering a promising solution to global ecological challenges and paving the way for a more sustainable future.
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Utilizing delayed expansion cemented paste backfill materials (DECPB) in mine backfill operations offers an effective solution to the challenge of achieving proper contact between the in-situ cemented backfill mass and the roof in mined-out areas (backfill-roof contact). This article presents recent progress in both theoretical and experimental research concerning DECPB. The types and mechanisms of expansive additives, including expansive agents, foaming agents, and expansive mineral materials relevant to DECPB, are explained, along with their impact on the expansive properties and mechanical characteristics of DECPB. The composition, material characteristics, preparation methods, and controlled release mechanisms of microcapsules are analyzed. Two methods for measuring the volume change rate of paste backfill slurries are discussed. Research indicates that an appropriate content of expansive additives not only enhances the expansive properties of cemented paste backfill materials but also contributes to improving their mechanical properties. Chemical foaming agents and bentonite are found to be more suitable for preparing DECPB compared to expansive agents. The delayed expansion of cemented paste backfill material is influenced by microcapsules, with their wall materials, preparation parameters, and the alkaline slurry environment affecting their controlled release mechanism. Developing and optimizing precise measurement devices and methods forfor the volume change rate of cemented paste backfill materials is essential requirement for studying the performance of DECPB. Combining delayed expansion technology with retarding techniques in cemented paste backfill materials can provide a reliable solution for achieving proper backfill-roof contact in the future.
Ultra-high-performance concrete (UHPC) as a kind of progressive cement-based material has attracted wide attention in the field of concrete engineering due to its superior performance. Nonetheless, the high early-age cracking potential of UHPC under restrained conditions due to the considerable autogenous shrinkage (AS) becomes one major impediment. Polypropylene fibers have been added to UHPC for increasing the tensile strength. However, the influence of polypropylene fibers on the cracking potential of UHPC under restrained conditions is not well studied. The restrained cracking behavior of UHPC incorporating polypropylene fibers was examined by the temperature stress test machine tests in the present study. To study the influencing mechanism of polypropylene fibers on the cracking potential of UHPC, workability, mechanical properties, shrinkage-induced stress, AS, and tensile creep were investigated. Results indicated that the incorporation of polypropylene fibers improved the tensile strength and flexural toughness, decreased the elastic modulus and AS, and increased the tensile creep of UHPC. The shrinkage-induced tensile stress obtained from test was reduced by 64.9%, 65.9%, 67.8%, and 67.5% attributed to creep when compared to the calculated theoretical stress for UHPC incorporating 0%, 1%, 2%, and 3% volume proportion of polypropylene fibers, respectively. As a result, the cracking potential of UHPC under uniaxial restrained condition estimated by the integrated criterion decreased by 21.7%, 60.7%, and 44.1% with the increasing volume proportion of polypropylene fibers from 0% to 1%, 2%, and 3%, respectively.
High-performance fiber-reinforced cementitious composites (HPFRCC) have shown benefits in improving infrastructure resilience but often compromises sustainability due to the higher upfront cost and carbon footprint compared with conventional concrete. This paper presents a framework to optimize HPFRCC for improving bridge resilience and sustainability. This research considers ultra-high-performance concrete and strain-hardening cementitious composite featuring high mechanical properties, ductility, and damage tolerance. This paper establishes links between resilience, sustainability, mechanical properties of HPFRCC, and HPFRCC mixtures. The investigated mechanical properties include the first crack stress, ultimate tensile strength, and ultimate tensile strain. With the established links, sustainability is maximized while resilience is retained by optimizing HPFRCC mixtures. The framework is implemented into a case study of a bridge that collapsed during construction. Results show that use of HPFRCC enhances resilience, and HPFRCC mixtures can be engineered to minimize the material cost and carbon footprint while retaining high resilience.
Ultra-high performance concrete (UHPC) combines advanced fibrous and cementitious material technologies to achieve high strength and exceptional durability. The material tends to have microscopic pores that prevent harmful substances such as water, gas, and chlorides from entering. UHPC can also achieve compressive strengths above 200 MPa and tensile strengths above 20 MPa. It also shows significant tensile strain hardening and softening behavior. Owing to all these characteristics, UHPC has excellent performance, making it a potentially attractive solution for improving the sustainability of construction components. Despite UHPC’s outstanding mechanical properties, superior toughness and ductility, and extraordinary durability, various challenges prevent its widespread use. At the same time, several challenges are currently being faced in the applications of UHPC, which include i) design aspects such as material properties; ii) production technology for large-volume and/or long-span elements with low workability, high spalling, and high shrinkage strains; and iii) unknown durability characteristics after the appearance of long-term concrete cracking. With a lack of industry experience, UHPC specialists face additional challenges in spreading hands-on practice to concrete industry professionals so that the latter can be well versed in applying this sophisticated concrete technology. For these reasons, a comprehensive literature study on recent development trends of UHPC should be conducted to determine its current status and prospects. This article scientifically reviews the current status, carbon capturing capabilities, sustainability aspects, challenges and limitations, and potential applications of UHPC. This state-of-the-art review is aimed at helping scientific researchers, designers, and practitioners widen the use of UHPCs in advanced infrastructure applications. This review will help specialists to develop the design guidelines to enable the widespread application of sustainable UHPC. In doing so, design engineers will be provided with an assurance to fully exploit the high strength and other special properties of UHPC and develop models that can efficaciously estimate the ultimate bearing capacity of UHPC sections under various loading conditions.
An experimental study was carried out to determine the properties of rice husk ash (RHA) and its effect on high-performance concrete's (HPC) mechanical and microstructural properties. RHA content was placed at 0–30% at 5% step intervals and a constant water-binder ratio (W/B) of 0.3. A slump flow test was carried out to measure the workability property of the fresh HPC. In contrast, the influence of RHA contents on compressive, splitting tensile, flexural strengths and microstructural properties were examined for the hardened HPC specimens. The X-ray fluorescence (XRF), scanning electron microscopy-energy dispersive x-ray (SEM-EDX), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy-Attenuate total reflectance (FTIR-ATR), Thermogravimetry analysis (TGA), Brunauer, Emmett and Teller (BET) specific surface area and laser diffraction particle size distribution (PSD) were used to access the feasibility of RHA in HPC. XRD and SEM/EDX techniques were conducted to investigate the hydration products and microstructure in hardened HPCs. The post-test examination showed increased compressive, splitting tensile and flexural strengths of HPC samples for a 10% RHA content mix, recording the highest compressive strength in all curing ages. As the curing ages increase, the microstructure of the samples with RHA becomes denser than the control due to the refinement of the microstructure by the RHA incorporated. The XRD and SEM/EDX confirmed the lower calcium hydroxides from pozzolanic reactivity and later formation of C–S–H. The results suggest that RHA can be used as a cement replacement for up to 10% in HPC to produce sustainable concrete.
