The Use of MaxRes for the Investigation of Partially hydrated Portland Cement Systems

Introduction

In the cement chemistry the following symbols are used for simplicity:

A for Al₂O₃ , C for CaO, H for H₂O, S for SiO₂ and S for SO₃. For example, tricalcium aluminate, 3CaO.Al₂O₃ becomes C₃A and gypsum, CaSO₄.2H₂O, becomes CSH₂.

The addition of water to Portland cement initiates the setting or hardening reaction, which binds the whole mass together. The hydration of Portland cement leads to the formation of different hydrates and is a very complicated process:

• Portland cement contains various components that take up water of crystallization at different rates.
• Many different hydrates, some of which are not stoichiometric, are formed.
• The degree of crystallinity of the hydrates is low.

In the first few hours after mixing water with Portland cement, C₃A reacts rapidly with the formation of a number of different calcium aluminum hydrates:

• 3CaO.Al₂O₃.6H₂O (C₃AH₆),
• 2CaO.Al₂O₃.8H₂O (C₂AH₈) and
• 4CaO.Al₂O₃.19H₂O (C₄AH₁₉)

The presence of calcium and sulfate in the aqueous phase (dissolved gypsum) causes C₃A to hydrate to ettringite (C₆AS₃H₃₂):

3CaO.Al₂O₃ +3CaSO₄.2H₂O+26H₂O⇒6CO.Al₂O₃.3SO₃.32H₂O

C₃A+3CSH₂+26H ⇒ C₆AS₃H₃₂

At the same time, a small amount of colloidal calcium silicate gel (CSH) is formed from the C₃S.

C₃S+nH₂O ⇒ C₃S.nH₂O (gel)

The interpretation of the thermogravimetric curves in the early stages of this hydration is made more difficult because the decomposition temperatures of CSH, ettringite and calcium sulfate dihydrate lie close together.

The thermogravimetric measurements were performed with a METTLER TOLEDO TGA/SDTA850. The adaptive event-controlled heating rate option (MaxRes [3 - 5]) was used to improve the separation of the dehydration processes.

^{The use of MaxRes for the investigation of partially hydrated Portland cement systems | Thermal Analysis Application No. UC 114 | Application published in METTLER TOLEDO Thermal Analysis UserCom 11}