Despite enormous interest in calcium silicate hydrate (C-S-H), its detailed atomic structure and intrinsic deformation under an external load are lacking. This study demonstrates the nanostructural deformation process of C-S-H in tricalcium silicate (C 3 S) paste as a function of applied stress by interpreting atomic pair distribution function (PDF) based on in situ X-ray scattering. Three different strains in C 3 S paste under compression were compared using a strain gauge, Bragg peak shift, and the real space PDF. PDF refinement revealed that the C-S-H phase mostly contributed to PDF from 0 to 20 A whereas crystalline phases dominated that beyond 20 A. The short-range atomic strains exhibited two regions for C-S-H: I) plastic deformation (0-10 MPa) and II) linear elastic deformation (>10 MPa), whereas the long-range deformation beyond 20 A was similar to that of Ca(OH) 2 . Below 10 MPa, the short-range strain was caused by the densification of C-S-H induced by the removal of interlayer or gel-pore water. The strain is likely to be recovered when the removed water returns to C-S-H. K E Y W O R D Scalcium silicate hydrate, deformation, portland cement, X-ray methods

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In recent years, nano-reinforcing technologies for cementitious materials have attracted considerable interest as a viable solution for compensating the poor cracking resistance of these materials. In this study, for the first time, titanium nanotubes (TNTs) were incorporated in cement pastes and their effect on the mechanical properties, microstructure, and early-age hydration kinetics was investigated. Experimental results showed that both compressive (~12%) and flexural strength (~23%) were enhanced with the addition of 0.5 wt.% of TNTs relative to plain cement paste at 28 days of curing. Moreover, it was found that, while TNTs accelerated the hydration kinetics of the pure cement clinker phase (C3S) in the early age of the reaction (within 24 h), there was no significant effect from adding TNTs on the hydration of ordinary Portland cement. TNTs appeared to compress the microstructure by filling the cement paste pore of sizes ranging from 10 to 100 nm. Furthermore, it could be clearly observed that the TNTs bridged the microcracks of cement paste. These results suggested that TNTs could be a great potential candidate since nano-reinforcing agents complement the shortcomings of cementitious materials.

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Considering the increase in research regarding environmental pollution reduction, the utilization of cementitious material, a commonly used construction material, in photocatalysts has become a desirable research field for the widespread application of photocatalytic degradation technology. Nano-reinforcement technology for cementitious materials has been extensively researched and developed. In this work, as a new and promising reinforcing agent for cementitious materials, the photocatalytic performance of titanium dioxide nanotube (TNT) was investigated. The degradation of methylene blue was used to evaluate the photocatalytic performance of the TNT-reinforced cement paste. In addition, cement paste containing micro-TiO2 (m-TiO2) and nano-TiO2 (n-TiO2) particles were used for comparison. Moreover, the effect of these TiO2-based photocatalytic materials on the cement hydration products was monitored via X-ray diffraction (XRD) and thermogravimetric analysis (TG). The results indicated that all the TiO2 based materials promoted the formation of hydration products. After 28 days of curing, the TNT-reinforced cement paste contained the maximum amount of hydration products (Ca(OH)2). Furthermore, the cement paste containing TNT exhibited better photocatalytic effects than that containing n-TiO2, but worse than that containing m-TiO2.

This study aims to elucidate the effect of heating on the local atomic arrangements, structure, phase transformation, and mechanical properties of synthesized calcium–silicate–hydrate (C–S–H). The alteration in the atomic arrangement of the synthesized C–S–H (Ca/Si =0.8) and the formation of crystalline phases that occurred in three distinct transformation stages of dehydration (105°C–200°C), decomposition (300°C–600°C), and recrystallization (700°C–1000°C) were investigated via powder X‐ray diffraction, 29Si nuclear magnetic resonance spectroscopy, and thermogravimetric analysis. Further, the deformation of the local atomic bonding environment and variations in mechanical properties during the three stages were assessed via pair distribution function analysis based on in‐situ total X‐ray scattering. The results revealed that the C–S–H paste before heating exhibited a lower elastic modulus in real space than that in the reciprocal space in the initial loading stage because water molecules acted as a lubricant in the interlayer. At the dehydration stage, the strain as a function of external loading exhibited irregular deformation owing to the formation of additional pores induced by the evaporation of free moisture. At the decomposition stage, the structural deformation of the main d‐spacing (d ≈ 3.0 Å) was similar to that of the real space before the propagation of microcracks. At the recrystallization stage, the elastic modulus increased to 48 GPa owing to the thermal phase transformation of C–S–H to crystalline β‐wollastonite. The results provide direct experimental evidence of the microstructural and nanostructural deformation behavior of C–S–H pastes after exposure to high temperature under external loading.

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Supplementary cementitious materials (SCMs), such as blast-furnace slag, 1 fly ash (FA), 2 natural pozzolans, 3 calcined clays, 4 and silica fumes, 5 have been investigated as partial replacements for Portland cement (PC) in concrete mixes. FA, a by-product of coal, is one of the main SCMs, and has been studied extensively. 6 Using FA as a partial replacement for PC allows for recycling of the industrial by-products, effectively reduces carbon emissions from cement production, and confers durability to concrete. 7 According to ASTM C618, there are two types of FA, namely Class F and Class C, which are defined based on their contents of silicates and CaO. While Class F is the main type of FA currently produced, it does not possess cementitious characteristics. 8 However, the siliceous and