**1. Introduction**

It has been well established that origins of the present use of supplementary cementitious materials (SCM) in the construction industry date to the ancient Greeks between 750 B.C. and 600 B.C. The Greeks blended the volcanic ash with the hydraulic lime to develop mortar. The baton of the Greek's knowledge had been passed on to the Romans; this enabled Romans to engineer the Roman aqueducts and the Coliseum and these structures speak till today [1]. Due to the use of SCM, the properties of hardened concrete are improved through hydraulic or pozzolanic activity. It is unique to use the word 'pozzolan' linked to the town of Pozzuoli, Italy where a large deposit of volcanic ash from Mount Vesuvius was found.

Based on the chemical characteristics of the SCM, these can be used either as an addition to the cement or as a partial replacement of the cement. In most of the countries, it is imperative to use the SCM due to the compelling reasons of carbon dioxide emissions due to conventional cement. Further, the SCM is used to replace a portion of the cement content for economical or property-enhancement reasons. The SCM can broadly be divided into two classifications based on their type of reaction, either hydraulic or pozzolanic. It is well known that pozzolanic substances react with water to form cementitious compounds. ASTM C125 outlines the pozzolanic material as 'a siliceous or siliceous and aluminous material'. It also explains that the finely distributed particles react in the presence of water. On another note, SCM can be categorized based on: (i) source, (ii) chemical compositions and (iii) particle characteristics. The first term 'source' aligns the classification on the origin or source, and this provides an ideal way of creating a classification system. Normally, the label 'source' is not implicated with the location where the material is obtained, rather it is dealing with the formation or the process in which the material is attained. Generally, the SCM is split into two classes, namely natural and artificial SCM. The classification of the SCM is shown in **Figure 1** [2].

In general, naturally occurring SCMs are divided into two categories, namely sedimentary and volcanic materials. During the cooling of the molten materials, if the volcanic materials such as tuffs, zeolites and pumice cooled at a faster rate, then these materials can have pozzolanic properties. Furthermore, the faster cooling is essential for the silica to be in amorphous form.

The artificial SCM can be classified typically as industrial by-products and waste materials. In most of the Asian countries, rice is the major crop that produces millions of tons of rich hush. The burning of rice husk results in rice husk ash (RHA). The major oil producing crop, namely palm oil also produces large quantities of wastes that include palm oil fuel ash (POFA), palm oil clinker powder (POCP) and eco-processed pozzolan (EPP).

**Figure 1.** *Classification of SCM [2].*
