If we are to monitor the chemical processes in cementitious materials, then pH assays in the pore solutions of cement pastes, mortars, and concretes are of key importance. However, there is no standard method that regulates the sample-preparation method for pH determination. The state-of-the-art of different methods for pH determination in cementitious materials is presented in this paper and the influence of sample preparation in each case. Moreover, an experimental campaign compares three different techniques for pH determination. Its results contribute to establishing a basic criterion to help researchers select the most suitable method, depending on the purpose of the research. A simple tool is described for selecting the easiest and the most economic pH determination method, depending on the objective; especially for researchers and those with limited experience in this field.
The measurement of pH provides relevant information on the status of both living and inert systems. In living systems, pH plays an important role in various contexts such as within the human body and the soil. For instance, the pH range of the epidermis is between 4 and 6.5. Its protective function prevents dehydration through the skin and functions as a barrier to microorganisms (
It is likewise of interest in the study of cementitious materials. For example, pH is of interest when studying setting and hardening processes, the use of hydraulic binders in cementitious materials with a low pH, and hydraulic binders as a complement to other tests in the diagnosis of pathologies such as decalcification.
The hydration of cementitious materials involves a sequence of chemical reactions between the solid components of the cement and water (
Cementitious materials with a low pH are also used in mixes that contain waste products with both inorganic and organic contaminants. At a high pH, of around 12 and 13, inorganic contaminants are precipitated as insoluble hydroxides, as the minimum solubility of those hydroxides ranges between 8 and 11. However, highly alkaline systems are not suitable for organic contaminants, as the most effective method for fixing them requires the presence of microorganisms, which can grow and survive within a pH range of 5.5 to 8.5 that is required for biodegradation processes (
Finally, decalcification of the cement paste is provoked by chemical attacks such as carbonation, leaching, and aggressive sulfates (
Leaching is a process which takes place when concrete is exposed to poorly mineralized or acidic water. The phenomenon consists of the dissolution of calcium and hydroxide ions out of the matrix, which increases its porosity and causes other changes in the material (
In contrast, sulfate attack refers only to the negative effects of the chemical reactions in which sulphate ions are present (
According to Grubb et al. (
Several methods have been proposed in the literature, such as pore pressing, dispersion in distilled water of crushed and ground samples, full immersion of the sample in distilled water, in-situ leaching (ISL) in small sample cavities, etc. However, there are no studies that compare the different preparation procedures, while assessing any influence that they may have on the pH of the pore solution. So, the selection of the method for the pH assay of cementitious materials and its regulation would be useful, as neither a standard sample-preparation method, nor a method to determine the pH of pore solutions exists at present.
A state-of-the-art approach of the aforementioned methods is therefore examined in this paper, in which the pH of hardened cementitious materials is analyzed and each method, its influence on the results, and its advantages and drawbacks are discussed. Furthermore, an experimental program was conducted to compare three of those methods, so as to determine whether the sample preparation procedure influenced the pH assay: full immersion in distilled water, dispersion in distilled water of crushed and ground samples, and superficial measurement by means of flat-surface pH electrodes.
The use of indicators such as phenolphthalein or bromothymol blue has been fully described in the literature. Phenolphthalein has traditionally been used to determine the altered zones of cementitious materials, as it dyes materials with a pH of over 9 in a purple-red color.
Chemical changes involving the use of phenolphthalein to determine the drop in pH mainly refer to carbonation due to environmental CO2 (
Carbonation front determined with phenolphthalein.
The use of phenolphtalein is an easy and cheap indicator and therefore the most common method for visual determination of carbonation. However, it provides no accurate information on the pH of the material. Several authors have proposed more accurate methods (
Furthermore, it could be of interest to study the process of pH reduction in terms of a combination of indicators.
Alkaline pH values indicators.
The use of pH indicator strips is extensive in areas such as medicine and biology. For instance, Ericson and Bratthall (
However, the use of this simple technique is less common for cementitious materials. The literature contains few works on their use in building materials; those that do refer mainly to pH verification following accelerated carbonation processes (
As with pH indicators, the use of pH strips for simple verification of the carbonation state of a concrete sample is both cheap and easy. The method simply involves placing a pH strip in contact with a few drops of distilled or deionized water on a concrete surface. However, this method should only be used for verification of alterations in the state of a material. Grubb et al. (
Islander et al. (
The use of digital equipment yields more accurate results than colour-coded methods. These modern methods are now the most widely employed to estimate the pH of a heterogeneous and mainly solid material such as concrete. Pore-water expression (PWE) is considered the most accurate index and the reference method. Proposed by Longuet et al. (
Pore-pressing equipment (1. Plastic tube for liquid drain; 2. Support cylinder; 3. Platen; 4. Die Body; 5. Piston assembly).
