STONE RESTORATION PRACTICE IN PALESTINIAN TERRITORIES:
A CASE STUDY FROM JERUSALEM
part 4
Porosity Test of the Hand-made Mixtures
The same
method was applied to determine the density (dry & bulk) and the optimum
absorption ratio for hand-made mixtures prepared by using various combinations
of materials: crushed stone, silica sand, hydrated lime, hydraulic lime, and
white cement. The main intention is to test the compatibility of these mixtures
in terms of porosity with both the existing stones and the new stones to be
used for the restoration works. For this purpose, a sieve analysis is conducted
for: (1) the crushed stone obtained from the area of A’nata due to the
results of the porosity analysis obtained and shown in Table (3); and (2) for the silica sand.
The aim of these tests is to determine the appropriate particles size for use
in all samples that will be subjected for lab testing. For this purpose,
procedures were as follows:
-
460 gr. of crushed stone was washed with water and
then dried in an electric oven for 24 hours.
It was re-weighed after drying, yielding a reading of 390 gr. The sample
was then put in a sieve (No. 200) and shaken in a mechanical shaker for 10
minutes. The results of this are shown in (Table 3) and (Fig 5), and they quantify
the gradation of particle sizes within the sample of crushed stone which was
used.
-
360 gr. of silica sand was washed with water and
dried in an electric oven for 24 hours. It was then re-weighed after drying,
showing then a weight of 300 gr. Then it was put in a sieve (No. 200) and
shaken in a mechanical shaker for 10 minutes. The results are shown in (Table 4)
and (Fig 6), and they show the gradation by particle size of the sample of the silica
sand which was used.
The lab
tests were performed in 3 groups in accordance with the materials being used to
prepare the mixtures for testing. The ingredients for each group were mixed in
different proportions by volume. Then the mixtures were placed in plastic
containers after adding adequate amount of drinking water. Each of the wet
mixture samples was then poured into a mold which had been numbered and exposed
to the natural atmosphere in the laboratory. After allowing one week for
drying, the porosity of the samples was measured, as follows: the hardened samples
were completely saturated with water; the weight and volume of each sample was
measured; the samples were dried in an oven at 105º C (±5º C) for 24 hours;
after drying, each sample was weighed; finally the weight of the lost,
evaporated water was calculated. The volume of each mixture was obtained
through variable units by volume for each of the materials being used in the
preparation of the samples for testing. The results are shown in Tables (7 to
9), with the porosity expressed as an absorption ratio. The three groups for
lab testing are as follows:
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Seven samples were prepared using various
combinations of fine crushed stone, hydrated lime paste and white cement (Table
7). The first 4 samples in this table provide the results of various mixes of
fine crushed stone with hydrated lime paste, which historically, presents the
oldest building hand-mixtures used for construction purposes in the country. At
that time, the stones were obtained locally, crushed to reach the necessary gradation
and then mixed with hydrated lime, grog and/ or ash to obtain the desired homogeneity
and hardening. During the testing of these 4 samples, the observations stated a
slight slowdown in the process of hardening of the mix. Thus, the other 3
samples were prepared by adding white cement- used widely in the present
restoration practice- to the mixture. As a result, the process of hardening
became faster, but no significant deviation have been observed either in the
absorption ratio or the density (Table 7).
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Then, ten samples were prepared using different
combinations from: silica sand, hydrated lime paste white cement and hydraulic
lime (Table 8). The use of silica sand for
construction purposes in the Palestinian territories dated back to the intense European
influence at the end of the nineteenth century and first decades of the
twentieth century- colonial settlement movement and the British Mandate. At
that time, the Portland cement started being introduced in the various building
activities, and the use of silica sand obtained from the coastal area of
Mandate Palestine turned into a popular practice in the preparation of the
concrete material. Gradually, the silica sand replaced the fine crushed stone
and turned out to be a basic component mixed with lime paste and a very small
amount of stone’s powder, grog or ash, for the preparation of different filling
mixtures applied in the building construction including in the restoration
practice. After the division of Mandate Palestine into two states (in 1948),
the Gaza and the coastal area of the
Mediterranean became inaccessible for the residents of Jerusalem and the silica
sand started being brought from the region of Swelyh/ Jordan.
