# pH Measurement of Cement-Based Materials: The Effect of Particle Size
## 1 Introduction
Pore solution extraction (PSE) method described in [PERSON] and [PERSON] [22] is commonly referred by researches to extract pore solution from the CBMs for further analysis. [PERSON] et al. [23] introduced in-situ leaching (ISL) method while ESL is the most widely applied method for pH measurement for CBMs. Each method has its advantages and disadvantages. However, there are insufficient studies about variables affecting each measurement method. Among different pH measurement methods, the ex-situ leaching (ESL) method is economic and simple [24]. ESL has good repeatability and reproducibility [20; 25]. Figure 2 illustrates the steps and considerations for each stage of the ESL method; these parameters are inconsistent among researchers, varying from specimen preparation to measurement. Given the lack of a standardised method for ESL, experimental errors may occur due to the wrong selection of variables. As shown in this figure, the first step of measuring pH  
Figure 1: pH measurement methods categorised by different types of sample preparation. is to prepare fine particle sizes from the specimens. Table 1 lists the particle sizes used by different researchers [13; 19; 25; 26] in their pH measurement. [PERSON] [26] measured the concentration of OH\({}^{-}\) ions (in mmol/L) for cement paste to calculate pH. The equation of pH = 14 + log[OH] was used in his study. The results showed that smaller particle sizes had a higher OH\({}^{-}\) concentration, indicating a higher pH value (more alkaline). [PERSON] and [PERSON] [19] tested concrete from blended cement with \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{1}{c}{**Specimen Particle Size**} & \multicolumn{1}{c}{**Reference**} \\ \hline \multirow{2}{*}{Powder from drilling} & [PERSON] et al. [6], \\  & [PERSON] and [PERSON] [16], \\  & [PERSON] and [PERSON] [27] \\ \hline \(<\)63 μm & [PERSON] [10] \\ \hline
150 μm & [PERSON] and [PERSON] [24] \\ \hline \(<\)297 μm & [PERSON] et al. [28] \\ \hline \multicolumn{2}{l}{Grind until \(<\)80 μm, do not use a sieve} & [PERSON] et al. [20] \\ \hline \multicolumn{2}{l}{Crush and grind using a vibratory disc mill at} & [PERSON] et al. [29] \\ \multicolumn{2}{l}{1500 rpm for 30 s, then sieve to \(<\)80 μm} & \\ \hline \(<\)500 μm, 500 μm-1 mm, 1–2 mm & [PERSON] [26] \\ \hline \(<\)75 μm, 75–12 μm, 125–250 μm, 250–500 μm, 500 μm-1 mm, 1–2 mm, 2–4 mm, 4–8 mm, 8–16 mm, \(>\)16 mm \\ \hline \(<\)850 μm, 850 μm-2 mm, \(>\)2 mm & [PERSON] et al. [13] \\ \hline \(<\)74 μm, 74–149 μm, 149–297 μm, 297–595 μm & [PERSON] et al. [25] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Particle sizes used by different researchers in the ESL method. Figure 2: Summary for stages of the ESL method. a solid-to-solvent ratio of 1:1, while [PERSON] et al. [13] tested concrete samples without fly ash, with a solid-to-solvent ratio of 1:2. Table 2 shows the pH values of concrete in two different studies when different particle sizes were used. Specimens with bigger particle sizes had lower pH. Therefore, fine particles should be used for a more accurate measurement of pH [19]. However, preparing specimens into very fine particles requires more energy and time, compared with using bigger particle sizes. [PERSON] [26] reported that the effect of the particle size of Portland cement paste with a high solid-to-solvent ratio of 1:1 is relatively smaller than those with the ratios of 1:2, 1:5 and 1:50. This study selected the ESL method to standardise a quantitative pH measurement method for CBMs to cover a wider pH range than phenolphthalein measurement. This work aims to determine the effects of using different particle sizes on the pH value of cement mortar and understand the possibility of using large particles (within the range of standard sand particle sizes) instead of using very fine particles for pH measurement. Similar studies were conducted on concrete and paste materials [13; 19; 25; 26], and the results revealed that large particles had lower pH than very fine particles.