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This paper aims to investigate the feasibility of the incorporation of nanosilica (NS) in ultra-lightweight foamed concrete (ULFC), with an oven-dry density of 350 kg/m3, in regard to its fresh and hardened characteristics. The performance of various dosages of NS, up to 10 wt.-%, were examined. In addition, fly ash and silica fume were used as cement replacing materials, to compare their influence on the properties of foamed concrete. Mechanical and physical properties, drying shrinkage and the sorption of concrete were measured. Scanning electron microscopy (SEM) and X-ray microcomputed tomography (µ-CT) and a probabilistic approach were implemented to evaluate the microstructural changes associated with the incorporation of different additives, such as wall thickness and pore anisotropy of produced ULFCs. The experimental results confirmed that the use of NS in optimal dosage is an effective way to improve the stability of foam bubbles in the fresh state. Incorporation of NS decrease the pore anisotropy and allows to produce a foamed concrete with increased wall thickness. As a result more robust and homogenous microstructure is produced which translate to improved mechanical and transport related properties. It was found that replacement of cement with 5 wt.-% and 10 wt.-% NS increase the compressive strength of ULFC by 20% and 25%, respectively, when compared to control concrete. The drying shrinkage of the NS-incorporated mixes was higher than in the control mix at early ages, while decreasing at 28 d. In overall, it was found that NS is more effective than other conventional fine materials in improving the stability of fresh mixture as well as enhancing the strength of foamed concrete and reducing its porosity and sorption. [ABSTRACT FROM AUTHOR]

Highlights • An evaluation of the use of waste magnetite powder (MP) as a cement replacement. • Evaluation of thermal resistance and γ-ray shielding properties of cement pastes. • Replacement of cement with MP improves compressive strength of unheated specimens. • Addition of MP decrease mass loss, strength loss and crack propagation of specimens. • MP addition improves γ-ray shielding properties of specimens at elevated temperature. Abstract In this contribution, the effect of waste magnetite powder (MP), obtained from a rock crushing process, as an additive to cement pastes exposed to elevated temperature and gamma-ray radiation, is evaluated. Cement pastes with different magnetite powder to cement ratios, up to 50% by mass, and exposed to temperatures of 200 °C, 300 °C, 450 °C and 600 °C, were prepared. The thermal conductivity, mass loss, compressive strength and shielding efficiency against gamma-rays (137Cs) of the specimens were determined and the results compared with theoretical values obtained according to the WinXCom program, at different energy levels. The results demonstrate that the addition of MP significantly increases paste density, thermal conductivity, as well as the specific heat of cement stone. Moreover, the inclusion of MP contributes to strength improvements in both heated and unheated states. Likewise, the presence of MP leads to a decrease in mass loss and surface cracking on exposure to high temperature. In conclusion, it is argued that the incorporation of MP results in a significant improvement in gamma-ray shielding properties, as compared to plain cement pastes. [ABSTRACT FROM AUTHOR]

Abstract This study aims to investigate the effects of carbon nanotubes and carbon nanotube-silica core-shell structures, on the behaviour of cement pastes exposed to high temperature (300, 450 and 600 ° C). Pristine multi-walled carbon nanotubes (MWCNTs) were coated with a solid nanosilica (NS) shell, to form a core-shell nanostructure (MWCNT/NS). The cement pastes were incorporated with three different nanomaterial contents equal to 0.125, 0.25 and 0.5 wt.-% of cement. The results demonstrate that incorporation of an optimum amount (0.125 wt.-%) of MWCNT/NSs, is much more beneficial than the incorporation of MWCNTs, for improving the properties of unheated and heated cement pastes. Silica shell improves the binding ability between cement matrices and nanotubes, which is reflected in specimens' compressive strength retention, as well as in decreased micro-cracking. However, exceeding the optimum amount of MWCNTs and MWCNT/NSs can lead to an agglomeration of nanomaterial, thus decreasing the thermal resistance of cement pastes. Graphical abstract Image 1 Highlights • Proposal of a green method of carbon nanotube-nanosilica (MWCNT/NS) core shell structure synthesis. • The use of MWCNT/NSs is more beneficial than the use of pristine MWCNTs, for compressive strength improvement. • MWCNT/NS structures present better mechanical and microstructural performance under elevated temperature. • Incorporation of MWNCT and MWCNT/NS decreases the cracking of specimens under elevated temperature. [ABSTRACT FROM AUTHOR]

This study investigates the effects of Fe 3 O 4 and Fe 3 O 4 /SiO 2 nanoparticle admixtures on the behavior of cement mortars exposed to elevated temperatures. Pristine nanoparticles of magnetite (Fe 3 O 4 ) were coated with a solid silica shell (via the Stöber method), to form a core-shell nanostructure (Fe 3 O 4 /SiO 2 ). The cement mortars were incorporated with an optimized ratio of 3 wt% Fe 3 O 4 and 5 wt% of Fe 3 O 4 /SiO 2 admixture. The specimens were exposed to elevated temperatures of 200, 300, 450, 600 and 800 °C. After cooling, the mass loss and flexural and compressive strengths of the specimens were determined. The results demonstrated that the presence of magnetite-silica (Fe 3 O 4 /SiO 2 ) nanostructures is much more beneficial for improving the thermal resistance of cement mortars than pristine Fe 3 O 4 nanoparticles. The silica (SiO 2 ) shell in the nanoparticles improves residual compressive strength and prevents crack extension in cement mortars exposed to elevated temperatures. The investigations undertaken in this study were supported by optical and scanning electron microscopic studies. [ABSTRACT FROM AUTHOR]

