Title 40 of the Code of Federal Regulations is a compendium of rules pertaining to environmental protection, written primarily by the Environmental Protection Agency (EPA). These environmental regulations stand some 19 inches wide on a library shelf and are enclosed by 15 volumes of everchanging information. Each of these volumes contains amendments made to the rules since the last publication of that volume, so the effective dates of new regulations can be difficult to ascertain. Interpreting certain regulations has been difficult for many, the cost of complying with them high, and the eventual impact on the national economy foreseen as gloomy. Few, however, would deny the benefits of a well-planned environmental protection plan and few would sympathize with private corporations that flagrantly ignore regulations, pollute their local environment and, when identified, sustain heavy fines from the EPA.

With problems like acid rain, air and groundwater pollution, global warming, ecological system degradation, municipal, industrial, and nuclear waste (and the federal regulations designed to control them), the need for important materials science research and development is great. These opportunities can be compared in magnitude with those provided by the Manhattan Project, during the 1940s and 1950s, and by the semiconductor revolution in later years. The difference between these past projects and the current situation is the way in which funds are appropriated for the research and how the work is performed. Rather than independently funded research performed by multiple organizations, work on environmental problems is witnessing cooperation among industry, government, academia, and the public, with a consequent savings of research monies. This cooperation is often achieved with a consortium of organizations such as SEMATECH or the National Center for Manufacturing Sciences.

Concrete is the most widely used man-made product; in the United States we use about two tons per year per person. It has been estimated that the replacement value of concrete structures is over six trillion dollars, and that over one trillion dollars needs to be spent in the repair and rehabilitation of these structures over a 20-year period. By contrast, the resources devoted to the materials issues of the industry have been meager. This neglect may in part explain why the development of the science of cementitious materials had lagged behind that of other materials. The situation has been changing rapidly in recent years, however, and the articles in this issue have been selected to illustrate some of the advances that are being made.

One key to the resurgence in research in cement-based materials is the realization of the pressing need to upgrade our deteriorating infrastructure. Another is the realization that a materials science and engineering approach is needed to provide a solid knowledge base to underpin new technologies. Chemistry took us a long way in the 1950s and 1960s, but an appreciation of how chemistry and processing could combine to create desirable micro-structures that would achieve desirable bulk properties came much later. It is significant that, with the noticeable exception of the Pennsylvania State University, cement research in universities in the United States previously was funded through civil engineering departments. Similarly, in Europe and Japan, research was conducted primarily in engineering faculties, although there were programs in a few chemistry departments. Now, however, on both sides of the Atlantic Ocean, research groups are well established in materials science departments. It is noteworthy that on an NSF Science and Technology Center devoted to advanced cement-based materials is funded through the Division of Materials Research.

Many of the useful properties of Portland cement concrete stem from chemical reactions between cement and water, which cause the slow transformation of the initial fluid suspension into a rigid solid. Because this transformation does not begin immediately, concrete may be processed—mixed, transported, consolidated into a mold, and given a smooth surface — before it becomes rigid (i.e., sets), which typically occurs a few hours after initial mixing. As a result of the need to maintain a workable suspension during processing, the flow properties of cementitious systems have seen considerable attention. However, control of workability remains poorly understood, in part because the relationships between flow and process-ability are not clear. Recent advances have been made in understanding the flow of nonreactive suspensions, and the objective of this article is to explore how these advances provide new insights concerning the behavior of cementitious materials.

There are two motivations driving investigations of cement rheology. The first is that understanding flow properties is the key to better processing. The second is that paste rheology can be used to gain a fundamental understanding of the initial cement microstructure and of the variables influencing early cement hydration reactions.

It has been well-recognized for many years that the particle-size distributions of the cement and the grading of the aggregates play an important role in determining the properties and characteristics of cement and concrete products. DSP (densified with small particles) type cements and concretes, to a certain extent, MDF (macro-defect-free) cements, and optimized concretes are recently recognized outstanding examples of the application of this principle. The preset characteristics of the cementitious slurry are also strongly influenced by these factors. Both the workability of the fresh material, and the microstructure development are controlled to a considerable extent by these geometric parameters.

Two seminal works in the areas of continuous particle size distributions and particle packing are those of Andreason and Furnas, respectively. Furnas deals mainly with discrete systems and Andreason with continuous distributions. As early as 1907, the concept of idealized particle packing was being used to optimize cements and concretes. Figure 1a shows an idealized cross section of a simple cubic packing of monodispersed spheres. This system has a maximum packing density of 0.65%. In an ideally packed system of discrete size ranges, the size of the next smallest particles would be such that they just fit in the gaps between the largest size particles, and so on for subsequent particle sizes; this system is represented schematically in Figure 1b. Not only the sizes but also the relative numbers of particles are important; Figures 1c and 1d show systems where some fraction of the smaller and larger particle sizes, respectively, are missing. Figure 1e shows a system where the size of the second largest particles is too large to fit into the gaps between the largest particles, resulting in a lower packing efficiency. Thus, both the particle size and fractions are important when considering packing efficiency.

Cement-based materials are random composite materials over many length scales. In common with any random material, the two main problems that must be solved to obtain quantitative theoretical understanding of microstructure-property relationships are: (1) what actually is the microstructure, and (2) given enough knowledge of a micro-structure, how can properties be calculated?

