Variables Affecting the pH Measurement of Cement Mortars

PAYAM Shafigh, SUMRA Yousuf, ZAINAH Ibrahim

(1. Centre for Building, Construction & Tropical Architecture (BuCTA), Faculty of Built Environment, University of Malaya, 50603 Kuala Lumpur, Malaysia; 2. Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia; 3.Department of Building and Architectural Engineering, Faculty of Engineering & Technology, Bahauddin Zakariya University, 60000 Multan, Pakistan)

Abstract: A quantitative pH measuring method has been used to measure the pH of pure and blended cement mortars. The blended cement mortars incorporating supplementary cementitious materials (SCMs)such as fly ash (FA), ground granulated ballast furnace slag (GGBFS) and palm oil fuel ash (POFA) were used.Moreover, different variables affecting the pH values of CBMs such as temperature of sample solution, quantity of sample powder, dilution ratio and temporary storage of sample during pH measuring process have been studied for all cement mortars.

Key words: mortar; cementitious materials; pH value; dilution ratio; temperature

1 Introduction

The cement-based materials (CBMs) such as paste, mortar and concrete are highly alkaline in nature and start their life at a high pH of about 13[1-3]. The basic reason of high pH value of CBMs is the presence of portlandite that is produced by cement hydration process due to reaction of di-calcium silicate (C2S)and tri-calcium silicate (C3S)[4-6]. The high pH value of CBMs is the fundamental reason for their strength,sustainability, durability and defect-free long service life[7,8]. The major durability issues such as cracking and spalling of concrete cover, corrosion, chloride penetration, carbonation, alkali leaching and sulphate attack are related to the reduction in pH value of concrete[9-11]. On the other hand, high pH value of concrete causes alkali silica reaction, porosity and moisture related damages in concrete structures[2,12,13]. Therefore,knowing the pH of concrete for the durability assessment of concrete structures is primarily important for their defect-free long service life[14].

Due to key importance of pH of CBMs in the durability and long lasting of building structures, an accurate, standard and reliable pH measurement method is required. However, in the present literature, very less attention has been given to the pH of CBMs and its measuring methods. There are some methods proposed by researchers for the pH measurement of CBMs such as pH testing strips[15,16], direct pH measurement using glass pH electrode[17-19], expression method using pore solution of concrete[20-22],in-situleaching method[23,24],ex-situleaching method[25-27]and pH measurement by optical fiber pH sensors[28-30]. Based on the literature,the authors find the quantitative pH measurement method more appropriate, practical, economical and applicable for pH measurement of CBMs. In this method, the pH measurement is done using a probe and pH meter by making solution of powdered sample of CBMs with distilled water in various dilution ratios. The dilution ratio of 1:2 was recommended more appropriate and practical for measuring pH of CBMs[7].

In each method applied for pH measuring of CBMs, there are some factors affecting this measurement such as the type of CBMs, quantity of sample, dilution ratio and temperature of the solution. In addition,temporary storage of powdered sample in plastic during pH measuring process and time taken for pH testing of the prepared sample affect pH test results[3,7]. The influence of temperature of sample solution, quantity of sample and dilution ratio on the pH of cement mortars was assessed. The mortars incorporating ordinary Portland cement (OPC), fly ash (FA), ground granulated ballast furnace slag (GGBFS) and ground and treated palm oil fuel ash (GPOFA and TPOFA) up to 30 %,were used. In addition, the OPC mortar was used to evaluate the influence of temporary storage of sample in plastic and open air, and time taken for measuring pH on the pH test results of cement mortars.

2 Experimental

2.1 Materials

The OPC with a specific gravity of 3.12 was used in all the mixes. The specific surface area (SSA) of OPC based on the Brunauer, Emmett and Teller (BET)test was measured to be 2 667.24 m2/kg. The class F FA had specific gravity of approximately 2.29. The color of FA was whitish grey. According to the BET test, the SSA of FA was measured to be 2 858.6 m2/kg. The GGBFS had specific gravity of approximately 2.83. The color of GGBFS was off-white. According to the BET test, the SSA of GGBFS was measured to be 3 197.2 m2/kg.

The POFA used in this study was G-POFA and T-POFA. Raw POFA was taken from a factory situated in Selangor State, Malaysia. Moisture was removed from raw POFA by heating it in an oven at 105 ± 5 ℃for 24 hours. Then, coarse residues of POFA were separated by sieving through a 300 mm sieve. After this,sieved POFA was crushed in a Los Angeles machine to decrease the particle size[31,32]. The time of machine was set to run for 16 hours using an electric motor at 33 rpm to grind around 6-8 kg POFA. The loss on ignition (LOI)content in POFA was decreased by placing it in a furnace at 600 ℃ for 2 hours. After that, the T-POFA was ground again to get finer particles. The SSA of G-POFA and T-POFA based on BET test method was measured to be 4 940 and 7 400 m2/kg, respectively. The specific gravity of G-POFA and T-POFA was measured to be 1.81 and 1.98, respectively.

