Resistance of Pastes from Carbonated, Low-Lime Calcium Silica Cements to External Sulfate Attack.

This paper presents the results of a study on the evaluation of resistance of pastes from carbonated, low-lime calcium silica cements to external sulfate attack. The extent of chemical interaction between sulfate solutions and paste powders was assessed by quantifying the amount of species that leached out from carbonated pastes using ICP-OES and IC techniques. In addition, the loss of carbonates from the carbonated pastes exposed to sulfate solutions and the corresponding amounts of gypsum formed were also monitored by using the TGA and QXRD techniques. The changes in the structure of silica gels were evaluated using FTIR analysis. The results of this study revealed that the level of resistance of carbonated, low-lime calcium silicates to external sulfate attack was affected by the degree of crystallinity of calcium carbonate, the type of calcium silicate, and the type of cation present in the sulfate solution.

Additive Manufacturing and Performance of Architectured Cement-Based Materials.

There is an increasing interest in hierarchical design and additive manufacturing (AM) of cement-based materials. However, the brittle behavior of these materials and the presence of interfaces from the AM process currently present a major challenge. Contrary to the commonly adopted approach in AM of cement-based materials to eliminate the interfaces in 3D-printed hardened cement paste (hcp) elements, this work focuses on harnessing the heterogeneous interfaces by employing novel architectures (based on bioinspired Bouligand structures). These architectures are found to generate unique damage mechanisms, which allow inherently brittle hcp materials to attain flaw-tolerant properties and novel performance characteristics. It is hypothesized that combining heterogeneous interfaces with carefully designed architectures promotes such damage mechanisms as, among others, interfacial microcracking and crack twisting. This, in turn, leads to damage delocalization in brittle 3D-printed architectured hcp and therefore results in quasi-brittle behavior, enhanced fracture and damage tolerance, and unique load-displacement response, all without sacrificing strength. It is further found that in addition to delocalization of the cracks, the Bouligand architectures can also enhance work of failure and inelastic deflection of the architectured hcp elements by over 50% when compared to traditionally cast elements from the same materials.

Cements in the 21st Century: Challenges, Perspectives, and Opportunities.

In a book published in 1906, Richard Meade outlined the history of portland cement up to that point1. Since then there has been great progress in portland cement-based construction materials technologies brought about by advances in the materials science of composites and the development of chemical additives (admixtures) for applications. The resulting functionalities, together with its economy and the sheer abundance of its raw materials, have elevated ordinary portland cement (OPC) concrete to the status of most used synthetic material on Earth. While the 20th century was characterized by the emergence of computer technology, computational science and engineering, and instrumental analysis, the fundamental composition of portland cement has remained surprisingly constant. And, although our understanding of ordinary portland cement (OPC) chemistry has grown tremendously, the intermediate steps in hydration and the nature of calcium silicate hydrate (C-S-H), the major product of OPC hydration, remain clouded in uncertainty. Nonetheless, the century also witnessed great advances in the materials technology of cement despite the uncertain understanding of its most fundamental components. Unfortunately, OPC also has a tremendous consumption-based environmental impact, and concrete made from OPC has a poor strength-to-weight ratio. If these challenges are not addressed, the dominance of OPC could wane over the next 100 years. With this in mind, this paper envisions what the 21st century holds in store for OPC in terms of the driving forces that will shape our continued use of this material. Will a new material replace OPC, and concrete as we know it today, as the preeminent infrastructure construction material?