# pH Measurement of Cement-Based Materials: The Effect of Particle Size
## 1 Introduction  
Calcium silicates (C\({}_{3}\)S and C\({}_{2}\)S) are the main cementitious compounds in cement. The hydration and hydrolysis of calcium silicates produces calcium hydroxide (Ca(OH)\({}_{2}\) or CH) [1]. About 25% of the structural component of cement paste is CH, which increases the pH of cement-based materials (CBMs) to more than 12 [2]. The high pH of concrete will enable the formation of a passive protective film on the embedded steel. This thin oxide layer formed on the steel can prevent the metal atoms from dissolving and reduce the corrosion rate to an insignificant level [3]. However, carbonation, CH leaching, or the presence of ion chloride reduces the amount of CH and other alkalis in concrete [4]. When the pH of concrete is reduced to less than 11.5, the passive protective film for the embedded steel will be unstable and the steel will be corroded at the rate of at least 1000 times higher [3, 4, 5]. Phenolphthalein solution is typically used as an indicator to test the depth of carbonation from the concrete surface because it changes from pink to colourless when the pH is less than 9.5 [6]. However, this method is not accurate enough to evaluate the durability of reinforced concrete, where the pH near the embedded steel is more than 11.5 [4, 5]. A more accurate pH measurement method is needed to evaluate the durability performance of concrete materials, particularly for concrete containing supplementaryceentitious materials (SCMs). [PERSON] et al. [7] reported that concrete containing SCMs is more vulnerable to carbonation than ordinary Portland cement concrete. [PERSON] et al. [8] also reported that mortars and concretes made of ground granulated blast-furnace slag (GGBFS) cement exhibited higher carbonation rates, particularly when they were poorly cured and had high GGBFS content. Researchers have investigated the pH of CBMs for different purposes. Most studies are related to the passive protective film of embedded steel bars and aim to compare the pH among different CBM mixtures [9; 10; 11], to monitor the pH profiles over time [12] and at different depths [6; 13; 14; 15], to determine the rates of pH changes [16] and the pH profiles of CBMs due to early carbonation curing [15; 17]. Meanwhile, [PERSON] [18], and [PERSON] and [PERSON] [19] measured the pH of a concrete surface for floor covering purposes. [PERSON] et al. [20] analysed methods for pH measurement for low-pH CBMs (pH about 11) used in a geological repository. Few studies have used pH testing and phenolphthalein testing and determined their relation. [PERSON] and [PERSON] [15] conducted phenolphthalein and pH measurement tests at different depths of the concrete surface at days 1 and 28 of different concrete mixtures, when the concrete samples were under early carbonation curing. At day 1, the pH test results at a carbonation depth of 10-25 mm showed higher variance than those at other depths of 0-10 and 25-50 mm. However, at day 28, no carbonation was found on the samples by using the phenolphthalein test, consistent with the pH test. The author suggested that this finding may be due to the existence of transition zones. [PERSON] [6] measured the apparent pH (not true pH) value of different carbonated concrete mixtures at different depths of the concrete surface. The measurements were conducted weekly, for a period of 6 weeks. At the 6 th week, a phenolphthalein test was carried out to identify the average carbonation depth. The results showed that, at the average carbonated depth measured by phenolphthalein, the apparent pH value was within 11.3-11.7. [PERSON] et al. [21] stated the lack of standardisation for pH measurement for concrete and urged the development of a profoundly reproducible and repeatable standard method for fresh and hardened concrete. Researchers typically applied their own consistent measurement methods across different samples for pH comparison (Figure 1).