# pH Measurement of Cement-Based Materials: The Effect of Particle Size
## 2 Research Methodology
### Experimental Procedure  
Test cubes were left to surface dry for about 15 min and crushed into pieces that could pass through sieve mesh no. 4 (4.75 mm). Throughout the test, CO\({}_{2}\) contamination on the specimens was minimised by keeping a distance from the operator's exhalation or other sources [23]. [PERSON] et al. [25] reported insignificant pH difference (standard deviation of \(\pm\)0.03) among paste powder samples exposed to air for 0, 1, 5, 10, 20, 30, 60 and 120 min before mixing with deionised water. The specimens and liquid used were maintained at room temperature throughout the test. The specimens for pH measurement were sieved to obtain at least 50 g for each category as shown below:  
1. Mesh no. 8--to obtain particle size between 2.36 and 4.75 mm diameter;
2. Mesh no. 16--to obtain particle size between 1.18 and 2.36 mm diameter;
3. Mesh no. 30--to obtain particle size between 600 \(\upmu\)m and 1.18 mm diameter;
4. Mesh no. 50--to obtain particle size between 300 and 600 \(\upmu\)m diameter;
5. Mesh no. 100--to obtain particle size between 150 and 300 \(\upmu\)m diameter;
6. Base tray--to obtain particle size between less than 150 \(\upmu\)m diameter.  
[PERSON] et al. [13] and [PERSON] et al. [29] milled the specimens to measure the pH of CBMs. For comparison, about 100 g of crushed specimens were ground using planetary ball mills. The majority of these specimens passed through the 150 \(\upmu\)m sieve after the milling process. Figure 3 displays the specimens with different sizes for the test. Fine powder appeared lighter in colour than large particles.  
Each category of the specimens was stored in individual, sealed plastic bags. Before the test, the pH meter was calibrated with standard buffer solutions of 4.01, 7.01 and 10.01, with an offset of \(\pm\)30 mV and slope ranging between 92% and 110%.  
In a 50 mL self-standing centrifuge tube, 10 \(\pm\) 0.1 g of the specimens was added to 20 \(\pm\) 0.1 g of fresh distilled water, followed by magnetic stir bar, before tightening the screw cap on the tube. The tube was placed on the magnetic stirrer to stir without heating for 5 min of leaching. The suspension was filtered through a quantitative ashless filter paper Grade 40 by using a Buchner funnel filter to obtain at least 10 mL of the solution. The filtered solution was immediately poured into a clean 50 mL self-standing centrifuge tube for pH measurement. The solution level was higher than the ceramic junction and stirred during the measurement. The meter was set to log the reading in stability-accurate mode to avoid human error. For each sample, three readings were recorded, and the mean value was taken. The test was conducted randomly until three samples were tested for each particle size. The estimated time for the experiment (after the specimens were separated into respective sizes) was as follows: preparation of the suspension: within 5 min; stirring and leaching: 5 min; filtering: within 1 min and pH measurement: within 5 min.  
Figure 4 summarises the variables set for the benchmark methodology to test the effect of different particle sizes. [PERSON] et al. [20] and [PERSON] et al. [25] applied 5 min of mixing with magnetic stirrer for leaching 10 g of powder, with the solid-to-solvent ratio of 1:1. The solid-to-solvent ratio of 1:2 for the benchmark methodology was set instead of 1:1, given that [PERSON] et al. [13] commented that this ratio was practical for the measurement. In brief, 20 g of water was chosen so that the filtered solution in the 50 mL tube was higher than the ceramic junction of the electrode during pH measurement. The suspension was filtered with Grade 40 filter paper to protect the electrode from physical damages [13; 29]. A Bushner funnel was also used to prevent carbonisation during filtering.  
Figure 3: Samples stored in individual, sealed plastic bags.