This article serves to introduce the May 2004 issue of MRS Bulletin on Construction Materials: From Innovation to Conservation. By volume, building materials are by far the most widely used type of materials. The most common construction materials—concrete and wood—are paradigms of complex and hierarchical materials, with a microstructure extending quasi-continuously down to the nanoscale. In the past, most improvements have been obtained by modifying the microstructure at the largest scales, for instance, by reducing the macroporosity. Recent advances in our understanding of the interactions and microstructure development show that the major levers for improvement from now on will rely on surface and colloid science and the science of complex materials, often at the nanoscale. This can lead to remarkable properties, such as self-compaction and ultrahigh strength, and even new functionality, such as self-cleaning through photocatalysis. Construction materials face a wide range of challenges today, many of which are linked to the need for more sustainable development: reducing the consumption of raw materials, reducing the energy used in processing, and increasing service life. In many parts of the world, there is also an increasing need to repair, rehabilitate, and conserve old buildings. The articles in this issue touch on these challenges as well as the advances being made in construction materials through materials research.

The introduction of a new generation of dispersants in concrete allow this material to exhibit self-compacting properties in its fresh state and high durability and mechanical strength in its hardened state. These properties translate into many practical advantages for the construction field.Two of the most important are reducing the ecological impact of this sector of industry and reducing the labor-intensive work associated with placing ordinary concrete by vibration. In this article, it will be shown that knowledge of colloidal science has proven essential in the development of this new generation of dispersants for concrete. Indeed, the polymer molecules used in these dispersants are specifically designed to induce steric repulsion between cement particles, reducing their agglomeration and allowing high workability of fresh concrete prior to setting. While the linkage between interparticle forces and the rheological behavior of cement pastes is still only semiquantitative, recent advances in the modeling of concrete rheology show very promising results in terms of handling aggregates with a wide distribution of particle sizes and shapes. However, accurate modeling requires reliable input on the interaction of the dispersant with the hydrating cement at the molecular level, which is identified as a future research challenge.

Unlike other porous materials such as sandstone, brick, or porous glass, the interatomic bonding continuity of cement-based materials like concrete is far from obvious. When scrutinized at the micro- or nanoscopic level, the continuity of the ionic–covalent bonding in the solid phase is interrupted almost everywhere by water molecules or liquid water films. The same situation is found in set plaster.Yet, plaster and cementitious materials are able to withstand stresses of the same order of magnitude as rocks. Molecular simulation studies and direct-force measurements by atomic force microscopy provide strong arguments for predicting that short- and medium-range surface forces mediated by partially or totally hydrated calcium ions are the essential components of cement strength, with additional contributions from van der Waals and capillary forces. This provides a clue for understanding the nano- and mesostructure of cement-based materials and new levers for improving their properties.

Ultrahigh-performance concretes (UHPCs) are obtained by optimizing several technologies: minimizing the amount of water added, using superplasticizers and a wide particle size distribution, and packing the particles to improve fluidity with minimized water additions and to optimize load-carrying capacity. Fibers can be incorporated to increase ductility, leading to ultrahigh-performance fiber-reinforced concretes (UHPFRCs). Such enhanced concretes can approach the compressive strength of steel, with a remarkable improvement in durability. UHPCs offer new solutions for innovative construction, especially in aggressive environments.

While the primary function of concrete is structural, its pervasiveness in our society lends it to other functions and creates the need for it to maintain its integrity and aesthetic quality. Therefore, concrete with added functionality–for example, self-cleaning characteristics and the ability to remove pollutants–is desirable. Heterogeneous photocatalysis (e.g., gas–solid or liquid–solid catalytic processes caused by light irradiation) by semiconductor particles or coatings has now reached a high level of development and is a promising technology for the reduction of global environmental pollutants. Among the various semiconductor materials, TiO2 in the form of anatase has attracted wide interest, due to its strong oxidizing power under near-UV radiation, its chemical stability when exposed to acidic and basic compounds, its chemical inertness in the absence of UV light, and the absence of toxicity. TiO2 has proved very effective in the reduction of pollutants such as NOx, aromatics, ammonia, and aldehydes. Surprisingly, the use of TiO2 in combination with cementitious materials has shown a favorable synergistic effect in the reduction of pollutants. These new materials have already found relevant applications in self-cleaning building walls and in the reduction of urban pollutants.

The process of heating and compressing wood to improve its properties or reform it to a new shape has been known for decades. Such improvements are usually accompanied by “shape memory,” where the deformation produced by compression is not permanent, and the material recovers when re-moistened and heated. The combination of densification and a thermo-hydro-mechanical (THM) treatment can transform wood into a new material with improved mechanical properties, decreased sensitivity to moisture, increased durability, and no shape-memory effects. This article presents the principles of combined densification and THM processing, the products and experimental results, the origin of the shape-memory effect and its elimination by THM treatment, and the potential use of THM-processed densified wood in construction applications.

Earth construction is the oldest and most universal architectural expression on our planet. The most ancient human settlements were built with raw earth, and even today at least 30% of the world's population still lives in earthen homes. The conservation of earthen architecture depends of two aspects of a global strategy. One part involves intervention to preserve existing structures, and the second involves continuing to use the construction methods and materials in contemporary and future building practices. Traditionally, earth construction has been approached in an empirical way. Only recently have those involved in the field felt it necessary to develop a specific materials research attitude. This has resulted in major technological progress over the last 20 years. The wide range of building systems emphasizes the technological diversity of earth construction. This article presents a historical perspective, along with a review of the characteristics of earth as a building material, stabilization processes, and construction methods, and a report on an ongoing research program aimed at identifying and characterizing the fundamental binding mechanisms responsible for cohesion properties in earthen construction.