Local mining sand having maximum grain size of 4.75 mm and specific gravity of 2.68 was used in this study. In order to get good workability for POFA blended cement mortars, an aqueous solution of modified polycarboxylate copolymers was used as superplasticizer (SP) with a density of 1.09 kg/m3. The water from the pipeline of the laboratory was used to mix and cure the samples. The chemical composition of OPC,FA, GGBFS, G-POFA and T-POFA was determined by X-ray fluorescence spectrometry (XRF) and shown in Table 1.

Table 1 The chemical compositions and loss on ignition (LOI)of OPC, FA, GGBFS, G-POFA and T-POFA /wt%

2.2 Mix proportions and mixing procedure

In the experiment, nine different mortars were produced. The control mix was prepared with cement to sand ratio (c/s) of 1:3 and water to cement ratio (w/c) of 0.665. FA and GGBFS was used to replace 30%of OPC weight in two of the mixes. For G-POFA and T-POFA cement mortars, the replacements were up to 10%, 20% and 30% to produce 6 mixes. Table 2 represents the mix proportions of all mortars in one batch.All the mortars had a flow of 220±10 mm. However,for G-POFA and T-POFA mortars, additional water and maximum dosage of SP (up to 1.8%) was used to achieve the same flow. It is due to the nature of POFA as it has unburned carbon that absorbs more water and SP resulting in less workability. Same flow was achieved because of the same application, workability and usage of all the prepared mortar mixes.

Table 2 Mix proportions of mortars in one batch

Table 3 The pH test results of all mortar samples at the ages of 2 and 28 days

Mixing was initialized by dry mixing of binder and sand for 2 minutes. Then, a mixture of SP and approximately 70% of mixing water were added and mixed for 3 minutes. Subsequently, the remaining water was added and mixed for 5 minutes to achieve a homogeneous mixture. Finally, workability was determined using flow table test.

Fresh mortar was then cast into 50 mm cube steel moulds in two layers. Each layer was compacted using vibrating table. One day after casting, all the cube samples were de-moulded. The samples were cured under water curing (WC) at room temperature. The curing was continued until the specimens were used for compressive strength test at the ages of 2, 7 and 28 days. The pH testing was conducted at the ages of 2 and 28 days.

At the age of 28 days, the effect of temperature,quantity and dilution ratio of the sample powder was determined for all cement mortars. In addition, the control sample was tested at the age of 28 days to determine the effect of time and temporary storage of sample in open air and plastic during pH testing on the pH values.

2.3 Test methods

Flow table test was used to control the workability of the mixtures. All mixtures were maintained in a good workability with a flow of 220±10 mm. The calculated compressive strength for each mix was the average of four tested samples. The compressive strength measurement was done using ELE testing machine with capacity of 3 000 kN and pacing rate of 0.5 kN/sec. The compressive strength test has been carried out according to the standard[33].

For pH measurement, initially, the inner portions of the cube were taken after crushing the specimen with compression machine. Then, these portions were grinded using grinding machine[8]. The 20 g of prepared powder was used for the pH measurement. It was mixed with 40 g of water with dilution ratio of 1:2 as recommended by the authors[7]. This solution was stirred using magnetic stirrer for 15 minutes. Then, the solution was filtered using number 40 filter paper with 110 mm diameter. Finally, three pH readings were taken by digital pH meter. The reported pH of each mix was the average of three values. The prepared solution was not stirred during measurement process. For each time of the pH reading, the pH value was recorded after pH meter showed a stable reading. It should be noted that the digital pH meter was calibrated before each pH measurement using two buffer solutions of pH 7.01 and 4.01.

The effect of temperature of sample solution for all mortars was observed at the age of 28 days. The pH was measured by varying the temperature of sample solution from 26-40 ℃ at a regular interval of 1 ℃. For determining the effect of quantity of sample and dilution ratio on pH of mortars, sample sizes of 5, 10 and 20 g were diluted with distilled water by dilution ratios of 1:1, 1:2, 1:3, 1:4, 1:5, 1:6 and 1:7, respectively.

To determine the effect of time duration and storage of sample in open air and plastic bag on the pH values of mortars, the control sample was tested at the age of 28 days. Some part of the sample powder was tested immediately, and its pH value was recorded. The remaining powder was divided into two parts. One part was stored in plastic bag and the other part was stored in open air in the laboratory environment at room temperature. The pH of these two samples was measured after 10 days. These 10 days were taken as maximum delay period that may occur in pH measurement process due to any reason such as time taken for sample crushing, grinding or any kind of mechanical problem.

3 Results and discussion

3.1 Compressive strength and the pH value

The compressive strength test results of all mortars at the ages of 2, 7 and 28 days are presented in Fig.1. It is clear from the results that the compressive strength of all blended cement mortars decreased at the early age of 2-days than control mix. It was due to the incorporation of FA, GGBFS, G-POFA and T-POFA as expected by the previous researchers[34,35]. A significant increase can be observed in strength of all mixes with curing time, particularly in T-POFA mortar mixes. The compressive strength of GGBFS-30, G-POFA-10 and G-POFA-20 mixes were almost comparable with each other at the age of 7-days. Moreover, the compressive strength of FA-30 mix was less than control and all other blended mixes due to its slow pozzolanic reaction as described by the previous studies[36,37].