Exposure to elevated temperatures has detrimental effects on the properties of cementitious composites, leading to irreversible changes, up to total failure. Various methods have been used to suppress the deterioration of concrete under elevated temperature conditions. Recently, nanomaterials have been introduced as admixtures, which decrease the thermal degradation of cement-based composites after exposure to high temperatures. This paper presents a comprehensive review of recent developments related to the effects of nanoparticles on the thermal resistance of cementitious composites. The review provides an updated report on the effects of temperature on the properties of cement-based composites, as well as a detailed analysis of the available literature regarding the inclusion of nanomaterials and their effects on the thermal degradation of cementitious composites. The data from the studies reviewed indicate that the inclusion of nanoparticles in composites protects from strength loss, as well as contributing to a decrease in disruptive cracking, after thermal exposure. From all the nanomaterials presented, nanosilica has been studied the most extensively. However, there are other nanomaterials, such as carbon nanotubes, graphene oxide, nanoclays, nanoalumina or nano-iron oxides, that can be used to produce heat-resistant cementitious composites. Based on the data available, it can be concluded that the effects of nanomaterials have not been fully explored and that further investigations are required, so as to successfully utilize them in the production of heat-resistant cementitious composites. [ABSTRACT FROM AUTHOR]

The scope of the study is to test the influence of nanocomposite silica-titania (mSiO 2 /TiO 2 ) core-shell structures on the mechanical and bactericidal properties of cement mortars. Nanocomposites are widely applied in many fields of science however, cement-based composites are mainly modified with single type of nanostructures. The goal of the presented work is to obtain a nanocomposite which exhibits the positive properties of both nanomaterials, while diminishing the negative impacts of each of the nanomaterials applied separately. The study shows that application of silica-titania (mSiO 2 /TiO 2 ) core-shell nanostructures simultaneously demonstrated the properties of nanosilica and titanium dioxide and thus the nanomaterial behaved not only as a filler to improve the microstructure and activator to promote pozzolanic reaction, but also exhibited self-cleaning and bactericidal properties when exposed to UV light. The observed improvement of compressive strength was caused by the presence of nanosilica (acting as a core) while titanium dioxide (shell) exhibits photocatalytic activity. [ABSTRACT FROM AUTHOR]

Pristine nanoparticles of magnetite were coated by solid silica shell forming core/shell structure. 20 nm thick silica coating significantly enhanced the chemical and thermal stability of the iron oxide. Chemical and thermal stability of this structure has been compared to the magnetite coated by mesoporous shell and pristine magnetite nanoparticles. It is assumed that six-membered silica rings in a solid silica shell limit the rate of oxygen diffusion during thermal treatment in air and prevent the access of HCl molecules to the core during chemical etching. Therefore, the core/shell structure with a solid shell requires a longer time to induce the oxidation of iron oxide to a higher oxidation state and, basically, even strong concentrated acid such as HCl is not able to dissolve it totally in one month. This leads to the desired performance of the material in potential applications such as catalysis and environmental protection. [ABSTRACT FROM AUTHOR]

In this contribution the effect of nanosilica (NS) on the behavior of cement mortars containing quartz aggregate and two types of heavyweight aggregates (magnetite and barite) exposed to elevated temperature will be investigated. The cement mortars have been modified with nanosilica in quantities of 1%, 2%, 3%, 4% and 5% (by weight of cement). The samples were exposed to the elevated temperatures of 200 °C, 400 °C, 600 °C and 800 °C, respectively. The mass loss, flexural and compressive strength of the cooled specimens were determined. Additionally, by means of scanning electron microscopy (SEM) and optical microscopy the structure defects have been determined. The results clearly demonstrate that there is an optimal nanosilica content, in the cement mortar containing quartz and magnetite aggregate, improving the thermal resistance and preventing the crack extension (especially in the range of 200–400 °C). However, the study has also shown that cement mortars containing barite aggregate tend to crack and exhibit spalling as a result of low thermal resistance of the aggregate. [ABSTRACT FROM AUTHOR]

The recycling of waste glass is a major problem for municipalities worldwide. The problem concerns especially colored waste glass which, due to its low recycling rate as result of high level of impurity, has mostly been dumped into landfills. In recent years, a new use was found for it: instead of creating waste, it can be recycled as an additive in building materials. The aim of the study was to evaluate the possibility of manufacturing sustainable and self-cleaning cement mortars with use of commercially available nanomaterials and brown soda-lime waste glass. Mechanical and bactericidal properties of cement mortars containing brown soda-lime waste glass and commercially available nanomaterials (amorphous nanosilica and cement containing nanocrystalline titanium dioxide) were analyzed in terms of waste glass content and the effectiveness of nanomaterials. Quartz sand is replaced with brown waste glass at ratios of 25%, 50%, 75% and 100% by weight. Study has shown that waste glass can act as a successful replacement for sand (up to 100%) to produce cement mortars while nanosilica is incorporated. Additionally, a positive effect of waste glass aggregate for bactericidal properties of cement mortars was observed. [ABSTRACT FROM AUTHOR]

The utilisation of biochar, the carbon negative product of pyrolysis, reduces the carbon footprint of the cementitious composites as it possesses the potential to replace the consumption of Portland cement. In a systematic investigation, biochar was used as a partial cement replacement for up to 20 wt% in both cement pastes and mortars. A comprehensive experimental framework was conducted to evaluate the impact of biochar replacement on the performance of (i) cement paste in terms of hydration kinetics, rheology, strength development, porosity, and (ii) mortars in terms of mechanical, thermal, and transport properties. In addition, the durability of developed mortars, including freezing and thawing resistance, thermal resistance, acid (corrosion) resistance, flammability, and smoke production, were examined. The results revealed that lower replacement rates of cement with biochar (up to 5 wt%) do not substantially change the performance of cementitious composites. However, incorporating biochar in higher dosages (i.e., 20 wt%) influenced the hydration process, reduced flexural and compressive strengths by 49% and 29%, respectively, and increased the water absorption coefficient by 60% compared to control specimens. The same cement mortar demonstrated the most promising freeze-thaw (i.e., 98% relative residual compressive strength), acid resistance as well as considerably lower thermal conductivity. In addition, regardless of biochar content, mortars did not exhibit flammability. Therefore, this study demonstrated that despite specific technical issues, biochar can be successfully incorporated into high dosage to cementitious composite as an alternative binder with minimum environmental impacts to improve durability and insulating performance of mortars. • Biochar was used to replace up to 20% of the cement in the paste and mortar. • Biochar increases yield stress and decreases workability of cement-based composites. • Biochar lowers mechanical properties in ambient and elevated temperature conditions. • Oven-dry density and thermal conductivity of mortars were substantially decreased. • Improved durability e.g. freeze-thaw and acid resistance was reported. [ABSTRACT FROM AUTHOR]

