Potential application of geopolymers as protection coatings for marine concrete III. Field experiment

Previous studies have shown a high potential of using geopolymers as new inorganic coatings in protecting marine concrete. This article reports the results of the experiment on field application. Geopolymer coatings were in-situ applied on the surfaces of concrete accropodes along the coast. It was observed that the geopolymer coatings set within 4 hours, bound strongly with concrete and were able to resist the wave shock in the first tide rise. There was a modicum of calcite detected by X-ray diffraction (XRD) but no sulphate was found in the coatings within 6 months. One issue raised during in-situ application is the large shrinkage of the geopolymer paste under ambient condition. Micro-cracks on the surfaces were observed after 7 days although the MgO-based expansion agent and polypropylene (PP) fibers were added to withstand the shrinkage. The humidity of the atmosphere and the thickness of the coating layer are the two significant factors affecting the integrity of coatings. It was noted that the coating with a thickness of 5 mm at the tidal area, where contact with seawater periodically, exhibited the best integrity. To solve the problem of large shrinkage, it is recommended to use suitable aggregates in coating paste and to develop appropriate shrinkage reducing agents together with careful curing procedures.