Modification of gypsum composite binder via introduction of ground granulated blast furnace slag and waterproofing agent

With a lower carbon footprint and energy consumption, gypsum can be a sustainable alternative to cement as binder material. Nevertheless, the main shortcomings of gypsum, which are low compressive strength and poor water resistance have hindered such application. Therefore, this study aims to enhance the compressive strength and water resistance of gypsum composite binder by incorporating different contents of ground granulated blast furnace slag (GGBS) at 0 %, 10 %, and 20 %, and various dosages of waterproofing agent (WA) at 0.6, 1.2 and 1.8 L/100 kg binder weight. The study found that incorporating GGBS into gypsum composite binders leads to the formation of ettringite and C-S-H gel. These hydration products are beneficial for improving compressive strength and water resistance. Moreover, the addition of 0.6 L/100 kg binder weight of WA enhanced the water resistance of the gypsum composite binder without affecting strength. However, an excessive amount of WA (1.2 and 1.8 L/100 kg binder weight) inhibited the growth of gypsum crystals, resulting in reduced strength, higher water absorption, and poorer water resistance. In summary, the best performance was achieved with GGBS and WA at 20 % and 0.6 L/100 kg binder weight, respectively. At 28 days, the compressive strength reached 18.9 MPa, while the softening coefficient, water absorption, and water contact angle were 0.84, 10.7 %, and 123.34°, respectively, achieving satisfactory water resistance.