Cesium immobilization of high pH and low pH belite-rich cement under varying temperature.

Low-pH cement is being studied in radioactive waste repositories. The belite-rich cement (BRC) recently gained attention due to its higher CO2 sequestration and low pH attainment under carbonation exposure. Therefore, this study evaluated the effects of pH and temperature on cesium immobilization of BRC. High pH (12.6) and low pH (9.9) BRC were produced via air curing and carbonation treatment, respectively. The high and low pH BRC samples were placed in a leaching environment at 25 °C and 45 °C for 90 days. An inverse correlation between pH and cesium mobilization of BRC was observed. The high pH BRC achieved the lowest effective diffusion coefficient (4.05E-09 cm2/s), whereas the highest value (2.64E-07 cm2/s) was achieved in case of low pH BRC. The physicochemical and morphological properties unveiled the decrease in Si/Ca ratio of gel, precipitation of Ca2+ ions in calcite formation, and increment in pore structure connectivity (pore size > 100 nm) in low pH BRC. However, the high pH BRC demonstrated the high Si/Ca ratio in C-S-H gel, hydroxide phases and higher disconnected pores. Thermodynamic modeling revealed the presence of significant carbonated phases beyond 15% CO2 uptake. The findings contributed to the BRC's feasibility in developing nuclear waste storage facility.

Phase-Change Materials in Concrete: Opportunities and Challenges for Sustainable Construction and Building Materials.

The use of phase-change materials (PCM) in concrete has revealed promising results in terms of clean energy storage. However, the negative impact of the interaction between PCM and concrete on the mechanical and durability properties limits field applications, leading to a shift of the research to incorporate PCM into concrete using different techniques to overcome these issues. The storage of clean energy via PCM significantly supports the UN SDG 7 target of affordable and clean energy. Therefore, the present study focuses on three aspects: PCM type, the effect of PCM on concrete properties, and connecting the outcome of PCM concrete composite to the United Nations sustainable development goals (UN SDGs). The compensation of reduction in strength of PCM-contained concrete is possible up to some extent with the use of nanomaterials and supplementary cementitious materials. As PCM-incorporated concrete is categorized a type of building material, the large-scale use of this material will affect the different stages associated with building lifetimes. Therefore, in the present study, the possible amendments of the different associated stages of building lifetimes after the use of PCM-incorporated concrete are discussed and mapped in consideration of the UN SDGs 7, 11, and 12. The current challenges in the widespread use of PCM are lower thermal conductivity, the trade-off between concrete strength and PCM, and absence of the link between the outcome of PCM-concrete composite and UN SDGs. The global prospects of PCM-incorporated concrete as part of the effort to attain the UN SDGs as studied here will motivate architects, designers, practicing engineers, and researchers to accelerate their efforts to promote the consideration of PCM-containing concrete ultimately to attain net zero carbon emissions from building infrastructure for a sustainable future.

Influence of the Precursor, Molarity and Temperature on the Rheology and Structural Buildup of Alkali-Activated Materials.

This study presents an investigation of the effects of the precursor, alkalinity and temperature on the rheology and structural buildup of alkali activated materials. Here, 100% fly ash, 100% slag and blended mixes of fly ash and slag were activated by 4 M, 6 M, 8 M or 10 M (only for sodium hydroxide) solutions at 25 °C, 35 °C, 45 °C and 55 °C. The rheological properties were investigated to obtain the flow curves, viscosity, storage modulus, and loss factor of these materials. The results showed that for the presence of slag, a higher molarity of the alkali activating solution and a high temperature all caused greater interparticle force, leading to an increase in the shear stress and viscosity of the alkali activated materials. It was also observed that slag had the greatest effect on the increase in the storage modulus of the blended mixes. Furthermore, the higher alkalinity and temperature levels were instrumental in initiating the dissolution of fly ash and improving its rate of structural buildup. Moreover, the interdependence of various factors showed that the type of precursor, as well as the concentration of alkali activating solution, were the primary influencing factors on the polymerization process, as well as the rheological measurements of alkali-activated materials.

