*3.1. Compressive Strength*

After demolding, the waste gypsum mortar specimens were first maintained in water and air for comparison. When the age of curing reached 7 days, the appearance of the specimens in water curing was irregularly cracked (as shown in Figure 2), which revealed that during the hydration of the specimens, the calcium sulfate dihydrate, calcium hydroxide and calcium oxide of waste gypsum might still react with water to produce expansion, resulting in cracking of the specimen after water immersion. On the contrary, there was no significant difference in the appearance of the air-cured specimens. Obviously, the specimens with waste gypsum completely replacing the fine aggregates should be placed in the atmospheric condition so that the hydration reaction could grow continuously.

**Figure 2.** Appearance of the cracked specimen.

Figure 3 illustrates the compressive strength development of air-cured specimens at 7, 14, 28 and 56 days. The test results indicated a minimal difference between the four test values for each age, and the compressive strength of air-cured specimens increased with age, with compressive strengths of 4.71 MPa and 6.08 MPa at 28 days and 56 days, respectively. As waste gypsum, cement, and fly ash solidified in the specimen, a small number of components such as CaO and Ca(OH)2 that might react with CO2 were stabilized. C-S-H colloids continued to form more with age due to the solidification reaction with

cement and fly ash, resulting in higher compressive strengths in CLSM specimens. The strength results of specimens exposed to the atmosphere (by air curing) indicated that CO2 in the atmosphere would not react continuously with the waste gypsum in CLSM specimens, causing harmful phenomena such as surface layer cracking.

**Figure 3.** Compressive strength histograms.

### *3.2. Volume Stability*

Among the tests measured at 28, 35, 42, 49 and 56 days, the length change curve along with age is shown in Figure 4. The length measurement value of the specimen at 28 days was used as the initial value for calculating the volume change. It was found that when exposed to an atmospheric CO2 environment (atmospherically cured specimens), the length change of the specimens tended to be flat and did not change significantly. The average measured length change of the specimens was 0.0579% for 28 days of observation (age from 28 to 56 days). Based on the test results, it was assessed that CO2 in the atmosphere would not react continuously with the waste gypsum in the specimens to compromise the volumetric stability of the CLSM specimens.

**Figure 4.** Length change curves.

After the specimen had been air-cured for 56 days, it was immersed in natural seawater, tap water and placed in the atmosphere. The length and weight variations at different ages of specimens immersed in tap water and seawater are shown in Figures 5 and 6, respectively. The results indicated that the average length variation of the specimens in seawater was 0.27%. In comparison, the length variation of the specimens before and after immersion in tap water was 1.00%. It can be deduced that there was no significant difference between the lengths of the specimens before and after immersion in seawater and tap water. After 28 days of immersion in seawater and tap water, the weight variations of the specimens were 3.20 and 2.93, respectively. Based on these results, it can be concluded that the weight fluctuations were insignificant. In addition, there was almost no change in the weight and volume of the specimens exposed to the atmosphere. Comparative appearances of the three immersed and exposed specimens are shown in Figure 7.

**Figure 5.** Trend of length change between seawater and tap water (black line: seawater).

**Figure 6.** Trend of weight change between seawater and tap water (black line: seawater).

**Figure 7.** Appearance of the three immersed and exposed specimens.
