Factors affecting free chloride contents of hardened cement paste with a low water-to-binder ratio determined by rapid water extraction

This paper assesses the free chloride content in hardened cement paste with a low water to binder (w/b) ratio by a proposed water extraction method. Key parameters including extraction time, particle size of the sample, types and concentrations of chloride salts are thoroughly investigated to understand their effects on the extraction results. The findings reveal that the increase of extraction time from 1 to 20 min and the decrease of particle size from 0.315-1 mm to 0.045–0.08 mm exhibits limited influence on the hydration degree of cement paste with low w/b ratio during the extraction process. With the increase of extraction time, the extracted chloride content gradually increases. Beyond the 10-min mark, the extracted chloride content stabilizes gradually, indicating a stable status in the extraction process. The content of Friedel's salt remains relatively unchanged throughout the 10min water extraction, while physically bound chloride gradually releases into the extraction water. Almost all physically bound chloride in NaCl and CaCl₂ mixed specimens is desorbed within the 10min of extraction time when the particle size is set at 0.045–0.08 mm. However, the physically bound chloride in MgCl₂ blended specimens keeps relatively stable when the particle size varies, which is due to the improved chloride binding capacity by M-S-H. The rapid water extraction method is proved suitable for free chloride testing of hardened cement paste with low w/b ratio by controlling suitable parameters. Consequently, the particle size of 0.08–0.16 mm and extraction time of 10 min are recommended for conservative evaluation of free chloride content in hardened cement paste with low w/b ratio.