# A Review on Dehydration of C(-A)-S-H and Rehydration of Dehydrated C(-A)-S-H for Recycled Cement

## TL;DR

This paper reviews the dehydration and rehydration processes of C(-A)-S-H in cement, which are key to recycling and creating sustainable cement materials.

## Contribution

The paper provides a comprehensive review of the multi-scale effects of dehydration and rehydration of C(-A)-S-H for recycled cement.

## Key findings

- Thermal dehydration of C(-A)-S-H causes structural changes at atomic, chain, and microstructural levels.
- Rehydration of dehydrated C(-A)-S-H is a distinct dissolution–precipitation process, not a simple reversal.
- Dehydration-induced metastability enables the development of thermally activated recycled cement with renewed strength.

## Abstract

Calcium silicate hydrate (C(-A)-S-H) and its aluminosilicate counterpart (C-A-S-H) constitute the principal binding phases in Portland cement and blended systems, governing mechanical strength and durability. This paper presents a summary of the work related to dehydration of C(-A)-S-H and rehydration of dehydrated C(-A)-S-H. Their thermal dehydration, a key process for cement recycling, induces profound multi-scale transformations: at the atomic level, it alters calcium and aluminum coordination environments and disrupts chemical bonding; at the chain-structure level, it causes depolymerization of the silicate/aluminosilicate networks; and at the microstructural level, it leads to changes in nanoscale particle morphology, aggregation state, and pore structure, creating a metastable, defect-rich, high-energy state distinct from the original C(-A)-S-H. The subsequent rehydration of this dehydrated C(-A)-S-H, which is not a simple reversal but a distinct dissolution–precipitation process, enables microstructural reconstruction and restored reactivity upon contact with water. This rehydration capacity is fundamentally exploited in thermally activated recycled cement—a novel binder concept that leverages dehydration-induced metastability for renewed strength development. Understanding these interconnected processes, influenced by factors like temperature, humidity, rate, and aluminum content, is critical for advancing sustainable cement technology, enabling the design of high-performance recycled cement and concrete, and facilitating the recycling of cementitious materials.