# Waste Cork in Metakaolin-Geopolymer Matrix: Physico-Mechanical Characterization
## 1 Introduction  
Geopolymers have been extensively studied as building materials. These types of materials were developed from an aluminosilicate precursor and an alkaline agent as a greener substitute for ordinary Portland cement (OPC) [1]. After hardening, a semi-crystalline amorphous material similar to concrete is produced [2]. The geopolymerization reaction can take place at room temperature or even at higher temperatures, depending on the final product to be obtained [1]. The strength of geopolymers varies according to the conditions under which the geopolymerization takes place [3]. Geopolymers can be produced from a variety of aluminosilicate natural or industrial precursors in the form of powders or slurries [1]. The most common sources of aluminosilicates used for geopolymer production are kaolin, metakaolin and by-products such as fly ash [4], granulated blast furnace slags [5], ladle slags [6]. The use of these last three types of partially amorphous aluminosilicate precursors to produce geopolymer cements significantly reduces the amount of CO\({}_{2}\) of construction projects by 80%, compared to OPC [1]. The powdery precursors for geopolymeric materials should be rich in alumina (Al\({}_{2}\)O\({}_{3}\)) and silica (SiO\({}_{2}\)) contents, preferably in the reactive amorphous form [1; 2; 3; 4], for fast dissolution when in contact with the alkaline agent. These alkaline activators must be carefullyselected in terms of type (NaOH, rather than KOH and/or sodium silicate) and concentration (typically from 6 to 10 M) in order to produce the proper degree of dissolution of the precursor to assure the correct development of compressive strength in hardened geopolymers [1]. To obtain a better dissolution of the aluminosilicates present in the various classes of raw material, many authors work at pH values higher than 13 [1; 2]. The result of geopolymerization [1; 2] is a semi-crystalline amorphous substance composed of solid phases of aluminosilicates assembled on the basis of SiO\({}_{4}\)[4\({}^{-}\) and AlO\({}_{4}\)[5\({}^{-}\) bonds as a tetrahedra that forms a 3D structure. Geopolymers have many characteristics in addition to eco-sustainability. As an example, geopolymers are excellent fireproof materials and are resistant to corrosive environments. In addition, geopolymers have excellent compressive strength and thermal properties. Geopolymers are also impermeable to water [1]. A degree of resistance to fracture propagation can be obtained with the addition of natural fibers [7; 8] and synthetic fibers [9], etc., although compromising the fire resistance. To improve fire resistance while maintaining the lightweight nature of the material for non-structural composites, waste cork was added to the matrix. In previous works [10; 11; 12; 13; 14], the use of cork has been discussed, but few articles have focused on the use of cork waste in mortars and concrete [15]. In this study, powdery waste from agglomerated cork bottle caps (CW) was introduced into metakaolin (MK) geopolymer fresh paste in increasing amounts from 2.4 to 9 wt%. Cork waste was taken from the manufacturing process of agglomerated cork bottle caps and, as they contain residues of binders and additives for cork particles, namely polyurethane glue and paraffin, they seem to be suitable for a cold process such as geopolymerization. The introduction of cork residue as a filler in a geopolymer matrix could reduce water consumption during the production phase and prolong setting times, thus leading to a more extended geopolymerization process, as reported by [PERSON] et al. [15]. Furthermore, the beneficial properties of cork could be exploited, including their lightness, impermeability to liquids and gasses, neutrality and chemical resistance, resistance to biological corrosion, thermal insulation properties, acoustic and anti-vibration properties, incombustibility, elasticity, durability, neutrality for health and mechanical resistance.