Ageing of cementitious materials: Pathfinding study on ageing of cementitious materials for concrete structures

Owners of concrete structures for civil infrastructure face ageing objects in their area of ownership. These objects are subject to physical ageing. The mechanism behind ageing in concrete - or more generally in cementitious materials - is not known yet, which hinders to some extent the assessment of the remaining service life. Well-known mechanisms in cementitious materials like carbonation and reinforcement corrosion are recognised and quantified when assessing a structure. Ageing – an intrinsic materialspecific change in a material over time – is still largely unknown.

The problem statement is as follows: “Ageing, a material-inherent decay phenomenon, in concrete (or cementitious materials in general) is not understood well. It is yet unknown to what extent ageing plays a role in degradation phenomena and in limitations of service life. The use of binder materials with very low clinker content (for sustainability reasons) increases, while the effects of these binders on the long-term performance are insufficiently known. This lack of knowledge may result in – from a durability point of view – vulnerable designs of concrete structures because of poor prediction of the long-term performance of the structures.”

A literature study was performed about ageing of different materials. In material disciplines of plastics, metals and alloys and microprocessors, ageing is well understood. Gradients in the material lead to diffusion phenomena (transport or flow of energy) that may lead to changes in the material. These inherent changes in the material are interpreted as ageing.

In this study it is attempted to trace ageing phenomena in cementitious materials. Two research tracks are considered to focus on ageing. In the first track the focus was on microscopic scale deformations in cementitious material when exposed to cycles of wetting and drying and how the response of the material to those cycles changed over time. The second track consists of an investigation of changes in the mineralogy o f cementitious material over time.

Deformations in the material were studied in an ESEM and with an optical microscope. Ageing phenomena in cementitious materials occur at a slow rate. To accelerate ageing, it was decided to expose the specimens to humidity changes with or without temperature changes. Due to the humidity changes large gradients occurred in the specimens, which may lead to changes in the cement paste. Specimens were tested in four test cycles with approximately 15 weeks in between. Between tests, the specimens were stored under different climate conditions (dry, wet, temperature cycles and wet-dry cycles).

Changes in the mineralogy were investigated by X-ray diffractometry (XRD). To detect changes, a reference test and a test on exposed specimens was conducted. The latter were stored in four different climate conditions (dry, wet, temperature cycles and wet-dry cycles) for about 40 weeks before testing.

The study was carried out on binders commonly used in the Netherlands (CEM I, CEM III/B, CEM III/B with fly ash and CEM III/C), with the former as a reference and the latter as a look-through for application of new binders.

In this study it was attempted to find changes in the deformational behaviour at the microscopic scale over time in cement paste upon exposure to changing relative humidity (from 50% RH to 100% RH and back). No changes in the deformational behaviour of the material were found that could be attributed to ageing. However, interesting findings were done with regard to the order of magnitude of the microscopic scale deformations in cement paste of the specimens.

From the XRD study it appears that at the exposure conditions used in this study, CEM I is a binder which is less prone to change than blended cements (CEM III) when exposed to similar exposure conditions. The observed changes were, however, not interpreted as ageing.