The Evaluation of Cementitious Pore Solution Composition and Electrical Resistivity Using X-ray Fluorescence (XRF)

Interest in performance specifications has been growing in the civil and construction industry in the past decade. One major focus area has been on understanding how to prolong the service life of concrete structures, since repair and rehabilitation of existing infrastructure have cost many trillions of dollars. Deterioration mechanisms such as corrosion can shorten the service life of a structure and are typically determined by the moisture and ionic species ingress into the concrete, or, in other words, the transport properties of the concrete. Ionic transport in concrete can be described using the formation factor, which is defined as the ratio of the resistivities of the concrete and the pore solution. Therefore, there is significant value in rapid and simple methods to measure these electrical properties. Measuring the resistivity of concrete, or bulk resistivity, is relatively straightforward; however, measuring the pore solution resistivity is more complex since extracting pore solution from hardened concrete is rather challenging. The pore solution resistivity value may be assumed from literature, directly measured using a resistivity meter, or computed from the pore solution composition using different chemical analysis methods. The objective of this thesis is to investigate the use of X-ray fluorescence (XRF) as a chemical analysis method to obtain the chemical composition of the pore solution which enables the calculation of pore solution resistivity. The first part of this study focuses determining the feasibility of using XRF to assess the chemical composition of the main ionic species in simulated pore solutions and to calculate the pore solution resistivity from the chemical composition. Two analysis methods were explored: the solution method and the fused bead method. The measured ionic concentrations were compared to theoretical concentrations; the calculated resistivities were compared to measured resistivities using a resistivity meter as a direct measurement. The results from this study showed that XRF can accurately detect the ionic composition of simulated pore solutions and can be used to accurately calculate the pore solution resistivity using both methods of analysis. The second part of this study focuses on measuring the ionic concentrations and calculating the resistivity of expressed pore solutions. The influence of test parameters such as sample size and storage time on the composition and resistivity was also studied. The calculated resistivities were compared to measured resistivities using a resistivity meter as a direct measurement. Chemical composition and resistivity determined using XRF were also compared with an online pore solution conductivity calculator developed at the National Institute of Science and Technology (NIST). The results from this study showed that the calculated resistivities from XRF match the measured resistivities from the resistivity meter. Therefore, it can be concluded that XRF can be used to accurately calculate the electrical resistivity of pore solutions. Chemical compositions determined from the XRF matched the ones determined from the NIST calculator after 24 hours of expressed age (but not earlier), since the NIST calculator neglects sulfate and calcium, which are present in significant amounts in pore solutions before 24 hours. In conclusions, the results from this thesis indicate that XRF is a potential alternative to time consuming methods which are currently used to determine the pore solution composition that can then be used to predict resistivity. This method could potentially bring benefits in terms of time and cost reductions, since XRF is a device commonly used in the cement industry.

Damage in cement pastes exposed to MgCl2 solutions

Magnesium chloride (MgCl2) reacts with cement pastes resulting in calcium leaching and the formation of calcium oxychloride, which can cause damage. This paper examines the damage in different cement pastes exposed to MgCl2 solutions. Volume change measurement and low temperature differential scanning calorimetry are used to characterize the formation of calcium oxychloride. Thermogravimetric analysis and X-ray fluorescence are used to quantify calcium leaching from Ca(OH)2 and C-S-H. The ball-on-three-balls test is used to quantify the flexural strength reduction. Calcium oxychloride can form in cement pastes exposed to MgCl2 solutions with a (Ca(OH)2/MgCl2) molar ratio larger than 1. As the MgCl2 concentration increases, two-stages of flexural strength reduction are observed in the plain cement pastes, with the initial reduction primarily due to calcium leaching from Ca(OH)2 and the additional reduction due to the calcium leaching from C-S-H (at MgCl2 concentrations above 17.5 wt%). For the cement pastes containing fly ash, there is a smaller reduction in flexural strength as less Ca(OH)2 is leached, while no additional reduction is observed at high MgCl2 concentrations due to the greater stability of C-S-H with a lower Ca/Si ratio. The addition of fly ash can mitigate damage in the presence of MgCl2 solutions

