• Introduction

Fragments of debonded external plasters were submitted by Themewerks Pte Ltd for petrographic examination to check the composition of the plasters to determine if the plasters contain glass fibres or if the plasters are made up of glass fibre reinforced cement (GFRC). As requested only one random sample from the fragments submitted was selected for the examination. See figure 1 below.

  • Methodology
    • Visual ExaminationThis was performed to obtain a first impression of the homogeneity of the samples and its constituents, to look for symptoms of damage and secondary reaction, and also to select an area on the samples for preparation of ground and polished thin section.
    • Microscopic AnalysisThe microscopic analysis was performed on the thin section prepared with a polarizing and fluorescent microscope (PFM) under transmitted and reflected light. For preparation of the ground section, a small block of the sample was cut and ground to attain a smooth finish. For preparation of a thin section, a small block was sawn from the core sample, glued to an object glass and impregnated with an epoxy resin containing a fluorescent dye. After hardening of the epoxy, a thin section with a surface area of approximately 33 x 55 mm and a thickness of 25 to 30 mm was prepared for PFM analysis.Through the examination of the ground section, assessment was made on the homogeneity of the plaster, compaction and types and distribution of the aggregates. Under transmitted light, the various components (type of cement and aggregates), air voids content, compaction pores and damage phenomena in the samples were identified. Under reflected light, the fluorescent microscopy made it possible to study the homogeneity of the mix and the cement paste and capillary porosity.
  • Findings
    • Macroscopic examination
      • The plaster is made up of grey cement and fine aggregate which are predominantly quartz sand. The cement matrix was generally light grey and the plater appeared to be well compacted. Dispersion of the aggregates appeared to be good and the cement matrix was notably homogeneous.  At the underside of the plaster, it would appear that the plaster had debonded cleanly from the substrate. The surface of the plaster had been finished with a coloured layer.
    • Microscopic analysis
      • The plaster is made up of grey Portland cement binder and the fine aggregates are predominantly quartz with traces of calcite. There is no evidence of any glass fibres in the plaster. The plaster is well mixed and the fine aggregates are well distributed.
      • The plaster is fully carbonated and there are traces of secondary ettringite formation within the pores of the plaster. Distribution of the secondary ettringite however is not uniform but rather sporadic. This indicates that there could have been localised ingress of water into plaster causing leaching.
      • Under fluorescent microscopy, the capillary porosity of the matrix appeared to be very homogeneous and high. No sign of segregation of excessive leaching could be seen. No crack was observed under the microscope.
      • The surface of the plaster had been finished with an amorphous coating.

Below are the petrography microscopic images taken.

The left micrograph shows a typical Portland cement binder and fine aggregates well mixed and with low compaction void. This sample however appeared to be air entrained and possibly a pre-packed system. The right micrograph shows the presence of belite and alite, which are typical of Portland cement binder.

Photomicrograph on the left shows that the plaster is fully carbonated. In the right photo, traces of secondary ettringite could be seen in the air void of the plaster. This is characteristically caused by water ingress into the plaster leading to leaching. The secondary ettringite however appeared to be sporadic.

Left photomicrograph shows the presence of a coating on the surface of the plaster. Under fluorescent microscopy, the binder is very homogeneous with high capillary pores as seen in the right photomicrograph.

  • Conclusion
    • The plaster sample selected from the fragments of debonded plaster samples submitted is not made up of glass fibre reinforced cement. There no evidence of glass fibres found in the sample analysed.
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