Effect of Microsilica and CFRP Fibers on Mechanical and Durability Properties of Ground Granulated Blast Furnace Slag–Based Geopolymer Concrete

Concrete is the most widely used building material in construction industry worldwide and its constituents are easily accessible everywhere. However, cement industry, as the producer of the primary binder of concrete, is one of the effective sources of environment degradation. Cement production needs extraction of mineral resources and burning fuel and causes extensive greenhouse emission due to disintegration of raw materials. Cement production alone is responsible for 7% of global CO2 emission with estimated annual growth of 4%. Toward environmental sustainability, one way is partially or totally replacing cement by waste or byproducts of other industries such as fly ash, ground granulated blast-furnace slag (GGBS), waste water, metakaolin, and silica fume. Geopolymer is a cementitious material with comparable characteristics to those of ordinary cement produced by alumina- and silica-rich waste materials. Therefore, it does not require energy-intensive and pollutive calcination process. Geopolymerization is formed by reaction of silica-alumina under an alkaline solution which creates three dimensional Si-O-Al-O polymeric chains to attain compressive strength, compared to the ordinary cement which develops calcium silicate hydrates (C-S-H) as the main adhesive. Extensive research has conducted on geopolymer concrete. However, more investigations are needed to better understand characteristics of geopolymer concrete containing additives. Fibers are proved to have a positive effect on mechanical strength of concrete. As well, fillers such as microsilica can improve mechanical and durability of concrete. Moreover, most studies in this area are focused on fly ash-based geopolymers and the investigations on GGBS-based geopolymer are rare in the literature. In this study, mechanical and durability of GGBS-based geopolymer concrete containing CFRP fibers and microsilica is investigated. Different concrete samples with 0-3% CFRP fibers and 0-10% microsilica are prepared and experimentally tested. Sodium Hydroxide (NH) and Sodium Silicate (NS) solutions are used as alkali activators. 8 M NH as well as NS with 14.7 Na2O and 29.4 SiO2are used with the NS/NH ratio of 2.5. Since no standard exists for mix design of geopolymer concrete, proposed mix design by Venkatesan and Pazhani (2015) is used. Alkaline to binder ratio of 0.4 is selected with 430 kg/m3binder.The specimens were tested after 28 days of curing. Next, mechanical and durability tests including compressive strength, tensile strength, ultrasonic pulse velocity, water absorption, RCPT, and acid resistance are conducted on the samples. Also, microstructure of the geopolymer concrete is investigated. Results of experimental tests show that, compressive and tensile strength of geopolymer samples decrease by adding microsilica. However, 5% microsilica is the best value to enhance mechanical properties of geopolymer concrete. On the other hand, microsilica can enhance durability properties of geopolymer concrete so that adding 5% microsilica causes moderate improvement of water absorption and chloride penetration. The greatest impact of microsilica is on acid resistance by which adding 5% microsilica resulted in 67% improvement of compressive strength loss. However, unlike the microsilica, CFRP fibers have detrimental effect on mechanical properties and durability of the geopolymer concrete since adding fibers can yield disruption of concrete integrity .On the other point of view, microstructure study shows that all the specimens exhibit micro cracks that can inversely affect the performance of concrete. Also, SEM images show that there is not a strong bond between CFRP fibers and binder paste which yields lower performance of concrete specimens containing fiber.