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Tidig hydratation.: Försök med acceleratorer inklusive nanosilika
RISE, SP – Sveriges Tekniska Forskningsinstitut, CBI Betonginstitutet AB.
2008 (Swedish)Report (Refereed)
Abstract [sv]

The reactions of the cement with water are a time dependent complicated interaction between dissolution of cement, forming of cement hydrates and the pore solution chemistry. When an accelerator is added above all the pore solution chemistry is affected, which in turn will affect the other reactions._x000D_ _x000D_ Concerning the early cement hydrations one must distinguish between what controls the start of the acceleration period and what controls the intensity of the acceleration period. In the project to begin with different salts and their functions were studied but this was extended with nanosilica as it proved to be a very interesting accelerator._x000D_ _x000D_ Concerning the salts it is known that CaCl2 is very effective both to shorten the induction period and to accelerate the hardening. Experiments with different salts showed that one must distinguish between cat- and anions. Basically calcium and the other alkali earth metals barium and strontium shorten the induction period independently of anion. Also some aluminate cations may give this effect. This depends probably on the fact that these ions are incorporated in the cement hydrates and thereby accelerate the forming of the hydrate shell so that the induction period can be finished and the acceleration period can start. The anions affect mainly the intensity of the acceleration period (the hardening). The chlorides are the most effective, but also nitrates and thiosulfates are effective. Also alkali hydroxides intensify the acceleration rate._x000D_ _x000D_ Silica specifically in form of nanosilica showed to be an effective accelerator both regarding the shortening of the induction period and as a hardening accelerator. Nanosilica is chemically precipitated amorphous silica in suspension with particles in the nano size. The experiments showed that its effect as accelerator depends on the particle size and pH in the pore solutions. Smaller particles and higher pH are more effective. Higher pH is achieved with a high alkali cement or by introducing alkali during mixing of the cement paste or concrete. An effective additive to achieve this without getting alkali silica reactions is lithium hydroxide._x000D_ _x000D_ The effect of nanosilica was different in different Portland cements but increased generally by adding alkali or calcium based products. In a coarser low alkali cement a greater amount of nanosilica is needed to get a clear acceleration effect than in a finely ground cement with higher alkali content._x000D_ _x000D_ Calorimetric measurements were made also at 10°C. BaCl2 gave a strong acceleration effect. Also nanosilica accelerated and the effect was stronger by adding alkali or calcium based products even at this temperature._x000D_ _x000D_ Some concretes were mixed to see the effect of accelerators of the early strength. A concrete with high amount of rapid hardening cement was casted in heat insulated moulds to utilise the hydration heat. After six hours 2 % CaCl2 and 1.5 % nanosilica respectively gave a compressive strength of about 33 MPa. 1.0 % nanosilica + 0.5 % LiOH gave a compressive strength of about 35 MPa after six hours. The concrete containing LiOH gave a much lower 28 day compressive strength than the reference concrete without accelerator._x000D_ _x000D_ Nanosilica gave for ordinarily cured concretes containing a class II cement with much lower cement content only a marginal effect at the compressive strength._x000D_

Place, publisher, year, edition, pages
CBI Betonginstitutet , 2008. , 85 p.
Series
CBI rapport, ISSN 0346-8240 ; 2008:3
Keyword [sv]
tidig hydratation, acceleratorer, kalorimetri, nanosilika, salter, induktionsperiod
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:ri:diva-2862OAI: oai:DiVA.org:ri-2862DiVA: diva2:960467
Note
648 Går att beställa: kontakta eva.lundgren@cbi.se (400kr)"Early hydration. Experiments with accelerators including nanosilica" 978-91-976070-3-2Available from: 2016-09-07 Created: 2016-09-07Bibliographically approved

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