Carbonation and Alkalinity in Alkali-Activated Concrete and Mortar

What is it about?

This paper investigates the impact of CO2 concentration and duration of exposure on the carbonation, alkali content, and pH of alkali activated concrete (AAC) and mortar produced from a ggbs-based alkali activated cementitious material (AACM). The phenolphthalein indicator test method is used to measure the depth of carbonation, and the alkali content, mineralogy, and pH of the AAC and AAM are determined by XRD, XRF, EDS, and pH analysis. The study shows that the pH at the carbonated zone of AAC and AAM is significantly high, indicating sufficient alkalinity to prevent reinforcement corrosion due to carbonation. The alkalis Na+, K+, Al3+, and Mg2+ are more abundant in AAC and AAM, boosting their alkalinity relative to the control PCC and PCM. The phenolphthalein test is more sensitive for detecting the carbonation of alkalis Ca(OH)2 and C-S-H, which are predominant in PCC and PCM. The study also reports findings on the total drying and carbonation shrinkage, carbonation rate, and compressive strength over long-term exposure.

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Why is it important?

This research is important because it investigates the influence of CO2 concentration and duration of exposure on the carbonation, alkali content, and pH of alkali activated concrete (AAC) and mortar produced from a ggbs based alkali activated cementitious material (AACM). This study provides valuable insights into the chemical and mineralogical characteristics of AAC and AAM under accelerated and natural carbonation, which can aid in the development of durable AAC mixes and steel-reinforced structures. Key Takeaways: 1. The high pH at the carbonated zone of AAC and AAM indicates sufficient alkalinity to prevent reinforcement corrosion due to carbonation. 2. The alkalis Na+, K+, Al3+, and Mg2+ are more abundant in AAC and AAM, boosting their alkalinity relative to the control PCC and PCM. 3. The phenolphthalein test is more sensitive for detecting the carbonation of alkalis Ca(OH)2 and C-S-H, which are predominant in PCC and PCM. 4. The long-term depth of carbonation is lower in AAC than the control PCC under ambient CO2 exposure, but it is greater under accelerated carbonation (5% CO2) when kinetically unstable reactions occur, and bicarbonate is also formed. 5. The release of alkali content from the coarse aggregate increases the pH of both AAC and PCC. 6. AAC and PC samples exposed to natural carbonation have higher compressive strength than those exposed to accelerated carbonation. 7. The 28-day compressive strength of AAC and AAM is influenced by porosity, which decreases with increasing molarity of activator. 8. Accelerated carbonation in AAC under high CO2 exposure starts by the rapid formation of sodium carbonate, which converts to bicarbonate at later stages. These reactions do not occur under natural CO2 exposure, where carbonates do not convert to bicarbonate.