# Carbon Dioxide Uptake by Brazilian Cement-Based Materials
## 3 Method to Estimate the Carbon Dioxide Removals Due to Carbonation
[40], the carbonation resistance in mortar and concrete is mainly governed by the carbon dioxide buffer capacity per volume of cement paste. In addition, the (water/CaOreactive) ratio is an indicator to evaluate such carbonation resistance. This indicates that the higher the cement content in the mortar or concrete, the better the carbonation resistance; and the lower the water/CaOreactive ratio, the better the carbonation resistance. Masonry mortars usually are made with high water/cement ratios (w/c about 0.60), whereas concretes normally are made with lower w/c ratios [41], as low as 0.45 in some cases depending on the exposure class [42; 43]. Furthermore, when the water-to-cement ratio is increased, the capillary porosity is increased. Capillary porosity is the main factor negatively affecting the compressive strength of cement-based materials, in addition to the carbon dioxide coefficient. The minimum cement content in concrete for the exposure class XC 1 is 260 kg/m\({}^{3}\) (C20/25 concrete with 20-25 N/mm\({}^{2}\)), while for the exposure class XA 1 is 360 kg/m\({}^{3}\) (C35/45 concrete with 35-45 N/mm\({}^{2}\)) [41]. By contrast, there are different grades of cement mortar (M0.5-M7.5), which is the minimum required for 28-days compressive strength (0.5-7.5 N/mm\({}^{2}\)). The ratio of cement to sand ranges generally from 1:3 to 1:8 for the mortar to be workable and strong. Proportions of the Portland cement mortar for masonry work with brick/stone as the structural unit range from 1:3 to 1:6 (cementsand); for architectural work it is about 1:6 and for flooring work the cementsand proportions range from 1:4 to 1:8. Similar proportions can be found in the American Standard Specification for Mortar Cement [44]. It is essential to point out that mortar involves, first and foremost, the use of low-clinker cements. For instance, the clinker content in European masonry cements must be greater than or equal to 25% (MC5) or to 40% for the rest of the masonry cements (MC12.5-MC22.5) [45], leading to a low carbon dioxide buffer capacity. Accordingly, this indicates that concrete even with a low cement content exhibits a better carbonation resistance than masonry mortars. With regard to the clinker content in Portland cements commonly used in structural concrete, this value can total up to 95% [46]. Concrete is a very strong structural building material composed of water, cement, and sand just like mortar. Nevertheless, concrete also has some coarse aggregates that makes it stronger and more durable than mortar. Mortar, which is composed of water, cement, and sand, has a higher water-to-cement ratio than concrete and, therefore, a higher capillary porosity. Mortar is used as the glue to hold bricks, blocks, etc., together, whereas concrete is mainly used for structural elements. The main factor influencing carbonation is the pore system which affects the carbon dioxide diffusivity through the mortar and concrete. The carbonation rate is controlled by the penetration of carbon dioxide into their pore structure by diffusion. Therefore, the pore size distribution influences the rate of transport. Factors affecting the diffusion rate include the type and amount of cement, the porosity of the mortar or concrete, and the curing conditions, among other factors [47]. The decrease in the capillary porosity, achieved by lowering the water-to-cement ratio, further decreases the carbonation rate. Therefore, a linear relationship between porosity and accelerated carbonation has been found [48]. In addition, adequate curing conditions are required to avoid this unwanted effect [47]. Most of the worldwide concrete design guides include the minimum conditions for the curing of concretes but not for mortars [42].