• The effects of micro-Bi 2 O 3 and nano-Bi 2 O 3 powders were compared. • Incorporation of high dosage of nano-Bi 2 O 3 results in lower strength loss when compared to micro-Bi 2 O 3. • Addition of both Bi 2 O 3 powders enhances the gamma-ray shielding capability of pastes. • Addition of nano-Bi 2 O 3 improved the slow neutron attenuation ability of cement pastes. • Both Bi 2 O 3 powders are effective in producing lead-free cementitious composites with improved shielding properties. This study presents a comparison of the effects of micro-Bi 2 O 3 and nano-Bi 2 O 3 powders on the mechanical, microstructural and gamma / neutron shielding properties of Portland cement pastes. Cement pastes with various ratios of cement (up to 30 wt-%) replaced with Bi 2 O 3 micro and nanoparticles were prepared. Consistency, compressive strength, water accessible porosity and mercury intrusion porosimetry tests were performed, in order to characterize their engineering properties. In addition, experimental and theoretical evaluations of γ-ray and neutron shielding properties were performed. The results showed that the incorporation of Bi 2 O 3 powders gradually leads to a decline in cement pastes' compressive strength and an increase in their porosity, as the amount of powder is increased. However, the deterioration rate varies depending of type of powder used, in favor of nano-Bi 2 O 3. Gamma attenuation tests results indicate that the addition of Bi 2 O 3 powders enhances the shielding capability of pastes, in the energy range of interest (0.08–2.614 MeV). However, the effects of particle size on γ-ray attenuation are negligible in that energy range. Slow neutron attenuation study showed that the addition of nano-Bi 2 O 3 improved the shielding ability of cement pastes, with enhancements ranging between 15.3% and 25.5% for samples with 5 and 30 wt-% nano-Bi 2 O 3 , respectively. In conclusion, this study shows that the addition of nano or micro-Bi 2 O 3 is effective in producing lead-free cementitious composites with improved γ-ray and neutron shielding properties. Unlike gamma shielding, nano-sized Bi 2 O 3 has a better ability to attenuate neutrons, in comparison to their micro-sized counterparts. [ABSTRACT FROM AUTHOR]

This paper explores the new potential strategy of using fine recycled aggregates (fRA) derived from waste 3D printed concrete (3DPC) as a substitute for cement in additive manufacturing. This study hypothesizes that fRA can optimize mixture design, reduce cement content, and contribute to sustainable construction practices. Experimental programs were conducted to evaluate the fresh and hardened properties, printability window, and buildability of 3DPC mixes containing fRA. Mixes with replacement rates of cement with fRA by 10 vol%, 20 vol%, 30 vol%, 40 vol%, and 50 vol% were produced. A comprehensive experimental protocol consisting of rheological studies (static and dynamic yield stress), dynamic elastic modulus determination (first 24 h of hydration), flexural and compressive strengths (2 d and 28 d), and an open porosity test was performed. The obtained results were verified by printing tests. In addition, an economic and environmental life cycle assessment (LCA) of the mixes was performed. The results indicate that up to 50 vol% cement replacement with fRA is feasible, albeit with some technical drawbacks. While fRA incorporation enhances sustainability by reducing CO2 emissions and material costs, it adversely affects the printability window, green strength, setting time, and mechanical properties, particularly in the initial curing stages. Therefore, with higher replacement rates (above 20 vol%), potential optimization efforts are needed to mitigate drawbacks such as reduced green strength and buildability. Notably, replacement rates of up to 20 vol% can be successfully used without compromising the overall material properties or altering the mixture design. The LCA analysis shows that reducing the cement content and increasing the fRA addition results in a significant reduction in mix cost (up to 24%) and a substantial decrease in equivalent CO2 emissions (up to 48%). In conclusion, this study underscores the potential of fRA as a sustainable alternative to cement in 3D printed concrete. [ABSTRACT FROM AUTHOR]

• 3D printable mortars containing different dosages of nanosilica (NS) were developed and evaluated. • NS accelerates significantly the setting and hardening of printable mortar, while reducing its open time. • Increment of yield stress, together with an increment in NS dosage, was found to have occurred. • Optimal NS dosage results in improvement of the compressive strength and pore structure. • Incorporation of NS in the mixture resulted in improved buildability and decrement in pore anisotropy. This study presents the experimental results of an investigation on the effects of nanosilica (NS) on the material characteristics of printable mortars used for additive manufacturing. Printable cement mortars based on Ordinary Portland Cement, limestone filler and silica sand were modified with different dosages of nanosilica (from 2% to 6% by weight of binder) and its influence on their hydration, rheological, mechanical and transport properties was assessed. The study showed that NS accelerates significantly the setting and hardening of printable mortar, while reducing its open time. Moreover, an increment of yield stress, together with an increment in NS dosage, was found to have occurred. The incorporation of an optimal NS dosage results in a noticeable increase in the compressive strength and alteration of the pore structure as determined by the MIP measurements. Moreover, transport properties of the produced mortar are significantly improved due to incorporation of NS. In addition to the microstructure refinement, Micro-CT and scanning electron microscopy (SEM) studies revealed that 3D printed mortars exhibit pore anisotropy in accordance with the printing direction. However, incorporation of NS in the mixture resulted in improved buildability, thus decreasing pore anisotropy. [ABSTRACT FROM AUTHOR]

