C-A-S-H gels formed in alkali-activated slag cement pastes. Structure and effect on cement properties and durability

¹ Eduardo Torroja Institute for Construction Sciences (IETcc-CSIC), Madrid, Spain
² Nanostructured and Eco-efficient Materials for Construction Unit, Associated Unit Labein-Tecnalia/CSIC.IETcc, Spain
³ Institute for Building Materials-ETH Zurich, Switzerland
⁴ Labein-Tecnalia, Derio, Bilbao, Spain
⁵ Basque Country University UPV/EHU, Bilbao, Spain
⁶ Universidad Rey Juan Carlos, Móstoles, Madrid, Spain

Abstract

The development of alternatives to traditional Portland cement produced with more eco-efficient processes (lower energy consumption and CO₂ gas emissions) is an item on climate change and innovation agendas. Alkaline cements and concretes are an effective alternative to traditional cements.

The structure of the C-S-H gel in Portland cements consists mostly of 14-nm tobermorite (with a chain length of five) and jennite (2-link chain). The mechanical properties of C-S-H gels can be explained in terms of the three types of packing found in these gels: low density (LD), high density (HD) and ultra-high density (UHD).

The main reaction product in alkali-activated slag (AAS) cements is a C-A-S-H gel, which adopts different structures depending on the nature of the alkaline activator. When the activator is a NaOH solution (4 % Na₂O by slag weight), the C-A-S-H gel formed has an intermediate structure between 14-nm tobermorite with a chain length of five links and 11-nm tobermorite with 14 links. When the activator is a waterglass solution (4 % Na₂O by slag weight), traits characteristic of 14-nm tobermorite with 11-link chains and 11-nm tobermorite with a chain length of 14 co-exist in the structure of the C-A-S-H gel formed. This densely packed structure (with three HD states) yields excellent mechanical properties. Like the C-A-S-H gels obtained in NaOH (4% Na2O)-activated AAS paste, the C-A-S-H gels forming in AAS gels activated with waterglass have no UHD states.

The structure and composition of these C-A-S-H gels determine strength development in AAS mortars and concretes as well as their resistance to aggressive chemicals.