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Phase development, heat of hydration and expansion in a blended cement with b-hemihydrate and anhydrite as calcium sulfate

Evju, Cecilie LU (2002)
Abstract
Expansive cements are utilised in special applications to improve properties such as cracking caused by drying shrinkage. Several types of expansive cements exist and the main hydration product responsible for the expansion is ettringite, [Ca<sub>3</sub>Al(OH)<sub>6</sub>]<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·26H<sub> 2</sub>O. The importance of the cement phase containing alumina is well known, in this study the source of sulfate is investigated. Pastes containing 50 wt% calcium aluminate cement, 25 wt% Portland cement and 25 wt% calcium sulfate mixed with water at a water to solid weight ratio of unity have been investigated. The calcium sulfate was b-hemihydrate,... (More)
Expansive cements are utilised in special applications to improve properties such as cracking caused by drying shrinkage. Several types of expansive cements exist and the main hydration product responsible for the expansion is ettringite, [Ca<sub>3</sub>Al(OH)<sub>6</sub>]<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·26H<sub> 2</sub>O. The importance of the cement phase containing alumina is well known, in this study the source of sulfate is investigated. Pastes containing 50 wt% calcium aluminate cement, 25 wt% Portland cement and 25 wt% calcium sulfate mixed with water at a water to solid weight ratio of unity have been investigated. The calcium sulfate was b-hemihydrate, anhydrite or mixes of the two. The phase development was studied by in-situ synchrotron X-ray diffraction, the heat of hydration by isothermal calorimetry and the dilatation was followed by a unidimensional dilatometer. The microstructure of hydrated pastes was investigated by scanning electron microscopy.



The source of calcium sulfate was shown to be important to the kinetics of ettringite formation and the expansive properties of ettringite. In the paste with b-hemihydrate, ettringite forms continuously over a time period of approximately three hours parallel to the formation and re-dissolution of gypsum. The last 50% of the ettringite formation is followed by an expansion caused by interacting spheres of ettringite crystals growing radially on the surfaces of the cement grains. When anhydrite is utilised, the ettringite formation is slower and not accompanied by expansion. The kinetics of ettringite formation and the expansive properties can be controlled by blending the two calcium sulfates. When 75 and 50 wt% of the calcium sulfate was b-hemihydrate, the kinetics of ettringite formation can be described as a linear addition of the kinetics in the pure pastes. In the remaining mixed paste, containing 25 wt% b-hemihydrate, the phase development differed from the other two mixes and sulfate-AFm was detected. Addition of lithium carbonate accelerates the dissolution of anhydrite and appears to alter the hydration reactions at high additions. Tartaric acid retards all stages of hydration in all pastes.



The utilisation of dynamic corrections in isothermal calorimetry has been investigated. A correction with two time constants has been found successful in order to study the very early reactions in cement hydration. (Less)
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author
opponent
  • Prof Justnes, Harald, SINTEF, Norway
organization
publishing date
type
Thesis
publication status
published
subject
keywords
dilatation, isothermal calorimetry, in-situ synchrotron powder diffraction, ettringite, calcium sulfate, Shrinkage compensating cement, expansive cement, Materiallära, Material technology, materialteknik
pages
120 pages
publisher
Truus Frid, Material Chemistry, Lund University
defense location
Hörsal B, Center for Chemistry and Chemical Engineering
defense date
2002-11-01 13:15
ISBN
91-628-5367-8
language
English
LU publication?
yes
id
b44a1ccb-409b-40a8-9e38-e0152eb6612f (old id 465123)
date added to LUP
2007-10-14 15:08:40
date last changed
2016-09-19 08:45:11
@phdthesis{b44a1ccb-409b-40a8-9e38-e0152eb6612f,
  abstract     = {Expansive cements are utilised in special applications to improve properties such as cracking caused by drying shrinkage. Several types of expansive cements exist and the main hydration product responsible for the expansion is ettringite, [Ca&lt;sub&gt;3&lt;/sub&gt;Al(OH)&lt;sub&gt;6&lt;/sub&gt;]&lt;sub&gt;2&lt;/sub&gt;(SO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;·26H&lt;sub&gt; 2&lt;/sub&gt;O. The importance of the cement phase containing alumina is well known, in this study the source of sulfate is investigated. Pastes containing 50 wt% calcium aluminate cement, 25 wt% Portland cement and 25 wt% calcium sulfate mixed with water at a water to solid weight ratio of unity have been investigated. The calcium sulfate was b-hemihydrate, anhydrite or mixes of the two. The phase development was studied by in-situ synchrotron X-ray diffraction, the heat of hydration by isothermal calorimetry and the dilatation was followed by a unidimensional dilatometer. The microstructure of hydrated pastes was investigated by scanning electron microscopy.<br/><br>
<br/><br>
The source of calcium sulfate was shown to be important to the kinetics of ettringite formation and the expansive properties of ettringite. In the paste with b-hemihydrate, ettringite forms continuously over a time period of approximately three hours parallel to the formation and re-dissolution of gypsum. The last 50% of the ettringite formation is followed by an expansion caused by interacting spheres of ettringite crystals growing radially on the surfaces of the cement grains. When anhydrite is utilised, the ettringite formation is slower and not accompanied by expansion. The kinetics of ettringite formation and the expansive properties can be controlled by blending the two calcium sulfates. When 75 and 50 wt% of the calcium sulfate was b-hemihydrate, the kinetics of ettringite formation can be described as a linear addition of the kinetics in the pure pastes. In the remaining mixed paste, containing 25 wt% b-hemihydrate, the phase development differed from the other two mixes and sulfate-AFm was detected. Addition of lithium carbonate accelerates the dissolution of anhydrite and appears to alter the hydration reactions at high additions. Tartaric acid retards all stages of hydration in all pastes.<br/><br>
<br/><br>
The utilisation of dynamic corrections in isothermal calorimetry has been investigated. A correction with two time constants has been found successful in order to study the very early reactions in cement hydration.},
  author       = {Evju, Cecilie},
  isbn         = {91-628-5367-8},
  keyword      = {dilatation,isothermal calorimetry,in-situ synchrotron powder diffraction,ettringite,calcium sulfate,Shrinkage compensating cement,expansive cement,Materiallära,Material technology,materialteknik},
  language     = {eng},
  pages        = {120},
  publisher    = {Truus Frid, Material Chemistry, Lund University},
  school       = {Lund University},
  title        = {Phase development, heat of hydration and expansion in a blended cement with b-hemihydrate and anhydrite as calcium sulfate},
  year         = {2002},
}