Concrete has played a pivotal role in shaping the modern world’s infrastructure and the built environment. Its unparalleled versatility, durability, and structural integrity have made it indispensable in the construction industry. From skyscrapers to long-span bridges, water reservoirs, dams, and highways, the ubiquitous presence of concrete in modern society underscores its significance in global development. As we stand at the crossroads of environmental awareness and the imperative to advance our societies, the sustainability of concrete production and utilization is becoming a new engineering paradigm. The immense demand for concrete, driven by urbanization and infrastructure development, has prompted a critical examination of its environmental impact. One of the most pressing concerns is the substantial carbon footprint associated with traditional concrete production. The production of cement, a key ingredient in concrete, is a notably energy-intensive process that releases a significant amount of carbon dioxide (CO2) into the atmosphere. As concrete remains unparalleled in its ability to provide structural functionality, disaster resilience, and containment of hazardous materials, the demand for concrete production is increasing, while at the same time, the industry is facing the urgency to mitigate its ecological consequences. This special publication investigates the multi-faceted realm of concrete sustainability, exploring the interplay between its engineering properties, environmental implications, and novel solutions, striving to provide an innovative and holistic perspective. In recent years, the concrete industry has witnessed a surge of innovation and research aimed at revolutionizing its sustainability. An array of cutting-edge technologies and methodologies has emerged, each offering promise in mitigating the environmental footprint of concrete. Notably, the integration of supplementary cementitious materials, such as calcined clays and other industrial byproducts, has gained traction to reduce cement content while enhancing concrete performance. Mix design optimization, coupled with advanced admixtures, further elevates the potential for creating durable, strong, and eco-friendly concrete mixtures. Concrete practitioners will gain an advanced understanding of a wide variety of strategies that are readily implementable and oftentimes associated with economic savings and durability enhancement from reading these manuscripts. The incorporation of recycled materials, such as crushed concrete and reclaimed aggregates, not only reduces waste but also lessens the demand for virgin resources. Furthermore, the adoption of efficient production techniques, along with the exploration of carbon capture and utilization technologies, presents an optimistic path forward for the industry. This special publication aspires to contribute to the ongoing discourse on concrete sustainability, offering insights, perspectives, and actionable pathways toward a more environmentally conscious future.

In the late 1980s, an innovative hydration-stabilizing admixture was introduced to help concrete producers effectively extend the working time of fresh concrete mixtures for challenging applications, particularly, in hot weather or long time-to-discharge applications. The hydration-stabilizing admixture also provided concrete producers with a means of managing returned concrete to address environmental issues associated with concrete waste. In recent years, admixtures that allow concrete producers to convert returned concrete into a very low-strength granular material that can be used for construction backfill, road base, or in other applications have been introduced. Together with the hydration-stabilizing admixture, concrete producers can now use chemical admixtures to significantly reduce concrete waste. In this paper, the operational and sustainability benefits of the hydration-stabilizing admixture and a new one-component engineered polymer admixture that facilitates the beneficial reuse of returned fresh concrete are presented and discussed.

Entraining tiny and stable bubbles into cementations mixtures and concrete is becoming more and more important with the complex composition of cement and concrete. Surfactants as air-entraining agents are important concrete admixtures that intentionally create a number of functional air voids in concrete. In this study, nonionic surfactants appear to be a stabilizing agent for ionic surfactants to improve the bubble stability in fresh concrete. The surface tensions and foam properties of their solutions, and the air contents and bubble size distribution of the fresh cement mortars were determined. The results show that nonionic surfactants are introduced into the interface for co-assembly, the electrostatic repulsion between ionic surfactant molecules is effectively diminished and making the arrangement on the interface more stable. The blend of nonionic and ionic surfactants induced smaller bubble formation in aqueous solutions, which also have increased bubble stability in cement mortars. So, it is of great practical significance to blend nonionic and ionic surfactants to improve the air-void stability in concrete.

A special concrete was used to erect the MAXXI building in Rome designed by Zaha Hadid and her team with long, inclined, curvilinear walls. Due to the very congested reinforcements, the original concrete issued by Zaha Hadid and her team was self-compacting concrete (SCC). However, irregular cracks -caused by the restrained drying shrinkage- appeared on the surface of this concrete a few days after removing the formworks. On the other hand, due to aesthetic reasons, neither saw cuts in the hardened concrete to produce regular contraction joints -carried out to avoid the irregular cracks caused by a restrained drying shrinkage- were accepted by the Architects. Therefore, a special 3-SC mixture was developed and used; it is characterized to be: - a self-compacting concrete based on the use of an acrylic superplasticizer, a viscosity modifier to avoid the bleeding risk, and a special particle size distribution of the aggregates; - a self-compressive concrete due to the use of a CaO-based expansive agent; - a self-curing concrete based on the use of a shrinkage-reducing admixture (SRA). This concrete called 3-SC, because it is 3 times “self”, was very successful in producing a crack-free concrete surface even in the very long, curvilinear, and inclined walls: after 18 years of building the long, inclined, curvilinear walls of the MAXXI museum have been carefully examined and during the last inspection their surface resulted to be still sound and crack-free. However, just before the building’s inauguration in 2009, in very few areas some micro-cracks were observed on the concrete surface and considered to be dangerous for the future of the building. Therefore, the concrete surface was treated with a transparent varnish in order to avoid the ingress of the aggressive humid air to protect the steel reinforcements from the corrosion promoted by the carbonation process.

The application of a novel superabsorbent polymer (SAP) as a multifunctional chemical admixture for concrete properties is expected to contribute to the overall durability and sustainability of concrete structures. SAPs are well known to quickly absorb and retain a significant amount of water within the concrete matrix as a means of providing internal curing. However, the rate of water uptake can significantly affect the rheology of fresh concrete such as reduced flowability. This paper introduces a novel SAP that features slow water absorption and swelling behavior, and its resulting impact on both fresh and hardened concrete properties. The novel SAP has been shown to delay swelling for several hours in cement filtrate, followed by a predictable absorption of water over a 24-hour period comparable to conventional SAP. The delayed swelling effect observed with the novel SAP eliminates the need for additional water to obtain a similar flowability, but with a very slight increase in viscosity, compared to a concrete mixture without SAP. Moreover, the internal curing capability of the novel SAP can result in an increase in both early age and long-term compressive strengths, improved freeze-thaw resistance, and a reduction in autogenous shrinkage under sealed and air curing conditions.