Microstructures hydrated cements

Concrete is a composite material composed of cement paste with interspersed coarse and fine aggregates. Cement paste is a porous material with pore sizes ranging from nanometers to micrometers in size. The large pores are known as capillary pores and the smaller pores are gel pores (i.e., pores within the hydrated cement gel). These pores contain water and within the water are a wide variety of dissolved ions. The most common pore solution ions are OH", K+ and Na+ with minor amounts of S042" and Ca2+. The microstructure of the cement paste is a controlling factor for durable concrete under set environmental exposure conditions. 

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

Concrete is a composite material made of aggregates and the reaction product of the cement and the mixing water, i. e. the porous cement paste. The structure and composition of the cement paste determines the durability and the longterm performance of concrete. Concrete is normally reinforced with steel bars. The protection that concrete provides to the embedded steel and, more in general, its ability to withstand various types of degradation, also depends on its structure. This chapter illustrates the properties of the most utiHsed cements and the microstructure of hydrated cement pastes. Properties of concrete and its manufacturing are discussed in Chapter 12. 

Figure 1.4 Example of microstructure of hydrated cement paste (scanning electron microscope)...

Korb, J.-R, Monteilhet, L., McDonald, P.J., and Mitchell, J. 2007. Microstructure and texture of hydrated cement-based materials A proton field cycling relaxometry approach. Cem. Conor. Res. 37 295-302. Koskela, H., Kilpelainen, L, Heikkinen, S., and Cahsqc, L.R. 2003. An application of a Carr-PurceU-Meiboom-Gill-type sequence to heteronuclear multiple bond correlation spectroscopy. J. Magn. Reson. 164 228-232. Kozlov, RV. and Burdygina, G.I. 1983. The structure and properties of solid gelatin and principles of their modification. Polymers 24 651-666. 

The dimensions of the abovementioned constituents are from 0.1 pm for single crystal depth up to 100 pm for crystal needle length. These constituents are mixed together forming a dense and very irregular structure, partly crystallized and interlocked, which also contains non-hydrated cement grains and water in different forms. In Figure 6.5 the microstructure of hardened cement paste is shown schematically in successive phases of hydration. 

Parrot, L. (1985) Mathematical modelling of microstructure and properties of hydrated cement , NATO ASI Series E Applied Science, 95 213-28. 

Examining the hydrated cement paste matrix reveals that there are two primary huilding blocks that make up its microstructure calci-um-siUcate-hydrate (C-S-H) and calcium hydroxide. The C-S-H takes the form of very small crystals packed closely together to form a very dense structure. The calcium hydroxide, on the other hand, forms much larger, layered, plate-like crystals. These crystals do not pack weU and tend to exhibit weakness between layers due to poor bonding. Ultimately, it is the calcium hydroxide that represents the weak link in both strength and permeability of the hydrated cement paste. 

Recently, there has been a significant interest in the development of computer-based models for the microstructure, hydration, and structural development in cement-based materials. Factors such as composition and shape of cement particles, w/c ratio, and curing conditions have been considered for obtaining mass and volume fiiaction of phases in cement... 

Electrochemical methods appear to have distinct advantages in the study of cement hydration. Methods involving potential measurement (including pH, zeta potential, and selected ion potential), conductivity measurement, and a.c. impedance measurement provide useful information related to both ion concentration of pore solution and microstructural change in hydrating cement paste. The early hydration and setting behavior of OPC-CAC and OPC-Hydrated-CAC paste systems can be determined using these techniques. 

Gebauer, j. Harnik, a. B. 1975. Microstructure and composition of the hydrated cement paste of an 84 year old concrete bridge construction. Cement and Concrete Research, 5, 163-170. 

Korb, J.-P, L. Monteilhet, P. J. McDonald and J. Mitchell. 2007. Microstructure and texture of hydrated cement-based materials A proton field cycling relaxometry approach. Cem. Concr. Res. 37, 295-302. 

Gas adsorption is often used to follow the evolution of surface area and porosity of hydrated cement with time and this is correlated afterwards with the degree of hydration or other engineering properties. To measure both parameters at the time of interest, it is required to stop the hydration while preserving the microstructure. 

Drying techniques and solvent replacement are the most used methods to stop hydration by removing the capillary water from hydrating cement samples. As summarised in Table 10.3, most of these methods have been shown to modify in some degree the microstructure of hydrated cement... 

In this section, a protocol for analysis of cementitious materials by nitrogen adsorption is proposed and degassing conditions for preserving the microstructure of anhydrous and hydrated cement samples are described. When using these recommended sample preparation conditions, the obtained values have shown to be reproducible and reliable. 

Solvent exchange (hydration stoppage) Removal of water from hydrated cement samples by exchange of water by an organic solvent. Upon immersion, the solvent replaces the (free) water in the sample. Upon complete exchange, the solvent is removed by evaporation. Isopropanol is most frequently used as solvent. Solvent exchange is usually preferred for microstructural studies. 

This edited volume provides the cement science community with a state-of-the-art overview of analytical techniques used in cement chemistry to study the hydration and microstructure of cements. Each chapter focusses on a specific technique, not only describing the basic principles behind the technique, but also providing essential, practical details on its application to the study of cement hydration. Each chapter sets out present best practice, and draws attention to the limitations and potential experimental pitfalls of the technique. Databases that supply examples and that support the analysis and interpretation of the experimental results strengthen a very valuable ready reference. 

Many cements show a shoulder or more definite peak (3) at about 16h. This has often been associated with the conversion of AFt into AFm phase, but comparison with the microstructural evidence shows that this is incorrect (P31) it is associated with the renewed formation of ettringite. A further, less distinct shoulder (4) has been associated with hydration of the... 

The evidence from microstructure, calorimetry and other sources suggests that the hydration processes of cement and C3S are essentially similar. There are important differences in the nature of the early product and in where the C-S-H formed in the middle stage of reaction begins to deposit, but in both cases it would appear that the early reaction slows down because of the deposition of a layer of product, which either isolates parts of the anhydrous surfaces from the main solution or allows the concentrations close to those surfaces to rise to values approaching the theoretical solubilities of the anhydrous compounds. In both cases, the initiation of the main reaction and the kinetics in its acceleratory phase appear to be controlled by the nucleation and growth of C-S-H. 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

---