Effect of thermal curing process on geopolymer concrete properties

Metakaolin (MK) which has pozzolanic characteristics and fly ash (FA) which is a thermal power plant waste were used as main raw materials in this study. The metakaolin used in the study was obtained by calcination of kaolin clay at 700 °C obtained from NG Kütahya. The fly ash was supplied from Kütahya Tunçbilek Thermal Power Plant. Sodium hydroxide (NaOH) and sodium silicate solution (Na2SiO3) (SS) were used as alkaline activators in the preparation of the samples. Aggregates used in the production of concrete were of three different classes (0-5 mm, 5-15 mm and 15-22 mm) and obtained from a special furnace in Afyon. FA and MK used as raw materials in the preparation of the geopolymer concrete samples were mixed in different proportions with alkaline activators (NaOH and SS) to achieve the best reaction. The mixtures prepared in a laboratory-type Pan mixer were placed in 10 x 10 x 10 cm metal molds with vibration technique. Afterwards, the samples were kept in the laboratory environment for one day and then placed in an oven in order to carry out the curing processes. The geopolymer concrete samples were cured at different temperatures (60 and 90 oC) and for different periods of time (5 and 24 hours) in order to apply the thermal curing processes. The samples were removed from the molds after the completion of the curing processes. Physical properties of the geopolymer concretes such as water absorption, apparent porosity, bulk density and apparent density, and their ultrasonic velocity values were determined using 7-day samples. Afterwards, compressive strength tests (7 days) were carried out according to TS EN 12390-1standard. A 200 tons concrete press was used for the compressive strength tests. The results indicate that the compressive strength values of the samples increase with an increase in curing times and FA addition rate. The highest compressive strength values were obtained from B5-24 concrete samples with 40% FA addition cured at 60 oC (30.1 MPa) and A5-24 geopolymer concrete samples with 40% FA addition cured at 90 oC (31.4 MPa). In light of this data, it can be concluded that the geopolymer concretes produced in the study are applicable, however, the properties of durability should be investigated further.