Cements meniscus

This shrinkage mechanism occurs only in pores within a fixed range of sizes. In pores larger than 50 nm the tensile force in the water is too small to cause appreciable shrinkage and in pores smaller than 2.5 nm a meniscus cannot form [122]. The amount of cement-paste shrinkage caused by surface tension depends primarily on the water-cement ratio, but it is also affected by cement type and fineness and by other ingredients (such as admixtures, and supplementary cementing materials) which affect pore size distribution... 

Dunham R.J. (1971) Meniscus cement. In Carbonate Cements (ed. O.P. Bricker) pp. 297-300. AAPG Stud, in Geol. No. 19. John Hopkins Univ. Press, Baltimore, MD. 

Figure 11.8 Photomicrographs of meteoric cements. (A) Low-magnesium calcite cement with meniscus fabrics in cayrock. Constituent grains are Halimeda fragments. Harry Jones Point, Turneffe Islands, Belize. Diameter of picture is 1 mm. (B) Blocky and bladed low-magnesium calcite line a dissolution cavity in cayrock. Cay Bokel, Turneffe Islands, Belize. Diameter of picture is 1.2 mm. (C) Close-up of same sample. Diameter of picture is 750 ptm.

Micritic fabric, meniscus cements, circumgranular cracking, crystallaria, alveolar textures, grain dissolution... 

Calcite cements associated with these concretions are dominantly micritic and exhibit circumgranular cracking, alveolar, micrite-spar and meniscus microtextures. Radial spar microtexture is present locally, characterized by bladed radial spar formed around a micritic nucleus (Fig. 1OD). 

Type 2 tabular units lack original sedimentary structures and are often associated with reddened clays and clayey sands from overbank fine (OF), palaeosol (P) and interdune (ID) deposits (Table 1 Fig. 6). Micritic calcite is the main cement, and micrite-spar textures, grain dissolution, alveolar structures, circumgranular cracking and meniscus cement are common. Type 2 tabular units are subdivided by outcrop morphology into massive, platy, wavy bedded, fractured and laminar types. 

Cements near the vadose end-member are associated with typical vadose features however, these features are less apparent than in type 2 tabular units, and sparry void filling cements are sometimes more abundant than micrite. As stated previously, micrite-spar cement textures could have initially formed in the vadose zone as pendant and meniscus envelopes around grains or groups of grains (Jacka, 1974 Reeves, 1976 Warren, 1983). Upon burial, these initial vadose cements would provide sites for further calcite precipitation and the unfilled voids could subsequently be filled with sparry calcite in the phreatic zone (Jacka, 1970 Funk, 1979). The vadose contribution to cementation may have been overlooked in the past because of this overprinting. 

When water comes into contact with a porous material such as concrete, it is absorbed rapidly by the underpressure in the pores caused by what is called capillary action. This action depends on the surface tension, viscosity, and density of the liquid, on the angle of contact between the liquid and the pore walls and on the radius of the pore. In concrete, the contact angle is small due to the presence of molecular attraction between the liquid and the substrate (that is, between water and cement paste). Under these conditions, a drop will spread on a flat surface, while the meniscus of a capillary pore will rise above the level of the surrounding liquid and be concave towards the dry side in Chapter 14 we will see how this aspect can be changed by hydrophobic treatment. 

The pore size distribution in cement paste can be also determined from the Defay s relation. Unking the radins of meniscus and the freezing point of water in the capiUary [61]. The information dealing with the continuity of pores system can be derived from the hysteresis loop between the heating and cooling. However, because of many assirmptions, the results are only approximate the most accurate relate to the larger pores. In Fig. 5.29 the pore size distribution measured with this method, and with merciuy porosimetry is shown [61]. 

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