Special Issue: "Microstructures and Durability of Cement-Based Materials".

Cement-based materials play an irreplaceable role in building and sustaining our society by meeting the performance demand imposed on structures and sustainability. Cement-based materials are no longer limited to derivatives of Portland cement, and appreciate a wider range of binders that come from various origins. It is therefore of utmost importance for understanding and expanding the relevant knowledge on their microstructure and likely durability performance. This Special Issue "Microstructures and Durability of Cement-Based Materials" presents recent studies reporting microstructural and durability investigation revealing the characteristics of cement-based materials.

Characteristics of Preplaced Aggregate Concrete Fabricated with Alkali-Activated Slag/Fly Ash Cements.

This study assesses the characteristics of preplaced aggregate concrete prepared with alkali-activated cement grout as an adhesive binder. Various binary blends of slag and fly ash without fine aggregate as a filler material were considered along with different solution-to-solid ratios. The properties of fresh and hardened grout along with the properties of hardened preplaced concrete were investigated, as were the compressive strength, ultrasonic pulse velocity, density, water absorption and total voids of the preplaced concrete. The results indicated that alkali-activated cement grout has better flowability characteristics and compressive strength than conventional cement grout. As a result, the mechanical performance of the preplaced aggregate concrete was significantly improved. The results pertaining to the water absorption and porosity revealed that the alkali-activated preplaced aggregate concrete is more resistant to water permeation. The filling capacity based on the ultrasonic pulse velocity value is discussed to comment on the wrapping ability of alkali-activated cement grout.

Mechanical Properties, Microstructure, and Chloride Content of Alkali-Activated Fly Ash Paste Made with Sea Water.

The aim of the present study is to investigate the potential of sea water as a feasible alternative to produce alkali-activated fly ash material. The alkali-activated fly ash binder was fabricated by employing conventional pure water, tap water, and sea water based alkali activating solution. The characteristics of alkali-activated materials were examined by employing compressive strength, mercury intrusion porosimetry, XRD, FT-IR, and 29Si NMR along with ion chromatography for chloride immobilization. The results provided new insights demonstrating that sea water can be effectively used to produce alkali activated fly ash material. The presence of chloride in sea water contributed to increase compressive strength, refine microstructure, and mineralogical characteristics. Furthermore, a higher degree of polymerization on the sea water-based sample was observed by FT-IR and 29Si NMR analysis. However, the higher amount of free chloride ion even after immobilization in sea water-based alkali-activated material, should be considered before application in reinforced structural elements.

Structural evolution of binder gel in alkali-activated cements exposed to electrically accelerated leaching conditions.

The structural evolution of a binder gel in alkali-activated cements exposed to accelerated leaching conditions is investigated for the first time. Samples incorporating fly ash and/or slag were synthesized and were exposed to electrically accelerated leaching by applying a current density of 5 A/m2. The leaching behavior of the samples greatly depended on the binder gel formed in the samples. The N-A-S-H type gel abundant in fly ash-rich samples showed some extent of dissolution upon accelerated leaching, while slag-rich samples underwent hydration of the anhydrous slag after leaching. The obtained results are discussed in view of the degradation of the binder gel induced by accelerated leaching, and their potential performance under repository conditions where groundwater-induced leaching is the main durability concern.

Utilization of Calcium Carbide Residue Using Granulated Blast Furnace Slag.

The solidification and stabilization of calcium carbide residue (CCR) using granulated blast furnace slag was investigated in this study. CCR binding in hydrated slag was explored by X-ray diffraction, 29Si and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and thermodynamic calculations. Mercury intrusion porosimetry and and compressive strength tests assessed the microstructure and mechanical properties of the mixtures of slag and CCR. C-A-S-H gel, ettringite, hemicarbonate, and hydrotalcite were identified as the main phases in the mixture of slag and CCR. The maximum CCR uptake by slag and the highest volume of precipitated solid phases were reached when CCR loading in slag is 7.5% by mass of slag, according to the thermodynamic prediction. This feature is also experimentally observed in the microstructure, which showed an increase in the pore volume at higher CCR loading.