Fig.1 Compressive strength results of all mortars at the ages of 2, 7 and 28 days

The pH values of all mortars at 2 and 28 days are presented in Table 3. A decreasing trend in pH is evident from the results with age, as expected by the researchers[38,39]. The pH values of mortars containing FA, GGBFS, G-POFA and T-POFA were lower than control mix, which is consistent with the findings of previous studies[35,40,41]. However, no significant reduction was observed in the pH values of all mortars until day 28. The reasons include the slow pH reduction and processing time. Therefore, continuous monitoring is necessary to determine the changes in the pH of CBMs.

3.2 Effect of temperature of solution on the pH value

The pH of all mortars was measured by varying the temperature of their solutions at the age of 28 days between 26 and 40 ℃ at a regular interval of 1 ℃. With the increase in temperature, the pH of all types of mixes demonstrated a decreasing trend, which is similar to the results of previous studies[7,27]. However, difference in the pH values was small, approximately less than 0.20.

The comparison of the curves obtained by the pesent and previous study[7]are shown in Fig.2. According to the latter, temperature effectively influenced the pH values, whereas in the present study, the effect was minor. However, the difference in both works lies in the temperatures used; the previous study was conducted on low temperature range of about 21.5-24.5 ℃,whereas, in the present study, the temperature of solution varied from 26-40 ℃.

Fig.2 The pH versus temperature (control mix and the previous study)

Based on the analysis of results, it can be concluded that the influence of temperature on pH strongly depends on the type of material. For each type of material, a linear relationship with strong correlation between temperature and pH was determined (Figs.3-11).However, any strong correlation could not be found valid for all types of materials.

3.3 Effect of quantity of sample and dilution ratio on the pH value

The effect of dilution ratio and quantity of sample on the pH value was determined for control and 30 %FA, GGBFS, G-POFA and T-POFA cement mortars.The dilution ratios were taken from 1:1 to 1:7 for 5, 10 and 20 g of sample powder. The obtained results are shown in Figs.13-17, respectively.

The results showed an increasing pH trend as dilution ratios decrease. In other words, the pH of a mix increased with the increase in the amount of sample (solute) in the prepared suspension. However, the difference was almost less than 0.1 for all types of mortars. The obtained results were consistent with the results of previous studies[7,27], which suggested that a dilution ratio of 1:2 is the most practical ratio for pH measurements. In conclusion, the effect of the sample quantity was not significant. Therefore, any amount of sample can be used in the proposed method.

Fig.3 The pH versus temperature (control mix)

Fig.4 The pH versus temperature (FA-30 and control mixes)

Fig.5 The pH versus temperature (GGBFS-30 and control mixes)

Fig.6 The pH versus temperature (G-POFA-10 and control mixes)

Fig.7 The pH versus temperature (G-POFA-20 and control mixes)

3.4 Effect of duration and sample storage on the pH value

The effect of time duration and storage of sample in open air and plastic bag on pH values of mortars was determined for control sample at the age of 28 days.The obtained findings are presented in Table 4. The results showed that the pH value immediately measuredat 28 days after compression test was smaller than the pH value measured after 10 days. In conclusion, the type of storage has no significant effect on the pH values. Previous studies recommend measuring the pH of cement pastes with SCMs on the same day because the carbonation of samples and pH reduction will occur even if the samples are stored in closed plastic containers[3].

Table 4 Effect of time duration and storage of sample on the pH value of control mortar

Fig.8 The pH versus temperature (G-POFA-30 and control mixes)

Fig.9 The pH versus temperature (T-POFA-10 and control mixes)

Fig.10 The pH versus temperature (T-POFA-20 and control mixes)

Fig.11 The pH versus temperature (T-POFA-30 and control mixes)

Fig.12 The pH versus temperature (all G-POFA mixes)

Fig.13 The pH of control mix versus dilution ratios and sample sizes

4 Conclusions

The aim of this study was to investigate different factors affecting pH values of cement mortars. For this purpose, factors such as temperature of sample solution, quantity of sample and dilution ratio were investigated. In addition, the effect of time duration and temporary storage of sample in open air and plastic during pH testing was determined. The following conclusions can be drawn.

a) There is a decreasing trend in the pH of cement mortars with increasing temperature of their solutions.However, difference in the pH results was less than 0.20 for the age of 2-28 days.

Fig.14 The pH of FA-30 mix versus dilution ratios and sample sizes

Fig.15 The pH of GGBFS-30 mix versus dilution ratios and sample sizes

Fig.16 The pH of G-POFA-30 mix versus dilution ratios and sample sizes

Fig.17 The pH of T-POFA-30 mix versus dilution ratios and sample sizes

b) During process of powder making and the pH measurement, temporary storage of sample in plastic or air for 10 days does not affect the obtained pH results.

c) There is not a single strong correlation between the pH and temperature of solution for all types of cement mortars.

d) There is an increasing trend in the pH of cement mortars by decreasing dilution ratios. However,the difference was almost less than 0.1 for dilution ratios from 1:1-1:7.