Influence of Plasticizers and Superplasticizers on Chemical Shrinkage of Cement
H. Justnes, E. J. Sellevold, D. Van Gemert, A. Van Gemert,
and F. Verboven
Appears on pages(s):
cement paste; plasticizer; superplasticizer
Total and external chemical shrinkage have been followed for a number of cement pastes until 48 h. Total chemical shrinkage is believed to roughly be proportional to degree of hydration, while the differences in external chemical shrinkage give an impression on how prone the mixture may be to micro-cracking. The difference between total and external shrinkage result in contraction pores. This study is part of a larger on-going study focusing on the early volume change of binders of high performance concrete. Variables in the present part of the study have been plasticizer (sodium lignosulphonate) and super-plasticizer (sodium salts of sulphonated melamine -formaldehyde condensate and naphthalene sulphonate - formaldehyde condensate) and water-to-cement ratio (0.30, 0.35, 0.40, 0.45 and 0.50). A few mixes were also made with a combination of plasticizer/super-plasticizer, a common practical concept. The total plasticizer/super-plasticizer dosage was 1% by weight of cement, and the cement chosen was a high strength portland cement commonly used for high performance structures. Total chemical shrinkage could be used to monitor the retardation of the cement setting by for instance lignosulphonate and the acceleration of hydration rate thereafter. The hydration development of pastes with plasticizers (LS) or super-plasticizers (SNF and SMF), as measured by total chemical shrinkage, seems virtually independent of the water-to-cement ratio in the range 0.30-0.50 during the first 48 hours. Only the hydration for the paste with the lowest w/c may be slightly accelerated immediately after setting, but reached slightly lower values at 48 h. Lignosulphonate (LS) retard setting more than the super-plasticizers naphthalene sulphonate - formaldehyde condensate and sulphonated melamine - formaldehyde condensate. The flattening-out-level of the external chemical shrinkage was only marginally influenced by w/c, with non-systematic individual differences being in the order of 0.1 ml/l00 g cement. The flattening-out- level of the external chemical shrinkage was independent of admixture type at w/c = 0.40, and the same as the reference without admixtures.