Belite strength

Increase in triclinic alite, increase in gamma belite strength not well-developed Reducing conditions (Uchikawa, 1992)... 

The reaction, like that of alite, is imprecise, and the chemical equation is not balanced. Belite reacts slowly, taking about a year to harden and is responsible for the long-term strength of concrete. The heat liberated, approximately 250 J per gram of powder, is not as great as that liberated in the reaction of alite and, because the reaction is slower. 

Belite reacts with water at a slower rate than alite but the end product is the same. It takes about 2 days for the hardening process to get started and about a year to be completed. The mechanical strength of fully hydrated belite is similar to that of hydrated alite. 

The final product of the reaction of ferrite with water is not known but its contribution to the ultimate strength of the paste is modest. Like alite and belite it is not attacked by dissolved sulfates. 

The setting reactions of the varions tricalcium silicate cements are very similar, as described in Chapter 8. They also resemble the setting of Portland cement. The initial setting involves the hydration of the alite (Ca SiO ) and belite (p-Ca SiO ) phases to form a poorly crystalline gel phase consisting of calcinm hydroxide in calcium silicate hydrate (approximate formula CajSi O ) [102], After the initial hardening, further condensation reactions occur which improve the strength and give rise to short silicate chains within the structure [103],... 

The clinker composition of the first group is a classic belite Portland cement. However, as it was proved experimentally in the case of raw materials, composed of chalk with high silica ratio, the high strength cements can be produced [130]. 

Stalk et al. [141-143] studied the possibility of belite cement activity improvement by qnenehing. A great impact of the cooling rate on the strength of mortar was proved, of elinker lime saturation factor below 90% (Fig. 9.31). 

The effects of sodium, potassium, and magnesium oxides on the strength of mortar were investigated by Ono, Hidaka, and Shirasaka (1969) and optimum percentages were established. These authors concluded that mortar compressive strength was related to abundances of alpha and alpha-prime belite. 

Using a somewhat different approach, Knbfel (1989) reliably predicted the 28-day mortar strength with a simple formula containing microscopically determined percentages of alite, belite, aluminate, and ferrite. The equation is F28 = 3(alite) + 2(belite) + aluminate - ferrite. F28 is termed the characteristic strength. The equation is designed for use within a cement plant where production conditions over the period of investigation are virtually identical. 

A paper by Takagi and Kawashima (1980) shows the effects of various kiln conditions on alite, belite, and interstitial phases. These authors present a clinker Character Index, a collection of parameters which, when plotted against compressive strength at 28 days, forms a sigmoidal curve. The Ono parameters figure significantly in the formulation of the Character Index, as seen in the following list of phase characteristics ... 

The microscopy of two clinkers produced within 10 seconds by electron beam radiation at 1.2 MEV was described by Handoo and others (1992). Alite and belite appear well formed, with crystal sizes of 10 to 16 pm. Mortar strength at 28 days was said to be 470 kg/cm (46.1 MPa). 

A multiregressional equation predicting the 28-day mortar cube strength was presented by Rao, Akhouri, and Sinha (1992), the data coming from rotary and vertical shaft kiln clinkers. The prediction has a standard deviation of 17.9 kg/cm, utilizing alite and belite percentages and average crystal sizes. 

Detailed microscopical characteristics of clinkers from vertical shaft kilns, compared to laboratory burns, are presented by Ahluwalia and Raina (1992). Alite and belite crystal sizes in plant clinkers averaged approximately 21 and 19 pm, respectively, possibly accounting for high mortar strengths (44.8 MPa at 28 days). 

Decomposition of alite to secondary belite and free lime decrease in compressive strength possible abnormal expansion Reducing conditions, coarse coal particles falling into raw mix (Hawthorne, Richey, and Demoulian, 1981)... 

Antes greater than 20 pm, with belite fringes Strength decreases about 1% (Tsuboi and Ogawa, 1972)... 

Large belite clusters Unstable ring formation, especially in sulfate-rich clinkers, if no large quartz grains in raw mix (Fundal, 1980) segregation of raw mix (DeLisle, 1979) poor strength development in oil-well cements (Reeves, Bailey, and McNabb, 1984)... 

Erosion of alite crystals to form surface belite pinhead belite in matrix Slow cooling alumina ratio above 2.5 up to 10% reduction in 28-day strength (Long, 1983)... 

Belite containing abundant alpha form, alite with high birefringence High temperature and quick cooling high strength (Ono, Kawamura, and Soda, 1968)... 

With the use of Ono s Method and microscopy of raw feed, clinker, cement, gypsum, and kiln dust, coupled with laboratory burns and sieve analysis of the raw feed, Vanisko (1978) demonstrated the deleterious effect of belite nests in development of mortar strength. He showed that coarse quartz was correlated with numerous large belite nests and, when the raw sand was separately and more finely groimd, mortar strengths jumped to 41 MPa. 

Total replacement of the raw feed silica with rice husk ash (Ghosh, Mohan, and Gandhi, 1992) resulted in more uniformity in the clinker silicate distribution, larger alite and smaller belite crystal sizes, and an approximately 11% higher 28-day strength (41.2 MPa), compared to a standard mix with 7% sand as the silica source. 

Zivanovic (1995) presented a description of clinkers made with silica fume, instead of quartz sand, as the primary silica component in the raw feed. The silica fume made up approximately 3.0 percent of the feed. Alite crystal size was reported to be from 50 to 200 pm and belite grains were said to be rounded and between 50 and 60 pm. No mention of belite nests was made. The cements made with silica fume or tuff and a particular marl, compared to other cements not made with these materials, did not require as much grinding and produced higher 28-day mortar strengths. 

Raw feed from the southwestern U.S., said to be unusually easy to burn, contains a clayey microcrystalline limestone and shale. The feed has only 3.2% retained on the 125-pm screen, with 15.2% >45 pm, and has <1.0% acetic-acid insoluble residue >45 pm, the latter composed of ordinary quartz with minor amounts of glauconite (a complex alkali-caldum-iron-magne-sium aluminosilicate). Belite nests are scarce and relatively small isolated belite is abundant. The clinker is finely crystalline, easily ground, and produces a high strength mortar (typically greater than 49 MPa). 

---