Cement chemistry and hydration reactions

Shrinkage and expansion of Portland cement systems

The chemical shrinkage of hydrating Portland cement systems, the resulting bulk volume changes and the potential consequences for hydraulic isolation and well integrity in oil and gas wells will be addressed in a series of articles.

The different articles are:

1. Introduction: background to cement chemistry and hydration reactions
2. Chemical shrinkage: what it is, how to calculate it and how to measure it?
3. Bulk volume changes: how do well conditions affect the bulk volume changes of cement and what are the consequences for zonal isolation?
4. Expanding agents: how can they be used to improve zonal isolation?

Cement chemical notation

In cement chemistry a special notation is often used to simplify the description of the cement compounds as many cement compounds can be expressed as a sum of oxides.

The common abbreviations of the oxides are:

Thus, tricalcium silicate, Ca₃SiO₅ can be written as 3CaO.SiO₂ which is simplified as C₃S

Portland cement components and their reactions

Portland cement is prepared by grinding Portland cement clinker with one or more forms of calcium sulphate, usually gypsum (CS̄H₂). The main oxide compositions of Portland cement are:

• CaO       60 – 70%
• SiO₂        18 – 22%
• Al₂O₃      4 – 6%
• Fe₂O₃     2 – 4%

Mineral Composition of Classical Portland Cement Clinker

When Portland cement (clinker with gypsum) is mixed with water the hydration of the different components at early times can be written as:

1. C₃S + 5.3H → C_1.7SH₄ + 1.3CH
2. C₂S + 4.3H → C_1.7SH₄ + 0.3CH
3. C₃A + 3CS̄H₂ + 26H → C₆AS̄₃H₃₂
4. C₄AF + 3CS̄H₂ + 30H → C₆AS̄₃H₃₂ +CH + FH₃

Equations 1 and 2 represent the hydration of the silicate phases.  The composition of the calcium silicate hydrate, written as C_1.7S̄H₄ in this example, can vary significantly depending on the hydration conditions.  CH is Portlandite.

Equations 3 and 4 represent the hydration of the aluminate phases in the case where there is still excess of sulphate ions present, which is more likely at early times.  The C₆AS̄₃H₃₂ is ettringite.