The literature lists a wide range of pressures applied to cement pastes and mortars to obtain a suitable amount of pore solution for the test procedure. Accordingly, the pressures applied to cement pastes varied between 345 MPa and 560 MPa, although it was not specified by all of the authors (
The diversity of the methods leads to a question on the possible effect of the pressure on the results. Constantiner and Diamond (
Traditionally, direct measurement of the pore solution has been considered a reference method. However, the technical and equipment-related costs are high and sufficient samples are not always easy to obtain. Therefore, effort had gone into the development of methods that reduce both costs and difficulty. In this sense, several variations of the ex-situ leaching (ESL) approach have been used. Basically, the aforementioned method corresponds to mixing deionized water and powder from crushing a known weight of material. However, there is an important question regarding the liquid-to-solid (L:S) ratio, the state of the sample (powdered, crushed, or in one piece) and the leaching period, which will all have significant influence on the pH results, due to the effect of dilution.
Regarding the L:S ratio, several authors used a ratio of 1, although other variables differed. For instance, Mori et al. (
In contrast, other authors have used a different L:S ratio and variations of the above methods. Webster and Loehr (
Pavlík (
Regarding the state of the sample, the results between powdered and non-powdered samples may differ significantly, which is also linked to the L:S ratio. As Li et al. (
Nevertheless, the concern regarding the dilution effect was first expressed by Sagües et al. (
3D scheme of the ISL method proposed by Sagües et al. (
However, variations of the ISL method such as the ex-situ leaching method have been used. Li et al. (
Up to this point, PWE, ESL and ISL have been identified as the main procedures for the analysis of pore solutions. When using those methods, other chemical analyses are performed as the purpose is usually more than a simple estimation of pH. Therefore, when the pH is the only parameter of interest, several authors have suggested the use of flat-surface electrodes. The basic procedure consists of pouring a drop of distilled water on a clean surface of the specimen and immediately placing the electrode in contact with the water. The main problem that remains is the sensitivity of the glass electrode and the danger of scratching in contact with the rough material. Islander et al. (
Similarly, Heng and Murata (
Comparisons between methods for a better understanding of their results have prompted multiple publications. Li et al. (
After comparing both the PWE and the ISL methods, Li et al. (
Grubb et al. (
An experimental program was performed to compare two different methods over 28 days. The selected methods were ESL with samples in powder and non-powder form, and the flat-surface electrode method proposed by Heng and Murata (
Cement pastes and mortar specimens were produced with Ordinary Portland Cement type I 52.5R and distilled water with a water to cement ratio (w/c) of 0.5. Silica sand was used in the production of the mortar specimens, according to standard UNE-EN 196-1:2005 (
Three cement-paste samples and three mortar specimens were individually immersed in distilled water with an L:S ratio of 10. As a non-destructive method, pH was measured at the different testing ages. Three other cement pastes and three mortar specimens per testing age were crushed and ground to powder until the material passed a #100 sieve. Then, 10 g of each sample were added to 100 ml of distilled water and vigorously shaken for 5 minutes. The pH assay was performed after one hour, under stirring. Finally, the other three cement-paste samples and three mortar specimens were used for pH assay with a flat-surface electrode. A drop of distilled water was poured on the surface of the specimen and the electrode was then positioned in place. Three different points were fixed on each face of the specimen, in order to monitor pH evolution of the same parts with the same specimens for all testing ages.
pH evolution of cement-paste samples (a) and mortar specimens (b) by means of ex-situ leaching of powdered samples (ESL (ps)), ex-situ leaching with non-powdered samples (ESL (nps)) and the use of a flat-surface electrode (FSE).
Comparing the curves, slight differences were obtained between the two variations of the ESL method, which were observed over the first 3 and 5 days for cement pastes and mortars, respectively. The different pH readings corresponded to the different times the solutions needed to reach equilibrium. The higher specific surface of the powdered particles in relation to the specific surface of the non-powdered specimens meant that equilibrium was reached earlier in the powder suspension, even though the L:S ratio was the same. Moreover, that fact is also a consequence of the lower amount of hardened cement paste in the cases of the mortar samples.
The results obtained for the FSE method showed similar values to those of the ESL method for the first 3 days and for the first 1 day, for the cement pastes and the mortars respectively. However, the evolution over time showed a drop in pH, as previously suggested, due mainly to the dissolution of the portlandite. The results presented above support the fact that proper planning and selection of the right method should always be done.
In this section, an analysis of the main advantages and drawbacks of the different methodologies will be presented. According to the above results as well as the experimental program, it is important to consider that all those methods are valid, although differences in terms of ease of testing, urgency of results, cost, accuracy, and monitoring evolution over time have been detected between them. Six different methods were analyzed: pH indicators, pH-indicator strips, PWE, ESL, ISL and FSE.
