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.

One of the issues of alkali-activated materials is their incompatibility with conventional organic admixtures, which means that they do not improve their properties as expected on the basis of their role in the Portland cement-based system or even have adverse effects. This is also true for hexylene glycol, on which the commercial shrinkage-reducing admixture is based. In previous studies, we observed that it can reduce the drying shrinkage of alkali-activated slag (AAS), but the main reason was its adverse effect on the hydration of AAS since the early ages and related coarse pore structure. In the present study, calorimetric studies showed that a great effect of hexylene glycol and some other alcohols is especially pronounced for a high silicate modulus (around two). This was confirmed by gelation experiments, in which the activating solution was mixed with calcium hydroxide, and organic additives were used. These tests revealed that the acceleration of gelation of silicates occurs for the same admixtures as the retardation of hydration and follows the same trend. A similar effect was observed for the gelation of silicate sol by NaCl solution, showing that the destabilization of silicates by organic additives is at the origin of the affected hydration of silicate-activated slag.

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.

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.

Retarders are very important during the production of cement-based materials. The delay in setting might be helpful in avoiding negative phenomena related to the long-term transport of the fresh concrete mix, unforeseen breaks in the transport, or laying of concrete. These admixtures prevent the local temperature rise of the concrete, and thus the formation of cracks and also extent the workability. Set-retarders provide a correct development of the microstructure and the undisturbed setting and hardening of cement which lead to higher strengths of cement-based materials. An investigation of the cement mortar with potassium methylsiliconate (MESI) applied as set-retarding admixture was carried out. Siliconates are a highly alkaline water solution of methylsiloxane resin in the potassium or sodium hydroxide. The study involved the cement paste and mortar with three dosages (1%, 2%, and 3% per cement mass) of organosilicon admixture. So far, the siliconates were not applied as admixtures for cement mortar or concrete. The mortar specimens were tested for compressive strength after 1, 2, 7, and 28 days and frost resistance after 25 freeze-thaw cycles. Moreover, the impact of the methylsiliconate admixture on the hydration (by isothermal calorimetry) and setting time of the ordinary Portland cement was also studied.