Lightweight aggregate concrete (LWAC) has relatively larger porosity than conventional concrete, mainly due to the incorporation of porous lightweight aggregates. The types of used lightweight aggregates are critical in determining the physical properties of LWAC, and it is therefore important to examine their effects on the durable characteristics of the material. To perform comparative analysis, the concrete mixture designs with two theoretical density classes were developed. The mixture composition for each class was constant and the only variable parameter was the type of the used lightweight aggregates-expanded glass (Liaver®), expanded clay (Liapor®), and foam glass (Ecoglas®). Accordingly, their pore characteristics and durability-related properties, such as sorptivity, open water porosity, and water penetration depth, were examined. To understand these phenomena, the permeable characteristic, tortuosity, was also calculated using a numerical approach incorporating X-ray micro-computed tomography. The examined results confirm that the durability characteristics of LWAC are strongly affected by the used aggregate types and are highly correlated with their pore structures. In terms of permeable characteristics, expanded glass is the most beneficial material among the used particles, and the systematic approach in this study can be used to examine the durability characteristic of LWAC. • Lightweight concrete with different aggregates and density classes were produced. • The pore characteristics and tortuosity were examined using micro-CT imaging. • Durability of lightweight aggregate concrete is affected by the aggregate pores. • Among the used materials, expanded glass has beneficial permeable characteristics. • A proper lightweight aggregate needs to be selected for the purpose of the use. [ABSTRACT FROM AUTHOR]

• Lightweight aggregate concretes (LWACs) with a targeted density of 850 kg/m3, and 450 kg/m3 were developed. • Cement was replaced with 1, 2, 5 and 10% nanosilica (by mass of cement). • Shrinkage, transport, mechanical and microstructural (2D and 3D) properties of concretes were evaluated. • Nanosilica has a beneficial effect in improving the mechanical and transport properties of concretes. • Nanosilica significantly improves the pore characteristics of lightweight concretes. This work is aimed at characterizing the effects of nanosilica (NS) on the properties of lightweight aggregate concretes with different densities. Lightweight aggregate concrete (LWAC) and ultra-lightweight aggregate concrete (ULWAC) with targeted oven-dry densities of 850 kg/m3 and 450 kg/m3, respectively, were produced. The mixtures were modified by replacing cement with nanosilica, in concentrations of 1, 2, 5 and 10 wt-%. For comparison purposes, control specimens containing either cement alone or cement with silica fume (SF) were also produced. Their mechanical properties, including flexural and compressive strengths and transport characteristics, were evaluated by measuring the water accessible porosity and water absorption coefficients of the concretes. In addition, the thermal conductivity and drying shrinkage, being important parameters of lightweight concrete, were characterized. The pore structure characteristics of the concretes were assessed using 2D and 3D image analysis techniques; namely, using an automated air void analyser and micro-computed tomography (micro-CT), respectively. The experimental results show that NS has a significant effect on improving the mechanical and transport properties of lightweight concretes and that the efficiency of NS is much higher than that of SF. Moreover, depending on dosage, NS was found to have a negligible or decreasing influence on the drying shrinkage of concrete, after 28 days of curing. Microstructural studies confirmed that NS significantly affects the pore characteristics of concretes, thus resulting in concretes with denser and stronger microstructures. [ABSTRACT FROM AUTHOR]

The performance of vacuum insulation panels (VIPs) is strongly affected by several factors, such as panel thickness, design, quality of vacuum, and material type. In particular, the core materials inside VIPs significantly influence their overall performance. Despite their superior insulation performance, VIPs are limited in their widespread use as structural materials, because of their low material strength and the relatively expensive core materials. As an alternative core material that can compensate these limitations, foamed concrete, a type of lightweight concrete with very low density, can be used. In this study, two different types of foamed concrete were used as VIP core materials, with their effects on the thermal behavior of the VIPs having been evaluated using experimental and numerical methods. To confirm and generate numerical models for VIP analysis, micro-computed tomography (micro-CT) was utilized. The obtained results show that insulation effects increase effectively when panels with lightweight concrete are in a vacuum, and both foamed concrete types can be effectively used as VIP core materials. [ABSTRACT FROM AUTHOR]

This study aims to more accurately investigate the pore size distribution of air voids in cement-based materials. For this purpose, micro-computed tomography (micro-CT) images were used to describe the inner structure of target materials, without damaging them. Together with the data obtained and the imaging techniques used, the pore structures of the specimens were visualized in 3D, with the pore size distributions being investigated using a volume-based method. The chord-length distribution, another approach to describe heterogeneous pore characteristics, was computed from 3D micro-CT images and compared with the conventional method. A RapidAir 457, an automated air void analyzer, was also used as a reference, with the results obtained being quantitatively and qualitatively compared using several curve fitting algorithms. The correlation between the pore characteristics and the mechanical properties of the specimens was examined, with the results indicating that the pore size distribution described using chord-length distribution is more effective than the conventional volume-based method. The results obtained can be utilized to investigate and predict material properties. • Two methods (2D and 3D) of pore size distribution investigation of concretes were analyzed • Concretes with different air-entraining agent dosages were produced • X-ray micro-computed tomography (micro-CT) method has been applied to obtain a 3D volumetric images of concretes • The correlation between the pore characteristics and the mechanical properties of the specimens was examined • Chord-length distribution can be effectively used to describe pore size distribution in 3D by incorporating micro-CT. [ABSTRACT FROM AUTHOR]

Seawater-mixed concrete (SWC) is a proposed solution for catering to the needs of developing nations facing extremely severe water stress. Recent research works advocate the feasibility of producing SWC by adding supplementary cementitious materials (SCMs) and alternative reinforcements without reducing the engineering properties of the same. However, limited information is available for optimising the type and amount of SCMs in binary and ternary blended SW-mixed cementitious systems for achieving desirable strength development and early-age hydration. A comprehensive study to understand the evolution of heat of hydration and strength up to 28 days was conducted on 31 binder compositions mixed with both fresh water (FW) and seawater (SW). Fly ash, slag, metakaolin, and limestone are the supplementary cementitious materials used with CEM I as a primary binder at a replacement level between 10 and 70%. Isothermal calorimetry results revealed an increase in total heat of hydration and a reduction in setting time with SW-mixed cement pastes compared to their FW-mixed counterparts. Similarly, a significant increase in strength between 0 and 50% was observed in SW-mixed cement pastes. Suitable binder combinations showing an increase in compressive strength and not a significant reduction in strength compared to the CEM I reference mix were identified using the strength improvement factor approach. Binary and ternary blended cementitious, consisting of fly ash, slag, and metakaolin at different replacement levels, are amongst the chosen binder combinations. [ABSTRACT FROM AUTHOR]

