materials Review Modification of Lightweight Aggregate Concretes with Silica Nanoparticles—A Review Karol Federowicz 1,* , Mateusz Techman 1, Myroslav Sanytsky 2 and Pawel Sikora 1,3 1 Faculty of Civil and Environmental Engineering, West Pomeranian University of Technology in Szczecin, al. Piastów 50a, 70-311 Szczecin, Poland; [email protected] (M.T.); [email protected] (P.S.) 2 Department of Building Production, Lviv Polytechnic National University, S. Bandera Str. 12, 79013 Lviv, Ukraine; [email protected] 3 Building Materials and Construction Chemistry, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany * Correspondence: [email protected]; Tel.: +48-91-449-48-14 Abstract: The use of lightweight concrete (LWC) for structural and non-structural applications has attracted great interest in recent years. The main benefits include reduced deadload of structural elements and generally lower production and transportation costs. However, a decrease in concrete density often leads to a decrease in strength and durability. Typically, concretes are mostly modified with mineral additives such as silica fume or fly ash. Because of the recent developments in nan- otechnology, research attention has turned to the possibility of improving concrete properties with nanomaterials, i.e., nano-SiO2. However, there are still certain issues with the dosage and efficiency of nanomaterials. Therefore, in order to establish the current state of knowledge in this field, this review gathers most recent results about the performance of LWC modified with nanomaterials. The review is divided into sections about the influence of nanoparticles on the fresh properties of concrete and their influence on the mechanical and durability characteristics. The paper studies in Citation: Federowicz, K.; Techman, M.; Sanytsky, M.; Sikora, P. depth the most common approach to nanomaterials in concrete technology and proposes areas for Modification of Lightweight further development. Aggregate Concretes with Silica Nanoparticles—A Review. Materials Keywords: lightweight concrete; strength; durability; nanosilica; nanoparticles 2021, 14, 4242. https://doi.org/ 10.3390/ma14154242 Academic Editor: Jean-Marc Tulliani 1. Introduction 1.1. Lighweight Concrete—General Consideration Received: 6 July 2021 Waste management is a crucial factor for decreasing environmental contamination. Accepted: 27 July 2021 One of the best solutions to minimize this issue is reusing and recycling the materials Published: 29 July 2021 and by-products from various branches of the industry. A good example of recycling is the production of fly ash (FA), where 19 M tons in 2012 were produced in just one Publisher’s Note: MDPI stays neutral European country—Turkey (3% of world’s total). It is projected that the amount of FA with regard to jurisdictional claims in production could grow by more than 30% by 2020 [1,2]. One of the many solutions to published maps and institutional affil- iations. prevent the depletion of natural resources and decrease environmental pollution is the production of artificial aggregates. This helps to dispose industrial waste and decrease the usage of natural aggregates in the building industry [3,4]. Among the many available types of artificial aggregates, the most popular are lightweight aggregates (LWA) such as Poraver (Germany), Leca (Denmark), or Liapor (Germany). The above mentioned Copyright: © 2021 by the authors. aggregates differ in terms of raw materials used for their production, water absorption, Licensee MDPI, Basel, Switzerland. and strength, but have one thing in common: a low density due to the high porosity, as This article is an open access article shown in Figures1 and2 . The concept of lightweight concrete (LWC) can be dated back distributed under the terms and conditions of the Creative Commons to 3000 years ago [5]. The Mediterranean region is filled with structures constructed with Attribution (CC BY) license (https:// LWC, including the famous Pantheon Dome built in the early Roman Empire [6]. Historical creativecommons.org/licenses/by/ LWC was made with natural lightweight aggregates such as pumice, diatomite, or scoria, 4.0/). Materials 2021, 14, 4242. https://doi.org/10.3390/ma14154242 https://www.mdpi.com/journal/materials Materials 2021, 14, 4242 2 of 23 Materials 2021, 14, x FOR PEER REVIEW 2 of 23 Materials 2021, 14, x FOR PEER REVIEW 2 of 23 grains of variablewith grains shape of and variable quality. shape The and av quality.ailability The of availabilitythese aggregates of these limited aggregates the limited the spreadgrains of LWCvariablespread to the shape of Mediterranean LWC and to thequality. Mediterranean Sea The basin av ailability[5]. Sea basin of [these5]. aggregates limited the spread of LWC to the Mediterranean Sea basin [5]. Figure 1. NaturalFigure LWA 1. Natural perlite: (LWAa) macro perlite: scale, (a) (macrob) micrograph scale, (b) to micrograph 32×, and ( cto) SEM32×, and micrograph (c) SEM tomicrograph 500×, adapted to with 500×,Figure adapted 1. Natural with LWA permission perlite: from (a) macro ref. [7]. scale, (b) micrograph to 32×, and (c) SEM micrograph to permission from500×, ref. adapted [7]. with permission from ref. [7]. Figure 2. Artificial LWA produced from expanded clay (aliven): (a) real scale, (b) micrograph to 32×, Figure 2. ArtificialandFigure (c) LWA 2.SEM Artificial producedmicrograph LWA from producedto 500×, expanded adapted from clayexpanded with (aliven): permission clay ( a(aliven):) real from scale, (ref.a) real [7]. (b )scale, micrograph (b) micrograph to 32× ,to and 32×, ( c) SEM micrograph toand 500 ×(c,) adaptedSEM micrograph with permission to 500×, from adapted ref. [wi7].th permission from ref. [7]. Lightweight concrete (LWC), according to the EN 206-1 [8], is defined as having a densityLightweight below 2000Lightweight concrete kg/m3. 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