Cement-rock reactions

The mineralogy and geochemistry of cement/rock reactions high-resolution studies of experimental and analogue materials... 

A core sample was loaned by the Building Research Establishment and has been examined by optical microscopy, BSEM and ATEM. The objective was to characterize cement paste gels and the products of cement/rock or cement/ aggregate reactions in greater detail than has been done before in materials of this age, principally by ATEM. It should be noted that an inherent sampling bias is incurred by sampling... 

Published experimental studies of mineral/cal-cium hydroxide reactions show that at low temperatures (below 110°C), the chief reaction products are calcium silicate hydrate (CSH) gels, while zeolites and feldspars are formed at higher temperatures and in the presence of alkalis NaOH and KOH. The phase identifications have however often been made by low resolution or bulk methods, neither of which are ideal for such material. Published results of numerical simulations are in broad agreement with those of experimental studies of cement/ rock interaction. These models predict that CSH gels will be replaced by zeolites and maybe feldspars as plume chemistry evolves. 

The ATEM characterizations of experimental reaction products and archaeological analogue material have shown that highly aluminous C-S-H phases precipitate due to cement/rock interaction. This contrasts with the results of theoretical models, in which all A1 is assumed to go into other phases, principally zeolites. Zeolite precipitation may therefore be overestimated by the models. 

In light of the small solubilities of many minerals, the extent of reaction predicted by this type of calculation may be smaller than expected. Considerable amounts of diagenetic cements are commonly observed, for example, in sedimentary rocks, and crystalline rocks can be highly altered by weathering or hydrothermal fluids. A titration model may predict that the proper cements or alteration products form, but explaining the quantities of these minerals observed in nature will probably require that the rock react repeatedly as its pore fluid is replaced. Local equilibrium models of this nature are described later in this section. 

The rock in question might contain a large amount of calcite cement, but the reaction path predicts that only a trace of calcite forms during burial. Considering this contradiction, the modeler realizes that this model could not have been successful in the first place there is not enough calcium or carbonate in seawater to have formed that amount of cement. The model in this case was improperly conceptualized as a closed rather than open system. 

Diagenesis is the set of processes by which sediments evolve after they are deposited and begin to be buried. Diagenesis includes physical effects such as compaction and the deformation of grains in the sediment (or sedimentary rock), as well as chemical reactions such as the dissolution of grains and the precipitation of minerals to form cements in the sediment s pore space. The chemical aspects of diagenesis are of special interest here. 

Concrete is made of cement aggregate and water mixed together to form a paste. The aggregate is usually a tiller material composed of inert ingredients such as sand and rocks. When water is added, the components of cement undergo a chemical reaction known as hydration. As hydration occurs, the silicates are transformed into silicate hydrates and calcium hydroxide (Ca OII 2), and the cement slowly forms a hardened paste. 

A few experiments have been successfully performed at low temperatures to simulate carbonate diagenetic processes for example, cements have been precipitated on skeletal carbonate sands in experimental reaction chambers designed to mimic vadose and phreatic meteoric cementation (Thorstenson et al., 1972 Badiozamani et al., 1977). These cements are remarkably similar in composition and morphology to those found in rocks cemented in the meteoric... 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

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