## 2 CO\({}_{2}\) Emissions by the Cement Sector  
About 65% of carbon dioxide emissions of the cement sector are due to the calcination of raw materials, mainly limestone calcination, as shown in Equation (1). The remaining 35% are emitted by the fuel combustion. Consequently, about 3% of the global anthropogenic CO\({}_{2}\) emissions can be attributed to the calcination process of the raw materials produced in clinker kilns [4]. The calcium oxide content in a clinker is 0.65 kg CaO/kg clinker; therefore, it is necessary to extract about 1.16 kg of limestone from the earth's surface to produce 1 kg of clinker. \[\text{CaCO}_{3}\rightarrow\text{CaO}+\text{CO}_{2} \tag{1}\]  
\[1\text{ kg}\to 0.56\text{ kg}+0.44\text{ kg}\]  
\[1.16\text{ kg}\to 0.65\text{ kg}+0.51\text{ kg}\]  
The largest source of the carbon dioxide emissions from the Portuguese cement industry is limestone calcination (62.4%-65.4%, 2005-2015 period). The remainder is accounted for by fuel combustion (34.6-37.6, 2005-2015 period). The content of CaO in clinkers amounts to 65%; then, the theoretical carbon dioxide emissions by calcination in the clinker production are 0.51 kg CO\({}_{2}\)/kg clinker. On the other hand, carbon dioxide emissions from fuel use depend on the type of fuel (petcoke, coal, fuel oil, alternative fuels, and so on). The global cement sector indicators in 2014, published by The International Energy Agency (IEA) [5], showed that the cement sector was the second biggest industrial carbon dioxide emitter, in general, and the first biggest industrial process emitter, in particular (63% CO\({}_{2}\) process and 37% energy-related carbon dioxide). In regard to energy use, the cement sector was the second biggest industrial coal user and the third biggest industrial energy user (61% coal, 13% electricity, 13% oil, 9% gas, 3% wastes, and 2% biomass). Therefore, the cement industry could play a big role in lowering carbon dioxide emissions. Particularly, carbon intensity might be reduced in several ways, such as lowering the clinker/cement ratio, increasing energy and process efficiencies, switching to fuels with lower carbon intensities, and carbon dioxide removal. In addition, after a period of time, part of these carbon dioxide emissions is absorbed by cement-based materials when the physicochemical process known as carbonation occurs. The chemistry of carbonation has been reviewed in some papers [6; 7; 8]. Carbon dioxide emissions per ton of cement have been lowered worldwide in the last few years [9]. Nevertheless, the production of cement is growing rapidly, achieving an amount of 4.65 billion tons in 2016 (Figure 2) [10]. Asia produces 80.5% of the world's production, while Europe-CEMBUREAU only produces 5.3%. Figure 3 shows the gross carbon dioxide emissions for grey clinker; the emissions shown are the direct carbon dioxide emissions, except for the on-site electricity production and the emissions from biomass fuel. Carbon dioxide emissions due to clinker production have been reduced in the last decade, as shown in Figure 3 [9], from more than 925 kg CO\({}_{2}\)/t clinker (North America) in 2005 to 825-850 kg CO\({}_{2}\)/t clinker in 2015. In particular, it should be highlighted that Portugal has experienced a significant decrease from 855 kg CO\({}_{2}\)/t clinker in 2005 to 821 kg CO\({}_{2}\)/t clinker in 2015 (Figure 3). Additionally, it should be pointed out that the European cement industry, in general, has considerably diminished its environmental impact [9; 10]. There was also a slight decrease in CO\({}_{2}\) emissions in several countries in the 1990-2003 period, when considering both calcination process and fuel combustion per ton of cement, as shown in Figure 4[11]. China, Spain, and Germany, among others, have exhibited a significant reduction of carbon dioxide emissions as consequence of their reduction of the clinker factor. That is, they followed a significant growing trend of blended cement production with an upward content of additions.