Metakaolin and fly ash based geopolymers compared with cementitious mortars of the same strength class: performances and corrosion behaviour of black and galvanized steel bars

Mechanical, morphological and durability aspects of three types of geopolymer mortars were compared with those of traditional cementitious mortars of comparable compressive strength. To this aim, mortars belonging to three mechanical strength classes (R1 ≥ 10 MPa, R2 ≥ 15 MPa and R3 ≥ 25 MPa according to EN 1504-3:2005) were tested and compared. Geopolymers were obtained with fly ash and metakaolin as precursors and a mixture of sodium silicate and NaOH or KOH as liquid activators. In order to obtain R1, R2 and R3 mortars cementitious mixtures were prepared by partial substitution of Portland cement with hydraulic lime, while geopolymers were prepared by varying the concentration of NaOH or KOH with a waterglass/hydroxide ratio always equal to 1. The obtained samples were characterized both in the fresh and in the hardened state. The first was evaluated by workability and density of the fresh mortar, the latter by compressive strength, dynamic modulus of elasticity, adhesive strength on ceramic surface, free and restrained shrinkage and by microstructural analysis such as SEM and mercury porosimetry. Durability aspects were also investigated through water vapor permeability, capillary water absorption, resistance to salt crystallization and corrosion of possible embedded rebars. Corrosion tests were carried out by measuring corrosion potential and corrosion rates of black and galvanized steel bars during both the curing period and wet-dry cycles in a chloride solution. At the same mechanical strength class, geopolymers shrink more than cementitious mortars if free while their low modulus of elasticity causes a lower shrinkage if mortars are restrained with bars. Pore dimensions affect the water vapor permeability, which resulted to be more pronounced in geopolymers than in cement mortars, and capillary water absorption with low values especially for fly ash geopolymers. During the first month of curing, the higher alkalinity of geopolymers matrix prolongs the active state of both black and galvanized steel bars. However, when exposed to a chloride solution, fly ash based geopolymers offer a higher protection to reinforcements than cementitious mortars