This study presents a new approach towards the production of sol-gel silica-coated Bi2O3/Gd2O3 cement additives towards the improvement of early mechanical performance and radiation attenuation. Two types of silica coatings, which varied in synthesis method and morphology, were used to coat Bi2O3/Gd2O3 structures and evaluated as a cement filler in Portland cement pastes. Isothermal calorimetry studies and early strength evaluations confirmed that both proposed coating types can overcome retarded cement hydration process, attributed to Bi2O3 presence, resulting in improved one day compressive strength by 300% and 251% (depending on coating method) when compared to paste containing pristine Bi2O3 and Gd2O3 particles. Moreover, depending on the type of chosen coating type, various rheological performances of cement pastes can be achieved. Thanks to the proposed combination of materials, both gamma-rays and slow neutron attenuation in cement pastes can be simultaneously improved. The introduction of silica coating resulted in an increment of the gamma-ray and neutron shielding thanks to the increased probability of radiation interaction. Along with the positive early age effects of the synthesized structures, the 28 day mechanical performance of cement pastes was not suppressed, and was found to be comparable to that of the control specimen. As an outcome, silica-coated structures can be successfully used in radiation-shielding cement-based composites, e.g. with demanding early age performances. [ABSTRACT FROM AUTHOR]

Nanoparticles were proposed as antibacterial cement admixtures for the production of cement-based composites. Nevertheless, the standards for evaluation of such admixtures still do not indicate which model organisms to use, particularly in regard to the further application of material. Apart from the known toxicity of nanomaterials, in the case of cement-based composites there are limitations associated with the mixing and dispersion of nanomaterials. Therefore, four nanooxides (Al2O3, CuO, Fe3O4, and ZnO) and seven microorganisms were tested to initially evaluate the applicability of nanooxides in relation to their further use in cement-based composites. Studies of nanoparticles included chemical analysis, microbial growth kinetics, 4- and 24 h toxicity, and biofilm formation assay. Nanooxides showed toxicity against microorganisms in the used concentration, although the populations were able to re-grow. Furthermore, the effect of action was variable even between strains from the same genus. The effect of nanoparticles on biofilms depended on the used strain. Gathered results show several problems that can occur while studying nanoparticles for specific further application. Proper protocols for nanomaterial dispersion prior the preparation of cement-based composites, as well as a standardized approach for their testing, are the fundamental issues that have to be resolved to produce efficient composites. [ABSTRACT FROM AUTHOR]

Recently, the recycling ofwaste glass has become aworldwide issue in the reduction ofwaste and energy consumption.Waste glass can be utilized in construction materials, and understanding its effects on material properties is crucial in developing advanced materials. In this study, recycled crushed and expanded glasses are used as lightweight aggregates for concrete, and their relation to the material characteristics and properties is investigated using several approaches. Lightweight concrete specimens containing only crushed and expanded waste glass as fine aggregates are produced, and their pore and structural characteristics are examined using image-based methods, such as scanning electron microscopy (SEM), X-ray computed tomography (CT) and automated image analysis (RapidAir). The thermal properties of the materials are measured using both Hot Disk and ISOMET devices to enhance measurement accuracy. Mechanical properties are also evaluated and the correlation between material characteristics and properties is evaluated. As a control group, a concrete specimen with natural fine sand is prepared, and its characteristics are compared with those of the specimens containing crushed and expanded waste glass aggregates. The obtained results support the usability of crushed and expanded waste glass aggregates as alternative lightweight aggregates. [ABSTRACT FROM AUTHOR]

• Magnesite was used as a precursor for synthesizing alkali-activated binder. • Magnesite was activated by sodium hydroxide, silicate and aluminate. • Adding lead glass sludge enhanced the polymerization of magnesium silicate hydrate. • Magnesite based binder recorded 28-day compressive strength of 9–53 MPa. • The prepared binder showed high efficiency in the stabilization of Pb inside sludge. The present paper focuses on synthesizing an alkali-activated binder, in which magnesium carbonate (MC) was the only precursor. MC was individually activated with sodium hydroxide (NaOH), sodium silicate (Na 2 SiO 3) and sodium aluminate (NaAlO 2). Increasing Na 2 O content (as NaOH) up to 10 wt% enhanced the compressive strength slightly. At constant Na 2 O content (10 wt%), the sample activated by (Na 2 SiO 3) recorded the highest compressive strength. Although the hardened mixture activated by NaAlO 2 showed the higher compressive strength compared to NaOH-activated sample, it still recorded strength lower than Na 2 SiO 3 -activated-mixture. XRD, TG/DTG, FTIR, and SEM techniques indicated that phase composition depends mainly on the Na 2 O source. 29Si MAS-NMR spectroscopy verified the formation of magnesium silicate hydrate with Q1, Q2, and Q3 silicate connectivity, when MC was activated with Na 2 SiO 3. Incorporating lead glass sludge (LGS) as a silicate source, enhances the formation of a cross-like silicate chain (Q3) coinciding with an improvement of the compressive strength and a retardation of Pb-leachability. As proved by 27Al MAS-NMR, Al inside hydrotalcite phase, formed within NaAlO 2 -activated-MC, has six coordination bonds. Activating MC with NaOH produces magnesium hydroxide (Mg(OH) 2) and thermonatrite Na 2 CO 3 ·H 2 O phases. Owing to the diversity of the hydration products, a wide range of 90-day compressive strengths (9–53 MPa) was recorded. A preliminary result showed that the CO 2 -emission generates from optimum mixture containing 30 wt% LGS is lower than resulted from Portland cement manufacturing. [ABSTRACT FROM AUTHOR]

