Preface to the Special Issue on Nano-Enabled Approaches for Sustainable Development of the Construction Industry

Concrete is the most consumed substance on Earth, and its production is expected to rise from 4.4 billion tonnes to 5.5 billion tonnes by 2050. However, its production contains cement, which comes with a massive environmental burden, contributing 8–10% of global anthropogenic CO₂ emissions [1]. With such widespread consumption, it is clear that aggressive research efforts are required to maximize the sustainability of concrete and limit the environmental burden of this material. Moreover, cracking in concrete is the most challenging issue in the construction industry [2]. This problem is an increasing difficulty that can lead to the corrosion of the steel reinforcement and thus to the possible failure of the whole concrete structure [3]. The construction sector urgently needs effective, cost-efficient, and sustainable green approaches to limit crack formation issues and ensure the sustainable and safe performance of the concrete industry.

Different strategies for repairing cracks, including surface coating [4] and the application of fillers [5], have been utilized to reduce the repair and rehabilitation costs of large-scale civil infrastructure. More recently, the introduction of applied biotechnology in the field of concrete has resulted in the development of a new domain called “microbial concrete” or “bio-concrete”. It is a microbial-based approach in which concrete structures are treated with bacteria to promote calcium carbonate precipitation in self-healing concrete [6]. The positive benefit of the biomineralization activity of several bacterial strains to promote concrete’s durability and crack healing has become a key research area during the past decade [7]. Still, the major challenges reside in the survival of the bacteria inside the harsh concrete environment. Extra attention must be focused on ensuring bacterial growth and repair of cracks, for example, by providing enough nutrients and space to grow as well as protection from the surrounding environment, being crushed, and high-alkaline media.

Nanotechnology has emerged as an innovative and transformative technology that offers inexpensive, safe, efficient, and sustainable options for problems related to concrete structures, including the increase in the strength and workability of self-healing concretes [8]. The higher reactivity and versatility of nanomaterials (NMs), along with novel properties and functionalities, enable the effective delivery of bacterial spores in a complex concrete environment. For example, Seifan et al. [9] applied iron oxide nanoparticles (FeO-NMs) in self-healing concrete as a protective vehicle for bacteria. Their findings indicate a lower absorption of water up to 26% in bio-concrete specimens compared to control samples. Similarly, applying 0.5% to 1% carbon nanotubes (CNTs) improved the tensile strength and reduced the porosity of asphalt concrete [10]. Nanotechnology seeks to take advantage of the unique properties of materials at the nanoscale, and extensive research has been carried out focusing on the use of NMs in the construction industry, obtaining such results as:

• Aluminium oxide NMs increase the serviceability of asphalt concrete [11];
• CNTs promote crack prevention in concrete [12];
• Titanium NMs contribute to the self-cleaning of concrete [13];
• Copper NMs advance the corrosion resistance of steel [14];
• Clay NMs increase the surface roughness and compressive strength of bricks and mortar [15].

Applying NMs improves the mechanical properties of concrete structures and overcomes physical and chemical deterioration. In the past, the journal Applied Sciences has focused on promoting the application of nano-enabled technologies in the construction industry. Relevant publications in recent years include the Special Issue “Nano-Modified Asphalt Binders and Mixtures to Enhance Pavement Performance” (2020) and individual research articles in various issues of the journal. Overall, the recent publications of the journal Applied Sciences include, for example, “Development and Evaluation of Nano-Silica Sustainable Concrete” [16]; “Graphene Nanoplatelets Impact on Concrete in Improving Freeze-Thaw Resistance” [17]; and “Effect of Graphene Nanoplatelet on the Carbonation Depth of Concrete under Changing Climate Conditions” [18]. The research opportunities in this field are enormous, and by introducing our Special Issue for Applied Sciences, entitled “Nano-enabled approaches for sustainable development of the construction industry”, we hope that relevant researchers will share their research work in this field, further expanding the knowledge and application of a nano-enabled construction industry. Furthermore, the current Special Issue and its reports will help improve our understanding of nano-enabled technologies in the construction industry and will provide a solid basis for developing next-generation nano-bio-induced self-healing concretes.