Cements grain coating

The admixture may react with cement constituents to precipitate insoluble products, and these may form slightly permeable films or coatings on the cement grains, acting as protective barriers with respect to further hydration. 

If substantial arsenic is present in a sandstone or conglomerate, it may occur in hematite or other iron (oxy)(hydr)oxide cements or coatings on mineral grains. Arsenic may also be enriched in sandstones, conglomerates, and other clastic sedimentary rocks if hydrothermal or other secondary sulfide minerals are present (e.g. St. Peter Sandstone, (Gotkowitz et al., 2001) arsenian pyrite cement in the Marshall Sandstone of the Michigan Basin, USA, (Kolker et al., 2000 Szramek, Walter and McCall, 2004)). Sections of the St. Peter Sandstone in eastern Wisconsin, USA, are enriched in sulfide minerals and contain about 500 mg kg-1 of arsenic. In contrast, unmineralized portions typically have <10 mg kg-1 (Gotkowitz et al., 2001). If present in sedimentary rocks, hydrothermal sulfide deposits often tend to concentrate in veins and faults more than the rock matrices. 

When acrylic polymers are added into a cement mortar mix, their spheres coalesce to form a continuous polymer matrix which coats the hydrating cement grains and aggregate. This polymer matrix acts as a barrier which helps to improve the hydration of the cement and also provides a polymeric network which increases the toughness and durability of the finished product. 

Calcareous soil Very weakly cemented or uncemented soil with small carbonate accumulations as grain coatings, patches of powdery carbonate including needle-fibre calcite, carbonate-filled fractures and small nodules... 

Authigenic clay minerals identified by SEM/EDS include minor amounts of kaolinite, chlorite, illite and mixed-layer illite/smectite, which form grain coatings, platelets or fibrous cements. 

Authigenic chlorite occurs as thin grain coatings and as fibrous cements, in both the Avalon/Ben Nevis and Catalina Sandstones, but rarely in the deeper reservoirs of Hibernia Field. It is one of the earliest cements in Hibernia Field, preceding early quartz cement. 

Fig. 6. Thin section photomicrographs of Norphlet Formation sandstones from Mobile Bay. (a) The tight-zone occurs at the top of the Norphlet, Almost all intergranular pore space is filled with quartz cement which encases grain-coating chlorite and pyrobitumen (black), (b) Porous reservoir sandstones occur beneath the tight-zone and are characterized by open intergranular pores that are lined with chlorite and pyrobitumen (black).

Fig. 9. General paragenesis for the Norphlet Formation at Fairway Field and surrounding areas. The timing of compaction, intergranular pressure solution and authigenic cements are plotted relative to the occurrence of grain-coating chlorite and pyrobitumen.

In all cases, the operations for preparing PCC are similar to those used for common concrete and mortar. Curing, however, is different. After only 1 day of moist cure, the surface of PCC work can be uncovered. The film already formed on the surface retains the necessary amount of moisture for the full hydration of the cement. In principle, after only a few days of air cure at around the normal ambient temperature (20°C), the PCC can be put in service. The polymer latex used for making a PCC must be able to form a film under ambient conditions, coat cement grains and aggregate particles, and form a strong bond between the cement particles and aggregates. 

The soluble alkali content in cement is of great importance for the aggregate expansion reaction. Sodium and potassium occur in cement in the form of sulphates and form the solid solutions in CjA and C2S (see Sect. 2.5.5.3). Sodium and potassium sulphates are very quickly dissolved. Alkalis from C3A solid solution are easily soluble too, while those from CjS are practically inactive, because of the slow CjS hydration. The release of alkalis is also affected by distribution of particular phases in the polymineral cement grain one can imagine C3A encapsulated in brownmil-lerite coating its reaction with water will be retarded. 

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