Effect of Carbonation on Abrasion Resistance of Alkali-Activated Slag with Various Activators.

The effect of carbonation on the abrasion resistance of alkali-activated slag (AAS) was investigated. Various activator sets were selected for synthesizing AAS specimens, and the compressive strength was measured before and after carbonation. The abrasion resistance of the specimens was measured in accordance with the ASTM C944 test method. The relationship between the mass loss caused by abrasion and compressive strength was analyzed to understand the effect of matrix strength on abrasion resistance. Test results showed that the decrease in compressive strength of AAS specimens by carbonation reduced their abrasion resistance. In addition, the abrasion resistance of AAS before and after carbonation was sensitively influenced by activator type. It can be concluded that additional caution is required when using AAS where abrasion may have occurred.

Calcined Oyster Shell Powder as an Expansive Additive in Cement Mortar.

The present study prepared calcined oyster shell powder having chemical composition and crystal structure of calcium oxide and lime, respectively, and investigated the fresh and hardened properties of cement mortar incorporating calcined oyster shell powder as an additive. The test results indicated that the hydration of calcined oyster shell powder promoted the additional formation of Ca(OH)2 at the initial reaction stage, thereby increasing the heat of hydration. In particular, the volumetric increase of calcined oyster shell powder during hydration compensated the autogenous shrinkage of mortar at early ages, ultimately leading to a clear difference in the shrinkage values at final readings. However, an excessive incorporation of calcined oyster shell powder affected the rate of C-S-H formation in the acceleratory period of hydration, resulting in a decrease in the compressive strength development. Meanwhile, the degree of flow loss was inconsequential and rapid flow loss was not observed in the specimens with calcined oyster shell powder. Therefore, considering the fresh and hardened properties of cement mortar, the incorporation of calcined oyster shell powder of approximately 3% by weight of cement is recommended to enhance the properties of cement mortar in terms of compressive strength and autogenous shrinkage.

Cesium and Strontium Retentions Governed by Aluminosilicate Gel in Alkali-Activated Cements.

The present study investigates the retention mechanisms of cesium and strontium for alkali-activated cements. Retention mechanisms such as adsorption and precipitation were examined in light of chemical interactions. Batch adsorption experiments and multi-technical characterizations by using X-ray diffraction, zeta potential measurements, and the N2 gas adsorption/desorption methods were conducted for this purpose. Strontium was found to crystalize in alkali-activated cements, while no cesium-bearing crystalline phases were detected. The adsorption kinetics of alkali-activated cements having relatively high adsorption capacities were compatible with pseudo-second-order kinetic model, thereby suggesting that it is governed by complex multistep adsorption. The results provide new insight, demonstrating that characteristics of aluminosilicate gel with a highly negatively charged surface and high micropore surface area facilitated more effective immobilization of cesium and strontium in comparison with calcium silicate hydrates.

An NMR Spectroscopic Investigation of Aluminosilicate Gel in Alkali-Activated Fly Ash in a CO2-Rich Environment.

The present study investigated aluminosilicate gel in alkali-activated fly ash exposed to a CO2-rich environment by means of NMR spectroscopy. The alkali-activated fly ash was exposed to an atmospheric CO2 concentration of 10% after curing at 80 °C initially for 24 h. Under high concentrations of CO2, highly reactive components Na and Al, which completely reacted within the first few hours, were unaffected by carbonation, while Si, with relatively slower reactivity, behaved differently. Despite a lower degree of the reaction in the carbonated sample, the monomeric silicates rapidly became of higher polymerization, meaning that exposure to high concentrations of CO2 caused Si to form a binding gel phase. Consequently, the carbonated sample possessed a higher amount of binding gel. The obtained results may be useful to understand the fundamental chemistry and behavior of aluminosilicate gel under high concentrations of CO2.