Pore waters cementation from

Another useful guide to the timing of cementation is estimation of the temperature of cementation from oxygen isotopic analysis and an assumed oxygen isotopic composition of. the pore water. From the temperature, cementation timing can be inferred from a time-temperature burial history. The present burial temperature defines the upper temperature limit of precipitation in much of the basin, where the sediment is currently at its maximum burial temperature. In the central basin, the pore water oxygen isotopic composition can be well constrained because only marine or evolved marine pore waters exist, and present-day values are known (Carothers Kharaka, 1978 Fisher Boles, 1990). For the central basin the pore water evolved from the initial Miocene marine value near zero (smow scale) to its present-day value near +4 (see Boles ... 

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. 

In the cement industry, the term hydration is used to describe a range of reactions between cement and water to produce a hardened product. A cement clinker particle is a multiphase solid having massive calcium silicate grains (50-100 pm) in a matrix of interstitial aluminate and ferrite. This is described as analogous to a distorted clay sequence, which traps regions of porosity-pore size distribution from nanometer to micrometer. 

Many cements used today are composites of Portland cement and industrial waste materials that can enter into the hydration reactions and contribute to the strength of the hardened product. These substances include pulverized fuel ash (PFA) from burning of pulverized coal in thermal power stations, crushed blast-furnace slag (Section 17.7), and natural or artificial pozzolanas—that is, volcanic ash and similar finely particulate siliceous or aluminosilicate materials that can react with the Ca(OH)2 in Portland cement to form hydrated calcium silicates and aluminates. As noted earlier, the solubility of Ca(OH)2 is such that the pH of pore water in Portland cements will be about 12.7, at which the Si-O-Si or Si-O-Al links in the solid pozzolanas will be attacked slowly by OH- to form discrete silicate and aluminate ions and thence hydrated calcium silicate or aluminate gels. 

The hydration product occupies more space than the cement from which it is formed, and the capillary pores were regarded as the remnants of the initially water-filled space. Their volume thus decreases, and that of the gel pores increases, as hydration proceeds. Evidence from water vapour sorption isotherms indicated that the hydration product was composed of solid units having a size of about 14 nm, with gel pores some 2 nm across (P34). The width of the capillary pores could not be determined from the available data, but they were considered to be generally much wider than the gel pores, though tending to become narrower as the water-filled space was used up, and thus in some regions indistinguishable from gel pores. 

Ratios of water/cement/pfa = 0.5 0.72 0.28. pfa glass reacted = 6.0%, CH content = 13%, both referred to the ignited weight of composite cement, pfa HP = in situ product from Pfa. Fe HP = hydrogarnet-type product from ferrite phase. Mg HP = hydrotalcite-type phase, excluding any present in pfa HP. Pfa res. = unreacted non-vitreous material from Pfa. Other components mainly C and PjO,. Other phases mainly insoluble residue, and alkalis present in or adsorbed on products or contained in the pore solution. Discrepancies in totals arise from rounding (T5). 

Hypothesis 2. Diffusion of DOC and sulfate from confining bed pore waters provides sources of electron donor (organic carbon) and electron acceptor (sulfate). Carbon dioxide produced by this reaction drives shell material dissolution/ calcite cement precipitation which can explain the major ion and carbon isotope composition of Black Creek aquifer water. 

Beachrock occurs predominantly on tropical to subtropical ocean coasts and islands (e.g. Russell, 1962 Krumbein, 1979 Scoffin and Stoddart, 1983), but it is also found in the Mediterranean (Alexandersson, 1972 El-Sayed, 1988 Strasser et al., 1989), the Black and Caspian Seas (Zenkovitch, 1967, pp. 183-186), South Africa (Siesser, 1974 Cooper, 1991) and as far north as 57° latitude (Knox, 1973 Kneale and Viles, 2000). Beachrock is also reported to form on the coasts of freshwater lakes (Binkley et al., 1980 Jones et al., 1997). Beachrock occurrences from polar regions, i.e. north and south of 60° latitude have not been reported. Collectively, this distributional evidence identifies warm climates with pore waters rich in calcium carbonate as essential cement-precipitation prerequisites for beachrock formation. 

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