Science and Technology of Concrete Admixtures

Pierre-Claude Aïtcin, Robert J Flatt

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9780081006931
• eBook ISBN: 9780081006962
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Science and Technology of Concrete Admixtures presents admixtures from both a theoretical and practical point-of-view. The authors emphasize key concepts that can be used to better understand the working mechanisms of these products by presenting a concise overview on the fundamental behavior of Portland cement and hydraulic binders as well as their chemical admixtures, also discussing recent effects in concrete in terms of rheology, mechanics, durability, and sustainability, but never forgetting the fundamental role played by the water/binder ratio and proper curing in concrete technology.Part One presents basic knowledge on Portland cement and concrete, while Part Two deals with the chemical and physical background needed to better understand what admixtures are chemically, and through which mechanism they modify the properties of the fresh and hardened concrete. Subsequent sections present discussions on admixtures technology and two particular types of concrete, self-consolidating and ultra-high strength concretes, with final remarks on their future.- Combines the knowledge of two leading authors to present both the scientific and technology of admixtures- Explains what admixtures are from a chemical point-of-view and illustrates by which mechanisms they modify the properties of fresh and hardened concrete- Presents a fundamental, practical, and innovative reference book on the topic- Contains three detailed appendices that can be used to learn how to use admixtures more efficiently

Part One

Theoretical background on Portland cementand concrete

Outline

1. The importance of the water–cement and water–binder ratios

2. Phenomenology of cement hydration

3. Portland cement

4. Supplementary cementitious materials and blended cements

5. Water and its role on concrete performance

6. Entrained air in concrete: rheology and freezing resistance

7. Concrete rheology: a basis for understanding chemical admixtures

8. Mechanisms of cement hydration

1

The importance of the water–cement and water–binder ratios

P.-C. Aïtcin Université de Sherbrooke, QC, Canada

Abstract

The concept of the water–cement and water–binder ratios is the base of concrete technology. This chapter explains the physical meaning of the water–cement (w/c) and water–binder (w/b) ratios, the nature of Portland cement, and how it hydrates. The numerical values of the w/c and w/b ratios are directly related to the distance separating the cementitious particles in a cement paste when the hydration process starts. These ratios directly impact the ultimate porosity, density, and mechanical properties of concrete. The lower these ratios are, the stronger, more durable, and more sustainable the concrete will be. Therefore, superplasticizers, which allow the production of concrete with lower water content without compromising workability, have opened the doors to radical improvements in concrete performance. In such concrete, there is not enough water for complete hydration, so compressive strength continues to increase as the w/c or w/b ratio decreases.

Keywords

Concrete durability; Concrete sustainability; Hydration; Superplasticizers; Water–binder ratio; Water–cement ratio

1.1. Introduction

It may be surprising to start a book on the science and technology of concrete admixtures by a chapter devoted to the importance of the water–cement (w/c) and the water–binder (w/b) ratios, before even explaining what Portland cement is and how it reacts with water. We agree with Kosmatka (1991) that the w/c and w/b ratios are the most important characteristics of concrete; they govern its properties in the fresh and hardened states and also its durability. It is therefore fundamental to understand their profound significance in order to optimize the use of concrete with the help of admixtures and thus to maximize its economic value while minimizing at the same time its carbon footprint.

Despite what many cement chemists think, the most important parameters that control concrete compressive strength are not:

• the cement dosage expressed in kg/m³

• the strength of the small cubes used to test the “cement compressive strength”

• the C₃S and C₃A content of the clinker

• the fineness of the cement, or

• the “gypsum” content.

In fact, the most important parameters have always been the w/c and w/b ratios, as was found in the nineteenth century by Féret (1892) for cement pastes and later on by Abrams, 1918 for concrete. As will be shown later, these ratios control the microstructure of the cement paste in both the fresh and hardened states, and consequently its rheology, mechanical properties, permeability, durability, and sustainability.

When batching concrete, each of its solid components is weighed; thus, it is common to express the w/c ratio as a mass ratio instead of a volumetric ratio. In this book, the symbols w/c and w/b will be used for the mass ratio, whereas W/C and W/B will be used for the volumetric ratios, according to American Concrete Institute (ACI) terminology. Note that the RILEM (Rassemblement International des Laboratoires d'Essais sur les Matériaux) terminology is the opposite.

It is always possible to transform the mass ratio into a volumetric one by taking into account the specific gravity of the cement. In this book, the theoretical specific gravity of Portland cement is taken as 3.14, which is an easy number to remember and a number very close to that of an actual “pure” Portland cement. The specific gravity of a blended cement containing a filler or supplementary cementitious materials with specific gravities different from 3.14 will be usually less than 3.14. In this chapter, it will be shown that w/c and w/b ratios are more than simple abstract numbers with an inverse relationship to concrete compressive—they do have a physical meaning.

1.2. The hidden meaning of the w/c

Using a sophisticated three-dimensional model, Bentz and Aïtcin (2008) demonstrated that the w/c ratio is a number directly related to the average distance between cement particles within a cement paste after it is mixed with water just before it begins to hydrate. This distance between the cement and/or binder particles influences the hydration conditions and the microstructure of the hardened cement paste and, consequently, its mechanical properties and durability.

To illustrate this fundamental concept, we will instead use a very simple two-dimensional (2D) model to provide a qualitative explanation of the meaning of the w/c. Consider four circular cement particles having a radius a that are placed at the corners of a square with a side equal to 3a, as represented in Figure 1.1. The minimum distance between these cement particles is their distance along the sides of the square: it is equal to a.

This arrangement of the cement particles can be represented by the unit cell shown in Figure 1.2.

Now calculate the W/C of this unit cell. The surface area of the unit cell is 3a × 3a = 9a². The surface of the cement particles is 4 × ¼ (3.14 × a²) = 3.14a².

The mass of the cement part...

Table of contents

• Cover image
• Title page
• Table of Contents
• Related titles
• Copyright
• About the contributors
• Woodhead Publishing Series in Civil and Structural Engineering
• Preface
• Acknowledgments
• Introduction
• Terminology and definitions
• Glossary
• Historical background of the development of concrete admixtures
• Part One. Theoretical background on Portland cementand concrete
• Part Two. Chemistry and working mechanisms
• Part Three. The technology of admixtures
• Part Four. Special concretes
• Part Five. The future of admixtures
• Appendix 1. Useful formulae and some applications
• Appendix 2. Experimental statistical design
• Appendix 3. Statistical evaluation of concrete quality
• Index