2.2.3. Water Absorption

In addition to compressive and splitting tensile tests, tests for WA were also performed on hardened concrete samples of all the concrete mixtures according to ASTM C948. To perform WA tests, 100 mm diameter and 50 mm thick concrete samples were cut from 91-day moist-cured concrete specimens. Samples of cut concrete were soaked in water at 21 ◦C for 24 h and then periodically weighed until the saturated-surface dry weight (SSD) stabilized. If less than 0.5% difference in weight occurred between successive SSD weight measurements, the weight was considered stable. The letter "B" refers to the most recent yielded weight of the samples. Afterward, the mass of the specimens suspended in water was measured to the nearest 0.01 g and labeled as "A". Concrete samples were oven-dried at 100−110 ◦C, and their weights were taken every 24 h. Upon achieving a weight loss of <0.5% of the last measured weight, the sample was cooled inside a vacuum desiccator at room temperature and weighed, which is referred to as "C". To calculate WA and AP, the following equations were used:

$$\text{Water Absorption (\%)}=(\text{B}-\text{C})/\text{C}\times100\tag{1}$$

$$\text{ApparentPorosity (\%)} = (\text{B} - \text{C}) / (\text{B} - \text{A}) \times 100\tag{2}$$

### 2.2.4. Casting and Curing of Paste Samples for Microstructure and Pore Structure Analysis

Paste samples for the control (100% cement) and other mixes with various amounts of WSA (binary) and blends of WSA with SF (ternary), as partial substitutes for cement (weight percentage), were prepared. Using a Hobart mixer (Hobart, IN, USA), a paste of standard consistency was obtained for all mixes. The freshly mixed cement paste was poured into small plastic containers (20 mm diameter and 50 mm height) immediately after mixing. The plastic containers were subsequently sealed and capped prior to curing. Following 91 days in the cured state, the specimens were dried with a solvent exchange method to inhibit the hydration process. Finally, the flaky slices and powder specimens were then washed with isopropanol for 15 min. In order to remove the isopropanol present in the paste samples, the samples were dried in an oven at 40 ◦C for 30 min. The samples were then sealed in plastic bags for storage before testing.
