A study on chloride-induced depassivation of mild steel in simulated concrete pore solution

The effect of chloride ions on passivity breakdown of steel in simulated concrete pore (SCP) solution was studied using electrochemical techniques. In this regards, the sensitivity of cyclic potentiodynamic parameters such as ΔE (difference between E_pit and E_rep), i_peak and AC-impedance parameters like R_ct, R_f, C_i, R_ad and C_ad to chloride ion concentration was investigated. Adsorption of OH⁻ ions on the metal surface in free chloride SCP solution and also displacement of those ions by Cl⁻ ions were demonstrated in high frequency part of Nyquist plots. In addition, a severe decrease in resistance of interfacial reaction (R_f+R_ct) was observed through breakdown of passive film in the [Cl⁻]/[OH⁻] ratio of 0.6. The interfacial capacitance, C_i, was abruptly raised when localized corrosion changed to general one.     

Alkali concentrations of pore solution in hydrating OPC

In order to study the pore solution, the release and binding of alkalis in a hydrating cement system have been studied. First, the binding factors for sodium and potassium as determined by Taylor [Adv. Cem. Res. 1 (1987) 5_16] and the corresponding distribution ratios as determined by Hong and Glasser [Cem. Concr. Res. 29 (1999) 1893_1903; Cem. Concr. Res. in press] are related to each other. It follows that the sorption of sodium is practically identical, whereas for potassium Taylor [Adv. Cem. Res. 1 (1987) 5_16] predicts a substantial lower degree of sorption. The concept of alkali release, pore solution decrease and sorption by formed C_S_H, is incorporated in the NIST hydration model (CEMHYD3D). Subsequently, the model is compared with OPC hydration experiments reported by Larbi et al. [Cem. Concr. Res. 20 (1990) 506_516]. Good agreement is obtained when the distribution ratios of Hong and Glasser [Cem. Concr. Res. 29 (1999) 1893_1903] are applied. The results suggest that C_S_H is the only binder of alkalis in hydrating OPC.     

The influence of potassium-sodium ratio in cement on concrete expansion due to alkali-aggregate reaction

In concrete containing potentially reactive aggregates, deleterious alkali-aggregate-reaction (AAR) can be prevented by the use of suitable mineral admixtures or by limiting cement content and alkalis (Na₂O-equivalent) of the cement. However, the Na₂O-equivalent of cement may not always accurately define the potential of cement to cause AAR. In this study, the potential reactivity of concrete produced with cements having similar Na₂O-equivalents but different K/Na-ratios has been measured and the composition of gel has been analyzed. Additionally, pastes and mortars have been produced to study the development of pore solution composition.The expansion of the concrete mixtures shows significant differences depending on the cement used. The different K/Na-ratio present in the cements is reflected in the pore solution of pastes and mortars and in the gel present in aggregates of the concrete mixtures. As the hydroxide concentration in the pore solutions of pastes and mortars produced with the different cements is nearly identical, the difference in K/Na-ratio has to be the reason for the observed differences in concrete expansion.     

Influence of lithium-based products proposed for counteracting ASR on the chemistry of pore solution and cement hydrates

Low- and high-alkali cement pastes were made with or without LiNO₃ or a Li-bearing glass. The [Li]/[Na+K] molar ratio was kept constant to 0.74. The specimens were stored at 23, 38, and 60 °C in sealed containers. After 3, 7, 28, and 91 days, their pore solutions were extracted and analysed, and their residual water contents were obtained by drying. The Li glass was found to react quite slowly, and the corresponding [Li⁺] in solution progressively increased with time, temperature, fineness (as-received glass vs. ground glass), and the [Na⁺+K⁺] concentration in solution. This glass increased the pH by about 0.1, and by about 0.2 after it was finely ground. In contrast, LiNO₃ decreased the pH by about 0.1, despite significantly increasing the [Na⁺+K⁺] in the pore solution. The higher the total %Na₂O_e content (including Li) in the original mixtures, the higher the total alkali content incorporated in the cement hydrates. The [Li⁺]-[Na⁺+K⁺] ratio in solution was about half of the initial ratio (0.74), while this ratio in the cement hydrates was always over 1.1. Li is the alkali most preferentially incorporated into the cement hydrates, while K is the least.     

