Compare the manner in which the aggregate particles become bonded together in clay-based mixtures during firing and in cements during setting.

During the firing process, clay-based mixtures undergo several physical and chemical changes that lead to the bonding of aggregate particles. The main components of clay are hydrous aluminum phyllosilicates (such as kaolinite, montmorillonite, and illite). When the clay is heated, water molecules are removed, causing clay particles to shrink and come closer together. This is followed by a series of chemical reactions that lead to the formation of new minerals, glass, and gas. As the temperature increases, the clay minerals (such as kaolinite) begin to decompose, and the aluminum silicates react with other elements in the mixture to form new compounds, such as mullite (3Al2O3·2SiO2). At higher temperatures, the remaining silicates melt, forming a glassy phase that fills the pores between particles. This glass acts as a binder, cementing the aggregate particles together and giving the fired clay its strength and rigidity.

Cement is made primarily of calcium silicates, which react with water in a process called hydration. During the setting of cement, various chemical reactions occur that lead to the bonding of aggregate particles. The primary reaction taking place during the setting of cement is the hydration of tricalcium silicate (Ca3SiO5) and dicalcium silicate (Ca2SiO4). When water is added to cement, these calcium silicates react with water molecules to form calcium-silicate-hydrate (C-S-H) gel and calcium hydroxide (Ca(OH)2). The C-S-H gel is a highly reactive substance that binds the cement particles together. The reaction also produces heat, which accelerates the rate of the reaction and formation of the C-S-H gel. As the C-S-H gel forms, it fills the spaces between the aggregate particles, bonding them together. Additionally, the calcium hydroxide produced during the hydration reaction can also react with other components in the cement mixture to form additional cementitious compounds, such as calcium aluminate or calcium sulfoaluminate, which can further enhance the bonding process.

In clay-based mixtures, aggregate particles are bonded together primarily due to the formation of new minerals, such as mullite, and the development of a glassy phase during the firing process. This process involves high temperatures and occurs over a longer period as the clay mixture is heated and cooled. On the other hand, in cement, the bonding of aggregate particles is achieved through the hydration of calcium silicates and the formation of calcium-silicate-hydrate gel. The setting and hardening of cement occur at ambient temperatures and happen relatively quickly compared to the firing process in clay-based mixtures. So, while both clay-based mixtures and cement involve chemical reactions to bond aggregate particles together, the mechanisms are different. Clay-based mixtures require high temperatures and new mineral formation, while cement hydration occurs at lower temperatures and forms new gel-like substances that bind the particles.