On the origin of cement setting, and how to control the properties of the fresh and hardened material

Abstract

Cement paste forms an elastic gel immediately after mixing cement and water. The gel’s strength rapidly increases during the dormant period. The cohesion of the gel originates from attractive forces among the hydrating surfaces and a network of nanoparticles of calcium-silicate-hydrate (C-S-H). These attractive forces do not vary throughout the dormant period, and cement paste stiffening can be interpreted as the result of an aging process occurring when cement paste is at rest. This aging process goes along with a refinement of the paste microstructure, with the large pores decreasing in volume. Shear interrupts the aging and brings back the system to its initial state, until the massive Portlandite precipitation occurring at the beginning of the acceleratory period initiates setting.

The microstructure of cement paste, which is at the core of both concrete strength and durability, is determined by the mechanisms of gel formation and by the coalescence of the cement and hydrates’ particles into flocs which span the entire space. This comes to say that the porosity, strength and toughness of the hardened paste result from the intensity of inter-particle forces and from the kinetics of particle aggregation. It is widely accepted, even if not completely understood, that chemical admixtures modify these forces. It is thus possible to control the properties of cement paste, both in its fresh and hardened state, by using chemical admixtures which interact with the hydrating surfaces and modify their interaction potential. Most of such applications are already in use and have been developed with a trial and error approach, however the understanding of the forces at the origin of cement cohesion, and the knowledge of how to modify them may lead to the development of new, more sustainable binders.