The Influence of Calcium Chloride on Flexural Strength of Cement-Based Materials

Calcium chloride (CaCl2), which is commonly used as a deicing salt, can react with calcium hydroxide (Ca(OH)2) in cement-based materials to form calcium oxychloride. This reaction causes damage that typically manifests itself as flaking of concrete pavements at the joints and leads to expensive repairs and a reduction of the service life. In this paper, cement pastes with different fly ash replacement levels were prepared to provide pastes with differing amounts of Ca(OH)2. Thermogravimetric analysis was used to quantify the Ca(OH)2 content in these pastes. Low-temperature differential scanning calorimetry (LT-DSC) was used to quantify the amount of calcium oxychloride formed when these pastes were exposed to CaCl2 solutions. The reduction in the flexural strength of these pastes saturated with different CaCl2 solutions was also measured. As the concentration of CaCl2 increases, the reduction in flexural strength increases. There is a lower flexural strength reduction in pastes with fly ash, because these pastes have lower Ca(OH)2 and form lower amounts of calcium oxychloride. The strength reduction is directly correlated to the amount of formed calcium oxychloride

Expression of Cementitious Pore Solution and the Analysis of Its Chemical Composition and Resistivity Using X-ray Fluorescence

The goal of this method is to determine the chemical composition and electrical resistivity of cementitious pore solution expressed from a fresh paste sample. The pore solution is expressed from a fresh paste sample using a pressurized nitrogen gas system. The pore solution is then immediately transferred to a syringe to minimize evaporation and carbonation. After that, assembled testing containers are used for the X-ray fluorescence (XRF) measurement. These containers consist of two concentric plastic cylinders and a polypropylene film which seals one of the two open sides. The pore solution is added into the container immediately prior to the XRF measurement. The XRF is calibrated to detect the main ionic species in the pore solution, in particular, sodium (Na ), potassium (K ), calcium (Ca ), and sulfide (S ), to calculate sulfate (SO4 ) using stoichiometry. The hydroxides (OH ) can be calculated from a charge balance. To calculate the electrical resistivity of the solution, the concentrations of the main ionic species and a model by Snyder et al. are used. The electrical resistivity of the pore solution can be used, along with the electrical resistivity of concrete, to determine the formation factor of concrete. XRF is a potential alternative to current methods to determine the composition of pore solution, which can provide benefits in terms of reduction in time and costs

Absorption and Desorption of Superabsorbent Polymers for Use in Internally Cured Concrete

Superabsorbent polymers (SAP) have been investigated as an additive for use in the manufacture of internally cured concrete. The ability of SAP to absorb and desorb fluid is important for the design of internally cured concrete mixtures. Internal curing research on lightweight aggregates (LWA) has typically focused on the absorption of water in the LWA internal curing agent. However, when SAP is used, the absorption test should be performed using a pore solution with a defined ionic concentration. To address the effect of the ionic composition of the pore solution on SAP absorption, pore solutions were extracted from fresh cementitious pastes, and their composition was evaluated using X-ray fluorescence. This study characterizes the absorption and desorption of a commercially available SAP, using both simulated and extracted pore solutions with a range of ionic concentrations. The teabag method was implemented to measure the absorption of the SAP. As expected, the absorption of the SAP decreased in solutions with higher ionic concentrations. In addition to studying solutions extracted from ordinary portland cement pastes, the effects of the inclusion of supplementary cementitious materials on the SAP absorption were studied. Results showed that the inclusion of supplementary cementitious materials had a relatively minor impact on the SAP absorption, primarily due to a dilution of the ionic concentration of the pore solution. This article examined the desorption of the SAP in two conditions: a reduction in the ambient relative humidity and after exposure of the SAP to solutions with a higher ionic concentration. It was observed that SAP-containing solutions with a higher ionic concentration had a reduced rate of desorption and a reduced overall desorption at a given relative humidity. In addition, moving the SAP from a solution with a lower ionic concentration to a more highly concentrated solution resulted in desorption. An equation was developed that expresses the SAP absorption as a function of the pH of the soaking solution. The expression was used to predict the desorption of SAP due to an increase in the ionic concentrations in a hydrating system. This equation was used to show that the desorption of SAP due to changes in the pore solution ionic concentration were significant during the first 72 hours. The findings and the techniques used in this study are meant to be used as an example for the characterization of SAP in concrete internal curing applications

