A structurally-consistent CASH+ sublattice solid solution model for fully hydrated C-S-H phases: Thermodynamic basis, methods, and Ca-Si-H2O core sub-model

Dmitrii A Kulik,George Dan Miron,Barbara Lothenbach

doi:10.1016/j.cemconres.2021.106585

Abstract

A new thermodynamic model, CASH+, is proposed, aimed at accurately describing equilibrium composition, stability, solubility, and density of C-S-H gel-like phases at varying chemical conditions. Taking advantage from recent atomistic and spectroscopic studies, this sublattice solid solution model allows incremental extensions to accommodate alkali, aluminum and other cations. This incrementality, achieved first time for a C-S-H solid solution model, means that all thermodynamic properties of endmembers and interaction parameters can be kept fixed in further extensions. This paper describes principles of how endmembers of CASH+ solid solution model can be constructed by permutating moieties assigned to different sublattices, and how the structural consistency of the model can be established. Initial standard thermodynamic properties of endmembers were estimated using predictive methods and PSI/Nagra and Cemdata18 chemical thermodynamic databases. The parameterized core CASH+ sub-model in Ca-Si-H2O system is shown to perform well in presence of liquid water at temperatures up to 90 °C.

Highlights

Calcium silicate hydrate (C-S-H) phases of variable composition determine the most relevant properties and the durability of hydrated cement pastes and concretes ([1,2] and references therein)
This paper aims at communicating main features of the CASH+ sublattice solid solution model to lay down the foundation for its initial parameterization and subsequent incremental extensions in companion publications
Possible cases of non-ideal mixing on sublattice sites. Because it was not known in advance what sublattice solid solution model variant (Berman or Compound Energy Formalism (CEF)) works better in the CASH+ model when compared with the experimental data, we have summarized in Table 6 their common or different features, as implemented in GEMS codes

Summary

Introduction

Calcium silicate hydrate (C-S-H) phases of variable composition determine the most relevant properties and the durability of hydrated cement pastes and concretes ([1,2] and references therein). There is an urgent need for accurate chemical thermodynamic prediction of stabil ity, density, solubility and composition of C-S-H (including water con tent and uptake of minor cations Na, K, Li, Al, Fe, Sr, Ba, U, ...), in response to changes in cement recipe, water/binder ratio, temperature, carbonation, leaching, and other factors. Resolving this problem is a major challenge in cement chemistry in general, and in the use of cement materials as a waste matrix or repository backfill in particular. The desire to make C-S-H solid solution models more consistent with the atomistic structure became more evident [4,6,7]

Objectives

Findings

Discussion

Conclusion