**Abstract**

In this chapter, we present our study of geopolymers and hybrid geopolymers synthesized with treated fly ash from eggshells (FAES) and sand from the dunes of southern Algeria using activators such as NaOH and Na2SiO3, respectively, in addition to the organic polymer polyethylene terephthalate (PET). Several parameters have been modified, such as alkali concentration and percentage of activators and PET, with the objective to improve the quality of the desired geopolymers and hybrid geopolymers. The main objective of this work is to study the use of waste PET in the matrix of this new material to replace Portland cement, which is widely used today, as well as develop ecological building materials that are durable and lightweight and prevent chemicals from attacking old structures. Through optical and electron microscopy, we studied the effect of the addition of PET on the structure of our geopolymer material and on the bond and interface areas between the aggregates and the matrix. The microstructural analysis discussed here refers to specimens containing 5% PET by weight. We observed that PET contents significantly altered the structure and morphology of the samples.

**Keywords:** fly ash of eggshells (FAES), sand dune, cement, microstructure, analysis, construction, Young's modulus, structure, silica fume
