# Study of Eco-Friendly Belite-Calcium Sulfoaluminate Cements Obtained from Special Wastes
## 1 Introduction  
The cement industry consumes huge amounts of natural raw materials and fuels, mainly fossil fuels and pet coke. It is one of the main contributors to climate change due to greenhouse gas emissions, mainly CO\({}_{2}\). In 2018, global cement production was about 3.99 billion tonnes, accounting for about 8% of all anthropogenic carbon dioxide emissions [1]. Ordinary Portland cement (OPC), a blend of clinker (about 95%) and gypsum, is the most used binder in the world. OPC clinker is usually obtained by heating a mixture (raw meal) of limestone (L, 80% by mass) [2] and clay (C) in a rotary kiln at about 1500 \({}^{\circ}\)C. According to [3], about 0.83 kg of CO\({}_{2}\) is released from the production of 1 kg of OPC; it is generated from both L thermal decomposition (about 60% of the total CO\({}_{2}\) emissions) and fuel combustion [4; 5]. To reduce its carbon footprint, the cement industry is exploring how to manufacture OPC with less impact on the environment through a switch to: (I) alternative fuels, (II) reduction in thermal energy demand, (III) use of non-carbonated CaO sources instead of L, (IV) decrease in clinker to cement ratio, (V) application of carbon capture and storage technologies to cement plants [6], and (VI) development of low-CO\({}_{2}\) non-Portland binders (e.g., alkali-activated, Mg-based cements, calcium sulfoaluminate (CSA) and belite-CSA (BCSA) binders) [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. BCSA cements represent a promising alternative to OPC as they exhibit comparable physical and mechanical properties [26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. The technical behavior of BCSA cements mostly depends on the ability of C\({}_{2}\)S and C\({}_{4}\)A\({}_{3}\)S (their main components) to generate, upon hydration, CSH (calcium silicate hydrates, at medium and longer periods) and C\({}_{6}\)A\({}_{3}\)H\({}_{32}\) (ettrinigite, at early ages), respectively. They are also characterized by eco-friendly features due to: (a) a lower synthesis temperature, (b) a reduced amount of L in the clinker generating raw mixture, (c) a decreased specific fuel consumption, and (d) easier clinker grindability [25; 29]. All these features allow a reduction in CO\({}_{2}\) of up to 30% [25]. BCSA cements are obtained at temperatures ranging between 1250 and 1350 \({}^{\circ}\)C by burning a raw meal composed of L, C, natural gypsum (G) and bauxite (B); natural raw materials can be effectively replaced by special wastes that are often difficult to reuse and/or landfill (e.g., blast-furnace slag, coal combustion ash from both traditional combustors and fluidized bed reactors, red mud, alumina powders, phosphogypsum and flue gas desulfurization gypsum). In particular, the use of alumina-rich wastes can lower the manufacturing costs of BCSA cements mainly depending on the use of the \"expensive\" bauxite [42; 43; 44; 45; 46; 47; 48; 49; 50; 51]. Water potabilization sludge (W) is a clayish residue produced in conventional drinking water treatment plants where suspended particles are eliminated by means of coagulation and flocculation processes using aluminum salts-based coagulants [52]. High quantities of sludge, representing about 1-5% of the total untreated water amount, are generated; therefore, huge amounts of W are produced worldwide (~10,000 ton/day) and most of them are landfilled. Until now, W has been used in the production of bricks, ceramics, lightweight aggregates and as an alternative raw material in the Portland cement manufacturing process [53]. Titanogypsum (T) is a sulfate-rich waste deriving from the production of titanium dioxide pigment, which is generally used in paints, plastics, papers, coatings, cosmetics, etc. The global production of TiO\({}_{2}\) pigments accounts for about 4 million tonnes/year; about one-half of TiO\({}_{2}\)-based pigments is produced by the sulfate process, the other half by the chloride route.