Ultra-high-performance concrete (UHPC) is a type of cement-based composite for new construction and/or restoration of existing structures to extend service life. UHPC features superior workability, mechanical properties, and durability compared with conventional concrete. However, some challenges limit the wider application of UHPC, such as low workability for large-volume production, high autogenous shrinkage, insufficient flexural/tensile properties, and unpredictable durability after concrete cracking. Therefore, this paper reviews the state-of-the-art technologies for developing UHPC mixtures with improved properties. This review covers the following aspects: (1) the existing design methodologies; (2) the typical ingredients (e.g., binders, aggregates, chemical admixtures, and fibers) for preparation of UHPC and the underlying working principals; (3) the technologies for improving and controlling key properties (e.g., workability, autogenous shrinkage, compressive performance, tensile/flexural properties, and durability); and (4) the representative successful applications. This review is expected to advance the fundamental knowledge of UHPC and promote further research and applications of UHPC.
Knowledge of the behavior of highly compacted expansive clays, as an engineered barrier, in disposal of high-level nuclear waste (HLW) systems to prevent the pollution due to migration of radionuclide is extremely essential. The prominent properties of globally and widely used bentonites have been extensively studied during past two decades. In China, GaoMiaoZi (GMZ) bentonite is the first choice as a buffer or backfill material for deep geological repositories. This review article presents the recent progresses of knowledge on water retention properties, hydromechanical behavior, and fractal characteristics of GMZ bentonite-based materials, by reviewing 217 internationally published research articles. Firstly, the current literature regarding hydrogeochemical and mechanical characteristics of GMZ bentonite influenced by various saline solutions are critically summarized and reviewed. Then, the role of osmotic suction π alongside the application of surface fractal dimension Ds is presented from the standpoint of fractal theory. Finally, the strength characteristics of GMZ bentonites using fractal approach have been discussed. Furthermore, this study sheds light on gaps, opportunities, and further research for understanding and analyzing the long-term hydromechanical characteristics of the designed backfill material, from the standpoint of surface fractality of bentonites, and implications of sustainable buffer materials in the field of geoenvironmental engineering.
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.
Rheological properties of ultra-high-performance concrete (UHPC) pastes significantly impact the flowability and segregation resistance of UHPC, thus are of great importance to investigate. This study investigated the coupled effect of the water-based nanoclay suspension and ambient temperatures on rheological properties of UHPC pastes. Firstly, the water-based nanoclay was proved to show good compatibility with used PCE-based HRWR. Besides, the nanoclay content varied from 0 to 0.20% and the evaluated ambient temperatures were 10 °C, 25 °C, and 35 °C. Results showed that the addition of nanoclay and elevation of ambient temperatures significantly increased plastic viscosity and dynamic yield stress of UHPC pastes. The thixotropy of UHPC pastes was also enhanced, which was quantified by a bi-linear model including initial fluctuations of static yield stress (τfloc) and thixotropy index (IA). The enhancement of rheological properties are closely related to the evolution of internal microstructure, including physical and chemical effects. The physical effects refer to the flocculation between particles: (1) the addition of nanoclay accelerated the flocculation by the electrostatic attraction forces; (2) the elevation of ambient temperatures accelerated the flocculation by promoting Brownian motions of particles. The chemical effects refer to the increased hydration reaction: (1) the nanoclay addition increased aluminates contents, thus promoting the hydration of C3A to generate ettringite; (2) the elevation of ambient temperatures increased the dissolution of C3A then promoted the hydration. More importantly, ettringite is the main hydration product to affect the rheological properties of UHPC pastes. This study revealed the coupled effect of water-based nanoclay and ambient temperature on rheology of UHPC pastes and will advance the large-scale application of the water-based nanoclay suspension.
Particle sizes and proportions of powders (e.g., cement and mineral admixtures) may play an important role in determining the early-age performance of ultrahigh-performance concrete (UHPC). Therefore, in this study, the effects of powder gradation on the fresh state, mechanical properties, and early-age shrinkage of UHPC were investigated. The results show the effects of dense powder gradation on fluidity and early-age strength, which could be related to the variation of hydration processes. Moreover, considering the effect of the dry environment on the autogenous shrinkage, accurate assessments of the shrinkage components were available, which further revealed that drying shrinkage may dominate the total shrinkage of UHPC at an early age. Besides, as the evaporation was combined with the pore structure, the critical pore diameters were calculated and employed to theoretically clarify the development of drying shrinkage.
Ultra-high-performance concrete (UHPC) is an emerging overlay material for the bridge deck rehabilitation. As a durable material, UHPC overlay can effectively extend the service life of the bridge. In this study, the highly thixotropic UHPC overlay was designed and developed to facilitate sloped overlay construction with minimum formworks. A new type of water-based and well-dispersed nanoclay suspension was utilized to enhance the thixotropy. Two indices, including initial fluctuation of yield stress (τ_floc) and thixotropy index (I_A), were proposed to evaluate the thixotropy of fresh UHPC. It was found that, as nanoclay contents increased from 0 to 0.20% (by mass of binders), the initial fluctuation of yield stress (τ_floc) increased from 392.7 Pa to 3290.1 Pa and the thixotropy index (I_A) increased from 9.7 Pa/min to 16.4 Pa/min, representing the significant enhancement on thixotropy. The highly thixotropic UHPC overlay showed good shape stability after the placement, but showing plenty of defects on the interface. It was shown that the bond strengths between thixotropic UHPC overlay and substrate are reduced by over 30%. To address this issue, the application of proper vibration index on highly thixotropic UHPC overlay was investigated. Results showed that the bond strength between thixotropic UHPC overlay and substrate was increased by 45% after the optimal vibration index. The SEM observation of the interface confirmed the increased air voids for highly thixotropic UHPC overlay and the reduced air voids after optimal vibration index. This study will promote the acceptance of highly thixotropic UHPC as an overlay.
