Robert J. Flatt

Conformation of Adsorbed Comb Copolymer Dispersants

Comb copolymers with an adsorbing backbone and nonadsorbing side chains can be very effective dispersants, particularly when a high ionic strength strongly penalizes electrostatic stabilization. For this reason, they have become essential components of concrete over the past decade. This article examines the steric hindrance characteristics of such polymers through the use of atomic force microscopy (AFM) on calcium silicate hydrate, the main hydration product of Portland cement. It is found that solution and surface properties (hydrodynamic radius, radius of gyration, surface coverage, steric layer thickness) and force-distance curves obtained during AFM measurements can be well described by a scaling approach derived in this paper. This represents the first real quantitative step in relating these properties directly to the molecular structure of such comb copolymer dispersants.

Complex concrete structures

Over the course of the 20th century, architectural construction has gone through intense innovation in its material, engineering and design, radically transforming the way buildings were and are conceived. Technological and industrial advances enabled and challenged architects, engineers and constructors to build increasingly complex architectural structures from concrete. Computer-aided design and manufacturing (CAD/CAM) techniques have, more recently, rejuvenated and increased the possibilities of realizing ever more complex geometries. Reinforced concrete is often chosen for such structures as almost any shape can be achieved when placed into a formwork. However, most complex forms generated with these digital design tools bear little relation to the default modes of production used in concrete construction today. A large gap has emerged between the possibilities offered by the digital technology in architectural design and the reality of the building industry, where actually no efficient solutions exist for the production of complex concrete structures. This paper presents construction methods that unfold their full potential by linking digital design, additive fabrication and material properties and hence allow accommodating the construction of complex concrete structures. The emphasis is set on the on-going research project Smart Dynamic Casting (SDC) where advanced material design and robotic fabrication are interconnected in the design and fabrication process of complex concrete structures. The proposed fabrication process is belonging to an emerging architectural phenomenon defined first as Digital Materiality by Gramazio & Kohler (2008) or more recently as Material Ecologies by Neri Oxman? 1. An overview is given that combine existing casting techniques with digital fabrication for the fabrication of complex concrete structures.The focus is set on Smart Dynamic Casting a technique that combines digital fabrication with slipforming and building material science.An overview of the experimental set up and procedure is given.Experimental prototype results are described.

Molecular design of comb-shaped polycarboxylate dispersants for environmentally friendly concrete

Concrete is the most widely used material in the world and, because of the large volume used, the production of cement, the main component of concrete, is responsible for a high CO2 emission. To reduce the quantity of CO2 emitted, one solution is to substitute a part of cement by supplementary cementitious materials, SCMs, such as fly ash. Because fly ash is largely inert in the first days of blended cement hydration, it is necessary to accelerate its dissolution by physical or chemical means to compensate the loss of mechanical strength in the early stage. The solution studied in this project is the alkaline activation by addition of NaOH that prevents the dispersive effect of PCE superplasticizers used in modern concrete with a low content of water. In this work, we investigated the influence of NaOH on hydration, rheology and mechanical strength of superplasticized blended cementitious systems. From the results and theoretical aspects of polymer adsorption, a simple criterion was established that defines which polymer structures are or are not compatible with alkaline activated systems.

Chemo-mechanics of salt damage in stone

Many porous materials are damaged by pressure exerted by salt crystals growing in their pores. This is a serious issue in conservation science, geomorphology, geotechnical engineering and concrete materials science. In all cases, a central question is whether crystallization pressure will cause damage. Here we present an experiment in which the crystallization pressure and the pore saturation are varied in a controlled way. We demonstrate that a strain energy failure criterion can be used to predict when damage will occur. The experiment considered is the most widely used means to study the susceptibility to salt crystallization, so quantification of this test has far-reaching implications.

Working mechanisms of water reducers and superplasticizers

Abstract The yield stress of cement suspensions depends on the balance between attractive and repulsive interparticle forces on the one hand and shear forces on the other hand. The effect of superplasticizers on yield stress comes from introducing a repulsive interparticle force that reduces the overall attractive force between cement particles, and consequently the yield stress. Early-generation superplasticizers were believed to disperse by inducing electrostatic repulsion, but because of their high ionic strength, cement suspensions can hardly be stabilized by electrostatic repulsion. The data presented here suggest that it is rather steric repulsion that must be considered at full surface coverage. Polycarboxylate ether or PCE superplasticizers are broadly accepted to act through steric hindrance. Here we describe the role of the molecular structure in the dispersive effect under conditions of full and incomplete surface coverage.

INTERACTION OF SUPERPLASTICIZERS WITH MODEL POWDERS IN A HIGHLY ALKALINE MEDIUM.

