3.3.1. Water Permeability

Figure 15 shows the water permeability results for all samples exposed to a water pressure of 500 ± 50 kPa for a period of time extended up to 72 ± 2h[66]. As can be seen from the figure, 100% OPC-SCC showed a 3 mm water penetration depth, whereas improved results showed water penetration resistance by FA and GGBS SCC. The SCC with 20.43% FA and MS replacement showed the same permeability as the reference concrete; however, there was a 67% reduction in water permeability when the replacement level increased to 29.5% FA and MS. The SCCs with 38.74% FA and MS and 38.74%, 52.6%, and 71.16% GGBS and MS showed zero water permeability. The results with zero water permeability can be related to the development of the hydration products with the homogeneous combination of the binder ingredients. The hydration products for FA with MS and GGBS with MS may be increased due to the pozzolanic reactivity acceleration. The permeability of concrete is a congregation of the size, shape, distribution, tortuosity, and continuity of the pores; overall, it is not a simple function. It has been reported previously that there is a good relation between concrete durability and maximum continuous pore radius [79]. In this investigation, it is suggested that different particle sizes for the fine sand with dune sand and large surface area for the particles of the cementitious materials in addition to the compactness of the hydration produced led to reduced pore size and cut off pore continuity. The mixture design mechanism priority targeted a great increase in the packing density by filling the micro- and nanopaste void–pore systems, which reduced the coefficient of permeability [80–83]. Moreover, the synergistic interaction of FA with OPC and MS or GGBS with OPC and MS may have refined the pore system generated in the cement gel that created and developed a very dense and complex structure inhibiting the penetration of water within the investigated duration of 72 h [84]. Improvement in concrete permeability may also be related to the superplasticizer effect, which is designed to lower concrete permeability, in addition to its different advantages with SCC.

**Figure 15.** Water penetration depth of SCC mixtures at 28 days.
