# Physico-Mechanical Properties of Alkali-Activated Based Composites Using Recycled Fire Fibers
## 1 Introduction  
In September 2015, the Summit for Sustainable Development approved Agenda 2030 for sustainable development (SD), which includes 17 Sustainable Development Goals (SDG) to end poverty, combat inequality and injustice, and address climate change [1, 2]. SDGs 11, 12, and 13 emphasize that SD is not possible without drastically transforming the way we build. This study focused on using more sustainable construction materials, such as composites with recycled tire fiber (TFs), and reusing industrial waste like spent fluid catalytic cracking catalyst (FCC) from the petrol industry and rice husk ash (RHA) to reduce the ecological footprint by changes in production methods, waste reduction, and recycling. It is important to note that one of the primary greenhouse gases (GHGs) is carbon dioxide, and the Portland cement (PC) industry contributes 7% of the total carbon dioxide emitted to the atmosphere by human activity [3]. This makes it a very polluting activity. Complete PC substitution for alkali-activated systems represents a significant reduction in the carbon footprint [4]. The progressive increase in the amount of rubber waste generated from used tires (UTs) has resulted in an environmental problem. According to reports from major tireand rubber product manufacturing associations, global annual tire production is around 1.4 billion units, the equivalent to an estimated 17 million tons of UTs a year. The dynamic growth in the number of UTs is well-exemplified by the EU, where production increased from 2.1 million tons in 1994 to 3.3 million tons in 2010, and the annual cost of its disposal in EU countries has been estimated at around EUR 600 million [5]. In 2020, the production of 4.2 million tons of rubber tires was estimated [6]. Waste generation is inevitable and it is therefore essential to have rational environmental management systems that minimize waste generation and maximize reuse and recycling. Alternatively, recovering energy and materials from discarded tires, known as valorization, is crucial [7; 8]. In the EU, producers operate according to the Extended Producer Responsibility (EPR) system [9], which covers about 15 countries. This appears to be the most suitable and robust system for addressing and resolving issues that arise from UTs in a sustainable manner by achieving a 100% recovery rate. In Spain, SIGNUS is responsible for ensuring environmentally sound UT management [10]. An example in Latin America is Brazil, where 18.3 million tires were produced in 2003. The demand for UTs (retreads) is very high due to low purchasing power and the high price of new tires. In recent years, the importation of UTs has increased, with up to 7 million tires in 2007 [11]. Retreading is an alternative in UT management and utilizes tire casings by replacing worn treads and shoulders to reconstruct the original structure [12]. During this valorization process, waste is generated in the form of TFs and particles in the scraping stage. The reaction of an aluminosilicate mineral mixed with a highly concentrated alkaline activating solution (based on a mixture of sodium hydroxide and an alkaline silicate such as sodium or potassium silicate) produces a structure called a geopolymer [13; 14]. This type of alkali-activated cement can provide a similar performance to conventional PC- based binders, and offers an additional environmental advantage by reducing GHG emissions [15]. This study analyzed the valorization of TFs as a lightweight aggregate in mortar based on alkali-activated cement (AAC). Previous studies conducted on PC concrete with TFs report decreased bulk density as the TF contents increase [16; 17; 18]. Additionally, the loss of mechanical strength in flexural and compressive strength has been identified, even with use of the fibers [19; 20; 21; 22; 23; 24; 25]. This consequence can be attributed to the poor or nonexistent adhesion between the cement matrix and rubber [24; 25; 26]. Other properties, such as thermal properties [27], have drawn the interest of some researchers.