**Cesare Signorini 1,\*, Antonella Sola 2, Sumit Chakraborty <sup>3</sup> and Valentina Volpini <sup>4</sup>**


**Abstract:** Cement and lime currently are the most common binders in building materials. However, alternative materials and methods are needed to overcome the functional limitations and environmental footprint of conventional products. This Special Issue is entirely dedicated to "New frontiers in cementitious and lime-based materials and composites" and gathers selected reviews and experimental articles that showcase the most recent trends in this multidisciplinary field. Authoritative contributions from all around the world provide important insights into all areas of research related to cementitious and lime-based materials and composites, spanning from structural engineering to geotechnics, including materials science and processing technology. This topical cross-disciplinary collection is intended to foster innovation and help researchers and developers to identify new solutions for a more sustainable and functional built environment.

**Keywords:** cement; lime; sustainable materials; fibre-reinforced composite; recycled aggregates

Cement and lime have been the predominant binders in the construction sector since ancient times, owing to their worldwide abundance, affordability, and well-established physical and mechanical performance. By tuning the relative amounts of binders with aggregates, water, and additives, a variety of conglomerates can be designed to address specific service requirements. Cement and lime-based conglomerates range from finegrained mortars for plasters to structural concrete with or without fibre reinforcement for buildings, bridges, tunnels, pavements, girders, precast members, walls, screeds, etc.

Prompted by the necessity of developing smart and reliable infrastructures and more energy-efficient urbanised areas, new frontiers are opening to identify viable materials and methods with the inclusion of industrial by-products, alternative aggregates, and natural reinforcements [1]. The partial or total replacement of conventional cement and virgin mineral aggregates is pivotal to reducing the environmental issues and carbon footprint of customary conglomerates, since the conventional manufacturing chain is known to require a substantial amount of energy and resources. Further, intensive investigation is under-way to formulate novel inorganic composite materials for lightweight precast elements and for strengthening laminates, which are expected to transform present-day approaches to the preservation and restoration of historical buildings and architectural heritage [2–4]. As new materials with embedded functionalities are being proposed every day, scientists gain a deeper understanding of the relationship existing between composition, manufacturing, and in-service behaviour of cementitious and lime-based materials and composites.

This Special Issue aims to shed light on the latest research outcomes in this multidisciplinary field, spanning through the vast areas of structural, geotechnical, and environmental engineering, mineralogy and materials science, nanotechnology, fibre and textile technology, and design criteria. The high-quality peer-reviewed contributions gathered in this Special

**Citation:** Signorini, C.; Sola, A.; Chakraborty, S.; Volpini, V. New Frontiers in Cementitious and Lime-Based Materials and Composites. *Crystals* **2022**, *12*, 61. https://doi.org/10.3390/ cryst12010061

Received: 9 December 2021 Accepted: 21 December 2021 Published: 4 January 2022

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Issue include both experimental papers, which discuss lab-based activities and/or theoretical modelling, as well as review articles that extensively analyse the state-of-the-art within specific fields of interest. The topics covered in this Special Issue include fibre-reinforced composites (e.g., textile-reinforced mortar/concrete (TRM/TRC) [5,6] and fibre-reinforced concrete (FRC) [7,8]), sustainable cementitious conglomerates where the ordinary Portland cement (OPC) binder is replaced by recycled by-products [9] (silica fume, kiln ashes, biochar, slag, biomass ashes, geopolymers, alkali-activated concrete, etc.), with the possible implementation of self-healing properties. In addition, studies regarding the incorporation of recycled aggregates providing concrete with increased eco-compatibility and additional functional properties, such as thermal and acoustic insulation, are also presented [10,11]. Moreover, durability is a key issue, especially for emerging materials whose behaviour in the long run is still unknown. Long-term stability is still the subject of debate for some of the recycled constituents, such as plastic fibres coming from low-grade sources, and also for composite systems in which the unoptimised interphase between the reinforcement and the surrounding inorganic matrix may represent the weakest link [12,13]. Reliability becomes critical in harsh environmental conditions, e.g., in coastal areas or underground applications [14]. Therefore, updated guidelines are sought to regulate the safe usage of these structural materials.

In this Special Issue, readers will find a wealth of information regarding existing literature and emerging research in all areas of cementitious and lime-based materials and composites, as summarised in the following paragraphs.


ments are exposed to extremely harsh environmental conditions, which may represent a challenge for the adoption of newly designed composite materials. For this reason, the paper by Yao et al. [18] thoroughly investigates the long-term performance of a new cement-based composite system based on the synergistic interaction of polyvinyl alcohol and polypropylene-steel hybrid fibres. It is found that the combination of these two kinds of fibres enhances the impermeability of the wall, thus bringing in superior resistance to corrosion and freeze-thaw cycles;


as a function of source, type, or chemical and physical characteristics of the RCAs, as well as composition of the mixture, water content, curing conditions, and several other parameters. Makul et al. [24] also outline the open issues and future research that must be carried out to optimize the design of "green" composites. In conclusion, RCAs are a valuable resource that can be safely employed in traditional concrete, but their uptake to produce high-performance structural concrete, which must meet stringent mechanical standards, still requires additional efforts;


**Author Contributions:** Conceptualization, C.S., A.S., S.C. and V.V.; resources, C.S.; writing—original draft preparation, C.S.; writing—review and editing, A.S., S.C. and V.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** A.S. is supported by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Research Office through the "Science Leader in Active Materials" grant.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

