Fig. 4—Autoclave expansion apparatus and specimens (reprinted with permission from Ref 7).STRUBLE ON PHYSICAL PROPERTIES OF HYDRAULIC CEMENTS443
deleterious expansion in concrete, a mortar expansion test forpozzolan used in blended cement is included as part of C 595and C 1157. This method tests combinations of pozzolan andcement or clinker using the ASTM Test Method for PotentialAlkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method) (C 227). This test is usually used to detect thealkali-silica reaction between cement and aggregate, but in thiscase it is used with a nonreactive aggregate to measure expan-sive reactions between pozzolan and cement. For C 595, thetest is run with various proportions of pozzolan between 2.5 %and 15 % in order to detect excessive expansion due to anadverse proportion of pozzolan in combination with portlandcement, and the maximum expansion for all mixtures is 0.05 %at 91 days. For C 1157, the test is run with the proportion ofpozzolan used in the cement and 5 % more and less than thatproportion; and, although the specification describes the test,it does not list a maximum expansion level.RecommendationsSeveral of the standards relating to volume change need im-provement. Drying shrinkage is an important issue in concrete;it initiates in the hydrated cement and is affected by propertiesof the hydraulic cement, so it seems important to have a shrink-age requirement for hydraulic cements. The current restrictionof the autoclave expansion test, C 151, to portland cement is amajor problem, and the subcommittee responsible for this testis currently exploring tests for cements containing pozzolan tomeasure expansion due to CaO and MgO hydration associatedwith unsoundness in concrete. There should be a limit on ex-pansion due to ettringite in C 595. The test for cement-pozzolanexpansion would be easier to utilize if it were described morefully in a separate method, rather than included as part of theblended cement specification. Finally, although there is a limiton cement-pozzolan expansion in C 595, there is no similarlimit for fly ash used as a mineral component in concrete (C618) nor for hydraulic cements that contain fly ash, a puzzlinginconsistency.StrengthStrength is the property that is probably most important toengineers, both as a general indicator of concrete quality andto assure that the concrete will perform as intended duringdesign of the structure. Although concrete strength may bemeasured in tension, shear, or compression, compressivestrength is generally most important and most often specified.SignificanceIt is widely recognized that concrete strength depends on thestrength of cement paste, on the paste-aggregate bond, and onthe aggregate strength. For ordinary concrete, the strengths ofpaste and the paste-aggregate bond control concrete strength.