The water demand (acc. to EN 196-3) of blended cements containing ggbs can be slightly higher then the water demand of Portland cements. Generally there is no direct correlation between the water demand acc. to EN 196-3 and the water demand / the workability of the concrete, because the influence of the cement is superimposed by other influences (e. g. aggregate, grading, admixtures). Nevertheless the resulting particle size distribution of the limestone in interground Portland limestone cements exerts a very beneficial effect on the water demand of the cement [Opo92, Tsi92]. Figure 1 shows the water demand as a function of the limestone content for interground cements with the same 28 day compressive strength (approximately 52 N/mm²).

Figure 1: Water demand as a function of the slope and the limestone content in Portland limestone cement [Lud03]	Figure 2: Flow diameter as a function of time and the cement type [Lud03]

In Figure 1 it can be seen, that the water demand can decrease with increasing limestone content. Although the cements have to be ground finer with increasing amount of limestone the water-depleting influence of the more finely ground clinker fraktion is offset by the partial replacement of clinker with limestone. On the other hand, the particle size distribution of the Portland limestone cements in this example becomes flatter - i. e. has a decreasing slope - with rising proportion of limestone. As a consequence, the water-filled void volume between the ground clinker particles in the cement paste is reduced by filling with fine limestone particles. In practice the two effects can result in a better workability of the concrete. The representation in Figure 2 contrasts the flow diameter of fresh concrete using Portland limestone cement and Portland cement. Is is obvious, that the initial flow diameter rises and the stiffening of the concrete in this example is reduced when Portland limestone cement is used.

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