The first 3 samples in table
(8) show the results of the lab tests of mixtures from silica sand and hydrated
lime paste. The observations show that there was a visible slowdown in the
hardening process of these 3 mixtures and that more than one week were
necessary to obtain the desired hardening. Thus, in the samples from 4 to 6, the
white cement- which replaced the stone’s powder, grog or ash at a later stage
around the mid of the twentieth century- was added to the silica sand and the
hydrated lime. Furthermore, in samples from 7 to 10 the hydraulic lime- still
not widely applied in Palestine- replaced
the white cement. Again in this group, the process of the hardening became
faster in the samples from 4 to 10, but no significant deviations have been
observed either in the absorption ratio or in the density (Table 8).
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The third group of laboratory experiments has to test
a possible blend between the crushed stone and the silica sand considering the
following three issues: (1) the crushed stone material is available at ample amounts
in the Palestinian territories and it can easily be obtained at fine quality
and reasonable prices. (2) There is a plenty of wastes from the stone industry
in Palestine that can be re-used through possible future manufacturing of the hydrated
lime, and (3) after the establishment of the Palestinian National Authority in
1994, it became more difficult to acquire the silica sand in adequate
quantities, good quality and reasonable prices, due to the mobility
restrictions imposed by the Israeli authorities. For these reasons, it was decided to test different
samples obtaining a mixture of 78% crushed stone to 22% silica sand, hydrated
lime paste, hydraulic lime and white cement (Table 9).
This proportion between the crushed stone and the silica sand is based
on the results of the sieve analysis from Tables (3 & 4) that allow the
characteristics of the two materials, crushed stone and silica sand, to be
identified. The next step was mixing the
two materials together using various proportions (by weight), starting with 90%
crushed stone to 10% sand and reaching finally the ratio of 50% each of the two
materials. The ultimate aim of this step was to find the optimal ratio for
mixing the crushed stone and the sand. The results of the sieve analysis of the
various mixtures are shown in (Table 5), and the data from the final column
were used to produce the diagram in (Fig 7a). That in turn allowed the
researchers to make a comparison between the conducted sieve analysis of these
two local materials and the criteria established by the ASTM C136 - 06 Standard
Test Method for Sieve Analysis of Fine and Coarse Aggregates, shown in (Fig
7b). By comparing the two diagrams in Figure (7), it was noticed that the
gradation of the tested ratios of mixing crushed stone with silica sand corresponded
adequately to the standards accepted internationally. So, the next step
was to test the optimal density of the
mixed materials using the ratios stated in Table (5) (but neglecting the ratios
60/40 and 50/50, since they were far from the accepted standards). The results
expressing the optimal density are shown in Table (6) and the diagram drawn in
Figure (8). The curve shows that the optimal density of the mixture is
approximately 78% crushed stone to 22% sand. It is worthy to mention that the
use of silica sand will minimize the quantity of water needed to prepare the
mixture. Yet, the presence of the silica will help more for better cohesion of
the mixture.
After determining the optimal crushed
stone-sand density, 11 mixture samples were prepared using this optimal (78/22) stone-sand mixture.
In samples 1-4 only the hydrated lime
paste was added, in samples 5-8 the hydraulic lime replaced the hydrated lime
paste and in the last samples from 9-11 the white cement was added to the
stone-sand mixture and the hydrated lime paste. During the lab tests, the
observations stated a normal and acceptable process of hardening of all samples.
but again, no significant deviations have been observed in the average sums
either of the absorption ratio or the density (Table 9).
The results of the lab tests in the three groups show that all
samples retain absorption ratios higher than those for the stones from the
buildings in the Old City of Jerusalem. The same elucidation is valid for the density, which draws closer
to the density of the stones in these buildings. These are good indicators, as
the Common practice is the new stones and the mixtures better being more porous
and less dense than those existing ones (Table 10). The only problem was
related to the hardening time needed for each sample. This process of hardening
accelerates or slows down according as the materials used to prepare the
various mixtures. In this context, the weaker results were obtained from mixing
the silica sand only with hydrated lime. This fact explains the still
prevailing practice of using the white cement in the present restoration
activities in Palestine, to accelerate the hardening process without
encountering the future negative consequences of this practice on the quality
of the restoration. Yet, the blend between the crushed stone and the silica
sand proved to be an excellent alternative to the present practice, and have
its potential towards the economic development in the fields of the local stone
industry.
Other two major issues were encountered during the laboratory
tests: the difficulty of controlling the
amount of water added or lost in the mixtures, especially the water in the
hydrated lime; and the quantity of soft material that was accumulated in the
No. 200 sieve, which was around 15% instead of the 5% allowed under international
standards. Both problems affect the accuracy of the results in general, however
they don’t argue against the appropriateness of the methodology that was used.
...to be continue....
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