[Paper Review] Creep in reactive colloidal gels: a nanomechanical study of cement hydrates

Michael Haist, Thibaut Divoux+5 more|arXiv (Cornell University)|Aug 6, 2020

Concrete and Cement Materials Research68 references38 citations

TL;DR

The paper shows that creep in hydrating cement paste is governed by inter-particle slippage of C-S-H nanoparticles, with LD-C-S-H and HD-C-S-H behaving as a single granular phase whose packing density controls creep.

ABSTRACT

From soft polymeric gels to hardened cement paste, amorphous solids under constant load exhibit a pronounced time-dependent deformation called creep. The microscopic mechanism of such a phenomenon is poorly understood in amorphous materials and constitutes an even greater challenge in densely packed and chemically reactive granular systems. Both features are prominently present in hydrating cement pastes composed of calcium silicate hydrate (C-S-H) nanoparticles, whose packing density increases as a function of time, while cement hydration is taking place. Performing nano-indentation tests and porosity measurements on a large collection of samples at various stages of hydration, we show that the creep response of hydrating cement paste is mainly controlled by the inter-particle distance and results from slippage between (C-S-H) nanoparticles. Our findings provide a unique insight into the microscopic mechanism underpinning the creep response in aging granular materials, thus paving the way for the design of concrete with improved creep resistance.

Motivation & Objective

• Investigate the microscopic creep mechanism in aging cement paste under constant load.
• Link mechanical creep to microstructure evolution during cement hydration.
• Differentiate the roles of LD-C-S-H and HD-C-S-H in creep through nanoscale measurements.
• Quantify how packing density and inter-particle spacing influence creep behavior.

Proposed method

• Perform nano-indentation on a large set of cement paste samples at various hydration degrees to extract H, M, and C.
• Use 29Si NMR to determine degree of hydration and characterize C-S-H polymerization (Q1, Q2, Q2(1Al)).
• Map chemical composition and indentation properties to identify LD-C-S-H and HD-C-S-H phases via Gaussian Mixture Modelling.
• Employ micromechanical modeling to estimate packing density η from particle stiffness and phase composition.
• Correlate creep modulus C with inter-particle distance r/d and analyze porosity contributions via nitrogen adsorption and MIP.
• Provide a coordinated view of sub-micron mechanics, chemistry, and porosity during hydration.

Experimental results

Research questions

• RQ1 How does the creep response of hydrating cement paste depend on the degree of hydration?
• RQ2 Do LD-C-S-H and HD-C-S-H differ in their creep behavior, or can they be treated as a single granular phase?
• RQ3 What microstructural parameter (e.g., inter-particle distance) primarily controls creep in aged cement paste?
• RQ4 Is creep dominated by inter-particle sliding rather than nanoparticle deformation or internal polymerization of C-S-H?
• RQ5 How do porosity changes during hydration relate to the observed nanoscale creep mechanisms?

Key findings

• Creep in hydrating cement paste is mainly controlled by inter-particle slip between C-S-H nanoparticles.
• LD-C-S-H and HD-C-S-H phases show increasing H, M, and C with higher degree of hydration, and can be described by a common packing-density-driven framework.
• Packing density η increases with hydration degree, approaching a maximum near 0.75, and defines the mechanical response of both C-S-H phases.
• Creep modulus C increases roughly linearly as interparticle distance r/d decreases, indicating that creep is an intergranular phenomenon rather than nanoscale particle deformation.
• C-S-H dimers and chain lengths (Q1, Q2, Q2(1Al)) evolve with hydration, but early-age creep correlates with interparticle spacing rather than nanoscale polymerization.
• Porosity contributions from LD- and HD-C-S-H remain substantial and align with independent porosity measurements, reinforcing the link between microstructure, packing, and creep.