Titolo della tesi: CONTROL OF CRYSTALLIZATION PRESSURE IN CEMENTITIOUS MATERIALS USING A BIO-BASED INHIBITOR
The manuscript examines the effectiveness of citric acid (3H-cit) and its synthesized phosphorylated analog, phosphocitric acid (AFC), in reducing crystallization pressures induced by sulfate attack within cementitious matrices. Sulfate attack is a significant deterioration mechanism for concrete in aggressive environments, such as sulfate-rich soils and saline coastal areas, often presenting as expansive growth of secondary ettringite and salt efflorescence within the cement matrix. This study evaluates the incorporation of inhibitors both as internal additives within the cement paste, assessing their potential as ready-to-use formulations, and as external surface treatments. Controlled crystallization tests were employed to assess the performance of superficial treatments with inhibitors. Analysis of efflorescence morphology, pore structure retention, and surface integrity in treated samples reveals citric acid’s ability to promote the formation of a denser, less expansive salt layer with lower aspect ratios than naturally occurring efflorescence. Phosphocitric acid at a concentration of 10⁻⁵ M showed the highest efficacy, producing finely distributed, non-disruptive efflorescences and maintaining superior pore integrity, indicating that AFC is particularly effective at inhibiting crystallization without compromising material integrity. The higher deprotonation level of phosphocitric acid likely facilitates stronger coordination bonds with sodium sulfate cations, thereby reducing its affinity for adsorption onto the cement surface.
The interactions between both inhibitors and the hydration of Portland cement (PC), particularly with the tricalcium aluminate phase (C3A), are analyzed with an emphasis on the formation of alumina-ferric oxide-mono (AFm) phases. A comprehensive suite of analytical techniques was utilized, each offering complementary insights into the underlying mechanisms. Isothermal calorimetry, conducted in both in-situ and ex-situ setups, was used to monitor the heat evolution during PC hydration and its potential modulation by the inhibitors. Various factors, including mixing energy, neutralization reactions, dissolution processes of C3A and C3S, possible complexation of Al³⁺ and Ca²⁺ ions, and the potential precipitation of AFm-citrate, were considered as contributors to the observed heat release variations. Scanning Electron Microscopy (SEM) analysis displayed distinct morphological changes, with higher inhibitor dosages producing different crystal habits.
Further validation is provided by studying modifications in primary ettringite within synthesized systems, as well as observing altered morphologies in secondary ettringite formed in the presence of inhibitory additives. Notably, the aspect ratio of secondary ettringite crystals was significantly reduced due to the influence of inhibitors. X-ray Diffraction (XRD) spectra displayed substantial shifts in basal d-spacing across different samples, indicating that both inhibitors actively modify the crystal growth habit, primarily by limiting crystal growth along the c-axis, thus reducing the aspect ratio. This alteration in interlayer spacing, oriented perpendicularly to the primary growth axis, results in smaller, more compact crystals than reference ettringite.
The research also extends to a case study investigating citric acid’s efficacy in reducing salt crystallization damage within the concrete matrix under cyclic wetting and drying, simulating repetitive marine exposure. Mechanical stress simulations were conducted using a wave flume to mimic dynamic loads imposed by waves on coastal structures. During each experimental phase, concrete samples' physical and elastic properties were monitored through acoustic wave velocity measurements—specifically, P-wave and S-wave velocities—which allowed for calculating Young’s modulus (E) as a proxy for the material’s elastic properties. The wave simulation results showed that citric acid-treated concrete retains a higher level of structural integrity compared to untreated samples, which deteriorated more rapidly due to the combined effects of salt crystallization and mechanical stress.