Desertification has become a global environmental problem that poses great challenges to sustainable development in desert areas. Bentonite is a natural mineral with many environmentally-friendly traits, such as hygroscopicity, adsorbability, distensibility, cohesiveness, and pollutant-free. In this study, different amounts of bentonite were tested as windbreak and sand fixing materials in indoor and outdoor environments. When added to sandy soil, bentonite filled the interspaces between sand particles, bonded sand particles and formed sand grains aggregate. And it has significant effects on sand-soil physical properties and compression strength. Soil-water retention and soil physical properties were significantly improved at 1% bentonite (p<0.05). Additionally, the erosion rate was reduced by more than 99% when the proportion of bentonite was 0.2% bentonite. Whereas, at 2% bentonite, the shear strength was 0.26 kg⋅cm⁻² reached a significantly higher level than the biological crust (0.15 kg⋅cm⁻²) (p<0.05), and at 5% bentonite, the compression strength was 2.3 kg⋅cm⁻². Most importantly, the cost of raw materials ranged from about $1,000-5,000 per hectare since it is naturally occurring and can be used in low content ratios in the sand—a much cheaper alternative than other sand prevention measures.
The water-absorbent polymer will bring large holes and the internal-curing region for cement-based materials. These two effects will bring negative and positive effects on the compressive strength of the cement-based material, respectively. In this paper, a calculation model of the compressive strength of cement-based materials with water-absorbent polymers was proposed considering the influence of the large holes and internal-curing on the compressive strength. A method for determining the internal-curing width of water-absorbent polymers based on backscattering image analysis was established. Based on the internal-curing width, the compressive strength was predicted and compared with the results of experiments. In addition, the influences of the internal-curing width, size, dosage, mass swelling rate, and volume swelling rate of water-absorbent polymers on the compressive strength were analyzed. The results show that the internal-curing width of the water-absorbent polymers with a mass swelling ratio of 4.22 g/g is about 13 µm. Based on the internal-curing width obtained by characterization, the compressive strength obtained by model is 7 % greater than that obtained by experiments. Besides, the influence range of the internal-curing width, size, and mass swelling ratio of the water-absorbent polymers on the compressive strength is between 80 Mpa and 90 Mpa. The influence range of the dosage and the volume swelling rate of the water-absorbent polymers on the compressive strength is between 20 Mpa and 90 Mpa. The compressive strength is mainly related to the dosage and the volume swelling rate of water-absorbent polymers. The larger the dosage and volume swelling rate are, the smaller the compressive strength is.
Ultra-high performance concrete (UHPC) can be a good candidate for bridge deck overlay or repair materials of structural members due to its superior mechanical properties or durability. However, large autogenous shrinkage characterized by UHPC may be an obstacle to extending its applications. A novel internal curing (IC) agent, calcined bauxite (CB) aggregate, has been developed to effectively reduce the autogenous shrinkage of UHPC and meantime enhance its mechanical properties. This study further examines its efficiencies in solving shrinkage issues by restrained ring test and large-scale UHPC-concrete composite slabs. In restrained ring tests, when both no fibers, the IC UHPC matrix with CB aggregate shows smaller crack width (0.25 versus 1.0 mm at 9 d) and delayed (5.2 d versus 3.6 d) onset of cracking than the normal UHPC matrix; when having fibers, the normal UHPC shows micro-cracking while no cracking was detected in IC UHPC even observed at one year. Normal UHPC overlay shows more serious delamination and curling than the IC UHPC overlay, and it also shows a hairline crack on the side surface at midspan. Early-age exposure to drying at 1 d has a detrimental effect by exhibiting the most severe delamination and curling of the UHPC overlay, and reinforcement in UHPC layer is effective in reducing its shrinkage strain.
In this study, a comparative life cycle assessment (LCA) approach was adopted to scrutinize the environmental impacts of infilled cementitious composites (ICC) and ultra-high performance concrete (UHPC). UHPC, being prevalent for decades, encounters obstacles in application due to its high binder content, high heat generation, high material cost, and absence of coarse aggregate and macro steel fibers. To break these bottlenecks, ICC is developed based on fiber-particle packing theory, paste volume control and UHPC technology. To facilitate wider applications of ICC, LCA is performed on five categories, i.e., global warming (GWP), ozone depletion (OD), acidification (AP), eutrophication (EP), and photochemical ozone creation (POCP), according to International Organization for Standardization (ISO) 14040 and the Ecoinvent database. The ecological impacts of ICC can be further mitigated using supplementary cementitious materials and filler (e.g., 10% increase in the silica fume to cement ratio by mass can result in 5.8% drop in GWP). A heuristic design of ICC is proposed towards less environmental impacts (e.g., 32.4% drop in GWP).
Great autogenous shrinkage has become a common problem in cement-based materials with a low water-to-binder ratio (w/b). To limit the autogenous shrinkage in ultra-high performance concrete (UHPC) during the process of hydration and curing, sustainable use of recycled micropowder (RMP) made from autoclaved aerated concrete waste (AACW) to cure UHPC internally is proposed in this paper. The influence of RMP (at the levels 5%, 10%, and 15% by mass) with three particle size classes ranging from 0–600 μm on the autogenous shrinkage, hydration reaction, internal relative humidity, and compressive strength of UHPC were investigated. The results show that mixtures with different dosages and particle sizes improve the fluidity of UHPC, and its internal curing effect can effectively inhibit the early-age autogenous shrinkage of UHPC and maintain a higher internal humidity compared with the control group. The incorporation of RMP can advance the appearance of a hydration peak and increase the cumulative heat of hydration. Microscale analyses show that the curing water released from RMP promotes the hydration of the surrounding unhydrated cement. The generated hydration product filled the internal pores of RMP and weakened the negative effect of the pores introduced by RMP on the compressive strength of UHPC. Thus, RMP can be utilized as internal curing materials for UHPC to reduce the autogenous shrinkage.
Cement demand has steadily increased around the world, negatively impacting the environment by consuming a substantial amount of energy, depleting raw materials, and contributing to climate change. Supplementary cementitious materials (SCMs) can be a potential way to deal with environmental issues by providing an alternative to the cement industry, resulting in more efficient, sustainable, and long-lasting concrete while keeping the environment safe for subsequent generations. The effect of individual and combined integration of bentonite and silica fume as a partial replacement for cement in concrete mixes with polypropylene fibers (PPF) was investigated by determining the mechanical and durability properties of concrete mixes. Binary mixes, containing either bentonite or silica fume as a partial replacement of cement, were prepared along with ternary mix incorporating both the SCMs. Additionally, PPFs were added to the ternary blends in increments of 0.25%, resulting in nine distinct concrete mixtures. Compressive, splitting tensile and flexural strength tests were carried out to study the hardened concrete properties. UPV, sorptivity, porosity, water permeability, abrasion resistance, acid attack resistance, and chloride penetration tests were performed to examine the durability properties. The experimental results exhibited that the addition of silica fume and bentonite resulted in enhanced compressive, splitting tensile and flexural strengths. The durability properties of SCM concrete mixtures also improved compared to the control mix. The microstructure investigation depicted a denser concrete in the case of SCM incorporated mixes. PPF mixes showed satisfactory performance up to a certain percentage of PPF (i.e., 0.5%).
