2.4. Measurement of Contact Angle

Traditionally, the sessile drop method is applied to calculate the surface energy of solid materials. However, the testing process of the sessile drop method is greatly affected by temperature and humidity, leading to inaccurate measurement of contact angles [20,21,22,23,24,25]. The bubble method, by contrast, is insensitive to environmental disturbance [26,27,28], and was adopted herein to measure the contact angle of the geopolymer. Contact angles and surface properties of geopolymers were tested using an optical contacting angle instrument (OCA 20 model, manufactured by Data Physics Instruments, Filderstadt, Germany). Initially, the sample coated with the geopolymer was immersed in a transparent square water tank. A bubble of air or alkane was then released from a needle at a speed of 0.6 microliters per second, which was operated via the injection system of OCA instrument. Eleven types of alkane were used to produce the air bubbles; their chemical compositions and surface free energies are listed in [28]. Since the density of the chosen alkanes was lower than that of the water, the bubbles were well-formed and tightly attached to the test surface under the action of buoyancy. Moreover, as alkane is non-polar, all its surface energy comes from dispersion forces, which simplifies the boundary conditions. Deionized water was selected as the liquid phase, and its surface energy, polar and dispersion components were 72.8, 51.0, 21.8 mN/m [20]. A solid–water–alkane (air) three-phase system was formed after the bubble was perfectly attached to the surface. The contact angle between the alkane (air) and the solid surface was measured with the OCA instrument.

To ensure the accuracy of the test results, two points at different locations of the polished surface of each sample were selected, and the arithmetic mean values of the left contact angle and right contact angle were calculated. For each mix proportion of the geopolymer, three samples were tested. Throughout the test process, powder-free gloves were worn to reduce the contamination of samples. When not being tested, the samples were immersed in deionized water to avoid carbonation.