Rapid Chloride Permeability Test of Mortar Samples with Various Admixtures ^†

Abstract

This paper is focused on the permeability of cement composite samples with various admixtures. Permeability was examined by the rapid chloride permeability test due to the simple and relatively quick performance among the many methods. Permeability is one of the durability parameters considering the pore system of the composite structure. Ion diffusion provides information about inner pore structure by passing a charge through the sample. In real-life conditions, not only chlorides but also other ions can penetrate into the structure and cause corrosion. Various cement supplements were used as admixtures. The reference sample consisted of cement, fine sand, and water, while the rest of the samples consisted of 20% blast furnace slag, bypass dust, eggshells or recycled glass instead of the cement. The results showed lower permeability in samples containing blast furnace slag and eggshells and a higher charge passage in samples with recycled glass and bypass dust than for the reference sample.

1. Introduction

Concrete is exposed to different aggressive attacks due to various environmental conditions. The durability of concrete can be defined as the resistance to physical or chemical deterioration caused by environmental interaction. Coastal areas and materials in direct contact with water, acid rain, airborne ions, or ions found in de-icing salt are the main sources of aggressive ions that encounter the material, and the activity of aggressive ions causes the corrosion of the structure of the composite cement material. Aggressive ions penetration and transport through the concrete is a process that can take place through a number of mechanisms such as diffusion and convection, among others. When the concentration of ions on the outside of the concrete member is higher than on the inside, the diffusion process occurs [1,2,3,4]. Convection transport is only applicable to concrete structures that are in contact with liquid under a pressure head, such as liquid-retaining structures, and is expressed as the rate of flow of liquids through a porous material caused by a pressure head. Therefore, it depends on the pore structure and the viscosity of the liquids or gases involved. Dissolved ions and gases are therefore transported by convection with penetrating water into the concrete [5].

In the past, the durability of concrete used to be specified by maximum water–cement ratio and minimum cement content. The development of materials allowed for the use of various additives and admixtures, including secondary raw materials. This made it more difficult to determine the fundamentals of durability properties. Chemical deterioration is differentiated from physical deterioration by chemical interactions between constituents [6]. The case studies that Metha mentioned in [4] reaffirmed that the key to overall durability is the permeability of the concrete rather than its chemistry. The term permeability generally describes the durability of a material, specifically it describes the ability of liquid to move through the concrete.

To determine the permeability of the material, several methods can be used. Methods can be categorized into the three categories: diffusion test, migration test, and indirect test based on resistivity and conductivity. Among the useable methods are the following: European method EN 13396 (immersion test for repair products and system), Nordest methods NT BUILD 433 (immersion test) and NT BUILD 492 (rapid migration test), INSA steady-state migration test, multi-regime migration test, and resistivity test. The rapid chloride permeability test (RCPT) is method defined in two American standards, ASTM C 1202 [7] and AASHTO T 277 [8], widespread in the USA and Canada, and used in the present research. The methods used have been confirmed to be performance-based specifications, which also include measurements based on the principle of the transport of chloride ions, especially in conditions where the materials are exposed to the influence of the environment [4,9]. RCPT, also known as the Coulomb Test, was first developed by Whiting in 1981 [10]. The RCPT is often utilized for its easy performance and time efficiency.

It was found that several parameters, which were examined by appropriate test methods, were related to different mechanisms of chloride transport. These parameters included the following: chloride diffusion coefficient, capillary absorption, sorptivity, initial surface absorption, the volume of permeable voids, compressive strength, water permeability coefficients, chloride migration, water–cement ratio, etc. [4,11]. The rate of chloride ion ingress into concrete is primarily dependent on pore size, pore distribution, and the interconnectivity of the pore system. The pore structure depends on other factors such as mix design, the type of cement and other mix constituents, the use of supplementary cementitious materials, construction practices, concrete mix proportions, the degree of hydration, compaction, and curing conditions. Whenever the potential risk of chemical corrosion is impended, the concrete should be evaluated for chloride permeability [11,12,13,14,15].

This paper is focused on determining the permeability of various waste-based cement materials investigated by measuring the chloride ions penetration. Measured admixtures were selected to test traditional and non-traditional wastes. The test of ion transportation aimed to obtain more information about the material’s internal structure and porosity system, which will provide information about the permeability and durability of the material, by simulating an accelerated model scenario of aggressive ions entering the material from the environment.

2. Materials and Methods

The fundamental tenet of the test is the passage of a charge through the sample during the 6 h test. According to [16] measured specimens with the diameter of 100 mm and thickness of 50 mm (obtained by sawing the central part of 100 × 200 mm concrete cylinders) should be used.

A constant voltage of 60 V was passing through the sealed sample. The sample was in contact with solutions on both sides; one side had a NaCl solution, and the other side had a NaOH solution according to standard [16]. To prevent sample from leaking, the sample was sealed with silicone putty. The samples were in contact with the solutions through a circular cross-section (as shown Figure 1) with sample dimensions of 44 mm circular diameter (on average) and 28 mm mean diameter. This RCP test has been modified to smaller dimensions compared to the guidelines. On the one hand, due to the difference in the composition of the mixtures, a mortar sample was used instead of a concrete sample, and on the other hand, due to the repeated current trend, the time frame was consequently shortened to 90 min.

