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Knowledge of the durability and related properties of alkali-activated materials is a prerequisite for their application as building materials. However, to date only limited data exists in this regard. The present contribution focuses on durability-related transport properties of geopolymer-based materials. We present results of accelerated carbonation, rapid chloride migration (RCM) and air permeability measurements, together with strength and porosity data, for fly ash-based geoplymers, including formulations containing ground granulated blast furnace slag (GGBFS).
It was found that, despite comparable total porosities, the carbonation depths, the chloride migration coefficients and the air permeabilities of the mortars differed significantly. In general, the addition of GGBFS to the binders improved the performance (decreased transport coefficients); however, this was not found to be true for the air permeability in all cases. This latter effect can be explained by drying damage of the C-(A-)S-H gel in GGBFS-containing binders. On the other hand, low transport coefficients can also be achieved by optimization of the binder formulation without the addition of GGBFS, which is also reflected in the material’s air permeability. Thus, there exists no simple correlation between air permeability (of harshly dried mortars) and durability-related transport coefficients for the studied alkali-activated materials.
In addition, corrosion potential, polarization resistance data and current density-potential curves of carbon steel in two of the geopolymer mortars are presented. The corrosion potential vs. time curves in combination with polarization resistance values reveal that the steel reinforcement in geopolymer mortars assumed a passive state. However, this happened considerably later than for steel in CEM I-based mortars.
Geopolymers present a group of novel building materials exhibiting improved chemical resistance, fire resistance and lower CO2 emissions over traditional OPC-based materials.
Corrosion of steel rebars in concrete presents one of the main deterioration mechanisms limiting service life of the reinforced structures. The corrosion is accompanied by an expansion of the corrosion products causing high pressures, concrete cracking and finally spalling of a cover layer. Critical chloride concentration, loss of alkalinity and modeling of the steel corrosion are in researchers' spotlight for decades, however steel corrosion in geopolymer materials is insufficiently described and understood yet.
An optimized geopolymer mixture based on German hard coal fly ash activated with sodium hydroxide and sodium silicate solutions was selected for steel reinforcement-corrosion experiments. The formation of passive layer on the steel rebars is observed after approx. two weeks of hardening at laboratory temperature. However, alternative heat-treatment at 80°C for several hours leads to immediate formation of the passive layer as well as to a faster strength gain (80 MPa after 24h at 80°C).
Chloride diffusion coefficient, Na+ leaching as well as carbonation rate is identified on unreinforced geopolymer mortar samples. The data are used for simulations and design of the steel rebars-corrosion experiments. The effect of chloride ingress, Na+ leaching and carbonation of the geopolymer mortar on the steel-reinforcement corrosion is studied.
Corrosion of steel reinforcement in concrete is one of the major deterioration mechanisms limiting the service life of reinforced concrete structures. While for conventional (Portland cement-based) concretes a great amount of experience exists in this regard, the factors that determine the onset of reinforcement corrosion in alkali-activated materials are incompletely understood yet.
One aspect of corrosion protection is leaching and the accompanying changes of the concrete pore solution. In the present study, alkali-activated fly ash mortars with embedded carbon steel rebars were exposed to de-ionised water for periods up to 330 days, and the electrochemical response of the steel (free corrosion potential, polarisation resistance), the alteration of the mortar (ohmic resistance, mechanical strength, pore size distribution) as well as the pore solu¬tion composition were monitored.
Although substantial alkali leaching was observed, the pH of the pore solution remained at values sufficient to protect the embedded steel from depassivation. The mortar did not exhibit indications of significant deterioration. Thus, the present results suggest that leaching is not critical for pro¬tec¬tion of steel reinforcement in alkali-activated fly ash mortars and concretes.