*3.3. Natural Fibers*

Natural fibers, such as fibers of mineral origin like basalt [12,212,225,226], and fibers of plant origin, like sisal [72,227], as well as banana [228,229], can also be used as reinforcement of HPC and UHPC. Their main advantages are the fact that they are renewable and eco-friendly resources. The main disadvantages are the great variability of properties and the possibility of degradation in an alkaline environment, especially in the case of fibers of vegetable origin that need previous treatments to be applied in cementitious matrices [72,227,230]. Another important advantage of natural fibers, especially vegetable fibers, is related to the fiber's high adherence to the OPC matrix. The adhesion mechanisms of this type of fiber are related to the fibrous structure of the cellulose [72]. Some authors have carried out studies on the improvement of the adhesion properties of these fibers through treatments with alkaline materials, which attack the fiber surface, increasing the roughness and adhesion with the matrix [227,230].

#### **4. HPC and UHPC Produced with Alkali-Activated Cement**

Another type of HPC and UHPC that has emerged as a great potential for application in civil construction is based on alkali-activated cement [231]. This type of cement, unlike OPC, is composed of two materials: a powdery and amorphous component named as a precursor, and another liquid composed of an aqueous solution of a dilute alkali metal, named as an activator [232,233]. The types of precursors and activators will be presented in detail in the following sections; however, in summary, they must result in composites with similar characteristics to OPC-based HPC and UHPC: adequate flowability, high mechanical strength, and good durability.

From the point of view of aggregates, the same as conventional HPC and UHPC are used, namely: washed sand from rivers or crushed sand, as fine, as well as gravel or rolled pebbles, as coarse. Thus, the materials used as precursors and activators will be discussed in this topic, since these are the components that differentiate concretes produced from alkali-activated cement.

#### *4.1. Precursors: Overview*

Precursors, as highlighted, are very fine materials and are predominantly amorphous, which can have two basic chemical compositions: (i) rich in calcium or (ii) rich in aluminosilicates. To classify the material, the CaO/(SiO2 + Al2O3) mass ratio is calculated. If this ratio is greater than 1, the material is considered rich in calcium, as is the case with blast furnace slag and some types of fly ash. If the ratio is less than 1, the material is considered rich in aluminosilicates, giving rise to a subclass of alkali-activated materials known as geopolymers [234,235]. This is the case of metakaolin, ceramic waste, glass waste, and most types of fly ash.

The use of precursors, which totally replaces the application of OPC, presents as main benefits the possibility of applying residues and agro-industrial by-products from different sectors, producing environmental and economic advantages. This is also related to the fact that the OPC industry is highly polluting. Alkali-activated precursors and cement are considered alternative and eco-friendly binders [232,236,237], usually resulting in concretes with lower CO2 emissions than OPC-based concrete (around 10%, according to [238]). The accurate determination of the environmental impact associated with these binders—and consequently compare it with those associated with OPC—would require a detailed analysis which will not be addressed in this paper; nonetheless, further details can be found in [239–241].
