L-Ascorbic Acid as an Efficient Green Corrosion Inhibitor of Steel Rebars in Chloride Contaminated Cement Mortar.

Corrosion of reinforcement is a major problem regarding concrete durability. In new structures the corrosion onset can be delayed if additional protection methods are provided as is the case for the addition of corrosion inhibitors in the concrete mix. The main goal of this paper is the evaluation of the effect of the ascorbic acid (AA) as a green steel corrosion inhibitor in cement mortars contaminated by chlorides. Concentration levels of ascorbic acid, ranging from 0.5 to 10-3 mol/L, were added to the mixing water. Electrochemical methods, including corrosion potential (Ecorr), linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS), were employed to assess the corrosion rate of the steel embedded in the mortars. The corrosion inhibiting performance of ascorbic acid was compared with that of sodium nitrite. The interaction of the ascorbic acid with the hydrated cement matrix was also evaluated with differential thermal and thermogravimetric analysis (DTA/TG) and pH measurements. The results indicated that, depending on the ascorbic acid concentration, it can be an activator of the corrosion process or an effective corrosion inhibitor in a similar manner to sodium nitrite. A corrosion rate decrease was achieved with concentrations below 10-2 mol/L and the optimum content was 10-3 mol/L. Within this concentration range, the AA does not modify the hydration performance of the cement matrix.

Effect of Sulfate Ions on Galvanized Post-Tensioned Steel Corrosion in Alkaline Solutions and the Interaction with Other Ions.

Zinc protection of galvanized steel is initially dissolved in alkaline solutions. However, a passive layer is formed over time which protects the steel from corrosion. The behavior of galvanized steel exposed to strong alkaline solutions (pH values of 12.7) with a fixed concentration of sulfate ions of 0.04 M is studied here. Electrochemical measurement techniques such as corrosion potential, linear polarization resistance and electrochemical impedance spectroscopy are used. Synergistic effects of sulfate ions are also studied together with other anions such as chloride Cl− or bicarbonate ion HCO3− and with other cations such as calcium Ca2+, ammonium NH4+ and magnesium Mg2+. The presence of sulfate ions can also depassivate the steel, leading to a corrosion current density of 0.3 µA/cm2 at the end of the test. The presence of other ions in the solution increases this effect. The increase in corrosion current density caused by cations and anions corresponds to the following orders (greater to lesser influence): NH4+ > Ca2+ > Mg2+ and HCO3− > Cl− > SO42−.

Radiological Characteristics of Carbonated Portland Cement Mortars Made with GGBFS.

The objective of this study is to assess whether the carbonation process can modify the physicochemical characteristics of the natural radionuclides of the three natural radioactive series, together with 40K. Three mortar specimens with different percentages of ground granulated blast-furnace slag (GGBFS), cured under water for 1, 3, 7, 14, or 28 days, were subjected to a natural carbonation process. Activity concentrations for the solid and ground mortars were determined by gamma spectrometry and by radiochemical separation of isotopic uranium. The novelty of this paper relies principally on the study we have carried out, for the first time, of the radiological characteristics of carbonated Portland cement mortars. It was found that the chemical properties of the 3 mortar specimens were not affected by the carbonation process, with particular attention placed on uranium (238U, 235U, and 234U), the activity concentrations of which were equivalent to the 226Ra results and ranged from 5.5 ± 1.6 Bq kg-1 to 21.4 ± 1.2 Bq kg-1 for the 238U. The average activity concentrations for the 3 types of mortars were lower than 20.1 Bq kg-1, 14.5 Bq kg-1, and 120.2 Bq kg-1 for the 226Ra, 232Th (212Pb), and 40K, respectively. Annual effective dose rates were equivalent to the natural background of 0.024 mSv. In addition, it was observed that the variation rate for the 222Rn emanation was due primarily to the Portland cement hydration and not due to the pore size redistribution as a consequence of the carbonation process. This research will provide new insights into the potential radiological risk from carbonated cement-based materials. Moreover, the assessment that is presented in this study will convey valuable information for future research that will explore the activity concentration of building materials containing NORM materials.

Durability of Blended Cements Made with Reactive Aggregates.

Alkali-silica reaction (ASR) is a swelling reaction that occurs in concrete structures over time between the reactive amorphous siliceous aggregate particles and the hydroxyl ions of the hardened concrete pore solution. The aim of this paper is to assess the effect of pozzolanic Portland cements on the alkali-silica reaction (ASR) evaluated from two different points of view: (i) alkali-silica reaction (ASR) abatement and (ii) climatic change mitigation by clinker reduction, i.e., by depleting its emissions. Open porosity, SEM microscopy, compressive strength and ASR-expansion measurements were performed in mortars made with silica fume, siliceous coal fly ash, natural pozzolan and blast-furnace slag. The main contributions are as follows: (i) the higher the content of reactive silica in the pozzolanic material, the greater the ASR inhibition level; (ii) silica fume and coal fly ash are the best Portland cement constituents for ASR mitigation.

Chloride Induced Reinforcement Corrosion in Mortars Containing Coal Bottom Ash and Coal Fly Ash.

Coal bottom ash is normally used as aggregate in mortars and concretes. When it is ground, its characteristics are modified. Therefore, the assessment of its long-term durability must be realized in depth. In this sense, an accelerated chloride ingress test has been performed on reinforced mortars made of Portland cement with different amounts of coal bottom ash (CBA) and/or coal fly ash (CFA). Corrosion potential and corrosion rate were continuously monitored. Cement replacement with bottom and fly ash had beneficial long-term effects regarding chloride penetration resistance. Concerning corrosion performance, by far the most dominant influencing parameter was the ash content. Chloride diffusion coefficient in natural test conditions decreased from 23 × 10-12 m2/s in cements without coal ashes to 4.5 × 10-12 m2/s in cements with 35% by weight of coal ashes. Moreover, the time to steel corrosion initiation went from 102 h to about 500 h, respectively. Therefore, this work presents experimental evidence that confirms the positive effect of both types of coal ashes (CBA and CFA) with regard to the concrete steel corrosion.