**1. Introduction**

Geopolymers can replace cement in various forms of construction work and the manufacture of concrete and mortars, and they have very important mechanical characteristics given their three-dimensional aluminosilicate network [1]. The use of geopolymers instead of Portland cement is justified by the reduction in CO2 emissions and energy savings [2]. Dune sand is a widely available natural resource that can be integrated into the construction industry, which needs more development [3]. The process of geopolymerization of mixtures of sand and fly ash is used to produce a new material composed of very fine elements. Mortars composed of

modified polymers are building materials with excellent properties and can replace mortars based on Portland cement [4]. Polymers have been used to enhance the waterproof properties and modify the mechanical properties of concrete and mortars as well as reinforce adhesion [5]. The literature specifies that the characteristics of concrete and mortar modified by polymers depend mainly on the polymer percentage or on the ratio between the cement and polymer, that is, the ratio of the value of the mass of solid polymers contained in a polymer-based addition to the value of cement in a polymer, concrete, or modified mortar [6]. Among plastics, polyethylene terephthalate (PET) is the most widely used to produce products such as consumer goods, beverage bottles,and food packaging [7]. PET bottles have replaced traditional glass bottles for liquid storage due to their ease of handling, light weight, and possibility of storage [8]. Fly ash is a fine and powdery material produced from coal when generating electricity. When coal is used in a power plant, it is crushed into a very fine powder that will be blown into the furnace of the plant [9]. The hydrogen and carbon in the coal are depleted, leaving molten noncombustible particles rich in alumina and silica. After the solidification of these particles in the fly ash state, these very fine powders easily enter the atmosphere and pollute water and the air [10]; they can cause respiratory problems if not properly eliminated. In addition, fly ash deposited on leaves and plants in agricultural fields near electrical power plants can diminish crop yield. However, when used properly, fly ash can help to conserve natural resources [11]. The manufacture of Portland cement is a major contributor of CO2 gas emissions, thus any reduction in the use of cements will result in a reduction in greenhouse gas emissions, possibly reducing emissions to zero. One ton of CO2 is emitted for each ton of Portland cement produced [12]. Thus, replacing Portland cement with fly ash will eliminate CO2 emissions. If all the fly ash produced is used instead of carrier cement in the various construction works of buildings, roads, and bridges, it is estimated that the reduction in CO2 emissions will be equivalent to the elimination of 25% of vehicles worldwide [13]. This chapter summarizes the scientific advances in the preparation, fabrication, properties, and applications of fly ash of eggshells (FAES) and sand dune-based geopolymer and geopolymer hybrids. The production of mixed geopolymers and hybrid geopolymers is mainly based on alkali-activated geopolymerization, which can occur under mild conditions and is considered a cleaner process due to much lower CO2 emissions than that from the production of cement [14].
