# Acoustic Applications of a Foamed Geopolymeric-Architected Metamaterial
## 1 Introduction  
Several researchers have recently worked to create sustainable building materials. One of the fundamental prerequisites for sustainable construction methods and environmental impact assessments is using green building materials and their manufacturing process [1]. There is a direct correlation between the energy used to produce different building materials and the carbon dioxide emitted while producing the same materials. Both energy consumption and gas emissions, primarily carbon dioxide and monoxide, are rising [2, 3]. The European Commission recently adopted several significant measures to achieve (Directive 2002/49/EC as updated in recently amended by the delegated directive(EU) 2021/1226 of 21 December 2020) climate neutrality in the EU by 2050, including an intermediate target of a 55% net reduction in greenhouse gas emissions by 2030 [4; 5]. As a result, researching sustainable alternatives to ordinary cementitious materials is a rapidly evolving field focused on the potential use of alkali-activated geopolymers (GPs) as sustainable binders in the building and construction industries. An average reduction in greenhouse gas emissions during the life cycle of geopolymer concrete was estimated to be 62-66%, with a range of up to 80% compared to an equivalent amount of ordinary Portland cementitious binder used in reference concrete of similar strength [6; 7; 8]. Geopolymers are a type of aluminosilicate material synthesised by an inorganic polycondensation reaction (also known as \"geopolymerisation\") between solid aluminosilicate precursors, and alkaline solutions such as sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)), potassium silicate (K\({}_{2}\)SiO\({}_{3}\)), or highly concentrated aqueous alkali hydroxide. From the process perspective [9], alkaline metal salts and hydroxides [8] (i.e., sodium silicate and NaOH) are needed to dissolve the metastable aluminosilicate [8; 10] and catalyse the complex polycondensation reaction [11; 12] involving linear and cyclic silate oligomers to form an amorphous structural glass [9] to be used as new, durable [13; 14], and more environmentally friendly construction materials [15]. During the alkaline-activated dissolution, the chemical bonds of the kaolin metastable aluminosilicate are fragmented down into hydrated silicate [OSi(OH)\({}_{3}\)]\({}^{-}\) and aluminate [Al(OH)\({}_{4}\)]\({}^{-}\) ionic precursors [9; 16; 17; 18] involved in the creation of cyclic and linear salates [9]. Geopolymers are inorganic materials utilised in various fields, including building, repairing and restoring concrete structures, 3D printing, fire-resistant materials, biomaterials, and others [16; 17; 18]. In addition, they have very interesting properties like low shrinkage [19] and bond strength [20] for cement structure repairs, thermal stability [21], chemical and freeze-thaw resistance [21; 22], long-term durability [22; 23], and potentially reliable use in greener technologies [24] and recyclability [25]. Advanced applications like catalysts, moisture sensors, heat storage, pH regulators, immobilisation of heavy metal, and new ceramic biomaterial are also under study [25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. The polycondensation process flexibility allows the specialisation of ceramic-organic hybrids [32; 33] and composite materials [34; 35; 36; 37; 38] with specific rheological and viscoelastic and chemo-physical properties [10] such as slurry thixotropy or final glassy geopolymer mechanical properties and biocompatibility for their applications as speciality materials. Geopolymers, depending on the Si/Al and activating solution vs. metastable aluminosilicate solid/liquid ratios or the addition of fillers, can be tailored to have a variety of densities, ranging from high-density systems such as mortars and concretes to porous low-weight materials [39; 40; 41; 42; 43]. In particular, porous materials can be prepared by incorporating foaming agents into the geopolymer paste during consolidation.