# Water Impermeability of Self-Compacting Fly-Ash-Containing Concrete
## 3 Results and Discussion
Figure 3: Strength diagram of self-compacting fly-ash-containing concrete at water consumption is 180 kg/m\({}^{3}\), r = 0.41. Figure 2: Dependencies of water impermeability of self-compacting fly-ash-containing concrete on: (a) W/C at 1—D = 50 kg/m\({}^{3}\); 2—D = 150 kg/m\({}^{3}\); 3—D = 250 kg/m\({}^{3}\); (b) ash consumption at 1—W/C = 0.3; 2—W/C = 0.4; 3—W/C = 0.5; water consumption is 180 kg/m\({}^{3}\); r = 0.41; S = 390 m\({}^{2}\)/kg. The water impermeability of the self-compacting fly-ash-containing concrete of the studied compositions at 28 days, due to the choice of optimal values and technological factors, can achieve a value of 1.3 MPa. With the increasing age of concrete, water impermeability increases (Figure 5) due to irreversible changes in the structure of the pore volume and an increase in the solid phase in the cement stone. Increasing the dispersion of fly ash and its consumption has an extreme effect on the water impermeability of the concrete. Increasing the specific surface area of ash to 390 m\({}^{2}\)/kg causes a significant increase in water impermeability. Further increase in the specific surface area of ash decreases its positive effect. This can be explained by the influence of ash dispersion on the capillary porosity of concrete, which, after reaching a minimum value as a result of increasing cement hydration products, begins to grow due to the increasing water demand of the concrete mixture. The analysis of the mathematical model (Equation (7)) and the strength diagram based on it (Figure 6) also indicates the extreme influence of both the consumption and dispersion of ash on the concrete strength. The range of possible fly ash consumption rates narrows as both concrete strength and water-cement ratio increase, while the strength remains unchanged. If the fly ash consumption changes, the superplasticizer consumption must be adjusted. This is necessary to achieve the same concrete mix workability. Figure 5: Changes in water impermeability of self-compacting fly-ash-containing concrete over time. Figure 6: Strength diagram of self-compacting fly-ash-containing concrete at W/C = 0.4; water consumption is 180 kg/m\({}^{3}\); r = 0.41. The diagram analysis (Figure 6) suggests that selecting the optimal values of ash consumption and its dispersion is possible. If the minimum possible ash consumption is optimal, it increases with an increase in the W/C, corresponding to the data [25] for low-slump ash-containing concrete. However, for practical purposes, the optimal fly ash consumption can be the same over the entire range of concrete strengths. This consumption can be found analytically from Equation (7) by determining the partial derivative of X4: dfc/dX4. For the accepted conditions, X4 opt = 0.2; i.e., the optimal fly ash consumption is 170 kg/m\({}^{3}\). Increasing the dispersion of ash (by grinding it) should increase its chemical activity, which has a positive effect on the growth of concrete strength. On the other hand, an increase in the dispersion of ash causes an increase in the concrete mixture's air content and the mortar component's viscosity [26]. When it reaches values above the permissible ones, external stratification of the mixture may occur. It should be noted that the strength of the concrete is affected by the grain size distribution of the initial components. Excessive content of fine ash fractions can lead to the stratification of self-compacting concrete [27]. The cement consumption in self-compacting fly-ash-containing concrete at the optimum fly ash consumption and its optimum dispersion are much lower than in conventional plasticized concrete. For example, for concrete with the superplasticizer additive without fly ash to achieve a strength of 56 MPa at the age of 28 days (at a water consumption of 170 kg/m\({}^{3}\); W/C = 0.3 and r = 0.43), the cement consumption was 565 kg.