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Superplasticerarnas inverkan på cementpastans hydratation och flytförmåga
CBI - Cement- och betonginstitutet.
1998 (Swedish)Report (Refereed)
Abstract [sv]

In the investigation the two most common types of superplasticisers on the market, based on melamine and naphtalene respectively, have been investigated. The cement types used were a Swedish low alkali, sulfate resistant Portland cement, Std P Degerhamn and an ordinary Swedish Portland cement, Std P Slite. The first type is usually used for external constructions and the other for residential constructions. Std P Degerhamn is the type of cement that usually is used for high performance concrete, because it is only with this type of cement possible to achive good flowability at low water/cement ratios. These cements are two distinct types of Portland cement with different clinker composition. This makes not only a product testing possible but it gives also information about two different clinker and hydration systems. As well type of admixture as type of cement give in certain cases very different results what concerns flowability, heat flow, retardation, pore solution chemistry and influence on hydration products. _x000D_ _x000D_ Flowability was measured with a mini-slump cone and a paste viscosimeter. The tests with paste show that the required amount of superplasticiser, for both types of superplasticiser and cement, to achieve a certain flowability, is increased when the water/cement ratio is decreased. Individually they however differ. The greatest difference is that the naphtalene based superplasticiser in general is most efficient at low water/cement ratios (0.30). With melamine, Std P Slite cement and low water/cement ratios it is difficult to get a satisfying flowability undependent of dosage level. Std P Degerhamn cement gives better flowability than Std P Slite cement at the same dosage level of superplasticiser. This is probably due to the fact that Std P Degerhamn cement is more coarse grained and has a lower C3A content than Std P Slite cement. Because the effects of the superplasticisers are quite different at low water/cement ratios, it is reasonable to assume that they act by different mechanisms. _x000D_ The calorimeter measurements show that the cement hydration of Std P Degerhamn cement is more retarded than that of Std P Slite cement at enhanced dosage levels of superplasticiser. Both types of superplasticiser give almost the same retardation. _x000D_ The XRD investigations which give the clinker consumption at different ages, show mainly the effect of retardation, which in turn depends on type and dosage of superplasticiser. As for the calorimetric investigation the results show that the retardation is stronger in pastes made of Std P Degerhamn. The consumption of calcium silicates (C3S and C2S) becomes in time same as for the reference paste undependent of type of cement and superplasticiser. The essential difference between Std P Degerhamn cement and Std P Slite cement is that the Degerhamn cement has a high amount of C4AF (ferrite), while the Slite cement has a high amount of C3A. At the age of one they the ferrite reaction is as much retarded by naphtalene as by melamine. At later stages of hydration the melamine consumes more ferrite than the reference mixture and the mixture with naphtalene. The higher consumption of ferrite is not reflected in a higher ettringite formation, which indicate that the formation of monosulfate or an other type of ferrite hydrate is increased. A similar effect concerning C3A in the Slite cement can not be observed. At normal dosages of superplasticiser and for cement with normal C3A content the amount of ettringite formed in the hardened paste at late ages seems to increase. The opposite seems to be the case for cement with high C4AF content. The amount of calcium hydroxide (CH) in the hardened paste increases somewhat at low dosage levels of superplasticisers. At high dosage levels a decrease of the amount of formed CH at one day can be seen, because of the retardation of cement hydration. This phenomenon is more obvious in Degerhamn cement than in Slite cement. Concerning the amounts of especially ettringite and calcium hydroxide the results must be interpreted with caution because of the method of analysis (XRD). For instance the chrystal size affects the results. Other methods as thermogravimetry (TG) and differential scanning calorimetry (DSC) may give different results. _x000D_ The pore solution investigation reflects the dissolution of clinker compounds and the strength development by formation of cement hydrates. Moreover the superplasticisers affects directly the pore solution chemistry especially by giving an addition of sodium and sulfate ions. This can be seen in the pore solutions by increased amounts of alkalies and sulfates. The sulfate concentration decreases gradually by formation of ettringite and monosulfate, while the enhanced amounts of alkalies remains in the hardened cement paste. This is reflected in a permanent enhancement of hydroxyl ion concentration (OH-) and pH in the hardened cement paste. The superplasticisers generally increase the amount of calcium ions in the pore solution. This indicate that the superplasticisers directly influence the dissolution and precipitation mechanisms for the alite and belite components in the cement paste. The enhanced amounts of calcium in the pore solutions indicate that the calcium silicate hydrates bind less calcium or that the precipitation of calcium hydroxide and other products containing calcium is hampered. The precipitated amount of calcium hydroxyde is however often greater at low dosage levels at one day, indicating that the superplasticisers have changed the composition of the calcium silicate hydrates on the surfaces of the clinker grains. _x000D_ The retardation given by the superplasticisers is mainly determined by cement type which means clinker composition, fineness of cement and dosage of superplasticiser._x000D_ The superplasticisers affects mainly the dissolution of the clinker grains, but they also affect the precipitation mechanisms, especially that of calcium hydroxide and ettringite. _x000D_ The superplasticisers give an addition of both alkalies and sulfate in the cement paste, which affect both the hydration development and pH in the hardened concrete. _x000D_ The most essential information given by this investigation is that both superplasticiser, cement and water/cement ratio must be optimized to get the best effect. This is especially the case at low water/cement ratios. The superplasticisers also affect the cement hydration mainly by retardation, but also the formed reaction products are changed.

Place, publisher, year, edition, pages
Cement och Betong Institutet , 1998. , p. 82
Series
CBI rapport, ISSN 0346-8240 ; 1998:1
Keywords [sv]
Superplasticerare, värmeutveckling, cementhydratation, hydratationsgrad, porlösningskemi, flytförmåga
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:ri:diva-2859OAI: oai:DiVA.org:ri-2859DiVA, id: diva2:960464
Note
276 Går att beställa: kontakta eva.lundgren@cbi.se "The influence of superplasticisers on hydration and fluidity of cement paste."Available from: 2016-09-07 Created: 2016-09-07Bibliographically approved

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