Effect of Alkalis on Cementic Properties of some Sudanese Cements

Abstract

Portland cement consists mainly of major oxides which include: CaO ,SiO 2 , Al 2 O 3 and Fe 2 O 3 as well as minor oxides which include : SO 3 , TiO 2 , Mn 2 O 3 , MgO, Na 2 O and K 2 O. Minor oxides usually not to be more than a few percent of the mass of cement. Two of the minor compounds are of interest: the oxides of sodium and potassium, Na 2 O and K 2 O, known as the alkali oxides. This research aims to study the effect of alkali oxide, Na 2 O and K 2 O on some properties of some ordinary Portland Sudanese cements. Three samples of cements provided from Al salam, Atbara and Berber factories in the River Nile State, to be evaluated for their alkali levels in order to check the effect of cement alkalis on the mechanical properties. The physical properties investigated were compressive and flexural strength at (2 - 7 and 28 days), initial and final setting time, soundness, and expansion. At first samples were properly analyzed by flame-photometry to determine their alkalinity. Then two series of tests were performed, one series using Berber cement without alkali addition and also with the addition of 2%, 4% and 6% alkalis as sodium hydroxide solution, to check the effect of increased alkalinity on strength, setting and soundness. The second series was performed using the three cements (Berber, Atbara and Al Salam) but with the addition of 5% alkali as sodium hydroxide solution to each of the three cements for alkali reactivity testing. The results of chemical tests show that Berber cement is relatively of a lower alkali level (0.7%) than Al Salam cement which, in turn, is of lower alkali level than Atbara cement. The three cements, however, are of alkali contents greater than the ASTM C150 specifications of 0.6%. Results of the compressive and flexural strengths measurements for Berber cement shows that with more alkali addition there was more reduction in both, the compressive and flexural strength. Furthermore, the reduction in strength was more pronounced at the 28 th day strength, particularly, the compressive strength. Result of setting time test for Berber cement shows that with alkali addition there was more reduction in initial and final setting time and the microstructure of the cement will be changed and become weaker. Results of the expansion measurements for the three cements gave expansion less than ASTM. But with addition of 5% alkali Atbara and Al salam cements gave expansion greater than ASTM failure criterion of 0.05% while Berber cement was not. Depending on the results of the analyzed cement, the research shows that, the high the alkali content in final product can cause many difficulties and affects adversely on the cementic properties. Serious attempts to decrease alkali contents in Sudanese cement, hence, become necessary.