**4.1 Pozzolanic materials**

The material of pozzolan is stated as an aluminosilicate/siliceous materials which are finely ground style and chemically react with calcium hydroxide within the presence of moisture it creates calcium silicate hydrate (CSH) and other cementitious materials. Clay and shale, opalinc chert, volcanic ash, and diatomaceous earth are an example of natural Pozzolanas while fly ash, rice husk ash, blast furnace slag, coffee husk ashy, silica fume, bagasse ash, and metakaolin are an example of artificial Pozzolanas. Most pozzolans used today are mainly widely available byproduct materials. Since pozzolan has a variety of diversity, its chemical structure and contents also vary. Therefore, classifying Pozzolanas only depending on their chemical composition would be difficult. For this reason, ASTM C-618 classifies Pozzolanas depending on a performance basis as tabulated in **Table 1** [3–6].

The reason behind using Pozzolanas is the improvement found on both the hardened and fresh state concrete. Lowering of the thermal shrinkage and heat of hydration, increase in water tightness, decrease in the alkali-aggregate reaction, resistance to sulfate attack, better workability, and price effectiveness are some of the improvements achieved by using Pozzolanas blended with cement [7–9].

Partially or cement replacing materials are special construction materials either naturally occurring materials or industrial wastes/byproducts or agricultural wastes which could be castoff for concrete production. And they rely on activation of by-products while incorporating minimal amounts of cement are promising low carbon candidates that can potentially complement the globe's growing concrete industries by using the equivalent performance concept.

#### *Sustainability of Concrete with Synthetic and Recycled Aggregates*


**Table 1.**

*Chemical requirement for Pozzolanic materials.*

Due to the increase of awareness of environmental concerns and natural resource consumptions, the issue of energy saving has been gradually emphasizing by the public. Owning to the considerable use of concrete and cement material, the natural material resources associated with the construction sector have been continuously reducing in recent years. However, for each country particularly for developing countries like Ethiopia, concrete is the most significant material for fundamental and public constructions. Thus, an innovative and alternative concrete material, which possesses feasibility and practicality, is critical and significant for mitigating environmental impact and promoting energy-saving performance. For this purpose, the most communal and practice in real concrete production for the cement replacement are natural pozzolana, Diatomaceous Earth, Glass Residue, Silpoz, fly ash, Corn Cob Ash, Ground granulated blast furnace slag, Silica Fume, Highly reactive metakaolin, rice husk ash (RHA), Bagasse ash, Coffee husk ash, calcined termite hell, water hyacinth ashy, etc.

#### i.Natural pozzolans

Natural pozzolans originated from volcanic activities are worldwide available materials, with varied compositions and subsequently a varied performance. However, for the reason that huge content in amorphous silica, it is usually an excellent material to be used as cement replacement. Curiously, before the invention of ordinary Portland cement, volcanic ash and air lime mixtures were commonly used, with a good performance and proven durability. Difficulties in the usages of these products are lack of characterization and the varied composition of raw-material layers, sometimes within the same area. However, naturally occurring pozzolans are used successfully in cement composition and may be looked at with cement replacement potential with the replacing percentage of cement up to 20% by mass. The use of ash of volcano for concrete production helps to reduce chloride ion diffusivity of concrete, inhibiting the localized corrosion of steel and further concrete degradation. This addition also promotes lesser heat of hydration and higher setting time. The improved performance has been qualified to the refinement of the stomate structure and the pozzolanic action of volcanic ash. One of the probable weaknesses using of these natural pozzolana materials is their diversity. To minimize this problem, usually natural pozzolanic materials from different extraction heights are mixed before use [1–3, 6, 10].

#### ii.Diatomaceous earth

Diatomaceous earth, which is also acknowledged as diatomite/fossil flour, is a sedimentary material mainly constituted by diatom outer shells. This very
