Cement paste microstructure

A comprehensive overview of the properties of superabsorbent polymers, specific water absorption and desorption behavior of superabsorbent pol5uners in fresh and hardening concrete, the effects of the superabsorbent pol5uners addition on the rheological properties of fresh concrete, the changes of cement paste microstructure and mechanical properties of concrete have been reviewed (42). 

Diamond, S., Cement Paste Microstructure - An Overview at Several Levels in Hydraulic Cement Pastes-Their Structure and Properties, Conference, University of Sheffield, p. 334 (1976)... 

Korpa, A., and R. Trettin (2006). The influence of different drying methods on cement paste microstructures as reflected by gas adsorption Comparison between freeze-drying (F-drying), D-drying, P-drying and oven-drying methods . Cement and Concrete Research 36(4) 634—649. 

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. 

The essential step is the efficient grinding and blending of raw materials. The final properties of cement strongly depend on its mineral composition so that raw composition and firing conditions are adjusted, depending on the type of cement to be produced. The microstructure of the steel fiber-cement paste interface was studied by scanning electron microscopy (SEM). The interfacial zone surrounding the fiber was found to be substantially different from the bulk paste further away from the fiber surface. The interfacial zone consisted of... 

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

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). 

Unless a wet cell or cryo stage is used, the fine microstructure is much altered by dehydration in the instrument (J10,S41). However, localized drying occurs in any paste even before it is placed in a high vacuum, as soon as the RH falls below saturation. The water is lost initially from the wider pores, which are probably represented disproportionately on fracture surfaces. The state of the cement paste in a practical concrete may thus vary on both a macro and a micro scale between dry and saturated. 

The recent microstructural evidence (Section 5.3.1) gives no indication that a membrane or other product distinct from that formed later is formed during the initial reaction in C3S pastes, though a gelatinous coating is formed in cement pastes, which show an induction period similar to that observed with C3S (Section 7.5.1). For C3S pastes, this evidence excludes hypothesis 2, and gives no positive support to hypothesis 1. It does not exclude the formation of an altered layer on the C3S surface, no more than a few nanometres thick. Tadros et al. (T28) postulated the formation of a SiOj-rich layer with chemisorbed Ca ", and Barret et al. (B63) that of a superficially hydroxylated C3S, formed by protonation of the 0 and SiO ions, balanced by loss of Ca ". ... 

The physical properties of concrete are only partly determined by those of the cement paste that it contains. We shall consider these properties only for hep and to the extent that chemical or microstructural studies have contributed to an understanding of them. Other aspects are treated elsewhere (e.g. Ref. M73). 

Harrisson ct til. (H4.H49) represented the results of X-ray microanalyses of individual spot analyses in all parts of the microstructure other than unreacted clinker grains on plots of Al/Ca ratio against Si/Ca ratio and of Mg/Ca ratio against Al/Ca ratio (Figs 9.1 and 9.2). If the analyses of the material formed in situ from the slag are excluded, the plot of Al/Ca against Si Ca is broadly similar to those obtained for pure Portland cement pastes, and may be interpreted in the same way (Section 7.2.5). 

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. 

G65 Grudemo, A., Strength-structure relationships of cement paste materials. Part I. Methods and bctsic data for studying phase composition and microstructure (CBI Research, 6 77), 101 pp., Swedish Cemcnl and Concrete Research Institute, Stockholm (1977) also private communication quoted by L.-O. Nilsson in Report TVBM-1003, Division of Building Materials, University of Lund, Sweden (1980). 

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)...

Some compositional features also have a strong influence on the mechanical strength of the concrete, in particular the wjc ratio. However, in particular in chloride-contaminated environments, the cement type is even more important. In previous chapters, the microstructure of the cement paste and the beneficial role of blast furnace slag and pozzolana such as fly ash have been outlined. The other most important factor is of course the thickness of the concrete cover, which will be discussed in Section 11.4. 

D. Btirchler, B. Elsener, H. Bohni, Electrical resistivity and dielectric properties of hardened cement paste and mortar , in Electrically based Microstructural Characterization,... 

Cement paste characteristics, for example, strength and permeability significantly depended on its nanostructure features in particular nanoporosity. In recent years, electron microscopy has been demonstrated to be a very valuable method for the determination of microstructure. Numerous studies on the influence of nano-SiO on the microstructure of plain cement mortar have been carried out. The results showed that nano-SiO particles formed a very dense and compact texture in the hydrate products and decreased the size of big crystals such as CH. In this chapter in order to study the microstructure of RHA mortar, with and without nano-SiO, a SEM was used. The microstrueture of the RHA mortar with 3% replacement of nano-SiOj and without nano-SiO at a euring age of seven days are presented in Fig. 5.5 and Fig. 5.6, respeetively. Results showed that the nano-SiOj particles improved the dense and compact microstructure of RHA and generated a more homogenous distribution of hydrated products. 

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