Mortar strength 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. 

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). 

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). 

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. 

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. 

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. 

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. 

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). 

A relatively "hard-to-burn" feed from the western U.S., with a very impure dolomitic limestone, has 6.7% >125 pm and an acetic-acid insoluble residue of 9.5% >45 pm, the latter comprised of quartz, feldspar, medium to finely crystalline igneous and metamorphic rock fragments, and an abundance of ferro-magnesian minerals (mainly amphiboles and pyroxenes). = 1.6% and = 4.5%. Belite nests (many with tightly packed crystals), solitary belite, and periclase are abundant in this fine- to medium-crystalline clinker. Nevertheless, a high-compressive strength mortar (44.8 to 48.3 MPa) is made, mainly because of the small alite size and the well-scattered solitary belite. 

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