PARTIAL CEMENT REPLACEMENT IN CONCRETE WITH RECYCLED GYPSUM FROM WASTE DRYWALLS

Abstract

Utilizing recycled gypsum powder from waste gypsum drywalls as a partial replacement for cement in concrete can address the problems that cement manufacturing process and waste gypsum drywall disposal could cause to the natural environment. For the first phase of this study, the impact of recycled gypsum powder content on the compressive strength of concrete cylinders in a short-term period is analyzed. As gypsum drywalls are typically covered with paper-based sheets, the recycled powder contains paper particles (hereafter called whole gypsum). In some cases, for this study, the recycled powder was sieved to remove the paper particles producing a fine powder (hereafter called fine gypsum). Five concrete mix designs which include 0, 10 and 20% of recycled fine gypsum powder and whole gypsum powder are considered for this study. A total of 45 cylindrical specimens were prepared in the first phase and three specimens of each mix design were tested after 7, 28, and 90 days of curing. For the second phase of this research, the durability of gypsum concrete specimens is evaluated using another 153 concrete cylinders. Specimens involving different gypsum content (0,10 and 20% of cementitious material mass) were exposed to five different environmental conditions namely air dry, freshwater, seawater, freshwater- air dry cyclic, and seawater-air dry cyclic. Specimens were tested in compression after 1000, 3000, and 6000 hours of exposure. It was revealed that whole gypsum particles could be as functional as fine gypsum particles as supplementary cementitious material as the former did not harm the properties of concrete noticeably. Also, the presence of gypsum in combination with fly ash enhances the mechanical properties of concrete if concrete specimens are submerged in freshwater or seawater and more than 3000 hours of exposure time passes. Wet and dry cycles also did not adversely impact the performance of concrete with gypsum content.