Effect of Activator Type, Curing Condition and Calcium Addition on the Strength and Microstructure of Fly Ash-Based Geopolymers

Abstract

The paper describes the synthesis and study of the mechanical characteristics and microstructure of geopolymers based on low-calcium fly ash (Class F) depending on the type of activator, temperature and humidity conditions of curing, and the content of an additional source of calcium. Sodium silicate solution with a silicate modulus SiO2/Na2O = 1-3 (Ms=1-3), a density of 1400 kg/m3, and a 12M NaOH solution were used as activators. Curing conditions included: (1) curing in an oven at 90 °C for 6 hours; (2) curing with steam at 90 °C for 6 hours. The heating rate was 0.5 deg/min, inertial cooling. During the experiment, it was revealed that for the investigated fly ash (FA), the most promising type of activator is sodium silicate solution with Ms=1, and the preferred curing mode is steam treatment since under these conditions and the type of activator the following were achieved: compression strength – 15 MPa, bending strength – 6 MPa, open porosity – 24%, water absorption – 15%, apparent density – 1830 kg/m3. Calcium oxide (CaO) was used as an active additive at dosages of 10, 15, and 20% by weight. With the introduction of 15% CaO into geopolymers activated by sodium silicate solution with Ms=1 and cured with steam, it was possible to increase their compressive strength by more than 2.5 times, the flexural strength increased by 17%, and the density by 9%, open porosity decreased by 25%, water absorption by 34%. It has been established that the curing conditions and the type of activator are the main factors that determine the properties and structure of FA geopolymers. A promising curing condition for alkali-activated FA is steam treatment at 90 °C since under these conditions it was possible to achieve strength twice as high as under heat treatment conditions. The introduction of a calcium component (up to 15% CaO) has a positive impact on the mechanical properties and durability of FA geopolymers.