Corrosion of ordinary Portland cement and metakaolin-based geopolymer immersed in nitric acid solution

Preventing the corrosion of cementitious materials in acidic environments is crucial for the safety and durability of infrastructure. The corrosion of specimens of ordinary Portland cement (OPC) and metakaolin-based geopolymer (MKG) in 1 mol/l nitric acid solution was investigated in this work. The changes in morphology, mass, compressive strength, pH, chemical composition, crystal phases and pores were compared and analysed. The surfaces of the OPC specimens were corroded into petaloid grooves due to the relatively severe reaction between the calcium hydrated products and the nitric acid solution. The increase in the porosity of the OPC specimens was greater than that of MKG specimens, whose main constituents reacted with the nitric acid solution relatively mildly. However, large quantities of fragments peeled away from the MKG specimens, and their mass and compressive strength losses were greater than those of the OPC specimens. The porosities of the MKG specimens were only slightly changed. The strength loss of the MKG specimens was mainly due to broken matrix structures and newly formed porous silicious frameworks.