An Atomistic Model Describing the Structure and Morphology of Cu-Doped C-S-H Hardening Accelerator Nanoparticles

Molecular Dynamics Simulation of Silane Inserted CSH Nanostructure

Herein, the toughening mechanism and effects of 3-(aminopropyl)triethoxysilane (3-APTES) intercalation in calcium-silicate-hydrate (CSH) structures were investigated through molecular dynamics simulations. CSH established a model using 11 Å-tobermorite to simulate the tensile properties, toughness, adsorption energy, average orientation displacement and radial distribution function of 3-APTES intercalation at different Ca/Si ratios under conditions of a CVFF force field, an NVT system, and 298 K temperature. Simulation results demonstrate that 3-APTES alters the fracture process of CSH and effectively enhances its tensile properties and toughness. The presence of 3-APTES molecules increases the energy required to destroy CSH, thereby increasing the adsorption energy of CSH crystals. Furthermore, 3-APTES molecules effectively increase the atom density within the CSH structure. As the Ca/Si ratio increases, Ca-O bond formation is enhanced, with noticeable aggregation occurring because of modification by 3-APTES within the CSH structure. This study found that 3-APTES organic compounds can effectively improve the tensile, toughness, adsorption and other properties of the CSH structure, and further improve the microstructure of CSH.

Recent Advances in C-S-H Nucleation Seeding for Improving Cement Performances

Reducing cement CO2 footprint is a societal need. This is being achieved mainly by replacing an increasing amount of Portland clinker by supplementary cementitious materials. However, this comes at a price: lower mechanical strengths at early ages due to slow pozzolanic reaction(s). This is being addressed by using accelerator admixtures. In this context, calcium silicate hydrate nucleation seeding seems to have a promising future, as it can accelerate cement and pozzolanic reactions at early ages, optimising their microstructures, without compromising late strength and durability performances. In fact, these features could even be improved. Moreover, other uses are low temperature concreting, precasting, shotconcrete, etc. Here, we focus on reviewing recent reports on calcium silicate hydrate seeding using commercially available admixtures. Current knowledge on the consequences of nucleation seeding on hydration reactions and on early and late mechanical strengths is discussed. It is noted that other features, in addition to the classic alite hydration acceleration, are covered here including the enhanced ettringite precipitation and the very efficient porosity refinement, which take place in the seeded binders. Finally, because the seeded binders seem to be denser, durability properties could also be enhanced although this remains to be properly established.

Detecting Early-Stage Cohesion Due to Calcium Silicate Hydration with Rheology and Surface Force Apparatus

Extremely robust cohesion triggered by calcium silicate hydrate (C-S-H) precipitation during cement hardening makes concrete one of the most commonly used man-made materials. Here, in this proof-of-concept study, we seek an additional nanoscale understanding of early-stage cohesive forces acting between hydrating model tricalcium silicate (C₃S) surfaces by combining rheological and surface force measurements. We first used time-resolved small oscillatory rheology measurements (SAOSs) to characterize the early-stage evolution of the cohesive properties of a C₃S paste and a C-S-H gel. SAOS revealed the reactive and viscoelastic nature of C₃S pastes, in contrast with the nonreactive but still viscoelastic nature of the C-S-H gel, which proves a temporal variation in the cohesion during microstructural physicochemical rearrangements in the C₃S paste. We further prepared thin films of C₃S by plasma laser deposition (PLD) and demonstrated that these films are suitable for force measurements in the surface force apparatus (SFA). We measured surface forces acting between two thin C₃S films exposed to water and subsequent in situ calcium silicate hydrate precipitation. With the SFA and SFA-coupled interferometric measurements, we resolved that C₃S surface reprecipitation in water was associated with both increasing film thickness and progressively stronger adhesion (pull-off force). The lasting adhesion developing between the growing surfaces depended on the applied load, pull-off rate, and time in contact. These properties indicated the viscoelastic character of the soft, gel-like reprecipitated layer, pointing to the formation of C-S-H. Our findings confirm the strong cohesive properties of hydrated calcium silicate surfaces that, based on our preliminary SFA measurements, are attributed to sharp changes in the surface microstructure. In contact with water, the brittle and rough C₃S surfaces with little contact area weather into soft, gel-like C-S-H nanoparticles with a much larger surface area available for forming direct contacts between interacting surfaces.

A Study on the Cement Gel Formation Process during the Creation of Nanomodified High-Performance Concrete Based on Nanosilica

One of the most science-intensive and developing areas is nano-modified concrete. Its characteristics of high-strength, high density, and improved structure, which is not only important at the stage of monitoring their performance, but also at the manufacturing stage, characterize high-performance concrete. The aim of this study is to obtain new theoretical knowledge and experimental-applied dependencies arising from the “composition–microstructure–properties” ratio of high-strength concretes with a nano-modifying additive of the most effective type. The methods of laser granulometry and electron microscopy are applied. The existing concepts from the point of view of theory and practice about the processes of cement gel formation during the creation of nano-modified high-strength concretes with nano-modifying additives are developed. The most rational mode of the nano-modification of high-strength concretes is substantiated as follows: microsilica ground to nanosilica within 12 h. A complex nano-modifier containing nanosilica, superplasticizer, hyperplasticizer, and sodium sulfate was developed. The most effective combination of the four considered factors are: the content of nanosilica is 4% by weight of cement; the content of the superplasticizer additive is 1.4% by weight of cement; the content of the hyperplasticizer additive is 3% by weight of cement; and the water–cement ratio—0.33. The maximum difference of the strength characteristics in comparison with other combinations ranged from 45% to 57%.

Portland and Belite Cement Hydration Acceleration by C-S-H Seeds with Variable w/c Ratios

The acceleration of very early age cement hydration by C-S-H seeding is getting attention from scholars and field applications because the enhanced early age features do not compromise later age performances. This acceleration could be beneficial for several low-CO2 cements as a general drawback is usually the low very early age mechanical strengths. However, the mechanistic understanding of this acceleration in commercial cements is not complete. Reported here is a contribution to this understanding from the study of the effects of C-S-H gel seeding in one Portland cement and two belite cements at two widely studied water-cement ratios, 0.50 and 0.40. Two commercially available C-S-H nano-seed-based admixtures, i.e., Master X-Seed 130 and Master X-Seed STE-53, were investigated. A multi-technique approach was adopted by employing calorimetry, thermal analysis, powder diffraction (data analysed by the Rietveld method), mercury intrusion porosimetry, and mechanical strength determination. For instance, the compressive strength at 1 day for the PC (w/c = 0.50) sample increased from 15 MPa for the unseeded mortar to 24 and 22 MPs for the mortars seeded with the XS130 and STE53, respectively. The evolution of the amorphous contents was determined by adding an internal standard before recording the powder patterns. In summary, alite and belite phase hydrations, from the crystalline phase content evolutions, are not significantly accelerated by C-S-H seedings at the studied ages of 1 and 28 d for these cements. Conversely, the hydration rates of tetracalcium alumino-ferrate and tricalcium aluminate were significantly enhanced. It is noted that the degrees of reaction of C4AF for the PC paste (w/c = 0.40) were 10, 30, and 40% at 1, 7, and 28 days. After C-S-H seeding, the values increased to 20, 45, and 60%, respectively. This resulted in larger ettringite contents at very early ages but not at 28 days. At 28 days of hydration, larger amounts of carbonate-containing AFm-type phases were determined. Finally, and importantly, the admixtures yielded larger amounts of amorphous components in the pastes at later hydration ages. This is justified, in part, by the higher content of amorphous iron siliceous hydrogarnet from the enhanced C4AF reactivity.