This study aims to optimize the rheological properties of Ultra-high performance concrete (UHPC) based on the rheology pre-control strategy. To be specific, bentonite is employed as a rheology controller, and the UHPC skeleton is developed by the Modified Andreasen and Andersen (MAA) model. Then, the rheological properties, mechanical properties, and microstructure of the developed UHPC are evaluated carefully. The obtained results reveal that the addition of bentonite can effectively regulate the rheological properties of UHPC. More exactly, When the bentonite content rises from 0 wt% to 15.0 wt%, the flowability of the UHPC slurry decreases by 55.71%, while the static yield stress, dynamic yield stress, plastic viscosity and thixotropy increase by about 17.05 times more, about 5.57 times more, about 1.16 times more and about 0.04 times more, respectively. Due to the improvement of the rheological properties of the developed UHPC matrix, the orientation and distribution of the utilized steel fibres can be significantly improved, which could bring a maximum increase of about 60% for the UHPC flexural strength. Additionally, based on the microstructure analysis results, there is optimal bentonite content, which is beneficial for obtaining a much denser structure of UHPC composite.
Developing efficient and low-cost adsorbent for removing heavy metal ions from aqueous solution is of great significance for environmental protection. Herein, low quality natural bentonite was purified and sodium-modified to adsorb Pb²⁺ from the aqueous phase. The effects of initial pH value of the solution, type and amount of the adsorbent, contact time and initial Pb²⁺ concentration on the adsorption performance of sodium bentonite were systematically studied. The adsorption efficiency of sodium bentonite was significantly better than that of purified bentonite and natural bentonite toward Pb²⁺. Under the optimum adsorption conditions of pH=5, adsorbent dosage of 0.2 g, initial Pb²⁺ concentration of 400 mg/L, and adsorption time of 120 min, sodium bentonite can remove more than 99.94% of Pb²⁺. Adsorption behavior fitted well with Freundlich isotherm and pseudo-second-order kinetics adsorption equations. Adsorption process was endothermic and feasible. The adsorption pathways of Pb²⁺ on sodium bentonite are mainly including ion exchange, surface hydroxyl functional group capture, electrostatic attraction and chemical precipitation by CO3²⁻ in the pore channel to form PbCO3. This research is anticipated to give technical support for prompting the wider application of bentonite as an adsorbent material.
This study aims to clarify the absorption-desorption process of internal curing water by pumice in ultra-high-performance concrete (UHPC) system based on relaxation theory. More exactly, the dynamic migration process of water from pumice in UHPC and its micro/macro properties are analyzed by ¹H nuclear magnetic resonance, backscattering electron, nanoindentation, etc. The results reveal that the prewetted pumice could delay the water release time for 1 h compared with dry pumice (5.5 h), and can continue to absorb about 14% of free water from the end of the mixing process to the beginning of water release. Notably, the release rate of water from prewetted pumice in UHPC is up to 80%, which is beneficial to the alleviation of internal relative humidity decline rate and thus reduces the autogenous shrinkage of UHPC. In addition, the use of prewetted pumice improves the microstructures of UHPC, where compared with river sand, its transport distance of water entraining reaches at 35 μm–55 μm and its interfacial transition zone (ITZ) structure is also denser. Finally, a relaxation theory-based dynamic model is successfully established, which can be used to illustrate the water migration process and predict migration distance in the UHPC system.
Extensive studies have focused on superabsorbent polymer (SAP) as an internal curing admixture to reduce the autogenous shrinkage of ultra-high performance concrete (UHPC). However, studies on the effect of SAP content on the tensile properties of UHPC are still limited. As we all know, tensile performance of UHPC is an aspect that cannot be ignored. Therefore, this paper investigates the effect of SAP content on the shrinkage and tensile properties of UHPC. First, according to mercury intrusion porosimetry (MIP) analyses, the moderate addition of SAP can refine the pore structure of matrix due to its internal curing effect. However, excess SAP brought more space filled with capillary water, thus increasing the porosity of matrix. The results of shrinkage test showed that the autogenous shrinkage of UHPC decreased first and then increased with the increase in SAP content. While the drying shrinkage of UHPC increased when SAP content increased. When 0.15% SAP was added, the 7 days autogenous shrinkage and 28 days total shrinkage of UHPC were the lowest, 51.3% and 30.1% lower respectively than those of UHPC without SAP. In addition, the moderate incorporation of SAP improved the tensile properties. Since the internal curing effect of SAP promoted the hydration of expand agents (EA) and cement, the microstructure of matrix became denser, enhancing the bond between fibers and matrix. Thus, the tensile strength of UHPC was improved. However, excess SAP have negative effect on the tensile strength due to the negative effect on pore structure. Furthermore, the tensile strain capacity of UHPC increased first and then decreased with the increase in SAP content. Finally, the micromechanical principle was proven to be a feasible method to evaluate tensile strain-hardening behaviors of UHPC. The trend of pseudo-strain-hardening (PSH) index and tensile strain capacity of specimen remained consistent. Additionally, when the ratio of maximum fiber bridging stress and matrix tensile cracking strength of UHPC was more than 1.1 rathan than 1.3, which was required in the filed of engineering cementitious composites (ECC), UHPCs exhibited obvious strain-hardening response. This indicated that the adjustment of the strength criterion in the micromechanical principle will be the key to its effective and reasonable evaluation of the tensile strain-hardening response of UHPC.
This paper studied the influence of pre-saturated lightweight sand (LWS) on the mechanical and microstructural properties of UHPC cast with steel fiber alignment. The changes in hydration kinetics, porosity, nano-mechanical, and mechanical properties were studied. The LWS was used at 0–50% replacement volumes of total sand. Predominant fiber alignment was favored through a flow-induced casting method during casting of flexural prisms. Experiment results showed that the 28-d autogenous shrinkage was decreased from 450 to 275 μm/m with the LWS content increasing from 0 to 50%. The addition of 20% LWS led to maximum increases of 15%, 15%, and 20% in compressive strength, flexural strength, and T150, respectively, relative to UHPC made without any LWS. The use of 20% LWS combined with fiber alignment led to a synergistic effect of 45% and 40% on enhancing the flexural strength and T150, respectively, relative to UHPC without LWS and having random fiber orientation. The addition of LWS can enhance the cement hydration given the internal curing effect. Such enhanced cement hydration increased the percentage of high density and ultra-high density C–S–H from 50% to 75% and reduced the 28-d porosity from 12.5% to 9.5% with the use of 20% LWS. On the other hand, such internal curing can be overwhelmed by the introduced pores of LWS when excessive LWS was used, which led to significant increase in porosity of UHPC.
