*2.1. Materials and Mix Proportions*

The test materials in this study were ordinary Portland cement, fly ash, waste gypsum and water in cement mortar specimens, in which waste gypsum of different particle-size scales was used to completely replace the fine aggregates. The specific gravity of Portland cement is 3.15 and the fineness is 3690 cm2/g; the specific gravity of fly ash is 2.25 and the fineness is 9100 cm2/g. The main oxidation components of fly ash contained 54.98% of SiO2, 29.23% of Al2O3 and 3.38% of CaO. Waste gypsum was sieved into the particle size≦4.75 mm, and the particle size between 4.75 mm and 9.53 mm. The specific gravity of waste gypsum was 2.08 and the fineness modulus (less than 4.75 mm) was 3.58 in the ovendried condition. The main chemical components of waste gypsum were calcium sulfate dihydrate (47.90%), calcium carbonate (14.80%), silicon dioxide (4.31%), calcium hydroxide (21.64%) and calcium oxide (5.82%). The external appearance of waste gypsum, as shown in Figure 1, reveals grayish-white multi-angular-shaped particles. The test mixture is presented in Table 1. The water/cementitious ratio was fixed at 0.65, the amount of fly ash instead of cement was 33%, and waste gypsum (fine aggregates) was 20% between 4.75 and 9.53 mm.

(**a**) Waste gypsum (large particles) (**b**) Waste gypsum (small particles) **Figure 1.** Appearance of waste gypsum.


**Table 1.** Mix proportions (kg/m3).
