Ph. D. Thesis Colloquium
Name: Ms. Nikita Rao
Research Supervisor: Dr. Sheetal Kumar Jain
Title: Structural Evolution and Carbonation Mechanism in Cementitious Materials: Insights from Solid-State NMR
Date and Time: Monday, 17th August at 11:00 a.m.
Venue: Rajarshi Bhattacharyya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
The continuous increase in atmospheric carbon dioxide (CO2), the dominant long-lived greenhouse gas, has intensified the search for sustainable carbon mitigation strategies. Cement production accounts for nearly 8% of global anthropogenic CO2 emissions, motivating the development of alternative cementitious binders with lower carbon footprints and greater carbon sequestration potential.1,2,3 This has motivated extensive research into alternative binders, which can reduce reliance on Portland cement.4,5 While carbonation has traditionally been regarded as a degradation process due to decalcification of the principal binding phases, calcium silicate hydrate (C-S-H) and calcium aluminum silicate Hydrate (C-A-S-H), controlled carbonation can enhance early-age strength.6 However, the molecular mechanisms governing carbonation-induced structural evolution remain poorly understood because these materials are heterogeneous, poorly crystalline, and structurally disordered. Conventional characterization techniques generally provide only phase-level or long-range structural information, making it difficult to resolve the short-range structural rearrangements associated with carbonation. In particular, understanding the evolution of silicate chain connectivity, aluminum coordination, interlayer environments, and charge-balancing cations requires probing of local atomic structures. This thesis investigates the molecular-level effects of accelerated carbonation on soil-based alkali-activated binders (AABs) and synthetic C-S-H and C-A-S-H, mainly using multinuclear solid-state NMR.
The first part of the work examines carbon sequestration in the soil-based alkali-activated binders, where accelerated carbonation enhances early-strength through carbonate formation and microstructural densification. However, prolonged exposure to CO2 results in competing effects, including decalcification of binding phases and deterioration of the microstructure.6,7 Detailed NMR analyses reveal significant reorganization of the aluminosilicate framework during carbonation. Solid-state 27Al NMR demonstrates the conversion of tetra-coordinated aluminum (Al(IV)) to hexa-coordinated aluminum (Al(VI)), accompanied by increased structural disorder. 29Si NMR shows enhanced silicate polymerization with the formation of Q3 and Q3(1Al) species, while 23Na NMR identifies changes in the local hydration environment associated with the consumption of charge-balancing Ca2+ and Na+ ions during carbonate formation.8
To isolate the role of individual binding phases, synthetic C-S-H and C-A-S-H with varying Al/Si ratios are systematically carbonated to establish the influence of aluminum incorporation on silicate chain connectivity, structural stability, and carbonation behavior. In addition, europium (Eu3+) is incorporated into C-S-H as a luminescent structural probe to monitor the early stages of carbonation. The complementary use of solid-state NMR and photoluminescence (PL) spectroscopy enables tracking of the slightest local structural changes that precede significant phase transformations, providing new insights into the evolution of the calcium environment during carbonation.9 The evolution of C-A-S-H in the presence of secondary phases such as AFm is also examined.10 Overall, this work provides molecular-level insights into carbonation mechanisms in cementitious materials, advancing the rational design of durable, low-carbon binders and demonstrating the power of solid-state NMR and complementary spectroscopic techniques for probing disordered materials.
References:
Intergovernmental Panel on Climate Change, Climate Change 2022: Mitigation of Climate Change (Cambridge Univ. Press, 2022).
E. Van Roijen et al., Science 387, 176 (2025).
L. D. Ellis et al., Proc. Natl. Acad. Sci. U.S.A. 117, 12584–12591 (2020).
J. L. Provis and J. S. J. van Deventer, Alkali Activated Materials: State-of-the-Art Report, RILEM TC 224-AAM (Springer, Dordrecht, 2014).
J. L. Provis, Cem. Concr. Res. 114, 40–48 (2018).
P. Sahoo, N. Rao, S. K. Jain, and S. Gupta, npj Mater. Sustain. 2, 34 (2024).
A. Kunhi Mohamed et al., J. Am. Chem. Soc. 142, 11060–11071 (2020)
N. Rao et al., Research Square, rs-6428163/v1 (2025).
N. Rao et al., (manuscript under preparation)
N. Rao et al., (manuscript under preparation)