# Pore Structure Degradation of Different Cement
## 1 Introduction  
Acid rain can be produced by natural effects (e.g., volcanic eruptions), but it is human action that causes the most emissions of sulphur and nitrogen oxides (e.g., metallurgical industry, energy production processes, and heating system). These constituents dissolve in contact with atmospheric water and generate sulphuric and nitric acid, which subsequently fall to the earth's surface; this is known as acid rain. Moreover, the chemical substances that generate acid rain, before precipitation and due to the wind, can travel long distances from the actual polluting region [1]. Acid rain (pH \(<\) 5) causes the deterioration of construction material surfaces on which the pollutants are deposited. In addition, several studies show that acid rain generates a layer of gypsum on the substrate [2, 3] due to a chemical reaction of sulphuric acid with products of cement hydration (such as calcium silicate hydrate and calcium hydroxide) [4, 5], and with calcium carbonate [6] that usually comes from aggregates utilised in the manufacturing of cementitious materials. Since cementitious materials are very likely to be in contact with acidic environments (such as acid rain, infrastructures in contact with groundwater and sanitation networks) it is necessary to study admixtures that can improve the resistance of cementitious materials to aggressive environments and the effects of these admixtures. This study compares three admixtures that have already been used with good results in acid attacks, but have not been tested to identify which ones have better performance. The tested materials are nanosilica, zinc stearate, and ethyl silicate; these admixtures are utilised to increase the durability of cementitious materials, as explained below. Nanosilica has been employed as an alternative to silica fume in the manufacturing of cement-based materials [7]. [PERSON] et al. state that the reactions of nanosilica in concrete are similar to that of silica fume or micro silica as regards strength and durability improvements [8]. However, [PERSON] et al. [9] indicate that on using nanosilica as admixture instead of silica fume, a higher compressive strength is obtained, especially in the early stages. Nanosilica is currently being used in cementitious materials to increase: (i) resistance, due to its pozzolanic effect [10; 11]; and (ii) durability, due to the reduction of porosity and improvement of the microstructure of cement-basedmaterials [12]. A recent study analysed the effect of nanosilica on concretes exposed to sulphuric acid; the results indicated a better performance of nanosilica concretes, since their compressive strength was greater and mass loss was lower [10]. Water-repellent materials have been used extensively in the manufacturing of cementitious materials to prevent the absorption of water. Its use is normal when mortars must be preserved from water or rain. [PERSON] et al. [13] studied several metallic soaps (zinc stearate, calcium stearate, and sodium oleate) and powdered silicone on mortars; their work showed that when mortars are exposed to sulphuric acid, a layer of gypsum is formed and some disintegration takes place. Despite the previous effects, the water-repellent admixtures improved the mortars' durability in terms of acid attack resistance. Zinc stearate was also used as an admixture in adobes, achieving good results as regards acid attack resistance [14]. Calcium stearate was studied in cement pastes under sulphate attack, and an increase in resistance to sulphate attack was found [15]. Ethyl silicate (also called TEOS-tetraethyl orthosilicate) is a versatile product that has been used since the early 20 th century for various applications (e.g., stone consolidant, ceramic binder, adhesive, source of amorphous silica) [16]. Currently, it is one of the most used products in the consolidation of architectural heritage materials, such as bricks, stone, and lime mortars. Also, it can be used as surface treatment on cement-based materials for its pozzolanic effect [17; 18; 19].