A model for the C-A-S-H gel formed in alkali-activated slag cements

Abstract

For first time, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials. The study involved a comparison with the C-S-H gel forming in a Portland cement paste.

The structure of the C-A-S-H gels in AAS pastes depends on the nature of the alkali activator. When the activator is a NaOH, the structure of the C-S-H gel falls in between tobermorite 1.4 nm with a mean chain length of five, and tobermorite 1.1 nm with a mean length of 14. When waterglass is the activator the structure of the C-A-S-H gel is indicative of the co-existence of tobermorite 1.4 nm with a chain length of 11 and tobermorite 1.1 nm with a chain length of 14. This very densely packed structure gives rise to excellent mechanical properties.

Introduction

Alkali-activated slag (AAS) cement development has been the object of much research in recent years because of the energy and environmental advantages of its production over ordinary Portland cement (OPC) manufacture. Specifically, the development of these alkaline cements entails the re-use of industrial waste, the reduction of both energy consumption and a substantial decline in greenhouse gas emissions, essentially CO₂. These cements are obtained by mixing vitreous granulated blast furnace slag with highly basic solutions such as waterglass (Na₂O·nSiO₂·mH₂O + NaOH), NaOH or Na₂CO₃, among others. In terms of mechanical properties, AAS cements are comparable to OPC, particularly when waterglass is used as the activating solution.1, 2 At the same time, they exhibit greater durability when exposed to acid, sulphates or seawater than Portland cement systems.3, 4, 5 Earlier studies6, 7, 8 have shown, however, that AAS cements and concretes carbonate more readily than OPCs, and that one of the chief technological problems posed by these alkaline materials when waterglass is the activating solution is their high autogenous and drying shrinkage rate. When NaOH or Na₂CO₃, solutions are used, however, cement shrinkage is comparable to the rate observed in OPC.⁹

The properties exhibited by alkaline cements and concretes are directly related to the nature and structure of their main reaction product, C-A-S-H gel. Further to the literature,10, 2, 11 the C-A-S-H gel forming in AAS pastes, like the C-S-H gel in OPC pastes,¹² is made up of tetrahedrally coordinated silicate chains with a dreierkette structure, in which each chain consists of (3n − 1) tetrahedra. The C-A-S-H gel chains in AAS cements are longer (with up to 13 tetrahedra) than the C-S-H gel chains in OPC systems (three to five tetrahedra) and, unlike the latter, include aluminium in their structure, which replaces the silicon in bridging positions. This substitution of Al³⁺ for Si⁴⁺ generates a charge imbalance compensated by the uptake of Na⁺ ions in the gel. On the grounds of MAS NMR and BSE/EDX findings, a number of researchers10, 2, 11 have concluded that the nature of the alkaline activator used leads to differences in C-A-S-H gel structure and composition in these AAS cements. Fernández-Jiménez et al. and Brough and Atkinson10, 2 reported that the use of waterglass as an alkali activator induces the formation of a C-A-S-H gel with high Si Q² and Q³ or Q^Poly contents and the formation of long, intertwined chains. Fernández-Jiménez et al.¹⁰ likewise concluded that when the activator used is a NaOH solution, the C-A-S-H gel exhibits a high Si Q² unit content. These authors confirmed the formation of long linear chains with no Si Q³ units which they did, however, detect in the C-A-S-H gel when using Na₂CO₃ as the activating solution. Finally, this effect of the nature of the alkaline activator on C-A-S-H gel structure has been confirmed to induce differences in its chemical composition.¹³ To this respect, Ca/Si in C-A-S-H gel in AAS is substantially smaller than the C-S-H gel in OPC systems.

While atomistic simulation techniques have recently been used to describe the structure and properties of C-S-H gel in OPCs,14, 15, 16 they have not yet to be applied to analyse AAS. The present study for first time a combination of experimental and modern computational techniques to propose a new model for C-A-S-H gels found in waterglass- and NaOH-activated AAS cements. A structural comparison between the C-A-S-H and C-S-H gels is provided to facilitate the description.