Utilising wheat straw reinforced OPC-free composites in the construction industry requires efficient, eco-friendly pre-treatment coupled with surface functionalisation methods to turn it into a high-performance material. Herein, alkali-activated material (AAM) was used as an OPC-free matrix, while eco-friendly hybrid pre-treatment and surface functionalisations were applied to mitigate the surface quality deficiencies of wheat straw and improve its compatibility with low-carbon binders. Wheat straw particles were subjected to a mild physical pre-treatment (hot-water followed by steam) and surface functionalisation using attapulgite nanoclay and graphene nanoplatelets to improve their capacity as an effective reinforcing material in AAM. Comprehensive characterisation verified successful pre-treatment and surface functionalisation, which led to the improved interfacial bond between wheat straw and AAM. The best results were obtained for the AAM samples reinforced with pre-treated straw that was functionalised with attapulgite nanoclay (i.e., H+S-AT), in which the volume of permeable voids decreased by 18%, while compressive and flexural strength at 90 d increased by 41% and 27%, respectively, compared to the control sample. However, the effect on the thermal properties of the resulted composites was not significant. • Hybrid pre-treatment and functionalising method using nanomaterials is presented. • Surface functionalising methods with AT nanoclay and G nanoplatelets were tested. • The deterioration mechanisms of wheat straw reinforced AAM were explained. • Improved compatibility of straw particles in the alkali-activated matrix was found. • Attapulgite functionalisation of straw improves compressive and flexural strengths. [ABSTRACT FROM AUTHOR]

This paper deals with the hygric characterization of early 20th century machine-made clay bricks, representative of great number of historical buildings in north-eastern Poland. Heritage buildings have a high potential for adaptive reuse, which is strictly connected with an urge for knowledge about the properties of these existing building envelopes. To better understand the hygric behavior of historic buildings, various experimental laboratory tests, including density, water absorption, compressive strength and freeze-thaw resistance, were conducted. In order to assess the microstructural characteristics of the tested bricks, mercury intrusion porosimetry (MIP) and X-ray micro-computed tomography (micro-CT) tests were performed. These tests were conducted on clay bricks from historic buildings, as well as on those that are currently being produced, in order to identify the relationship between the materials used in the past and the replacements produced presently. This paper addresses the lack of systematic application of existing standards for evaluating the state of the conservation of historic bricks and for establishing the specifications for replacement bricks. The results of conducted study and further research will be the basis for creating a historic materials database. It would be a useful tool for selecting bricks that correspond with the historically used materials and help to maintain homogenous structure of the restored buildings. [ABSTRACT FROM AUTHOR]

This study investigates the addition of Poly-vinyl Alcohol (PVA) fibres and attapulgite nanoclay to alkali-activated materials (AAMs) with the aim of enhancing the mechanical performance and optimizing the printability and buildability of AAMs. The fresh properties of six mix formulations, including flowability, slump values, rheology, shape retention, and extrusion window, were evaluated. The best performing mixes, that exhibited optimal fresh properties, were 3D printed, and their mechanical performance, microstructure, and buildability were investigated. The addition of 1 wt.-% attapulgite nanoclay (i.e. A-1) showed the desirable fresh properties required for 3D printing, as well as providing sufficient mechanical reinforcement to the samples. The 3D printed A-1 samples showed an improved flexural and compressive strength by 43% and 20%, respectively, compared to both the casted and printed control mixes. Moreover, microstructure analysis, including SEM, Rapidair measurement, and micro-CT, provided evidence of the compatibility by showing the lowest pores anisotropy and mixture homogeneity, between attapulgite and AAMs. [Display omitted] • Poly-vinyl alcohol fibres and attapulgite nanoclay as additives for 3D printable alkali-activated materials • P0.25 and A-1alkali-activated composites were found to be the best performing sample for 3D printability and mechanical performance • The microstructure analysis showed the effectiveness of attapulgite and PVA fibres incorporation in filling the voids and crack bridging mechanisms [ABSTRACT FROM AUTHOR]

• Ultrafine slag was partially replaced by andesite igneous rock (AIR). • AIR enhanced the performance of alkali-activated slag at normal temperature. • The sample with 70 % AIR showed highest thermal resistance at all temperatures. • AIR transforms into solidified melt with high mechanical performance at 800–1200 °C. • Activated slag-AIR can be used as refractory material. According to the literature, the engineering properties of alkali-activated slag (AAS) generally decrease as temperatures rise up to 1200 °C. Although the addition of certain amorphous aluminosilicate additives can increase the residual strength of AAS, it still exhibits lower performance compared to samples cured at ambient temperature. Hence, there is a significant need to identify an effective additive that can greatly enhance the thermal stability of AAS at elevated temperatures. In this study, the contribution of andesite igneous rock (AIR) in improving the performance of alkali-activated ultrafine slag (AAUS) at normal and elevated temperatures was evaluated. The addition of AIR at a weight percentage of 10 % enhances the performance of humidity-cured AAUS through its filling and nucleating effect. However, increasing the AIR content beyond 90 wt% negatively impacts the physical and mechanical performance of AAS due to the low reactivity of AIR towards alkali activation. Nevertheless, higher AIR content enhances the tolerance capacity of AAUS to temperatures as high as 1200 °C. AIR acts as a microaggregate in the activated system, reducing the contraction of AAUS at temperatures below 1000 °C. At higher temperatures, the unreacted AIR undergoes transformation into solidified melt and thermally-stable pyroxenes. This results in microstructural compaction and an increase in compressive strength, particularly when the AIR content exceeds 50 wt%. [ABSTRACT FROM AUTHOR]

