Gehlenite, formation

As noted in Section 3.3.1, the decarbonation of calcite is greatly favoured by intimate mixing with quartz or clay minerals. Under these conditions, much CO2 is lost before any free lime can be detected. The formation of C2S as an early product is well established, but the situation with the calcium aluminate phases is more complex, no one phase being dominant as an initial product (W11). The aluminate or aluminosilicate phases most often reported as early or intermediate products in laboratory experiments with pure chemicals or raw meals have been CA, CjjA, and, less frequently, gehlenite (W12,D6-D8,W13,L8,L9,I6,C9,C10,R10). CjA can form by 850°C (L9,C10) but seems usually to be a later product. Ferrite phase forms readily, and is initially of low Al/Fe ratio (D7,R10). 

In system F calcite was used as a calcium source for the formation of gehlenite. The reaction did not proceed as completely as with calcium acetate, as inferred from two observations 1) some of the calcium remained as calcium oxide upon decomposition of... 

In systems G and H, sodium sulfate was used as a sodium source mixed with kaolinite and calcium acetate. Sodium sulfate melts at approximately 884 C. Therefore, at 750 C no interaction between kaolinite and sodium sulfate was seen (systems G and H). In system G nepheline was formed at 1000 C, with none of the sulfur released from the sodium sulfate involved in the formation of any new high temperature minerals. However, in system H additional feldspathoids (i.e., gehlenite and hauyne) other than nepheline were formed. In this case, sulfur was involved in forming new high temperature minerals. 

Gehlenite is an intermediate phase formed at the temperature range of 800-1200 °C by some authors is mentioned. Starting from the temperature 950-1000 °C the formation of ferrites C2(A, F) can be observed. 

In the mixtures of lime and natural pozzolana Sersale [27] found the hydro-gehlenite C2ASHg. It was formed, apart from hydrogamet at later age, from 70 to 150 days, when the amount of combined calcium hydroxide changed from 46 to 60% [28], The formation of hydrogamets was found also by Takemoto and Uchikawa [23]. 

In mixes with Portland cement as the main constituent, the setting is due to a rapid formation of ettringite and the hydration of calcium aluminate cement. The hydration of the calcium silicates has little influence on the setting process, but contributes to the subsequent strength development (Gu et al., 1994). In addition to the phases formed in the hydration of pure Portland and calcium aluminate cements, stratlingite (gehlenite hydrate, C2SAHg) may also be formed in the hydration process. 

C decomposition of CaCOj formation of gehlenite (C2AS) as a transitional... 

Han H-W, Huang C-Y, Liu HS (1999) Formation of gehlenite (Ca2Al2Si07) by carbothermal reduction of carbonized rice hulls. J Ceram Soc Jpn 107 1115-1120... 

In case of the alumina-chromia brick RESISTAL RK30SR below the several mm thick reaction zone the highest microstmctural changes were observed in the area 0-2 mm from the hot face. In that brick area pore filling infiltration, recrystallization of Cr-comndum bearing matrix, corrosion of Zr-mullite and formation of (Na)-Ca-Al-silicate of type gehlenite (Ca2Al2Si07) took place. Infiltration of the microstmcture can be traced over the whole polished section (0-20 mm from the hot face). 

There has been a continued interest in producing cement clinker at lower temperatures than what has been the normal practice. A study of binary and ternary systems containing limestone, alkaline basalt and fluorite has been conducted to study the possibility of obtaining belite and alite at lower temperatures.[ 1 The reaction was followed by DTA, DTG, and TG. Several endothermal and exothermal peaks were obtained. De-carbonation occurred at about 65 0°C, and a peak at 1145°C could be linked to the formation of C12A7 and gehlenite. The peak at 1170°C was attributed to the formation of belite, and that at 1235°C to the formation of alite. 

In the heating of a kaolin-lime mixtures, gehlenite (C2AS) and a small amount of J8-C2S have been detected at 900°C. At 1050°C, formation of C12A7 occurs. At the temperatures of 1100°C and 1400°C, C3A and C3S are formed respectively. In the thermograms of Ca(0H)2-Si02 mixtures, several endothermal and exothermal effects result that can be attributed to the formation of /3-C2S, 7-028, Ca(OH)2, etc. 

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