Influence of Cement Composition on Volume Stability of Mortar


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Title: Influence of Cement Composition on Volume Stability of Mortar

Author(s): S. Kelham

Publication: Special Publication

Volume: 177


Appears on pages(s): 27-46

Keywords: alkali; clinker; curing; fly ash; mortars; prisms

Date: 1/1/1999

The volume stabilities of mortars have been monitored for periods up to 5 years. In addition to controls maintained at 20°C, prisms were subjected to high temperature curing regimes, typically 12 hours at 70, 80 or 9OO C. Curing temperature was found to be the dominant factor in determining whether significant (>O. 1%) expansion took place during subsequent water storage at 20°C. Most of the mortars expanded after curing at 90°C but very few at 80°C and none at 70°C. Analysis of the results for mortars cured at 90°C indicated that expansion increased with cement fineness and levels of alkali, C3 A, C3 S and MgO and showed a maximum at a cement SO3 content of -4%. The pessimum SO3 content increased with cement alkali content. The incorporation of ground limestone accelerated expansion but did not affect the ultimate value. Cement replacement levels of 30% or 40% with siliceous fly ash and 30% or 50% with ground granulated blastfurnace slag appeared to prevent expansion. A sample of Type IS cement gave only moderate expansion. The mortars that expanded after curing at 80°C had high alkali levels, through the incorporation of KOH or K2 SO4 in the mixing water. At 20°C no long-term expansions were observed, despite the inclusion of cements with total SO3 levels up to 5.6% and clinker SO3 levels up to 2.6%. The Type K cement expanded by -0.05% within the first 7 days and was then stable. Cements based on a high C3 A clinker gave early expansions (-56 days) which increased as the cement SO3 content increased and as the specific surface area decreased, although all expansions were <0. 1%.