• Katoite (KT) binder is fabricated by mixing dolomite powder with sodium aluminate. • Both KT and unreacted dolomite powder completely decomposed at 1000 °C. • Thermally-exposed KT powder can be rehydrate to yield layered-double hydroxide. • Using silicate waste powder enhanced firing resistance but inhibit the rehydration. This study investigated the effect of red-clay brick and perlite wastes (RCBW and PW) on the performance and rehydration of katoite-based binder after exposure to elevated temperatures. The KT-binder was produced by combining dolomite waste with sodium aluminate. The exposure of KT binder to temperatures up to 1000 °C (CA-1000) caused a significant compressive strength loss associated with the decomposition of KTand other phases into mayenite, sodium aluminate, and lime. The CA-1000 acts as a one-part alkali-activated binder, as it can re-interact with water to yield a layered double hydroxide-based binder with 1-day compressive strength of 9.7 MPa and volume expansion of 160%. The individual inclusion of 50 wt% RCBW and PW improved the thermal resistance of the CA-binder but decreased its ability to rehydrate, as they hindered the reformation of sodium aluminate and mayenite through the formation of lower hydraulic calcium/sodium aluminosilicates. [ABSTRACT FROM AUTHOR]

This study presents the results of an experimental investigation of the effects of nanosilica (NS) on the strength development, transport properties, thermal conductivity, air-void, and pore characteristics of lightweight aggregate concrete (LWAC), with an oven-dry density <1000 kg/m3. Four types of concrete mixtures, containing 0 wt.%, 1 wt.%, 2 wt.%, and 4 wt.% of NS were prepared. The development of flexural and compressive strengths was determined for up to 90 days of curing. In addition, transport properties and microstructural properties were determined, with the use of RapidAir, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) techniques. The experimental results showed that NS has remarkable effects on the mechanical and transport properties of LWACs, even in small dosages. A significant improvement in strength and a reduction of transport properties, in specimens with an increased NS content, was observed. However, the positive effects of NS were more pronounced when a higher amount was incorporated into the mixtures (>1 wt.%). NS contributed to compaction of the LWAC matrix and a modification of the air-void system, by increasing the amount of solid content and refining the fine pore structure, which translated to a noticeable improvement in mechanical and transport properties. On the other hand, NS decreased the consistency, while increasing the viscosity of the fresh mixture. An increment of superplasticizer (SP), along with a decrement of stabilizer (ST) dosages, are thus required. [ABSTRACT FROM AUTHOR]

[Display omitted] • Printable heavyweight mixes with maximum dry density of 3500 kg/m3 were developed. • Mix containing 60% of magnetite aggregate exhibited the highest green strength. • Mixes containing MA displayed an adequate fresh property performances for medium scale printing. • The incorporation of MA gradually increases the flexural and compressive strengths of both cast and printed samples. • A substantial improvement in gamma-ray and neutron shielding was reported in mixes incorporating MA. The main objective of this study is to develop 3D printed heavyweight concrete (3DPHWC) to produce elements with a dry density of up to 3500 kg/m3 by replacing natural aggregate (SA) with magnetite aggregate (MA) up to 100%. A comprehensive systematic study was conducted to thoroughly assess mixtures' mechanical properties, physical proficiency, fresh properties, and printing qualities. The inclusion of MA exhibited the desired fresh properties required for 3D printing and promising physical and mechanical properties. Evaluation of the mechanical properties of designed 3DPHWC indicates that replacing SA with MA increases both cast and printed samples' strengths. The 3D printed M100 sample achieved higher 28 days flexural and compressive strengths by 18 % and 20 %, respectively, compared to printed control mix (M0). Micro-CT study correspondingly demonstrated improvements in the composites' porosity, pore size, and pore morphologies. The linear attenuation coefficients (LACs) and half-value layer (HVLs) for slow neutron and gamma-ray were measured to assess radiation shielding characteristics. A significant performance improvement was obtained for slow neutrons by introducing the magnetite aggregate. Unlike slow neutrons, no significant difference was observed between cast and printed samples against γ-rays. Moreover, the effect of porosity on the shielding performance was discussed. [ABSTRACT FROM AUTHOR]

Highlights • The characteristics and properties of lightweight aggregate concrete and foamed concrete with the same density levels were investigated based on image analysis. • SEM images confirmed that the solid structures of the matrix in foamed concrete are relatively denser than that of lightweight aggregate concrete. • The micro-CT images demonstrated that foamed concrete has larger porosity than lightweight aggregate concrete when the material density is similar. • The proper use of lightweight aggregate concrete can be more beneficial in terms of having a material with better mechanical performance by minimizing the loss of the insulation effect. Abstract Lightweight concrete is a special type of concrete with low density and advanced insulation, mainly produced using lightweight aggregates or a cellular matrix. Concrete material made of lightweight aggregates is called lightweight aggregate concrete, while material made from a cellular matrix is generally called foamed concrete because of the pores introduced by a foaming agent. Both lightweight concrete types have different characteristics due to their different compositions. In this study, the material properties and characteristics of these lightweight concretes were investigated and compared. A series of foamed and lightweight aggregate concrete specimens with the same density level were produced, with their mechanical and thermal properties being evaluated using sensitive measurement tools. X-ray micro-computed tomography (μ -CT) and scanning electron microscopy (SEM) were used to characterize each material, using image-based techniques. The results brought to light the details of each lightweight concrete, at the microstructural level, in regard to their material properties and showed that the properly designed lightweight aggregate concrete can be more beneficial in mechanical performance by minimizing the loss of the insulation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Natural and calcined Halloysite clay additives were characterised. • Halloysite clay additives were incorporated to modify AAMs' printing properties. • Enhanced compatibility of calcined Halloysite clay additives in the AAMs was verified. • Buildability and mechanical properties were improved in CH-modified AAMs. This study investigates the effects of natural and calcined halloysite clay minerals ("NH" and "CH", respectively) on the performance of 3D printed alkali-activated materials (AAMs). Halloysite clay minerals are selected as they are low-cost and abundantly available. At first, different characterisation techniques were employed to characterise the NH and CH additives. Mechanical performance, extrusion window, and shape stability of several AAM formulations containing various dosages (0.5 wt% to 5 wt%) of the NH and CH additives were evaluated. The best-performing mixtures in terms of fresh and hardened properties namely, NH-1.5 and CH-1.5 mixtures (containing 1.5 wt% of NH and CH additives, respectively) were then selected for 3D printing. The results showed that the CH-1.5 mixture exhibited enhanced shape stability, buildability, and mechanical properties as compared to the control mixture. The flexural and compressive strengths of 3D printed CH-1.5 samples were 88% and 40%, respectively higher than those of the printed control samples. Using the CH-1.5 mixture, a twisted column with an intricate shape was printed to verify the suitability of the developed CH-modified AAM for the construction of complex structures. This study establishes the use of halloysite clay minerals as low-cost additives for enhancing the mechanical properties and printing performance of AAMs. [ABSTRACT FROM AUTHOR]