Hydration, Pore Solution, and Porosity of Cementitious Pastes Made with Seawater

AbstractUnreinforced concrete or concrete reinforced with noncorrosive reinforcement could potentially be mixed with seawater in locations where potable water is scarce. A fundamental understanding of the properties of concrete mixed with seawater is therefore essential. This paper analyzes the hydration kinetics, hydrate phases, pore solution, and porosity of cementitious pastes made with seawater and compares these results with the corresponding ones from pastes made with deionized water. Pastes were prepared with cement and with a 20% mass replacement of the cement with fly ash. Isothermal calorimetry (to study hydration kinetics), thermogravimetric analysis (to study the hydrated phase assemblage), X-ray fluorescence (to determine pore solution composition and electrical resistivity), and dynamic vapor sorption (to determine the pore size distribution) were performed on the paste samples. Seawater accelerates hydration kinetics at an early age; however, this effect is negligible at later ages. Friedel’s salt formation in systems with seawater at later ages is negligible [0.4% (by mass of paste) at 91 days]. The primary difference between the hydrated phases of pastes made with seawater and those made with deionized water appears to be the absorption of chloride in the calcium silicate hydrate. The pore solution in pastes made with seawater has higher sodium, chloride, and hydroxide ion concentrations. The concentrations of sodium, potassium, and hydroxide ions in pore solutions are lower in pastes with fly ash compared to pastes without fly ash. Pastes with seawater show a lower electrical resistivity than pastes with deionized water due to the higher ionic concentrations. Paste with seawater has a slightly finer pore structure compared to paste with deionized water

Activation Energy of Conduction for Use in Temperature Corrections on Electrical Measurements of Concrete

The formation factor obtained through electrical resistivity measurements is becoming a popular method to determine transport properties of concrete. Resistivity measurements are dependent on multiple factors, including degree of saturation, pore solution conductivity, and temperature. The Arrhenius equation is used to correct electrical resistivity for temperature effects using an activation energy of conduction (Ea-cond). This parameter has been measured on a wide variety of materials, including pore solutions, pastes, mortars, and concretes (with a variety of saturation states). The reported values of Ea-cond typically range from 9 to 39 kJ/mol. This article examines the factors affecting Ea-cond in order to select an appropriate temperature correction. In this study, Ea-cond was determined from data measured on various concrete mixtures used in transportation infrastructure applications as well as extracted and simulated pore solutions. The Ea-cond of pore solutions remains relatively constant (an average value of 13.9 ± 1.5 kJ/mol) for typical pore solutions and was slightly lower than the Ea-cond of saturated specimens (an average value of 15.8 kJ/mol). It was found that Ea-cond increases as the degree of saturation of the specimen is reduced. Drying increases the ionic concentration of the fluid in the pores; however, this does not explain the changes in Ea-cond. The effects of drying were determined to be primarily due to a change in the volume of the conductive fluid film in the concrete and in the connectivity of the fluid-filled pores. While it is better to directly measure the Ea-cond of a concrete mixture, this is not always feasible or practical. In such cases, for pore solutions, a value of 13.9 kJ/mol can be used, and for saturated concretes, a value of 15.8 kJ/mol can be used. For concretes with a varying degree of saturation, the Ea-cond can be estimated using the developed equation