# An Insight into the Chemistry of Cement--A Review
## 1 Introduction  
Cement is a hydraulic binder; it consists of a finely ground inorganic material which forms a paste when mixed with water, is able to set and harden because of numerous exothermic hydration reactions (and processes), and is thus capable of binding fragments of solid matter to form a compact whole solid [1; 2; 3]. After hardening, cement retains its strength and stability, even under the effect of water. Cement forms a composite defined as mortar when mixed with water and fine aggregate (i.e., sand), whereas it forms concrete when mixed with water, sand and gravel (i.e., small stones) [4]. Cement-based materials, such as concrete, have been used for many centuries, mostly in the construction and civil engineering fields, thus becoming the most widely used material, and the second most consumed resource on Earth [5]. Among the different types of cement, ordinary Portland cement (OPC) is the most widely used one [6].  
Even if the technology of cement seems quite well established, recent discoveries in nanotechnology and materials science (e.g., the discovery of graphene) opened the possibility of inducing novel smart functionalities in cement and concrete, allowing their use in advanced technological applications. Examples of such systems are self-healing systems [7; 8], health-monitoring systems [9; 10], conductive materials [11], water permeable materials [12], thermal energy storage materials [13], rubberized concrete [14; 15; 16; 17], and sustainable composites containing bio-based fillers [18; 19; 20].  
From the economic viewpoint, even if cement-based materials are low-cost products (with a maximum cost of 155 EUR ton\({}^{-1}\)), cements still represent raw materials of remarkable interest for the global market (i.e., cement accounted for 0.074% of total world trade in 2020). Countries leading in the worldwide exportation of cement in 2020 were Vietnam (USD 1.5 billion, covering almost 12% of the global market), Turkey (USD 1.3 billion, 10.3%), United Arab Emirates (USD 578 million, 4.7%), Thailand (USD 574 million, 4.6%), and Germany (USD 528 million, 4.3%) [21]. On the other hand, the largest global importers of cement-based materials are the US (USD 2.3 billion, 10.5%), China (USD 1.2 billion, 9.7%), Bangladesh (USD 513 million, 4.1%), Philippines (USD 464 million, 3.7%), and France (USD 452 million, 3.7%) [21].  
This document aims to provide a useful survey of the chemistry behind cement and cement-based materials. Particular attention has been dedicated to the key role played by water in the hydration reactions occurring in OPC, and the different types of compounds generated during this step. In this context, the recent literature [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41] clearly highlights the pivotal interest still present in the development of new advanced cement and cementitious materials. Even if cement is a well-consolidated material, the chemistry of cement (and the chemistry inside cement) remains very complex and not obvious. Therefore, in this study, the different chemical compounds generated during hydration and setting are clearly investigated, keeping an eye on their mechanisms of formation. Furthermore, the physical and structural role of water in driving the entire process of pores formation has been considered. Lastly, a dedicated section regarding the latest developments in the field of sustainable recycling for the construction industry has been proposed, with a special focus on the use of recycled (waste) materials to produce a new generation of \"green\" concretes and mortars.