An eco-friendly pre-treatment coupled with surface functionalisation were developed to enhance the quality of wheat straw particles to be used for development of high-performance polylactic acid (PLA) compressed strawboard. Eco-friendly hybrid pre-treatment (i.e., hot water followed by steam, H+S) and surface functionalisation processes employing attapulgite nanoclay (AT) and graphene nanoplatelets (G) were used to obtain an appropriate wheat straw surface quality while increasing its compatibility with the PLA matrix. The successful pre-treatment and surface functionalisation of wheat straw particles was verified through characterisation techniques, including SEM, FTIR, XRD, Raman spectroscopy, and TGA. Tensile strength and water absorption properties of compressed strawboards were examined to investigate the influence of pre-treatment and surface functionalisation of wheat straws. The maximum tensile strengths of 28 MPa and 27 MPa were recorded for 10 H+S-AT and 10 H+S-G samples, respectively, which are considerably higher than the value (i.e., 9.7 MPa) registered for the sample without pre-treatment and surface functionalisation (i.e., 10UN). The lowest water absorption after 24 h of immersion was registered for 10UN-G (i.e., 1.6%), which is 11% and 31% lower than the 10UN and 10 H+S samples, respectively. The effect is attributed to an improved interfacial bond between wheat straw and PLA matrix due to the graphene surface functionalisation, as evidenced by the SEM. • Dual enhancement of pre-treatment and surface nano-functionalising was proposed. • Pre-treatment and functionalising effects on the straw's structure were verified. • Crosslinking effect of surface functionalising agents is confirmed. • Maximum tensile strength of 28 MPa was registered for 10 H+S-AT. • The developed composites can be used in several applications, e.g., construction and automotive industry. [ABSTRACT FROM AUTHOR]

• Lead glass sludge (LGS) was prepared with no foaming agent addition. • Adding MgCl 2 caused a notable retardation in foaming process at all temperatures. • MgCl 2 enhanced LGS crystallization by the formation of diopside and cristobalite. • A significant decrease in Pb-leaching rate was achieved by adding MgCl 2. In the present work, different MgCl 2 dosages were admixed with lead glass sludge (LGS) to evaluate their effects on the performance of the produced LGS-foam at different elevated temperatures. The results revealed that adding MgCl 2 caused a negative impact on the bloating process, including porosity and volume expansion reduction, as well as bulk density and thermal conductivity increment. This was associated with the high affinity of magnesium for enhancing the crystallization of LGS through the formation of diopside and cristobalite minerals. Moreover, the speed up of softening temperature of LGS using MgCl 2 makes LGS completely softened before full dissociation of CaCO 3 , decreasing gas entrapping inside the softened LGS. Nevertheless, the use of MgCl 2 decreased the free Pb inside LGS through thermal/chlorination process. [ABSTRACT FROM AUTHOR]

It was shown that the uniform dispersion of nanoparticles into a cement matrix is a key issue in their application in the field. Therefore, here we compare two methods of incorporation of mesoporous silica nanospheres into cement using acetone (method A) or water (method B) as a dispersant, to reveal its influence on the mechanical properties of the formed composite. It was proven that in both cases, small amounts of silica nanoparticles agglomerates does not affect the mechanical properties of the cement mortars. However, the addition of mesoporous silica nanospheres to cement in an acetone solution is more effective and results in higher compressive and flexural strength than the sample of the composite formed from water solution of silica nanoparticles and cement. [ABSTRACT FROM AUTHOR]

The use of lightweight concrete (LWC) for structural and non-structural applications has attracted great interest in recent years. The main benefits include reduced deadload of structural elements and generally lower production and transportation costs. However, a decrease in concrete density often leads to a decrease in strength and durability. Typically, concretes are mostly modified with mineral additives such as silica fume or fly ash. Because of the recent developments in nanotechnology, research attention has turned to the possibility of improving concrete properties with nanomaterials, i.e., nano-SiO2. However, there are still certain issues with the dosage and efficiency of nanomaterials. Therefore, in order to establish the current state of knowledge in this field, this review gathers most recent results about the performance of LWC modified with nanomaterials. The review is divided into sections about the influence of nanoparticles on the fresh properties of concrete and their influence on the mechanical and durability characteristics. The paper studies in depth the most common approach to nanomaterials in concrete technology and proposes areas for further development. [ABSTRACT FROM AUTHOR]

This investigation studies the effects of hot water and hot air curing on the strength development, transport properties, and freeze-thaw resistance of mortars incorporating low-heat blast furnace slag cement and nanosilica (NS). Mortar samples were prepared and stored in ambient conditions for 24 h. After demolding, mortar samples were subjected to two different hot curing methods: Hot water and hot air curing (40 °C and 60 °C) for 24 h. For comparison purposes, mortar reference mixes were prepared and cured in water and air at ambient conditions. Strength development (from 1 to 180 days), capillary water porosity, water sorptivity, and freeze-thaw resistance were tested after 180 days of curing. The experimental results showed that both curing regimes accelerate the strength development of mortars, especially in the first seven days of hydration. The highest early strengths were reported for mortars subjected to a temperature of 60 °C, followed by those cured at 40 °C. The hot water curing regime was found to be more suitable, as a result of more stable strength development. Similar findings were observed in regard to durability-related properties. It is worth noting that thermal curing can more efficiently increase strength in the presence of nanosilica, suggesting that NS is more effective in enhancing strength under thermal curing. [ABSTRACT FROM AUTHOR]