Laboratory and field investigations of the influence of sodium chloride on alkali-silica reactivity

Concrete cylinders, 255 mm in diameter, were made with high- and low-alkali cements, a highly alkali-silica-reactive coarse aggregate, and subjected to various conditions at 38 °C: (1) immersion in 3% NaCl solution; (2) immersion in 6% NaCl solution; (3) humid air at 100% RH, and (4) 14-day cycles including 12 days in humid air, 2 days of drying, and 3 h in 6% NaCl solution. After 1 year, a number of cylinders were drilled to obtain dry powder samples from different depths, which were analyzed for total and soluble chloride and for soluble sodium and potassium. Concrete cores were also taken in a number of parapets and abutments, either exposed to deicing salts or not, on which chemical analyses were also performed on slices taken at different depths from the exposed surface. The results obtained suggest that making concrete with a low-alkali content is an effective way to prevent expansion due to alkali-silica reaction even for concretes exposed to seawater or deicing salts; this is attributed to the fact that the OH⁻ ion concentration in the pore solution, and then the pH, is decreased in the near-surface layers of concrete exposed to sodium chloride, which does not penetrate at depth in concrete.     

Ex situ leaching measurement of concrete alkalinity

A new technique involving ex situ leaching (ESL) of concrete particles in deionized water and chemical analysis of the resulting leachant was developed to determine the concrete pore water alkalinity. A water-to-concrete powder ratio no higher than 1 and a leaching time no less than 3 days were mostly used. The concrete pore water hydroxide ion concentration ([OH⁻]_pore) was then calculated based upon a protocol that included several assumptions. It was found that [OH⁻]_pore was proportional to the cement equivalent alkali content but relatively insensitive to the concrete water-to-cement ratio. Determinations of [OH⁻]_pore were compared with (1) results from parallel, conventional pore water expression (PWE) experiments using concrete from the same batch, (2) a theoretical estimate of this parameter based upon mix design and cement chemistry of the concretes employed for ESL, and (3) data from the literature. All of these comparisons either directly or indirectly confirmed the validity of the proposed method. However, applicability of the ESL method, as proposed, to concrete containing pozzolans or highly soluble inorganic salts (or both) needs to be further explored.     

Alkali mass balance during the accelerated concrete prism test for alkali-aggregate reactivity

The alkali mass balance was calculated in concrete specimens submitted to the storage conditions of the Canadian standard CSA A23.2-14A concrete prism test for expansion due to alkali-aggregate reaction (AAR). The alkali concentration of both the concrete pore solution expressed under high pressure and the water below specimens in storage pails (bottom water) was measured. Measurements were conducted over a 1-year period, which corresponds to the length of the above test. Two reactive aggregates were tested [Potsdam sandstone (PO) and Spratt limestone (SP)]. Each aggregate was incorporated in two concrete mixtures (mass concrete and structural concrete), for a total of four batches. Significant alkali leaching occurred at 38 °C while performing tests in high moisture storage conditions even though prisms were covered with plastic sleeves. After 52 weeks, the alkali loss ranged from 12% to 25% of the original Na₂O_e content of the concrete, depending on the mixture proportioning and the aggregate type. After estimation of the proportion of alkalis fixed in cement hydrates, it appears that about 23% to 39% of the original alkalis released by the cement are quickly sorbed on aggregate surfaces or have rapidly migrated inside aggregate particles, which may have been incorporated with time in the AAR product. After 52 weeks at 38 °C, the pore solution alkalinity expressed from mass concrete made with PO was 250 mmol/l, whereas the alkalinity was 270 mmol/l in mass concrete incorporating SP. Since prisms of both mixtures were still expanding at 1 year, these alkalinity values are above the thresholds required for sustaining AAR in these concrete mixtures.     

Estimating the electrical conductivity of cement paste pore solutions from OH, K and Na concentrations

A proposed method for estimating the electrical conductivity of cement paste pore solution at 25 °C is based on the concentrations of OH⁻, K⁺ and Na⁺. The approach uses an equation that is a function of the solution ionic strength, and requires a single coefficient for each ionic species. To test the method, the conductivity of solutions containing mixtures of potassium hydroxide and sodium hydroxide with molar ratios of 4:1, 2:1 and 1:1, and having ionic strengths varying from 0.15 to 2.00 mol/l were measured in the laboratory and compared to predicted values. The proposed equation predicts the conductivity of the solutions to within 8% over the concentration range investigated. By comparison, the dilute electrolyte assumption that conductivity is linearly proportional to concentration is in error by 36% at 1 mol/l and in error by 55% at 2 mol/l. The significance and utility of the proposed equation is discussed in the context of predicting ionic transport in cement-based systems.     

Porosity, pore size distribution and in situ strength of concrete

In this study, in situ strength of concrete was determined through compression test of cores drilled out from laboratory cast beams. The apparent porosity and pore size distribution of the same concrete were determined through mercury intrusion porosimetry, performed on small-drilled cores. The normal-strength concrete mixes used in the experimental investigation were designed to exhibit a wide variation in their strengths. To ensure further variation in porosity, pore size distribution and strength, two modes of compaction, two varieties of coarse aggregates, different levels of age, curing period and exposure condition of concrete were also introduced in experimental scheme. With the data so generated, an appraisal of the most frequently referred relationships involving strength, porosity and pore size of cement-based materials was carried out. Finally, a new empirical model relating the in situ strength of concrete with porosity, pore size characteristics, cement content, aggregate type, exposure conditions, etc., is presented.     