In the past few decades, physicists and chemists have studied microstructure/property relationships for atomically disordered materials like semiconductor and metallic glasses, while mechanical engineers have done the same for continuum problems of macroscopic inclusions of one material distributed in a matrix of a second material. With the large memories and fast computational speeds of late 20th-century computers, it is now possible to attack problems that lie between these two extremes, namely random grain problems like those of cement-based and ceramic materials. For these kinds of materials, the important randomness for most problems is at the grain or particle level, not the atomic level, and is complicated by the fact that their microstructure changes markedly on time scales of hours and days, either due to sintering (ceramics), or hydration reactions (cement-based materials). The following sections of this paper describe how microstructures are simulated and properties like electrical conductivity and elastic moduli are computed-mostly for cement-based materials, but with some mention of parallel work on ceramic materials.

The characterization of fracture behavior is a continuing challenge to the cement and concrete community. The performance of a material can be evaluated by its stress-strain response. For an ideally brittle material, elastic response is terminated when stress suddenly drops to zero, as shown in Figure 1a. However, cement-based materials are considered quasi-brittle because they respond nonlinearly prior to peak stress, and their stress gradually decreases after reaching a peak, as indicated in Figure 1b.

To make cement-based materials stronger and tougher, one needs to understand the fracture mechanisms associated with nonlinear stress-strain behavior and to characterize material fracture properties based on these fracture mechanisms. Three novel techniques are being used at the Center for Advanced Cement-Based Materials (ACBM) to detect the quasi-brittle nature of cement-based materials. These three techniques are laser holographic interferometry, acoustic emission, and microscopic surface analysis. This article summarizes both the fracture mechanisms in cement-based materials and the application of the three techniques to characterize and measure fracture properties.

The infrastructures of the United States and other industrialized nations are, to a large degree, based upon concrete. With such a pervasive application, the importance of accurately analyzing concrete structure failures and addressing durability problems is obvious, as is the timely recognition of premature structure deterioration, so that an appropriate recommendation for remediation may be made. Although concrete is one of the most important engineering materials in use today, it is perhaps also the least understood, primarily because it is chemically reacting and changing throughout its service life. The chemical and physical changes that occur in this material can have degenerative effects on its mechanical properties and, ultimately, on its durability. Understanding how these chemical reactions occur within the concrete and affect its physical characteristics is often not straightforward. For example, the reactions can be affected by internal components (cement, fine and coarse aggregate), mixing procedures, curing cycle parameters, and/or the choice of additives. External influences on these reactions include weather exposure, chemically aggressive environments, and climate-mitigating surface treatments. Furthermore, there are some deleterious internal reactions that tend to occur sequentially and others that may occur concurrently. The overlap of simultaneous reactions can make it difficult to isolate the initial occurrence and to identify the deleterious component of deterioration. To resolve such questions, it is necessary to understand the interrelationship among the concrete components and their processing, and to examine the material with a variety of complementary techniques and sample configurations.

Portland cement and Ca(OH)2, “slaked lime,” are representative of a family of materials that have conceptual and practical advantages as matrices for the immobilization of wastes. These are relatively proven construction materials with an extensive history of use in various ground-water regimes, and in various climates. The raw materials are widely available and they exhibit a reasonable amount of resistance to physical damage and attrition. They are also durable. Although modern Portland cements are only about 150 years old, numerous examples of Roman concrete made from slaked lime and volcanic pozzolanic ash survive; the Pantheon in Rome (circa A.D. 300) continues in regular use.

Satisfactory immobilization matrices can also be formed by incorporating other artificial waste materials, e.g. coal combustion fly ash, blast furnace slag, and natural materials such as calcined kaolin or siliceous volcanic ashes containing glass and zeolites. These materials react with Ca(OH)2 and cement to become an integral part of a cement matrix. Thus one waste (ash, slag) can be used to help immobilize another, and may actually improve the retentive capacity of a cementitious matrix.

Since cements require water to harden, they are tolerant of wet wastes and are especially useful for many effluents. In addition, they are low-cost, nonflammable materials with the potential to provide chemical and physical immobilization of wastes, but their set characteristics can be affected by certain material additions. Table I balances some advantages against disadvantages of the use of cement systems for waste immobilization.

The term “macro-defect-free” refers to the absence of relatively large voids (or defects) that are normally present in conventional cement pastes due to entrapped air or inadequate mixing. A decade ago, Birchall and co-workers developed a novel processing method that avoids the formation of these strength-limiting defects. This method, outlined schematically in Figure 1, consists of mixing hydraulic cement powder, a water-soluble polymer, and a minimal amount of water under high shear to produce a macro-defect-free (MDF) cement composite. Several cement/polymer systems can be processed by this flexible technique, although the calcium aluminate cement/polyvinyl alcohol-acetate (PVA) copolymer system is most common: MDF cements display unique properties relative to conventional cement pastes. For example, the flexural strength of MDF cement is more than 200 MPa as compared to values on the order of 10 MPa for conventional pastes. One can view MDF cements as a type of “inorganic plastic.” As is the case with plastic processing, fillers such as alumina, silicon carbide, or metal powders can be added to MDF cement to modify its performance properties (e.g., abrasion resistance, thermal or electrical conductivity, and hardness). The combined attractiveness of inexpensive raw materials and flexible, low-temperature processing has generated great interest in this new class of advanced cement-based materials.