Basic Mortar Analysis

HISTORIC PRESERVATION PROGRAM

Basic Mortar Analysis
HP 306 Lab Assignments

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 reasonable 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, a calcimeter analysis would be necessary to determine the amounts of soluble silicates present.

Equipment: analytical balance (0.01g precision), mortar and pestle, 250 ml beakers, 250 ml Erlenmeyer flasks, funnels, filter paper (417 coarse & 413 fine), 14% muriatic acid (or 15% HCl, or if lime mortar, 5% distilled white vinegar), water, drying oven / microwave oven, microscope, sieve set with U.S. Standard sieve numbers: 4, 8, 16, 30, 50, 100, 200.

Procedure:

1. Sampling

Photograph the mortar on the building before sampling. Note color, texture, and tooling marks.
Remove three 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.
Place each sample in a sealable poly bag and label with the following information: your name, sample number, building name, building location, sample location, mortar type, date of sampling.
Examine the samples and record the following characteristics: color range, texture, inclusions, hardness.

2. Sample preparation

Powder two of the samples with mortar and pestle; leave other sample for reference.
If samples are not dry, place in drying oven at 110ºC until dry or if humidity is low, air dry for 24 hours.
Weight each sample (W₁) and record weights.
Place samples in flasks and moisten with a small amount of water.
For each sample, using a pencil, label two pieces of filter paper (one grade 413 label "F" for fines and one grade 417 label "A" for aggregate) and your initials and sample number.
Weigh each piece of filter paper and record weight (W₂) in the data sheet.
Fold the filter papers into quarters and place in funnels. Position funnels so that it will drain into flasks.

3. Acid digestion

In a fume hood (or well ventilated place if using vinegar), add acid to the flasks with the samples to dissolve binder. Use as little acid as necessary, but sufficient to complete reaction. Observe reaction and record observations.
Add a few drops of acid to be sure reaction is complete.
Slowly add water to remaining to remaining sample.
Swirl flask to suspend the fines.
Slowly pour the liquid with suspended material through the fine (413 grade) filter, leaving the solid particles at the bottom of the beaker.
Repeat process until water is clear.
Dry the fines collected on the filter paper in the drying oven. (If using a microwave for this, set for about 5 minutes.)
Add more water to the flask.
Use a wash bottle to transfer all the sand to the second coarse (417) grade filter.
Dry the sand collected on the filter paper in the drying oven. (If using a microwave for this, set for about 5 minutes.)
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.
Weight filter paper with the dry sand (W₄) and record the weight on the data sheet.
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%.

4. Microscopic analysis

Examine the sand under a microscope. Record the characteristics (colors, particle shapes, sizes, etc.) and capture representative digital images of the sand particles.

5. Sieving

Re-weigh each sample. Record. Download Mortar Analysis Excel template
Clean sieves carefully. Weigh each sieve and the pan. Record.
Pass sample 1 through the sieves, shaking gently. Reweigh each sieve and pan.
Clean sieves.
Repeat sieving procedure.
Calculate the percent passing through each sieve.

6. Report

Summarize the results of your analysis in a professional mortar analysis report with text, graphs and images that characterize the mortars with sufficient detail to enable a mason to duplicate the samples. The particle size distribution of the aggregate should be presented in a table and as a graph as an appendix to the report.