Pozzolanic Reaction

MS Quartz and cristobalite

PFA Quartz and mullite peaks

RBD Quartz, hematite, anhydrite, gypsum, and anorthite

RHA Quartz and cristobalite

Tile Quartz, hematite, gismondine and gypsum

YBD Mullite and cordierite

2.5.2 Pozzolanic reaction

One of principal uses of pozzolans is in the production of concrete and mortars as partial replacements for cement. In the presence of water, cement release lime (also known as portlandite) in solution which reacts with available SiO2 and/ or Al2O3 to form calcium silicate hydrates(C-S-H), calcium aluminate hydrates(C-A-H) or calcium aluminate silicate hydrates(C-A-S-H) as shown in equations 2.6 and 2.7. (Dunstan, 2011). The pozzolanic reaction yields products that are similar to the hydration products of

The economic benefits of using pozzolans come from the fact that most pozzolanic materials are either naturally occurring materials or by-products from industrial processes meant for dumping. On the other hand cement production is an expensive process and it’s reflected on the expensive price of cement. Replacing cement by a significant quantity of pozzolans therefore saves cost and energy of both cement production and waste disposal (Malhotra & Mehta, 1996).

Hundreds of millions of tonnes of pozzolanic materials are generated every year (Malhotra & Mehta, 1996). These materials possess potential risk to the environment when inadequately dumped due to their containments of toxic and hazardous chemical elements. The recycling or reusing of these materials in the construction industry therefore provides a saver route for their disposal and hence improves environmental sustainability (Malhotra & Mehta,

On the other hand it is also comprised of heavy metals and pathogens which may pollute the soil and contaminate food products (McGrath, et al., 1994). Also, the biological content of sewage sludge tends to reduce amount of soil biomass and alter the soil biochemical activity (Häni, Siegenthaler, & Candinas, 1995). Such negative impacts have raised public concerns (Inguanzo, et al., 2002). As a consequence tighter regulations have been established to restrict the implementation of such practices such as the EU Council Directive 86/278/EEC which encourages the right application of sludge in agriculture but at the same time seeks to protect the environment and alternatively human’s health (Kirchmann, Börjesson, Kätterer, & Cohen, 2016).

These various methods posses risks to the environment and health to a greater or lesser extent. This has lead to the consideration of other means of managing sewage sludge such as sewage sludge treatment.

2.6.1 Sewage sludge