The inferior mechanical properties and durability of conventional lightweight concrete restrict its wider application. This study intended to design a novel lightweight ultra high-performance concrete (L-UHPC) using a mathematical approach, with a view to achieving low density and excellent performance. Two lightweight materials (i.e. micro-sized glass microsphere and expanded shale aggregates) were introduced in the mix design of L-UHPC. The statistical optimization via Central Composite Design was proved to be an effective method to produce the L-UHPC. The designed L-UHPC showed comparable or even superior functional and durability properties than normal-weight UHPC and high-performance cement mortar. The use of specified lightweight materials played prominent roles in ensuring the mechanical properties of L-UHPC. The high microhardness of paste matrix and the internal curing of lightweight aggregates contributed to the great durability of L-UHPC. The development of L-UHPC can provide a new solution for constructing high-performance lightweight structures and composites.
This research aims to clarify the intrinsic effect of microwave pre-curing on the hydration kinetics and microstructure development of Ultra-High Performance Concrete (UHPC). Six hours after casting, the UHPC samples are subjected to microwave pre-curing for 60–240 s. Then, the macro and micro characteristics of the cured UHPC are evaluated. The observed experimental results show that the microwave treatment can create a dense shell around the cured UHPC's periphery, which can resist the lateral deformation of the specimen and guarantee the excellent mechanical properties of the treated sample. Moreover, when steel fibres are included into UHPC, the reflection and shielding effect of the 3D fibres network can significantly block the promotion effect of microwave on the strength development of UHPFRC. Additionally, due to the high energy of microwave, high polymerization hydration products and advanced pore structure (micro and meso scales) can be observed in the pre-cured UHPC.
Water in the external environment of textile-reinforced concrete (TRC) migrates along the direction of internal directional fiber bundles, thereby damaging the TRC and thus increasing the permeability of the internal matrix. This paper studies the effects of concrete water–cement ratios of concrete, grid sizes and fiber bundle Tex content on the permeability of TRC on the macro and micro scales, and the roughness of concrete was evaluated by fractal dimension through a nuclear magnetic resonance test. Results show that the permeability of TRC increases with the decrease in the grid size of the textile, the increase in the Tex content and the increase in the water–cement ratio of concrete. In this paper, a TRC seepage prediction model is established based on the dimensional analysis method and permeability test. The increase of permeability will seriously affect the durability of building materials and will be unsuitable for strengthening and repairing buildings. Therefore, studying the increase of the permeability of TRC under the action of external pressure water is important.
This paper presents a comparative study on the effect of steel and polyoxymethylene fibers on the characteristics of Ultra-High Performance Concrete (UHPC). Firstly, based on the modified Andreasen & Andersen packing model, a UHPC with ultra-low cement content is produced. Then, the steel fibers and polyoxymethylene (POM) fibers are added into this UHPC matrix separately and together. The obtained experimental results show that the addition of POM fiber has limited contribution to the compressive strength of UHPC, while it has a positive influence on its flexural strength and high temperature resistance. The results of nanoindentation indicate that POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Based on the nanoindentation results, it can be found that the interfacial transition zone (ITZ) is about 50 μm in the case of UHPC with only steel fibers, while this value gradually increase to about 60 μm when POM fiber is added individually. This results further demonstrate that the POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Therefore, to efficiently apply the POM fiber in UHPC, it is suggested to combine it with steel fibers and its optimal content should be around 1% (vol.). It can be concluded that it is difficult for POM fiber to fully replace steel fiber in referencing UHPC, while the hybrid fibers should be a good choice to produce excellent UHPC composite.
This paper presents feasibility and benefits of utilizing off-specification fly ash (OSFA), which would have otherwise been landfilled, in preparing ultra-high-performance concrete (UHPC). Effects of mixture design variables, including OSFA content, water-to-binder ratio, and slag content, on compressive and flexural properties of UHPC were tested. Experimental results showed that UHPC with proper combination of OSFA and slag achieved desired compressive and flexural strengths, as well as low autogenous shrinkage and leachability of heavy metals. The underlying mechanisms of property development were investigated through isothermal calorimetry, thermogravimetric analysis, and X-ray diffraction. Results indicated that use of OSFA retarded hydration reactions, but incorporation of slag effectively suppressed adverse effects of OSFA. Economic and environmental analysis showed that use of OSFA greatly reduced the life-cycle cost, carbon footprint, and embodied energy consumption of UHPC. This study develops a new avenue for valorization of OSFA and development of cost-effective and eco-friendly UHPC.
The use of supplementary cementitious materials (SCMs) can improve the properties of concrete, reduce pressure on natural resources and CO2 emissions. However, certain SCMs are unable to meet the growing demand of construction sector. The presented study investigates the role of bentonite clay (BC) and its synergistic effect with silica fume (SF) as partial replacement of cement on strength, durability, and microstructure of concrete. Five different mixtures were prepared containing 0%, 7.5%, 15% and 22.5% (by weight) of BC as a replacement of cement whereas SF was kept constant at 10% replacement level. The experimental results showed that the addition of SF had a positive impact on compressive strength with increase in curing time i.e., a maximum value of 43.09 MPa was achieved. The bentonite based concrete requires excess curing to achieve high strength. The ultrasonic pulse velocity tests for all the mixes resulted in velocity value > 3.5 km/s which is deemed as “good” quality concrete. Durability testing depicted that the BC and SF significantly improved the pore structures and resistance against sulfate attack and chloride ingress. The proposed empirical formulations (using machine learning technique i.e., gene expression programming) were found accurate (correlation>0.9) and can be utilized for prediction of properties of concrete. The environmental impact analysis revealed that utilization of BC and SF can reduce the carbon emission by approximately 23% compared to control mix. It is recommended to conduct leachate analysis for each mixture prior to use. The proposed concrete mixtures were deemed technically, environmentally, and economically viable for application in construction sector.
