Suitability of phenolphthalein indicator method for alkali activated concrete

Alkali activated cementitious materials (AACM) concrete is a sustainable alternative to Portland cement PC concrete by reducing CO2 emitted during the production of PC by 60%. Fundamental investigations on alkali activated concrete (AACM) has been on their engineering properties with less emphasis on the service life and durability properties. The carbonation effect on concrete is crucial for its service life prediction. This paper investigates the suitability of phenolphthalein indicator method to determine the carbonation front in AACM. The geopolymerisation products of AACM are different from the hydration products of PC concrete and consequently, the carbonation process of these two types of concrete is expected to be different. Three mixes of AACM 1, 2, 3 and control PC concrete were produced and cured in water (20 ± 20C) for 27days and then in laboratory air (20 ± 20C, 65% R.H) for 42days. A total of twenty-four cylindrical specimens with 50mm diameter X 60mm depth were produced. All the specimens were exposed to 5% CO2 inside a carbonation chamber at 20 ± 20C and 65% R.H for 327days. Carbonation depths were determined by phenolphthalein indicator method on twelve specimens. Powder samples were obtained from the carbonated and non-carbonated zones of the other twelve specimens to measure the pH of the carbonated concrete and powder. Results show that the pH of the carbonation in AACM concrete and mortar are above threshold 9 while it was below 9 in PC concrete. This suggests that phenolphthalein indicator method is inappropriate to determine the carbonation of alkali activated cementitious materials (AACM) concrete