Issue 50

A.G. Lekatou et alii, Frattura ed Integrità Strutturale, 50 (2019) 423-437; DOI: 10.3221/IGF-ESIS.50.36 425 American Concrete Institute defines fly ash as “the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gases from the combustion zone to the particle removal system” [27]. The beneficial effect of FA on the corrosion performance of reinforced and bare concrete has been reported in several investigations [25,26,28-30]. Through the pozzolanic reaction, fly ash transforms Ca(OH) 2 from the cement hydration process into C-S-H (calcium silicate hydrate of variable stoichiometry 0.6-2.0CaO·SiO 2 ·0.9-2.5H 2 O, often also incorporating partial substitution of Al for Si), as follows [25]: Ca(OH) 2 + H 4 SiO 4 → CaH 2 SiO 4 ·2H 2 O (1) Many pozzolans may also contain aluminate or Al(OH) 4 - and silica that will react with Ca(OH) 2 and H 2 O to form mixed calcium aluminate silicate hydrates of complex formulas. In the presence of SO 4 2- , CO 3 2- , Cl - , calcium monosulfoaluminate phases (often with substitution of Fe for Al and/or other anions, such as OH - or CO 3 2- for SO 4 2- ) and calcium trisulfo- aluminate hydrate or ettringite (Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26H 2 O) sometimes with substitution of Fe for Al and/or CO 3 2- for SO 4 2- are formed [31]. The latter phases may lead to loss of mass and strength [26]. Papadakis has proposed a series of reactions describing the pozzolanic activity of a high-Ca fly ash with Portland cement [32]. C-S-H is more resistant than Ca(OH) 2 to attacks of aggressive species, such as sulphates and chlorides [30]. The formation of C-S-H causes a decrease in the hydration heat release, drying shrinkage, porosity and permeability of concrete [24,25]. Other reaction products can also fill the capillary voids in the concrete, thus reducing its permeability. One such product is the Friedel’s salt (3CaO·Al 2 O 3 ·CaCl 2 ·10H 2 O) [25]. The filler effect of the fine particles of FA also contributes to the decrease in the porosity and permeability of the concrete [26]. The above attributes of FA have been shown to increase the com- pressive strength of concrete after long term exposure to 3.5% NaCl [33]. However, not all types of FA benefit the corrosion resistance of concrete. Chousidis et al. noted that fly ash with significant amounts of clay minerals presents low resistance to chloride penetration and strength [25]. Additionally, Papadakis [32] concluded that the strength of Portland cement partially replaced by fly ash will exceed that of the cement only if the fly ash is richer in active silica as compared to the cement.. The existence of an optimum content of fly ash in concrete, as far as the corrosion performance of the steel reinforcement is concerned, was suggested in preliminary efforts [34,35]. This instigated a more integrated investigation of the effect of fly ash (FA) on the corrosion behavior of 304L rebars subjected to three different modes of accelerated testing: a) electro- chemical degradation in a solution simulating concrete having suffered severe attack by acid rain; b) electrochemical de- gradation in a solution simulating concrete subjected to a mild attack by AR; c) mechanical degradation of rebars embedded in cubes of concrete by salt spraying for 4 m. This work has been motivated by the limited information on the steel cor- rosion of reinforced concrete under acid rain and H 2 SO 4 -rich environments [28]. Corrosion studies and associated structural integrity studies on concrete combined with fly ash and reinforced with stainless steel rebars are even more rarely documented. The particular investigation is targeted to applications of ancient monument restorations in polluted urban environments and coastal environments. E XPERIMENTAL PROCEDURE Materials ebars of austenitic stainless steels (304L of nominal wt.% composition: 0.03% C, 18.00% Cr, 8.00% Ni, 1.00% Si, 2.00% Mn, 0.0045% P, 0.03% S, Fe bal. and 316L of nominal wt.% composition: 0.022% C, 17.31% Cr, 10.08% Ni, 2.02% Mo, 0.54% Si, 1.75% Mn, 0.0032% P, 0.0001% S, Fe bal.) having diameter of 6 mm and length of 2.5 cm were used for the electrochemical tests. 7×7×7 cm 3 cubes of ordinary Portland cement (OPC) mixed with pulverized fly ash (FA) from the Hellenic Public Power Corporation lignite mines in the Region of Western Macedonia and reinforced with 304L rebars of 6 mm diameter and 12 cm length were employed for the salt spraying test. FA is of alkaline nature, with CaO being the main component, also containing SiO 2 , Al 2 O 3 , SO 3 , Fe 2 O 3 and MgO [35]. These oxides form solid solutions, such as CaCO 3 , CaSO 4 , 3CaO·Al 2 O 3 , CaSiO 3 , 3CaO!Al 2 O 3 !Ca(OH) 2 ·8H 2 O, Ca(Mg,Al)(Si,Al) 2 O 6 , AlFeO 3 , CaFe 2+ SiO 4 , CaAl 8 Fe 4 O 19 , CaFe 2 3+ (SiO 4 ) 3 , Ca 3 SiO 5 , K 0.9 Ca 5.8 Al 18 . 7 Si 14.15 O 32 . Al 2 O 3 , SiO 2 in the forms of quartz and cristobalite, as well as CaO and Fe 2 O 3 , have also been detected. Electrochemical testing 304L rebar specimens of 2.5 cm length were subjected to potentiodynamic polarization testing. Their cut edges were mounted in epoxy resin and then encapsulated in PTFE leaving a surface of about 2 cm 2 to be exposed to the electrolyte. A standard three electrode cell (with Ag/AgCl as the reference electrode and a platinum gauze as the counter electrode) was R