This study aims to explore a new method to reduce autogenous shrinkage of ultra-high performance concrete (UHPC) by incorporating natural sisal fibers. The water absorption and desorption behavior of sisal fibers were firstly determined. Then, flowability, setting time, hydration heat, autogenous shrinkage, internal relative humidity, mechanical properties, fiber distribution, hydration performance, and microstructure of UHPC mixtures incorporating various volume content of sisal fibers were evaluated. The results show that sisal fibers can release the absorbed water with decreasing relative humidity in UHPC specimens. The addition of sisal fibers can restraint the cement hydration process and delay the setting times. The 7 days autogenous shrinkage of UHPC was reduced by 71.4% by adding 1.5 vol% sisal fiber due to the internal curing and reinforcing effect induced by sisal fibers. Incorporating sisal fibers can promote late hydration of UHPC, so the 28 days compressive strength was reduced by only 7.7%∼8.2%. Moreover, the obvious gap between sisal fiber and concrete can be observed from the backscattered electron images, which is caused by the contraction of sisal fiber after releasing water. Finally, the environmental and cost evaluation shows that the use of renewable sisal fiber as a shrinkage-reducing material can reduce the production cost and carbon footprint of UHPC, so it is of great significance to the sustainable production of UHPC.
Sugar cane bagasse ash (SCBA) is the final waste material in the sugar production chain. Due to the pozzolanic characteristic and the abundance, SCBA has been tried to utilized in construction materials. In this study, ecofriendly ultra-high performance concrete (UHPC) was developed with SCBA as a cement replacement. The effects of SCBA on the fluidity, setting time, compressive strength, flexural strength and autogenous shrinkage of UHPC were analyzed. Hydration heat evaluation, X-ray diffraction (XRD), thermogravimetric (TG) analysis, and mercury injection (MIP) were applied to reveal the effects of SCBA on cement hydration, hydration products and pore structure. The results indicate that the application of SCBA in UHPC as a cement replacement not only maintains the compressive strength but also improves the workability and decreases the autogenous shrinkage of UHPC paste. Compared to the control group, UHPC prepared at a 40% replacement rate obtained a good comprehensive performance. Autogenous shrinkage decreased by 24.48%, and the compressive strength was almost the same as that of the control group. This study verifies the technical feasibility of SCBA application as a cement replacement in UHPC and may further promote the utilization of agricultural byproducts in cementitious materials.
The fabrication of reactive powder concrete (RPC) has been largely restricted by the high cost of raw materials and the high energy-consuming curing method. In this work, dolomite powder was incorporated to fully replace silica sand as fine aggregates to minimize the material cost of RPC and realize green sustainable development in economic society. Moreover, sustainable reactive powder concrete (SRPC) with dolomite powder fine aggregates was cured in energy-saving mode by using low-energy ohmic heating (LEOH) curing to replace traditional high temperature steam (HTS) curing. The results showed that the curing temperature of LEOH cured sample reached up to 180 °C to reduce the curing duration within 3 h, and the implementation of 3 h LEOH curing could remarkably stimulate the compressive strength of SRPC sample to 105.8 MPa which was comparable to that cured by 3 days HTS curing. Microstructural analyses results indicated the advantages of LEOH curing on improving hydration degree and microstructure of SRPC samples with dolomite powder incorporation. A thorough economic analysis was conducted to further emphasize the quantitative effects of dolomite powder incorporation and LEOH curing on reducing the material and energy costs of SRPC construction. This work broadens the application scenario for RPC structure construction with higher efficiency and lower cost.
A strong and porous aggregate (calcined bauxite, CB) has been applied as novel internal curing (IC) agent in UHPC to counteract its high autogenous shrinkage. However, in view of intelligently UHPC mixture design to acquire satisfactory workability, mechanical properties, and low-shrinkage UHPC, the influences of CB particle size and IC water incorporation methods are not well explored yet, and thereby are investigated in this study. UHPCs with different particle sizes (1 ∼ 3 mm, 3 ∼ 5 mm, and 5 ∼ 8 mm) of CB and methods of prewetting CB or using dry CB with adding IC water during mixing were investigated, together with preparing a reference UHPC (no CB). Multiple microstructural characterization techniques were used to explore the underlying mechanisms. Results show that the UHPCs with any condition of CB aggregate all obtained promoted mechanical properties and significantly reduced autogenous shrinkage compared to the reference UHPC. The large CB led to the highest compressive strength increment while the small and medium CB resulted in similar increments; more shrinkage reduction (higher IC efficiency) was achieved with the medium or small CB (above 70% reduction) than with large CB (58% reduction). The prewetting practice resulted in slightly more strength increment than using dry CB with adding IC water during mixing, and these two practices reached similar IC efficiency. Microstructural analysis with TG test and BSE observation confirmed the promoted hydration in the UHPCs caused by the IC effect of CB aggregate; reduced total porosity was observed in these UHPCs with CB by MIP test.
To reduce the dead load of concrete structures, this study developed a high performance lightweight aggregate concrete (HPLAC) by combining the use of ultra high performance cementitious composite (UHPC) and different types of aluminosilicate lightweight aggregates (LWAs). The physicochemical properties of two types of LWAs (i.e. expanded clay and expanded shale) influencing the HPLAC were elaborated and compared. The composition distribution and micromechanical properties in the interfacial regions of paste and LWAs were revealed by elemental mapping and nanoindentation. The results showed that the incorporation of the clay LWAs or shale LWAs in the HPLAC led to similar density and thermal conductivity values, while the use of the shale LWAs induced a lower water absorption and higher strength of HPLAC as compared to the clay LWAs due to the fine pore structure and higher pozzolanic activity of the former. The internal curing effect provided by the pre-wetted shale LWAs was more efficient in enhancing the hydration of binder, and the Al dissolution from the shale LWAs further densified the interfacial bonding to form a dense rim surrounding LWAs, resulting in improved micromechanical properties at the interface. The X-ray CT results indicated that the adoption of UHPC was beneficial to preventing segregation of the LWAs and steel fibers in the HPLAC. By virtue of the physical-chemical interactions of LWAs, the synergetic use of UHPC and pre-wetted shale LWAs was able to produce an HPLAC with high structural efficiency, good thermal insulation, low autogenous shrinkage and permeability.
This paper presents a feasible way to design a low carbon footprint Ultra-High Performance Concrete (UHPC) with gold tailings based on the idea of full scale recycling. Due to the fact that the particle size distribution of the recycled gold tailings is normally wild, from micrometer to millimeter, it could be utilized to replace powders and aggregates in UHPC simultaneously, which can get rid of sieving process and improve the recycling efficiency. Therefore, in this study, a particle dense packing model is firstly employed to design the UHPC matrix with gold tailings, based on the full scale recycling point of view. Then, the fresh and hardened properties of the newly developed UHPC are investigated, including flowability, compressive strength, hydration kinetics, durability and microstructure. The tested results indicate that the idea of full scale recycling can guarantee a relatively high long-term compressive strength, low DRCM, and low leaching toxicity for UHPC. Additionally, the environmental evaluation further proves that the developed UHPC is a green building material with advanced properties and simple preparation process.