The amount of current passed through the sealed sample in mA at a constant voltage of 60 V ± 1 V was measured during a period of 90 min. Every minute, the highest and lowest value of the current in mA was recorded. To determine the permeability of mortar samples, the total charge (Q_(total)) passed through the mortars was calculated based on the maximum current detected within the 90 min measuring interval according to [16]:

$$Q_{\left(total\right)}={\int }_0^{t}Idt\hspace{1em}\hspace{1em}\hspace{1em}[\mathrm{C}]$$

(1)

where:

Q—Charge (C);

I—Current (A);

t—Time (s).

The calculations of the total charge (Q_(total)) according to the standard [4], considering the 360-min duration of the test, was comparable to the results of other studies. Q_(total) was established in accordance with the recurring current transition trend from the current charge achieved during 90 min.

The investigated preparations consisted of a reference sample (CEM) comprising a mix of Ordinary Portland Cement, fine sand, and water. The other samples had the cement component replaced by 20 wt.% of admixtures from secondary raw materials. The high chloride resistance of concrete depends on the type of cement and maximum water–cement ratio [17]. According to [4], concrete with a water-to-cement ratio of 0.38 reports a much lower permeability than the concrete with a water-to-cement ratio of 0.52. In the present research, the water coefficient was set to 0.5 for all mixtures, according to [18].

Blast furnace slag (BFS), bypass dust (BD), eggshells (ESs), and recycled glass (RG) were the secondary raw materials used in the cement composite samples as the 20 wt.% cement (CEM) supplement. Mixtures were taken from the bulk sample into round molds with dimensions of 44 × 28 mm and left for hydration for 28 days. After a period of 28 days, the samples were saturated with water for at least an hour before the RCP test to achieve a faster current increase.

3. Results and Discussion

The values of maximum passed current (I_max) through the individual samples measured during the 90 min experiment are reported in

Table 1. Measured maximum currents (I_max) per reference sample (CEM) and cement samples with supplements.

Sample	CEM	BFS	BD	ES	RG
Current Imax (mA) *	55.52	31.8	71.6	52.6	72.9

* Maximum current in milliamperes.

. These maximum values were used to calculate the charge transfer according to Equation (1).

A comparison of the results of ion penetration between composite samples with supplementation and reference composite sample showed lower permeability for samples with BFS and ES, but higher permeability for samples with BD and RG by more than 30% (Table 1). However, this measurement provided only preliminary input data of the samples. Measured samples contain pozzolanic and latent hydraulic materials, which are subjected to continuous hydration processes, hardening of matrix, and changes in the inner structure.

Figure 2 represents the charge passed through each sample measured during a 6 h test according to the standard test method [7].

The sample with blast furnace slag (BFS) substitute and with eggshells (ESs) had a lower ion transition than the cement reference sample (CEM). The low permeability of the BFS samples corresponded with the results of the study [19]. Samples with bypass replacement (BD) and glass (RG) showed the highest ion permeability. The results for cement kiln dust (BD) from study [20] corresponded with the results obtained in the present research.

To express the differences in ion penetration and thus permeability among the samples with individual cement substitutes and reference sample, the charge index (CHI) was calculated by dividing the values of calculated total charges Q_(total) of a sample with supplement and the reference sample:

$$CHI=\frac{Q_{\left(total\right)}\left(i\right)}{Q_{\left(total\right)}\left(ref\right)}$$

(2)

where:

Q_(total)(i) is the total charge calculated per individual sample (C);

Q_(total)(ref) is the total charge calculated for reference sample (C).

Figure 3 shows results of charge indexes of composites with supplementations in relation to the reference sample.

The charge transport does not depend only on the chlorides contained in the test solution, but also on the chlorides contained in the sample. Therefore, the sample with bypass dust content was optimized by interpolation calculation, and despite this, it turned out that the structure of the composite is the most permeable among investigated mixtures. This implies that the porous system of the structure is richer in pores filled with air or water, which allows for the passage of not only chloride ions in the pore solution of the composite but also the passage of other ions, for example, OH⁻, SO₄²⁻, Na⁺, Ca²⁺, etc., which participate in concrete corrosion [12].

4. Conclusions

The present work was focused on the estimation of the permeability of the cement composite material with different admixtures comprising secondary raw materials in comparison with a reference cement composite sample without admixture. The pore system of the material structure can lead to the corrosion of the material caused by ion penetration to the inner structure. To investigate the permeability of the material, the rapid chloride permeability test was used. RCPT is widely used test thanks to its relatively quick and simple performance. The results represent the amount of the passing charge transported through the sample under a constant voltage, which shows the permeability of the inner structure. The results showed a higher permeability for samples with bypass dust and recycled glass admixtures than the reference sample. On the other hand, a lower permeability than the reference sample was measured for blast-furnace-slag- and eggshell-based composites. The improvement in terms of decreasing the permeability was found for blast furnace slag samples by up to 30%.

Further investigations including the performance of X-ray fluorescence and diffraction analysis of the samples will provide more information about changes in the compound composition of the materials’ structure.