A discussion of the paper “Dynamic microstructural evolution of hardened cement paste during first drying monitored by 1H NMR relaxometry” by I. Maruyama, T. Ohkubo, T. Haji et al.

doi:10.1016/j.cemconres.2019.105928

Cement and Concrete Research

Volume 128, February 2020, 105928

https://doi.org/10.1016/j.cemconres.2019.105928 Get rights and content

Abstract

The recent paper “Dynamic microstructural evolution of hardened cement paste during first drying monitored by ¹H NMR relaxometry” authored by I. Maruyama et al. is discussed. Besides the observed transformation of relatively thin gel pores into interlayer pores, it is worth noting that the coarse gel pores and inter-hydrate pores become coarser during drying. Additionally, interlayer pores simultaneously contract during drying at beyond 40% RH. The drying shrinkage of cement-based material may be physically rooted in the remarkable contraction of C-S-H gel, which is also responsible to the considerably lower specific surface area accessible to nitrogen than that to water.

Introduction

Taking the advantages of ¹H NMR relaxometry, Maruyama et al. have monitored the dynamic micro-structural evolution of hardened cement paste (HCP) during the first drying process [1]. It is found that the water contents in capillary pores, inter-hydrate pores and gel pores continuously decrease, while the water content in interlayer pores first increases and then decreases after reaching its peak. Combining with the colloidal nature of C-S-H gel [[2], [3], [4], [5]] and some previous investigations on the micro-structural evolution of HCP during drying process [[6], [7], [8]], the first increase of interlayer water is attributed to the transformation of gel pores into interlayer pores. The movable C-S-H sheets shared by gel and interlayer pores are proposed to be responsible for this considerable transformation. However, the water distribution identified through the NMR relaxometry may be insufficiently analyzed by the authors. Moreover, these interpretations on the micro-structural evolution of HCP during first drying may need further enrichment. In this discussion, the pore-size resolved water removal process and thus micro-structural evolution of HCP during first drying are discussed to enhance the understanding on the behaviors of C-S-H gel sensitive to water.

Section snippets

Pore-size resolved water removal process

At the room temperature ca. 17 °C, the Kelvin radius in accordance with 80% RH is roughly 4.9 nm in the capillary regime [9, 10]. Thermodynamically, the coarse pores larger than a characteristic length adsorb only a thin water film on their walls, and other finer ones are all filled by liquid water with a meniscus of radius 4.9 nm at equilibrium [11, 12], even under the influence of complex pore shapes such as ink-bottle pores [13, 14]. Although the authors do not measure and report the

Micro-structural evolution of C-S-H gel and HCP

Physically, the transformation of gel pores into interlayer pores must be driven by the actions of capillary pressure, surface energy and possibly disjoining pressure [30, 31], etc. As the authors declare, the water removal from gel pores sharing movable C-S-H sheets with interlayer pores brings up the transformation of gel pores into interlayer pores, which could be also considered as the contraction of relatively thin gel pores. At the same time, the interlayer water is also under the action

Other considerations

From the micro-structural evolution of cement-based material, it may be confidently concluded that C-S-H gel will contract during water removal and will swell during water regain. This unique behavior of C-S-H gel is similar to the water sensitivity of reservoir rocks [[36], [37], [38]], which is similarly linked to the swelling and contraction of their clay minerals. The nanoscale pore structure of C-S-H gel and thus cement-based material is so sensitive to water that, great effort needs to be

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The financial supports from the National Natural Science Foundation of China (No. 51578194) and the Open Research Fund of Jiangsu Key Laboratory of Construction Materials (CM2018-07) for the current research work are gratefully acknowledged.

References (1)

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Citation Excerpt :
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DiscussionA discussion of the paper “Dynamic microstructural evolution of hardened cement paste during first drying monitored by 1H NMR relaxometry” by I. Maruyama, T. Ohkubo, T. Haji et al.