In this study, the hydration product, C-S-H density, and pore structure at different layers from the drying surface were measured. Besides, the zone affected by the released internal curing water from SAP was identified by assessing the nanoindentation modulus. The results found that the SAP affected the moisture distribution and changed the evaporation and self-desiccation rates in the inner and outer layers. Such imposed moisture gradients affected the micromechanical properties of UHPC at different layers due to the change in the hydration degree of the binder, the density of C-S-H, and the pore structure of the matrix. The addition of SAP improved the hydration dynamics and transferred the LD C-S-H into HD or UHD C-S-H. As the distance from the drying surface increased, the affected zone of SAP increased from 60 to 120 μm, showing the more effectiveness of SAP at inner layers.
Bentonite in its compacted form can be utilized as a liner material, provided the hydraulic conductivity and sorption capacity are within the permissible range, else the affirmative utility of the barrier system could be jeopardised. However, the presence of Zinc (Zn2+) in the leachate of the waste material can seriously affect the effectiveness of the bentonite as a liner. Although Zn2+ is present in significant amounts in leachates, no comprehensive study has been performed investigating its influence on the bentonite's sorption and hydraulic characteristics. The present study evaluates the change in surface morphology, hydraulic characteristics, and sorption capacity of two locally available Indian bentonites due to varying Zn2+ concentrations. FESEM, FTIR and EDX studies analysed the changes in surface morphology, adsorption patterns and exchangeable cations, respectively of both bentonites, before and after Zn2+ sorption. Tests were conducted in batch modes to optimise operation parameters like pH, initial concentration, dosage, and time of contact for both bentonites. The test results were fitted with different isotherm and kinetic models. Based on these, it was concluded that the Langmuir isotherm and Pseudo-second order kinetic models were the best fit for both bentonites. Furthermore, the hydraulic conductivity of both bentonites increased with increasing Zn2+ concentrations. It was also observed that bentonites having high cation exchange potential, swelling behaviour, and specific surface area displayed higher percentage removal and adsorption capacity at 2000 mg/L of the initial concentration of Zn2+ solution. The results from the study can aid in designing a liner system and select a suitable bentonite type for arresting pollutants in a waste disposal system.
Fresh concrete needs vibration to compact, fill the mould and reach a dense state. During the compaction process, coarse aggregates (CAs) tend to settle, affecting the homogeneity and eventually the long-term durability of hardened concrete. In this study, a 3-D, multi-phase numerical model for fresh concrete is developed for better understanding the CA settlement under vibration. The settlement rate of the CA in vibrated concrete is considered based on the Stokes law, and the calibrated rheological parameter of mixtures is determined by the segmented sieving method. The model prediction shows that the vibration time has the greatest effect on CA settlement, followed by the particle size of CAs, whereas the density of CAs and the plastic viscosity of mixtures contribute a little compared with the aforementioned factors. Through experimental tests, the validity of prediction results is well verified. The proposed model provides a new method to understand and estimate the settlement behaviour of CAs.
Superabsorbent polymer (SAP) is an effective internal curing agent, and its efficiency is highly dependent on its physical and chemical characteristics. This study investigates the effect of molecular structure, size, content and pre-treatment of SAP on its performance as an internal curing agent in ultra-high performance concrete (UHPC) through the measurement of internal relative humidity, heat of hydration evolution , autogenous shrinkage, and compressive strength. Two types of SAP (ionic and non-ionic) at three contents of 0.2%, 0.4%, and 0.6% were used at the dry state. The ionic SAP was used in two sizes of 471.3 and 95.1 lm to study the effect of SAP size on the efficiency of internal curing. On the other hand, selected ionic SAPs were prewetted by additional water before mixing (0.4% by mass). Results showed that non-ionic small particle size SAP exhibited a higher restraining effect on autogenous shrinkage. The incorporation of SAP can significantly increase the cumulative heat after 15 h, and thus increase the degree of hydration of cementitious materials in UHPC. However, the investigated SAP characteristics showed no significant effect on the cumulative heat of hydration within the tested period of 72 h. The addition of pre-wetted SAP increased the autogenous shrinkage and delayed the hydration, compared to the dry SAP particles with additional water. Finally, the autogenous shrinkage of UHPC was fitted with several published prediction models, and an optimized model was proposed to predict autogenous shrinkage with respect to internal relative humidity and heat of hydration.
Commonly used internal curing (IC) agents (super absorbent polymers or lightweight aggregate) to mitigate the higher levels of autogenous shrinkage in UHPC could possibly lead to a negative effect on the mechanical properties of concrete due to the increased void created by the agents, the not-well estimated added water for IC, or low strength of the materials. In this study, a new type of calcined bauxite (CB) aggregate was found to be stronger than normal aggregate and exhibit a porous structure. Different quantities of this aggregate and its powder product were applied to estimate their effect on the mechanical and shrinkage properties of non-fibrous UHPC matrixes and UHPCs with steel fibers. The maintained higher internal relative humidity and significantly reduced autogenous shrinkage (more than 65%) demonstrate that the use of either dry or prewetted CB aggregate can both provide IC effect in UHPC. It reveals that the dry CB aggregate absorbs water during mixing and releases them after setting. Both the compressive and tensile strength of UHPC mixtures were significantly improved by using CB aggregate as compared to the reference mixture, which is due to the actual lower effective w/b, enhanced properties of the ITZ, and increased hydration by internal curing. However, the powder product of CB did not provide IC effect, but a higher content of CB powder (600 kg/m³) helped enhance the strength of UHPC matrix while the one with 300 kg/m³ of CB powder obtained similar strength both compared to the reference mixture.
The study aims to improve flexural properties of ultra-high-performance concrete (UHPC) prepared with different steel fiber volumes by controlling the rheological properties of the suspending mortar. Welan gum and high-range water reducer were incorporated to control rheology. Optimum rheological properties led to enhanced fiber distribution. Fiber distribution coefficient was proposed and considered to characterize fiber dispersion and orientation. For UHPC with 1%, 2%, and 3% fiber volumes, the highest flexural performance was obtained with mortars having plastic viscosities of 36, 52, and 66 Pa·s, respectively. Beyond these values, further increase in viscosity resulted in greater air entrapment and lower mechanical properties. Prediction model for flexural strength that considers a correlation between fiber dispersion and flexural-to-tensile strength ratio of the UHPC was developed. Viscosity of UHPC mortar was shown to be a reliable indicator to determine flexural-to-tensile strength ratio, and hence predict flexural strength of UHPC.
