PENETRATION OF CHLORIDE IONS FROM AQUEOUS SOLUTIONS INTO CEMENT MATERIALS

The main problem of corrosion of reinforced concrete structures is the penetration of chlorides, which destroy the passivating layer of steel reinforcement and promote its electrochemical corrosion. This is especially relevant for hydraulic structures, transport infrastructure and other objects in contact with sea water or road reagents containing chlorides. The article examines the process of chloride ion penetration into cement materials. Analysis of preliminary studies showed that carbonate additives can both reduce chloride penetration by binding and increase it by changing the porosity of the cement structure. The aim of the work is to analyze the effect of carbonate additives on the depth of chloride ion diffusion and the strength of cement composites. Experimental studies were carried out on cement-sand mortars (cement: sand ratio - 1:3, water-cement ratio - 0.5). Cement-sand mortar specimens with different types of cement (Portland cement CEM I and cement with carbonate additives CEM II) were cured in three environments: seawater, 0.1 M NaCl solution and running water. Tests were carried out after 7 and 28 days to determine the compressive strength, flexural strength and chloride penetration depth. After 7 days of curing, specimens with CEM II cement showed higher flexural strength than specimens with CEM I. However, after 28 days, the strength of CEM I specimens became higher regardless of the environment. CEM II specimens demonstrated stable strength in seawater, whereas in running water and NaCl solution, the strength slightly decreased. Studies of the chloride penetration depth showed that specimens with carbonate additives (CEM II) have a greater chloride diffusion depth compared to specimens made of pure Portland cement (CEM I). This is explained by an increase in capillary porosity of cement composites due to the introduction of carbonates. In seawater, chloride diffusion was less intense due to the presence of other ions that could hinder their penetration. It was found that carbonate additives have a contradictory effect on the properties of cement composites. On the one hand, they promote the binding of chlorides into hydrated phases, and on the other hand, they increase the permeability of the material through modification of the pore structure. Cement with carbonate additives (CEM II) has a higher initial strength, but after 28 days it is inferior to cement without additives (CEM I). Carbonate additives increase the permeability of concrete for chlorides due to modification of the pore structure. Protection against chloride diffusion can be improved by using nanodispersed additives and optimizing the particle size distribution of cementitious materials.