关于胶凝C-S-H纳米颗粒的形成外文翻译资料

 2022-07-31 20:27:11

J Computer-Aided Mater Des

DOI 10.1007/s10820-006-9030-0

On the formation of cementitious C–S–H nanoparticles

H. Manzano · A. Ayuela · J. S. Dolado

Received: 3 June 2006 / Acceepted: 1 September 2006

copy; Springer Science Business Media B.V. 2007

Abstract This work explores from a theoretical viewpoint the underlying growth mechanisms which govern the formation of the most important hydration product present in cementitious environments, the so called C–S–H (calcium–silicate–hydrate) gel. Aiming at identifying the basic building blocks which make up the cementitious C–S–H gel, we have performed ab-initio calculations at the Hartree Fock (HF) level. Two different growth mechanisms have been identified depending on the amount of Si and Ca ions, which naturally lead to the appearance of tobermorite-like and jennite-like nano-crystals.

Keywords Cement-based materials · C–S–H gel · Tobermorite · Jennite · ab-initio calculations · Self-assembly

1 Introduction

The chemistry involved in the formation of cement-based materials is certainly complex and appealing. Just by pouring cement powder to water, a myriad of chem-ical reactions take place to form a rigid, complex and porous structure that is called cement paste. This cement paste is a multiphase material [1] which evolves over

H. Manzano · J. S. Dolado

Labein Centro Tecnoloacute;gico, C/Geldo Ed. 700,

Parque Tecnoloacute;gico de Bizkaia, 48160 – Derio, Bizkaia, Spain

A. Ayuela

Departamento de Fiacute;sica de Materiales, Facultad de Quiacute;micas,

Universidad del Paiacute;s Vasco, Unidad de Fiacute;sica de Materiales, Centro Mixto CSIC-UPV/EHU and Donostia International Physics Center (DIPC),

P.O. Box 1072, 20018 San Sebastian/Donostia, Spain

J. S. Dolado (B)

Nanostructured and Eco-efficient Materials for Construction Unit, Associated Unit LABEIN-Tecnalia/CSIC, 48160 Derio, Bizkaia, Spain e-mail: jsanchez@labein.es

H. Manzano et al.

time. Basically it can be viewed as a composite in which calcium hydroxide crystals (portlandite), aluminates, and non reacted cement powders are embedded into an amorphous nanostructured hydration product, the so-called C–S–H gel (Calcium sil-icate hydrate1). This gel is the most important hydration product of cement based materials. It constitutes about 60–70% of the fully hydrated cement paste, and is responsible for most of the properties of cement-based materials. From a composi-tional point of view, the C–S–H gel is often characterised by its Ca/Si ratio, which turns to be in the range that goes from 0.7 to 2.3. This variable composition2 explains why, though intensively characterised by techniques like SEM, TEM, NMR, etc [2], many features of the nanostructure of C–S–H gel remain unravelled.

Much of the existing knowledge on the nanostructure of C–S–H gel has been gained from structural comparisons with crystalline calcium silicate hydrates. In fact, several models have been proposed so far [1, 3–6] that draw structural analogies with toberm-orite and jennite crystals. Schematic representations of both crystals are depicted in Fig. 1a. From these models, C–S–H gel can be approximately viewed as a layered structure, in which calcium oxide sheets are ribbed on either side with silicate chains, and free calcium ions and water molecules are present in the interlayer space. This viewpoint based on layered crystal-like structures which resemble clay morphologies has also served as starting point for the few existing computational works which deal with the C–S–H gel. Most of them [7–9] have paid attention to the study of the cohe-sion of the C–S–H layers. Basically they have assumed structureless charged platelets of C–S–H embedded in a counterion-rich electrolyte solution, being the interlayer forces simulated either within the framework of the Deryaguin–Landau–Verwey– Overbeek (DLVO) theory [10] or by Monte Carlo simulations. The still fewer studies that account for atomistic level descriptions of the C–S–H gel [11–13] have similarly rested on crystalline tobermorite structures.

However, experimental evidence shows [2] that if the structure of the C–S–H gel is actually composed of tobermorite and jennite pieces, these ones should show multiple defects and imperfections. For instance, 29Si NMR experiments have revealed [4, 17] that only finite silicate chain lengths are known to exist. In fact, current state of the art admits that the C–S–H gel present at early ages could be a mixture of tobermorite-type and jennite-type dimeric structures (hereafter denoted by T2 and J2 respectively3), whereas that formed later approaches more to longer jennite-type morphologies [18]. Moreover, a recent model [19] has achieved to rationalize many scattered measure-ments regarding physical properties such as densities, porosities, or surface areas by assuming that the C–S–H gel is built up by aggregation of about 2-nm-sized particles. In this context it is clear that a much deeper knowledge on the processes that occur at the nanoscale of the C–S–H particles is needed. It seems to us that, complementarily to the approaches that resort on comparisons with crystalline species, a new one based on a complete bottom-up vision is required.<!--

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