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.
Fractal geometry has been successfully applied to the study of structures and transport properties of porous media due to its scale invariant property. A new fractal probability density (FPD) model is developed to characterize the pore structures of porous media in this paper. Mechanical press filtration (MPF) is performed on pre-dewatered sludge samples in order to achieve different porous structures. The pore structures and water contents of the dewatered porous sludge cakes are then measured by low-field nuclear magnetic resonance (NMR). The pore fractal dimensions are determined with the proposed FPD model based on NMR results. It has been found that the pore fractal dimension increases with porosity under fixed pore size range. The value of the pore fractal dimension of porous samples is larger than 2.6, which indicates highly heterogeneous pore structures. The present work may provide insight on the multiscale structures of porous media based on NMR technique.
Concrete produced from recycled concrete aggregates (RCA) has been evolved as a good substitute of natural aggregate concrete (NAC) because of associated environmental benefits and increasing attention towards sustainable development in recent decades. Despite recycled aggregate concrete (RAC) presenting numerous benefits, it has inferior properties compared to NAC. Researchers have reported that mechanical and durability performance of RAC can be improved by using supplementary cementitious materials (SCMs) such as fly ash, silica fume, metakaolin, etc. Low calcium bentonite is a naturally found pozzolana and can be used as partial replacement of ordinary Portland cement (OPC) in concrete. In this study, the effect of bentonite on mechanical and durability behaviour of both NAC and RAC was investigated and compared. RAC was prepared by using RCA as coarse aggregate. Bentonite was used as 0, 5, 10, 15, and 20% by mass replacement of OPC in both NAC and RAC. Workability, fresh density, air content, compressive strength, split tensile strength, water absorption, chloride migration coefficient, and acid attack resistance were investigated. Results of testing revealed that with the incorporated levels of bentonite, RAC showed significant improvement in the durability and later strength. Bentonite contributes more to strength and durability development of RAC than NAC.
This study presents an optimized design method in developing ultra-high performance concrete (UHPC) with high wet packing density, in which the multiply effects of solid and liquid phases on UHPC packing mode are considered. Specifically, a model based on D-optimal method is firstly established to assess the influence of solid granular particles and superplasticizer (SP) on the UHPC packing density. Based on the theoretical analysis, the mixture proportions can be optimized, and UHPC with high wet packing density could be produced. Then, to evaluate the reliability of the math-physical method in developing UHPC, its compressive strength and pore structure are tested. The obtained experimental results show that the designed UHPC contributes high packing density, leading to optimized pore structure and extraordinary compressive strength. The experimental verification further highlights that D-optimal method is a promising and effective approach to design UHPC, and eventually for the development of sustainable UHPC.
Highly efficient elimination of UO22+, Cu2+, Zn2+, Cd2+, Ni2+, and Pb2+ from wet-process phosphoric acid (WPPA) was investigated as a batch system using murexide-reinforced activated bentonite (MRAB), which was characterized by X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), and fourier tranform infrared spectroscopy (FT-IR) techniques. The experimental adsorption capacities of UO22+, Cu2+, Zn2+, Cd2+, Ni2+, and Pb2+ were 155.0, 180.0, 190.0, 143.0, 115.0, and 170.0 mg/g at 5 M H3PO4, respectively. The obtained data indicated that the sorption technique used its functionality to remove six metal ions after six reuse/cycles, wherein the MRAB could be regenerated using HCl. The elimination of real impurities from WPPA was assessed through the proposed procedure under optimum conditions using MRAB.
The shrinkage of bentonite-sand mixtures may degrade the performance of a clay barrier constructed in waste disposal facilities. In this study, changes to material properties occurring when bentonite was modified by addition of sand and exposure to different pore water salinities were considered. Paste-like bentonite-sand mixtures were free-desiccated in the laboratory. Sand addition could only reduce the volumetric shrinkage of material when the percentage of sand was larger than 30%. Changes in pore water salinity also may reduce shrinkage of the material because bentonite aggregates were flocculated by synthetic brackish groundwater based on results from particle and pore size analysis. For both effects on the bentonite, the reductions of material shrinkage can be considered theoretically by a packing phenomenon accompanied by the evolution of a mixed grain size distribution.
This research examined the solidification/stabilization process of using a cement binder with actual electroplating sludge. An extensive comparative analysis was done with the cement–sludge mixture before and after the application of cement additives (kaolinite clay, waste latex paint and calcium chloride). The characterization analysis using the Fourier transform infrared spectrophotometer and X-ray diffraction analysis indicated the development of hydration at different cement mixtures. Through a comprehensive assessment of various cement additives, this study showed that the utilization of the admixtures in the cement binder could surpass the required standard unconfined compressive strength (350 kPa). Moreover, calcium chloride as its chemical additive (1704.5 kPa) was able to further aid the strength development against the control concrete (1151.5 kPa) at 28 days. In terms of the leaching tests, the use of cement additives has proved to successfully immobilize the heavy metals (Ag, Cu and Ni) in the cement–sludge matrix based on the toxicity characteristic leaching procedure. The results revealed an effective leaching resistance that leads to satisfying the stringent regulatory leaching requirements of the United States Environmental Protection Agency and the European Union Council (Ag ≤ 5.0 mg/L, Cu ≤ 0.6 mg/L and Ni ≤ 0.12 mg/L).
In this study, optimization of autogenous shrinkage and microstructure for Ultra-High Performance Concrete (UHPC) based on appropriate application of porous pumice is addressed. The pumices with different water absorption rates are utilized to replace river sand by 10%, 20% and 30%, and its effect on the properties of the developed UHPC is investigated. The obtained experimental results show that the inclusion of pre-wetting pumice (particle size distribution is 0.6–1.25 mm) can significantly reduce the autogenous shrinkage of UHPC, without decrease the mechanical properties. Mercury Intrusion Porosimetry and Computed Tomography results indicate that the incorporation of 0.6–1.25 mm hydrous pumice can refine the pore structure and increase the fraction of interconnected pores. Environmental Scanning Electron Microscope, Super-high Magnification Lens Zoom 3D Microscope and Electron BackScattered Diffraction images revealed that the interfacial transition zone (ITZ) skeleton between hydrous pumice and UHPC paste is obviously improved.