HISTORIC PRESERVATION PROGRAM

HP 306 Basic Mortar Analysis

Goals:

The goals of this procedure are to determine three common components of an historic mortar: (1) the binder (typically acid-soluble calcium carbonate CaCO3), (2) the fines (pigments and impurities such as clays and aluminates); and (3) the sand or aggregate. This procedure uses acid digestion, filtration and microscopic analysis of the fines and aggregate.

Intent and limitations:

While this procedure may work reasonably well for characterizing mortars for much preservation work, there are two major limitations. First, this is not suitable for calcareous mortars that use limestone, shell, or coral sands, as these aggregates will dissolve in the acid. Second, the procedure is not suitable for determining the amount of cement that may be in a mortar. Thus for hard cement mortars that cannot be easily crumbled with fingers, an analysis procedure using a stronger acid and calcimeter would be necessary to determine the amounts of soluble silicates present.

Equipment:

personal safety equipment, analytical balance (0.01g precision), mortar and pestle, 250 ml beakers, 250 ml Erlenmeyer flasks, funnels, filter paper (417 coarse & 413 fine), acetic acid [CH3COOH] as 5% distilled white vinegar), water, drying oven or microwave oven, digital microscope, sieve set with U.S. Standard sieve numbers: 4, 8, 16, 30, 50, 100, 200.

Suitable eye protection should be worn through this procedure.

(Alternatively, hydrochloric acid [10% HCl or commercial muriatic acid] could be used for faster and more complete digestion, especially if cement may be present in the binder, however as HCl fumes could be hazardous this should only be done in a fume hood with proper safety procedures and equipment.)

Reaction:

To separate the binder from the aggregates and fines in a mortar sample, the binder is dissolved with acid, leaving the non-acid soluble materials behind. The difference between the original sample weight and the resulting weight of the aggregates and fines yields the weight of the binder that was removed by acid digestion.

In this procedure, the calcium carbonate binder of the mortar is dissolved in an aqueous solution of acetic acid to produce soluble calcium acetate, water, and carbon dioxide with the following reaction:

CaCO3 + 2CH3COOHaq → Ca(CH3COO)2 + H2O + CO2

The aggregate is removed by physical separation, the fines are separated by filtering, and the
calcium acetate, water, and carbon dioxide are discarded. The aggregate and fines are dried separately and weighed.

Procedure:

A. Sampling

  1. Photograph building feature with a record shot.
  2. Photograph the mortar in situ before sampling.
  3. Note and record the color, texture, and tooling marks or the mortar.
  4. Remove at least two samples of the same type of mortar from different nearby locations, each between 5 and 10 grams, (about the size of a pat of butter). To avoid damaging the building, sample from obscure, but representative places. If the mortar appears be applied in layers of different compositions (for example, there may be a soft bedding mortar and a harder, sometimes colored pointing mortar) separate samples by each type.
  5. Place each sample in a sealable poly bag. Label each with the following information: your name, sample number, building name, building location, sample location, mortar type, date of sampling.
  6. Examine the samples and record the following characteristics: color range, texture, inclusions, hardness.
  7. Allow samples to air dry for at least 24 hours in an open bag in a protected place.

B. Sample preparation

  1. Powder at least one of the samples with mortar and pestle (about 5 grams each); leave other sample for reference.
  2. If samples are not dry, air dry for another 24 hours or place in drying oven at about 110C until dry. If unsure if dry, weigh, then dry and weigh again. If the mass has been reduced, repeat the drying until a constant mass is recorded.
  3. Download Mortar Analysis Excel template.
  4. Save as in separately labeled files for each sample to be analyzed. (If computer is not brought to lab, print out mortar analysis template pages before class and enter data manually.)
  5. Label and weigh each flask to be used. (Be sure flasks are completely dry.)
  6. Add sample to flask and re-weigh to determine weight of sample (W1). Record weight and repeat for other samples.
  7. Add a small amount of water to dampen the samples and swirl. Allow to set a minute and observe whether the wash water is colored or opaque with suspended clay.

C. Filter papers

  1. For each sample, using a pencil, label one grade 417 filter paper with your initials and sample number.
  2. If your mortar is has pigments or clay, the wash water will be opaque. To collect this, prepare a second filter paper grade of 413. Label this "F" for fines.
  3. If the wash water is translucent, then use two pieces of 417 filter for each sample (one for the aggregare and one for the fines).
  4. Weigh each piece of filter paper and record weight (W2) in the data sheet.
  5. Fold the filter papers into quarters and place in funnels.
  6. Position the funnels so that each will drain into a beaker.

D. Acid digestion

  1. In a fume hood (or well-ventilated place if using white vinegar), carefully add acid to the flasks with dampened samples to dissolve the mortar binders. Use as little acid as necessary, but sufficient to complete the reaction. Observe the reactions and record these observations.
  2. Add a few drops of acid to be sure reaction is complete.
  3. Swirl flask to suspend the fines.
  4. If the mortar is pigmented or if the wash water is opaque, slowly pour the liquid with suspended material through the fine (413 grade) filter, leaving the solid particles at the bottom of the flask. If the wash water is translucent, then instead filter through the coarse (417) filter paper.
  5. Slowly add water to remaining to remaining sample.
  6. Repeat process until water is clear and acid has been washed away.
  7. Dry the fines collected on the filter papers in the drying oven (being careful not to burn the filter paper) or air dry in a protected place.
  8. Add more water to the flask.
  9. Use a wash bottle to transfer all the sand to the second coarse (417) grade filter paper and allow to drain.
  10. Dry the sand collected on the filter paper in the drying oven (being careful not to burn the filter paper) or air dry in a protected place.
  11. Weigh filter paper with the dry fines (w3). Subtract the weight of the paper (w2) to determine the weight of the fines. Record both values in the data sheet.
  12. Weight filter paper with the dry sand (w4) and record the weight on the data sheet.
  13. Express the amount of sand as a percentage of the whole sample. Express the amount of fines in the same manner. The amount of the dissolved binder is calculated by summing up the percentages of the sand and the fines and subtracting from 100%.
  14. Label and save the aggregate samples for the following procedures.

E. Microscopic analysis

  1. Examine the aggregates under a microscope. Record the characteristics (colors, particle shapes, sizes, etc.)
  2. Examine the fines. If they are pigments or are colored, match color to Munsell color notation system.
  3. Capture and save representative digital images of the aggregate particles.
  4. Transfer images to your digital account and remove images from lab computer.

F. Sieving

  1. Re-weigh each sample of aggregate. Record on Mortar Analysis Excel sheet.
  2. Clean sieves carefully. Weigh each sieve and the pan. Record.
  3. Pass sample 1 through the sieves, shaking gently. Reweigh each sieve and pan.
  4. Clean all sieves.
  5. Repeat sieving procedure for any other samples.
  6. Enter data in to Excel mortar analysis spreadsheet and calculate the percent passing through each sieve for each sample.

6. Report

  1. Summarize the results of your analysis in a professional mortar analysis report with a narrative text that describes the project and procedures and observations, along with graphs and images that characterize the mortars with sufficient detail to enable a mason to duplicate the samples.
  2. The data on the particle size distribution of the aggregates should be presented in a table and as a graph from the Excel spreadsheet in the appendix of the report.
  3. Submit the report in a professional manner with a cover and simple binding.

c. 2014. Prof. Thomas Visser, University of Vermont Historic Preservation Program