The role of alkali content of Portland cement on the expansion of concrete prisms containing reactive aggregates and supplementary cementing materials

This paper presents results covering the effects of alkali content of Portland cement (PC) on expansion of concrete containing reactive aggregates and supplementary cementing materials (SCM). The results showed that the alkali content of PC has a significant effect on expansion of concrete prisms with no SCM. When SCM is used, the expansion was found to be related to both the chemical composition of the SCM and, to a lesser extent, the alkali content of the PC. The concrete expansions were explained, at least partly, on the basis of the alkalinity of a pore solution extracted from hardened cement paste samples containing the same cementing blends. An empirical relation was developed correlating the chemical composition (Ca, Si and total Na₂O_e) of the cementing blend (PC + SCM) and the alkalinity of the pore solution. Results from accelerated mortar bar test (ASTM C 1260) and a modified version thereof are also presented.     

The pore solution phase of carbonated cement pastes

Samples of hydrated cement pastes were exposed to atmospheres with various carbon dioxide concentrations at relative humidities controlled by different saturated salt solutions. When carbonated throughout their thickness, as indicated by the phenolphthalein test, they were resaturated with water and subjected to pore solution expression and analysis. The effects of the various carbonating environments on the pore solution composition and on aspects of the pore structure and mineralogy of the carbonated products are reported. Implications regarding the likely effects of different accelerated carbonation regimes on the corrosion behaviour of steel in concrete are discussed. In particular, it is shown that the use of saturated sodium nitrite solution to control the relative humidity of atmospheres with high concentrations of carbon dioxide may cause an evolution of gaseous oxides of nitrogen, which can result in the contamination of the pore solution with nitrite and nitrate salts.     

Interactions between shrinkage reducing admixtures (SRA) and cement paste's pore solution

Shrinkage reducing admixtures (SRA) have been developed to combat shrinkage cracking in concrete elements. While SRA has been shown to have significant benefits in reducing the magnitude of drying and autogenous shrinkage, it has been reported that SRA may cause a negative side effect as it reduces the rate of cement hydration and strength development in concrete. To examine the influence of SRA on cement hydration, this study explores the interactions between SRA and cement paste's pore solution. It is described that SRA is mainly composed of amphiphilic (i.e., surfactant) molecules that when added to an aqueous solution, accumulate at the solution-air interface and can significantly reduce the interfacial tension. However, these surfactants can also self-aggregate in the bulk solution (i.e., micellation) and this may limit the surface tension reduction capacity of SRA. In synthetic pore solutions, SRA is observed to form an oil-water-surfactant emulsion that may or may not be stable. Specifically, at concentrations above a critical threshold, the mixture of SRA and pore fluid is unstable and can separate into two distinct phases (an SRA-rich phase and an SRA-dilute phase). Further, chemical analysis of extracted pore solutions shows that addition of SRA to the mixing water depresses the dissolution of alkalis in the pore fluid. This results in a pore fluid with lower alkalinity which causes a reduction in the rate of cement hydration. This may explain why concrete containing SRA shows a delayed setting and a slower strength development.     

The pH measurement of concrete and smoothing mortar using a concrete powder suspension

A simple method is presented to determine the pH value of concrete and fine smoothing mortar. The method is based on mixing powdered samples with a solvent and measuring the pH of the suspension. The parameters affecting the result are investigated. The validity of the pH results is tested by comparing the values measured from a suspension of pulverized specimen with the extracted pore solution. Measurements were done by using a pH electrode and by titration. The technique investigated furnishes results with good reproducibility and has been proven feasible. The pH value results of the proposed suspension method could be compared with the results of the extracted pore solution in two normal-strength concretes and the fine smoothing. Sufficient pore solution of the extra-air-entrained concrete and high-strength concrete could not be obtained in the reference method of extraction of the pore solution. The difference between the results of the suspension method and the extraction method was 0.07 pH units on the average with a standard deviation of 0.27 pH units when the powder-to-suspension ratio in the suspension method was 0.5.     

Reinforcing steel passivation in mortar and pore solution

Under field conditions, steel is embedded in concrete for a long period of time before chlorides penetrate. In studying the corrosion behaviour of steel in concrete, mortar or in simulated pore solution, it is essential to allow enough time for the steel to create a passive layer which is the subject of this study. This time is given to steel in chloride free concrete, naturally; while it should be provided to steel in synthetic pore solution, before adding chloride to the solution. For determining this time, samples were made with steel with different surface conditions: as-received with mill scales and sand-blasted. One set of steel bars (as-received and sand-blasted) were embedded in mortar and one set were immersed in synthetic pore solution. Corrosion of each steel bar was monitored every hour by LPR technique for total time of 300 h. Also, half-cell potential of steel bars was measured during that time. Results show that steel needs to be kept at least three days in synthetic pore solution and seven days in mortar to be passivated.     

