Contact point geometry governs structural build-up at rest in Portland cement-limestone blends

TL;DR

This study reveals how contact point geometry influences early stiffening in Portland cement-limestone blends, showing that particle shape and size affect the rate of hydration-induced stiffening.

Contribution

It demonstrates that early stiffening depends on contact geometry and particle characteristics, extending understanding of hydration mechanisms in cementitious materials.

Findings

Stiffening rate correlates with particle size and shape.

Contact geometry influences hydration product formation.

Mechanism remains consistent across limestone blends.

Abstract

The early stiffening of fresh cement paste plays a key role in shaping and stability during casting and 3D printing. In Portland cement systems, this phenomenon arises from the formation of calcium-silicate-hydrate (C-S-H), which stiffens grain-to-grain contacts. However, the role of powder characteristics such as particle size and morphology remains poorly understood. Here, we vary the fineness and grain shape by blending Portland cement with either coarse or fine limestone, leveraging the affinity of C-S-H to nucleate on limestone surfaces. By coupling calorimetry and rheometry, we relate the amount of formed hydration products to the increase in stiffness, and show that the mechanism of contact stiffening through C-S-H formation remains unchanged with limestone addition. Nevertheless, the rate of stiffening varies across blends. We find that these rates correlate with a characteristic length scale that captures particle size and shape. These results demonstrate that early stiffening depends not only on the amount of hydration products formed, but also on the geometry of the contacts where these products form, offering a framework for understanding more complex systems such as limestone-calcined clay cements.