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IntroductionThe incorporation of chemical admixtures in concrete is common practice, especially in high performance concretes. These admixtures are designed to have specific effects on certain properties of the concrete, and are expected to have little or no negative effects on other aspects. One such admixture is the superplasticizer that is added to concrete in order to increase its workability in the fresh state, eliminating the need for increasing the water content. The effects of superplasticizers on the long-term properties of the hardened concrete have been studied to a very limited extent (1, 2), and not much data is available on their effects on creep and shrinkage. In the present work, the influence of two different types of superplasticizers on the shrinkage of mortars is studied, along with another product belonging to an emerging class of chemical admixtures that are shrinkage reducing admixtures (SRAs). Work reported in the literature (3, 4) on concretes with SRAs clearly demonstrates the decrease in the drying shrinkage and a reduction in restrained shrinkage crack widths. The influence of an SRA, which also has a plasticizing effect in the fresh state, has been analyzed in the present work, especially in terms of the shrinkage of concrete under drying conditions. Effects of Superplasticizer Type on the Shrinkage of Cement Mortar MaterialsCement mortars with the same base composition (denoted as Ref) and different admixture incorporations were studied: two mortars with different superplasticizers (melamine and copolymer based), one with the melamine superplasticizer and an SRA, which also has a plasticizing effect, and another with the melamine superplasticizer and silica fume. A CEM I 52.5R cement, crushed limestone sand with a grain size range of 0-5 mm, and a polypropylene glycol based SRA were used. The water/cement and sand/cement ratios were 0.4 and 2, respectively. All the mortars with admixtures were designed to be flowable, requiring little or no compaction during casting. The mortars and the admixtures used are specified in Table 1, along with some of the properties obtained. The superplasticizer dosages are given as the solid superplasticizer/cement ratio, by weight, with the water content being accounted for in the water/cement ratio. The SRA and silica fume dosages are also expressed with reference to the cement content, by weight.

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