A theoretical model describing diffusion of a mixture of different types of ions in pore solution of concrete coupled to moisture transport

A theoretical model is established for diffusion of different types of ions in pore solution of concrete and the coupling to moisture flow and moisture content. Mass exchanges between ions in pore solution and solid hydration products in the concrete are also considered. The basic concepts behind the so-called mixture theory are used. The development of a mass balance principle for ions in pore solution is established. This principle accounts for (i) diffusion caused by concentration gradients of ions and gradients of the so-called internal electrical potential, (ii) convection, i.e. the effect on the motion of ions due to a motion of the pore solution in concrete, (iii) the effect on the concentration due to changes in the moisture content, and finally, (iv) the effect of mass exchange of ions between solid hydration products and the pore solution phase. The model is general in the sense that all different types of ions appearing in pore solution phase can be included and computed for during quite arbitrary boundary conditions.     

Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes

The effect of temperature on the hydration products and the composition of the pore solution are investigated for two Portland cements from 5 to 50 °C. Increased temperature leads to an initially fast hydration and a high early compressive strength. At 40 and 50 °C, the formation of denser C-S-H, a more heterogeneous distribution of the hydration products, a coarser porosity, a decrease of the amount of ettringite as well as the formation of very short ettringite needles has been observed. At 50 °C, calcium monosulphoaluminate has formed at the expenses of ettringite. In addition, the amount of calcium monocarboaluminate present seems to decrease. The composition of the pore solution mirrors the faster progress of hydration at higher temperatures. After 150 days, however, the composition of the pore solution is similar for most elements at 5, 20 and 50 °C. Exceptions are the increased sulphate concentrations and the slightly lower Al and Fe concentrations at 50 °C.     

Effect of water-to-cementitious materials ratio and silica fume on the autogenous shrinkage of concrete

This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement.The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.     

Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions

Electrochemical chloride extraction (ECE) is used for the rehabilitation of chloride-contaminated concrete. Anions such as chloride and hydroxide are pushed away from the cathode (reinforcement), and cations such as sodium, potassium and calcium are attracted to the cathode. During ECE an increase of the concrete resistance can be observed. The results of a petrography study on ECE-treated concrete are presented in this paper. It also investigates the influence of pore size on ion migration using a concrete substitute model with known pore size. Findings showed that the pore size and pore size distribution of concrete are altered due to ECE. It is therefore suggested that concrete acts as active migration medium in the migration process by releasing ions into the pore solution. Moreover, small pores hinder the migration of ions, which may partially be responsible for changes in concrete resistance.     

The current state of the accelerated concrete prism test

Expansions due to alkali-silica reaction (ASR) in the accelerated concrete prism test (ACPT-60 °C) show a significant reduction at 13 weeks compared to 52 week testing in the standard concrete prism test (CPT-38 °C). Previous work indicated that increased leaching, higher mass loss and a reduction in the pH were observed when temperature was increased from 38 to 60 °C. After further investigation the authors have revealed that non-reactive fine aggregate from certain sources combined with the same reactive coarse aggregate exhibited further reduction in expansion in the ACPT. Expansion data for a wide range of reactive coarse aggregates in 38 and 60 °C testing regimes is shown. Data investigating the Spratt reactive coarse aggregate combined with seven different non-reactive sands will be shown to demonstrate the dramatic effect of the non-reactive sand. Selected pore solution analyses will be given to further elucidate this issue.     

Transport of a surface-applied corrosion inhibitor in cement paste and concrete

Surface-applied corrosion inhibitors are a kind of repair material and usually contain an aminoalcohol and a component forming a salt with the aminoalcohol. According to the manufacturers, this type of inhibitor penetrates very rapidly into concrete; however, the transport mechanisms have not been sufficiently investigated so far. The major part of the study therefore focused on the transport of the ingredients of an inhibitor in cement paste and concrete, which contained an aminoalcohol and a phosphorous compound. It has been shown that the latter forms an insoluble calcium salt in the environment of cement and precipitates quantitatively. It is thus unable to penetrate from the outside into the alkaline concrete zone and cannot develop its inhibiting effect there. The aminoalcohol, on the other hand, is not bound by cement, but remains largely dissolved in the pore liquid, thus providing optimal conditions for high mobility. The analysis of the transport mechanisms involved has revealed that diffusion in the dissolved state is by far the most efficient transport mechanism. While basically the transport of the aminoalcohol via the gaseous phase is possible, it does play an inferior role only. Surprisingly, the substance had hardly been absorbed by concrete by capillary suction, but at first remained close to the concrete surface.