**1. Introduction**

The word plastic refers to any material which is made of synthetic or organic compound which has malleable properties. Polyethylene Terephthalate (PET) is being used for beverage and food packages all over the world, but the problem with the disposal of this non-biodegradable hydrocarbon materials has been a major challenge to the world. The major features which play a big role in the usage of plastic are malleable, light weight, impervious to water and low cost [1]. Recycling of plastic into construction materials can help in providing a cleaner environment. Much of the waste generated from this PET has ended up in water drains and eventually into the ocean. It is the belief of some school of thoughts that by 2050, there will be more plastics than fish in the ocean of the world if this trend is not quickly addressed [2]. Concrete bricks are a type of brick that is commonly used in low and medium cost housing development in developing countries since it is cheap and easy to produce. The conversion of the solid residue remaining after pyrolysis to bricks will add to sustainable value addition recycling of waste PET.

During pyrolysis the hydrocarbon bonds are broken down due to the heat applied. **Figure 1** shows the most common but crude way in which people have been disposing of plastic waste in Zimbabwe.

**Table 1** shows the different types of plastics and their symbols. The different plastic types have different uses affected by their chemical structure.

Waste utilization has become an attractive alternative to disposal given the scarcity of space for landfilling and its ever increasing cost; utilization of waste PET in concrete bricks is viable. The use of waste products in concrete not only makes it economical, but also helps in reducing disposal problems [3]. The three types of pyrolysis namely conventional/slow pyrolysis, fast pyrolysis and flash pyrolysis differ majorly in terms of temperature, residence time and products made. **Table 2** summarizes the differences in order to understand the process.

### **Figure 1.**

*Disposal of waste in Harare, Zimbabwe.*


**Table 1.** *Types of plastics and typical uses.* *Conceptualization and Design of a Small Pyrolysis Plant for the Sustainable Production… DOI: http://dx.doi.org/10.5772/intechopen.100053*


**Table 2.**

*Differences between types of pyrolysis [4].*


### **Figure 2.**

*Illustration of pyrolysis properties on end products [4].*

Hence from the table, as the temperature increases, the residence time decreases and the solid products become less. The amount of solid residue produced is determined by the type of pyrolysis and mainly affected by the heating rate. **Figure 2** clearly outlines the effect of heating rate and other pyrolysis factors on the products.

Hence from the figure it can be deduced that in order to obtain a higher yield of the solid residue or char, the heating rate should be slow.

### **2. Background and literature**

Plastic pyrolysis is a tertiary recycling method for disposing of waste and repurposing it in a sustainable manner [5]. Incineration and landfilling are two traditional methods for disposing of plastic waste in developing countries, both of which have shown to be unsustainable. Plastic waste is primarily disposed of in landfills or incinerated. Utilization of PET in bricks and flexible pavements has gained momentum over the past decade. The general failures linked with basic bituminous asphaltic concrete (BAC), which are commonly used in surfacing courses in bricks and flexible pavements include rutting, bleeding, polish, cracking and potholes. Repurposing of the waste PET plastic in raw BAC used in bricks and pavements will be a more sustainable waste management option since flexible pavements constitute over 90% of paved roads around the world [6]. Due to various environmental issues such as air pollution and soil contamination, as well as economic resistance due to space waste and disposal costs, these approaches are encountering significant popular resistance [7]. The conversion of PET to bricks would be a more sustainable method of recycling plastic waste.
