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Composition of mortar as a function of distance to the brick-mortar interface : A study on the formation of cured mortar structure in masonry using NMR, PFM and XRD

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Author: Brocken, H.J.P. · Larbi, J.A. · Pel, L. · Pers, N.M. van der
Type:article
Date:1999
Publisher: Delft Univ of Technology
Place: Delft, Netherlands
Institution: TNO Bouw
Source:Heron, 4, 44, 257-270
Identifier: 280438
Keywords: Architecture and Building · Binders · Brick · Carbonation · Curing · Interfaces (materials) · Mechanical permeability · Mortar · Nuclear magnetic resonance spectroscopy · Optical microscopy · X ray diffraction analysis · Air entraining agents · Polarizing-fluorescent microscopy (PFM) · Masonry construction

Abstract

The formation of cured mortar structure in masonry was studied using multiple experimental techniques. Starting with fresh mortar, nuclear magnetic resonance (NMR) was used to measure the water extraction during brick laying. After curing, the composition of cured mortar was investigated with polarizing and fluorescent microscopy (PFM) and X-Ray diffraction (XRD). Two typical mortars were investigated: a cement-lime mortar and a cement mortar with air entraining agent. The measurements indicate that the mortar composition (i.e. the contents of sand, cured binder and voids) and the contents of chemical substances of the cured binder (i.e. the contents of calcite and portlandite) change with distance to the brick-mortar interface. For the cement mortar with air entraining agent, the observations are explained by the enrichment of binder towards the brick-mortar interface, resulting from the local compaction and compression of the fresh mortar. In the cement-lime mortar such an enrichment of binder hardly occurs and the observations are explained by the intense carbonation that takes place. As a result, the chemical composition of the binders is very much different in both mortars. In the cement mortar with air entraining agent, near the brick-mortar interface the enrichment of cement and the low water content (resulting from the low water retentivity of this mortar), lower the water-to-cement ratio and as a consequence the cement is not fully hydrated. In the cement-lime mortar, because the Ca(OH)<sub>2</sub> content and the water content is higher, near the brick-mortar interface, a carbonated zone is formed which is hardly permeable for CO<sub>2</sub> (and probably water). The latter does not occur in the cement mortar, it remains permeable. The analysis of the experimental results have lead to the formulation of a conceptual model for the formation of cured mortar structure in masonry. Such a model may be helpful in analyzing the predicting the durability of mortars.