Differential scanning calorimetry study of ordinary Portland cement

doi:10.1016/S0008-8846(99)00128-3

Cement and Concrete Research

Volume 29, Issue 9, September 1999, Pages 1487-1489

https://doi.org/10.1016/S0008-8846(99)00128-3 Get rights and content

Abstract

The present work involves using differential scanning calorimeter (DSC) in an investigation of the thermal behaviour of hydration products in ordinary Portland cement as a function of age. The two-step loss of water from calcium silicate hydrate, dehydroxylation of calcium hydroxide, and decarbonation of calcium carbonate contribute respectively to the three major endothermic peaks in the DSC curves. Peaks due to the formation of ettringite and iron-substituted ettringite, C₄AH₁₃ and Fe₂O₃ solid solution, were also found. Some DSC observations were supplemented by X-ray diffraction analysis.

Introduction

In ordinary Portland cement (OPC), there are four major compounds: tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), and tetracalcium aluminoferrite (C₄AF). Both C₃S and C₂S react with water (H) to form calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) as their principal hydration products 1, 2 in the following manner: 2C₃S + 6H → C₃S₂H₃ + 3CH and 2C₂S + 4H → C₃S₂H₃ + CH.

The formula of C₃S₂H₃ is a rough approximation and more than one form of this hydrate is possible.

In the presence of gypsum, as in the case of OPC, C₃A undergoes a rapid reaction to form C₄AH₁₃, which reacts with calcium sulphate in gypsum to form ettringite. The fourth compound, C₄AF, reacts with gypsum to form iron-substituted ettringite [2].

Thermal analyses with differential thermal analysis (DTA), thermogravimetry (TG), and differential thermogravimetry (DTG) have been used in various studies of cement hydration for a number of years 2, 3, 4, 5. In the present work, this is further extended to using high temperature heat flux differential scanning calorimeter (DSC). X-ray diffraction (XRD) was also used for phase identification in samples studied with DSC.

Section snippets

Methods

An ordinary Portland cement cube was made with a water:cement ratio of 0.33. The cube was stored constantly under water at 20°C. Before each collection of DSC samples from the cube, the first 4–5 mm of the surface was removed by either grinding or filing. The sample was obtained from the surface then showing, either by chipping or filing enough to approximately half fill the DSC sample holder (giving a sample weight of ∼40 mg), and tested immediately in the DSC. Such a procedure helps to avoid

Results and discussion

The DSC data analysis software gives graphs of heat flow between sample and reference crucibles vs. temperature. Upward peaks show endothermic reactions and downward peaks show exothermic reactions. In this paper, only DSC curves obtained during heating are considered. No obvious pattern was observed in cooling curves.

A number of peaks were observed in DSC curves consistently, albeit they do not appear at all ages. The identification of these peaks are given in Fig. 1, Fig. 2. There are three

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PapersDifferential scanning calorimetry study of ordinary Portland cement

Abstract

Introduction

Section snippets

Methods

Results and discussion

Thermochim Acta

Cem Concr Res

Thermochim Acta

The Chemistry of Cement and Concrete

Papers
Differential scanning calorimetry study of ordinary Portland cement