Accuracy should not be a requirement when the purpose of the test is to determine the state of the material. Usually, pH indicators such as phenolphthalein are spread onto a cross-section of the specimen, so as to detect the carbonation depth. In this sense, indicators will provide a binary response: i.e. purple dye on non-carbonated areas and colorless dye on carbonated areas.
The results with pH-indicator strips may give a more specific range of pH values. Several ranges of pH values are covered by the industry from the whole pH range to limited ranges of less than 2 points of pH. However, the method is based on comparing the colors of a color-coded strip against a color chart, which means that it is, for example, less accurate than using a pH-meter. Strips are also a good alternative when the purpose of the study is to determine pH readings on an approximate basis.
The last four methods are considered the most accurate, as the readings are obtained with a pH-meter. However, there are significant differences between their advantages and drawbacks. PWE, ESL and ISL are the most suitable methodologies especially when the purpose of the study is not only pH determination of the pore solution. PWE is the most expensive method due to the equipment and requires previous sample saturation to obtain a large enough volume of the sample. Moreover, the volume of pore solution is low or null depending on the cementitious material. For instance, experiments to try to obtain the pore solution of Ordinary Portland Cement and Magnesium Phosphate Cement mortars by means of this technique were done in previous works (
The main drawback of the ESL methods consists of the dilution effect due to the L:S ratio leading to an underestimated pH reading. On the other hand, the hydration of anhydrous particles may produce overestimated readings. Therefore, it is really important to calibrate the method in case of using different materials than those previously studied. Nevertheless, as the experimental program has shown, although lower L:S ratios and sufficient leaching time should be applied, further tests comparing a wide range of significant parameters should also be performed. As was also evident from the experimental program, the leaching time should be determined depending on the specific surface of the sample.
Considering the analysis,
pH measurement selection tool
Methodologies | ||||||||
---|---|---|---|---|---|---|---|---|
Purpose | Criteria | pH indicators | pH strips | PWE | ESL (ps) | ESL (nps) | ISL | FSE |
Altered state of the material | Ease of testing |
5 |
5 |
3 |
||||
Complete chemical analysis | Ease of testing |
1 |
3 |
4 |
2 |
|||
Determining superficial pH | Ease of testing |
5 |
5 |
1 |
3 |
4 |
2 |
3 |
Range 1 to 5 being 1 the less suitable methodology and 5 the most suitable one.
Different piece of the specimen should be used;
Different area of the specimen should be tested; no super index: No special considerations.
The survey of 5 questions called
Does the process require more equipment than a pH meter?
Does the process take more than 5 minutes?
Does the method require any specific induction without considering calibration and the use of a pH meter?
In case of requiring the use of a pH meter, is it necessary to use any special electrodes?
Has the humidity content of the sample or specimen, prior to testing, affected the results?
The scores associated with the factor
In contrast, the
Finally, the scores for the factor ‘
Is it necessary to test a different piece of the sample or specimen?
Is it necessary to test a different measurement area of the sample or specimen when it is possible to use the same piece?
Should the time between measurements be longer than 1 hour?
Is it acceptable to obtain results that are not highly accurate? (The concept of “highly accurate” is related to the scores given to points 4 and 5 in the ‘Results accuracy” section)
Is it acceptable to use more equipment than a pH meter?
The pH of cementitious materials provides relevant information on the different chemical processes of cementitious materials. However, the selection of the method is frequently seen as a controversial step in research, especially for scientists new to the topic. Consequently, a review of the different methods is essential before a selection tool may be definitively proposed.
Six different methods have been reviewed: use of indicators, use of pH strips, pore water expression (PWE), ex-situ leaching (ESL), in-situ leaching (ISL) and the use of a flat surface electrode (FSE). The first two are mainly used to determine the state of the material while the other four provide accurate results.
PWE is the reference method due to its high accuracy, although it requires the use of specific equipment. Moreover, the costs related to both the equipment and the technique, promoted the development of the ESL method and the ISL method afterwards. According to the literature, the ESL method overestimates the pH value due to the L:S ratio. Hence, the proposal of the ISL method as an alternative. Finally, the FSE method was proposed to measure the surface pH value. All three - the ESL, the ISL, and the FSE methods-have been contrasted to verify the accuracy of their results. However, more in-depth research comparing all the methods for different purposes might provide a better overall picture.
In conclusion, the criteria proposed here aim to facilitate the selection of the most suitable method. The selection process is accomplished by a tool which scores the different methods depending on the purpose and considering factors such as ease of testing, urgency of results, test budget, accuracy of results, and the possibility of studying changes in the pH over time.
The authors of this paper wish to acknowledge the economic support received from the Spanish Ministry of Economy and Competitiveness through Research Project BIA2010-17478.