Leaching Kinetics of Iron-Rich Alkali-Activated Materials under Sulfuric Acid Attack: An <i>In Situ</i> Method Using Low-Field NMR Relaxometry

Proton nuclear magnetic resonance (NMR) relaxometry is applied to monitor the temporal changes of Fe³⁺ ion concentration in an aqueous solution by exploiting the paramagnetic behavior of Fe³⁺ ions. The nondestructive and noninvasive nature of NMR techniques allows us to observe <i>in situ</i> the leaching behavior of iron-rich alkali-activated materials (AAMs) in sulfuric acid solution. By calibrating the relation between proton relaxation rates and Fe³⁺ ion concentrations, we can quantitatively measure the real-time release of Fe³⁺ ions into the acid solution. Traditional methods for estimating the acid resistance of AAMs are ex situ, often resulting in additional error and being time consuming. The proposed in situ NMR method is a novel and efficient complementary technique to traditional methods for studying the kinetics of acid attack on iron-rich AAMs. With the high temporal resolution of NMR measurements, the kinetics of Fe³⁺ release can be described by a proposed model that considers the sample's geometry. Two iron-rich AAM samples with different calcium molar ratios are examined. It is found that in the "low Ca content" range (<20 mol %) AAM with higher calcium content has a lower apparent reaction rate constant, indicating greater resistance to sulfuric acid attack.