Model for the mechanical stress due to the salt crystallization in porous materials

R. M. Espinosa, L. Franke, G. Deckelmann

Research output: Contribution to journalArticlepeer-review

Abstract

This paper deals with the experimental investigation and the mathematical modelling of crystal growth in porous materials and resulting mechanical stress due to the crystallization pressure. Crystallization of potassium nitrate and of sodium sulphate was induced in two bricks by cooling down at constant rate. The measured temperatures describe indirectly the crystallization and the dissolution rates. Thus, the time-dependent amount of crystallized salt can be estimated by means of an energy balance in the reaction sample (impregnated with salt solution) and in the reference sample (impregnated with water). Therefore, the supersaturation ratio of the solution is also obtained at each point of time. The sample strain is measured continuously during crystallization. The results show that the mechanical stress due to the salt crystallization depends on both salt and pore structure. A mathematical model for the mechanical stress due to crystallization pressure was developed based on crystal growth kinetics considering the influence of pores with different size (called pore classes). The model considers that nucleation and crystal growth take place in large and small pores simultaneously and calculates the distribution of salt crystals in pores of different size and the resulting load surface of the crystals under stress. The estimation of two model parameters is necessary for the proposed computation model. It is assumed that the mechanical stress is caused by the crystallization pressure that crystals exert on the pore wall when growing in a supersaturated solution. Thus, the crystallization pressure is calculated as a function of the solution supersaturation. Finally, the average mechanical stress is obtained by means of a uniform section force in a defined volume. The results show that the material strain owing to the crystallization of KNO3 and Na2SO4 can be approximately estimated in the selected bricks by means of this new model. An improvement of the model is necessary for a better prediction of the mechanical stress, e.g., at drying conditions or for the simulation of the re-allocation of salt crystals from pores under stress into stress-free pores. In this work, the required properties of the salt solution are calculated using a thermodynamic model according to Steiger [Steiger M, Kiekbusch J, Nicolai A. An improved model incorporating Pitzer's equations for calculation of thermodynamic properties of pore solutions implemented into an efficient program code. Constr Build Mater 2007, accepted for publication], which is based on the Pitzer model [Pitzer KS. Ion interaction approach: theory and data correlation. In: Pitzer KS, editor, Activity coefficients in electrolyte solutions. Boca Raton: CRC Press; 1991. p. 75 [chapter 3]].

Original languageEnglish (US)
Pages (from-to)1350-1367
Number of pages18
JournalConstruction and Building Materials
Volume22
Issue number7
DOIs
StatePublished - Jul 1 2008
Externally publishedYes

Keywords

  • Crystal growth
  • Crystallization pressure
  • Salt crystallization
  • Supersaturation ratio

ASJC Scopus subject areas

  • Civil and Structural Engineering
  • Building and Construction
  • Materials Science(all)

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