# Influence of Sulphate Attack on Properties of Modified Cement Composites
## 3 Results and Discussion  
It is known the nature of the microstructure and composition of the solid phase play an important role in determining the physical and mechanical properties of cement matrix. The process of capillary diffusion is initiated when the samples are immersed in an aggressive solution. The sulphates penetrate into cement matrix and enter the exchange reaction with the reactively active components to form the products of the expansive phase. The titrimetric analysis shown in Figure 1 indicates that the solid phase of the control and electrically conductive concretes intensively absorb sulphate ions during the first 14 days in the volumes of 26.2 mL (the control composition) and 26.7 mL (the electrically conductive composition). According to previous research data, this volume of absorbed sulphate ions can be considered to be directly proportional to the volume of leaching portlandite [7, 41]. The correlation of the data on the absorption of sulphates with the loss of mechanical strength suggests that a significant leaching of portlandite from the solid phase leads to a decrease in the compressive strength of both compositions due to formation of pores and voids. By analyzing the rate of leaching of components of these compositions, it is possible to predict a significant loss of strength in the long term associated with sample decomposition. According to the data presented in Figure 1, and confirmed by the coefficient of variation, the electrical insulation composition demonstrates consistently high mechanical compressive strength for 28 days, in contrast to the control and the electrically conductive compositions. The volume of sulfates absorbed by the electro-insulating cement composite after 28 days was 20.2 mL. During the absorption of the solution, the appearance of micro- and macro-cracks in the sample was not detected. Analysis of the volume of the absorbed solution enables an assumption to be made about the optimality of the pore space in samples. The control composition and the conductive composition have a more developed  
Figure 1: The rate of absorption of SO\({}_{4}\)\({}^{2-}\) ions by concrete samples from a 1N aqueous solution of Na\({}_{2}\)SO\({}_{4}\). (The coefficient of variation in determining the strength was V = 4.3% (control composition); V = 5.2% (electrical insulation composition); V = 4.8% (electrically conductive composition). Rcom—compressive strength, MPa). pore structure that leads to a significant absorption of the aggressive solution. The electrical insulation composite is characterized by a low leaching rate of portlandite. The study of the electrical resistivity under the influence of hydrosulphate corrosion shows that a significant quantity of mobile ions appears as a result of the diffusion of an aggressive solution in the solid phase of cement matrix that, accordingly, reduces the electrical resistivity of samples. In the future, the expansive phase (minerals formed with significant volume, such as ettringrite) in the concrete structure reduces the volume of the pore space due to the formation of a low-conducting crystal-hydrate structure, which increases the specific electrical resistance of the compositions. It is necessary to note the preservation of the effect of the electrical insulation layer because the resistivity of the non-conducting composition is four-fold higher than the resistivity of the conducting composition on the 28 th day of the test (Figure 2). Changes in the physical, mechanical, and electrical properties of composites are caused by physical-chemical transitions with the formation and growth of expansive phases. Differential thermal analysis was used to determine the causes of changes in the physical and mechanical properties of the conductive composition, as shown in Figures 3 and 4.