Synopsis: It is broadly recognized that the adsorption of superplasticizers on cement particles is a key factor in determining the rheology of concrete. In order to avoid the problems linked to the hydration of cement, the adsorption of superplasticizers is often studied on unreactive model powders. However, in order for the model system to remain as close as possible to cement, the surface should have a similar charge and a similar chemical nature. Furthermore, the pH of the solution should be close to that of the hydrating cement (about 12.5). Under these conditions, cement has been shown to have a positively charged surface. The model powders used in this study were Mg(OH) 2 and dead burnt MgO, which have nominal isoelectric points of 12.0 and 12.4 respectively, and which are chemically similar to Ca(OH) 2 and CaO. The surface charge of such model suspensions was studied as a function of added superplasticizer. These were either commercially available or currently under development, ranging from strongly to very weakly ionic. Adsorption isotherms for two polymeric superplasticizers, with similar structures but with different ionic group spacing, have been measured for both MgO and Mg(OH) 2 at pH 12 and 11.3 respectively and between 10 and 40°C. Results showed a strong temperature dependence for the adsorption of the less ionic polymer on MgO.

Chemistry of chemical admixtures

Chemical admixtures are nowadays very important for concrete design. This chapter presents an overview of the chemical structures of different organic chemical admixtures, ranging from small organic compounds to large polymers having a certain polydispersity, and of both natural and synthetic origin. The choice is guided by the fact that this is where the real added value of molecular structure comes into play in terms of design of new or modified chemical admixtures. Such admixtures offer the greatest possibility to chemists to modify properties and target improved performance by specific exploitation of structure/property relationships. The overview gives a basis for better understanding of the working mechanisms of these admixtures.

Adsorption of chemical admixtures

Abstract The properties of most of admixtures come from their ability to adsorb on the surface of particles (as observed for water reducers, superplasticizers, and retarders) or on the liquid–vapor interface (e.g., for air-entraining or shrinkage-reducing admixtures). Their adsorption behavior depends not only on their chemical composition, molecular structure, and dosage but also on the characteristics of the adsorbent surface and the composition of the liquid phase. This is particularly important during cement hydration, when the formation of new phases and an evolving ionic activity may alter strongly adsorption efficiency of admixtures. In practice, the determination of an adsorption isotherm allows for comparing the adsorption behavior of admixtures. The most common way to quantify the amount of adsorbed molecules is the solution depletion method. This method and its limitations, as well as the modeling of the adsorption isotherm, are deeply discussed to show that adsorption measurements and their interpretation are not trivial.

Impact of chemical admixtures on cement hydration

Abstract Many chemical admixtures are known to retard cement hydration. This is an intentional effect of retarders. However, for many other admixtures such as superplasticizers, it is mostly an undesired side effect that becomes more and more problematic in modern concrete with reduced clinker content. In this chapter, the general mechanisms that affect the hydration processes and the behavior of most chemical admixtures are described. This includes possible modification of the rates of the dissolution, nucleation, and/or growth of various phases as well as destabilization of the silicate–aluminate–sulfate balance. Because of the importance of polycarboxylate ether (PCE) superplasticizers and the flexibility in their molecular design, a special section presents information concerning their impact on hydration. Finally, sugars are discussed because they are the most used set retarders and that important steps have been made in studying their impact on retardation.

A force field for tricalcium aluminate to characterize surface properties, initial hydration, and organically modified interfaces in atomic resolution

Tricalcium aluminate (C3A) is a major phase of Portland cement clinker and some dental root filling cements. An accurate all-atom force field is introduced to examine structural, surface, and hydration properties as well as organic interfaces to overcome challenges using current laboratory instrumentation. Molecular dynamics simulation demonstrates excellent agreement of computed structural, thermal, mechanical, and surface properties with available experimental data. The parameters are integrated into multiple potential energy expressions, including the PCFF, CVFF, CHARMM, AMBER, OPLS, and INTERFACE force fields. This choice enables the simulation of a wide range of inorganic-organic interfaces at the 1 to 100 nm scale at a million times lower computational cost than DFT methods. Molecular models of dry and partially hydrated surfaces are introduced to examine cleavage, agglomeration, and the role of adsorbed organic molecules. Cleavage of crystalline tricalcium aluminate requires approximately 1300 mJ m(-2) and superficial hydration introduces an amorphous calcium hydroxide surface layer that reduces the agglomeration energy from approximately 850 mJ m(-2) to 500 mJ m(-2), as well as to lower values upon surface displacement. The adsorption of several alcohols and amines was examined to understand their role as grinding aids and as hydration modifiers in cement. The molecules mitigate local electric fields through complexation of calcium ions, hydrogen bonds, and introduction of hydrophobicity upon binding. Molecularly thin layers of about 0.5 nm thickness reduce agglomeration energies to between 100 and 30 mJ m(-2). Molecule-specific trends were found to be similar for tricalcium aluminate and tricalcium silicate. The models allow quantitative predictions and are a starting point to provide fundamental understanding of the role of C3A and organic additives in cement. Extensions to impure phases and advanced hydration stages are feasible.