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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]

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]

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]

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 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]

• 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 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]

• 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]

• 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]

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]

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]