Effect of alkali hydroxide on calcium silicate hydrate (C-S-H)

Yiru Yan,Sheng-Yu Yang,George D Miron,Ines E Collings,Emilie L'Hôpital,Jørgen Skibsted,Frank Winnefeld,Karen Scrivener,Barbara Lothenbach

doi:10.1016/j.cemconres.2021.106636

Abstract

This paper investigates the effect of KOH and NaOH on C-S-H structure and solubility. Both KOH and NaOH have a similar effect, they increase pH values and silicon concentrations, and decrease calcium concentrations. At higher alkali hydroxide concentrations, more portlandite precipitates, while amorphous silica dissolves. This increases the Ca/SiC-S-H at low Ca/Sitarget but lowers the maximum Ca/SiC-S-H from 1.5 to 1.2 in 1 M KOH/NaOH. The amount of alkalis bound in C-S-H increases with increasing alkali hydroxide concentrations and is higher at low Ca/SiC-S-H. KOH/NaOH lead to a structural rearrangement in C-S-H, increasing the interlayer distance, number of layers stacked in c direction and shortening the silica chains. The mean chain lengths (MCL) estimated from FTIR and Raman spectroscopy agree well with the trends from 29Si NMR. Comparison with the independently developed CASH+ thermodynamic model showed a good agreement between the observed and modelled changes, including the shortening of the MCL.

Highlights

Cement, with an annual consumption of 4 billion tons, is the most used construction material [1]
The measured trends and concentrations agree well with other observations reported in the literature as summarized in e.g. the recent review of Walker et al [6], and with the changes modelled by using the CASH+ thermodynamic model [48], the model somewhat underestimates the Ca and overestimates the Si con centrations
The mean chain lengths (MCL) quantified from the Fourier Transformation-Infrared (FTIR) and Raman spectra agrees well with the results from 29Si NMR, indicating that a reliable qualitative analysis is possible based on FTIR and Raman

Summary

Introduction

With an annual consumption of 4 billion tons, is the most used construction material [1]. The main cement hydration product is calcium-silicate-hydrates (C-S-H), which makes the major contribution to the strength and other properties of hydrated Portland cement [2,3,4]. The structure and properties of C-S-H, as the most important component in hydrated cement, has been investigated in a large number of studies, see e.g. C-S-H has a layered structure, with calcium oxide layers sandwiched between silicate chains in a “dreierketten” arrangement. The two “pairing” silicate tetrahedra coordinate with Ca in the calcium layer, while the third silicate tetrahedron, the bridging Si site, links two pairs of silicate units. Bridging silicate tetrahedra are mainly present in C-S-H with low Ca/Si and largely absent at high Ca/Si. Between the layers of Ca–O sheets and silicate chains, an interlayer region is present, which contains water molecules, Ca2+, and possible alkali ions (Na+ and/or K+)

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