Catarata Congénita

The degree of capillary porosity, which was determined on 7 and 28 day old paste

samples, enabled the effect of temperature on the early stages of the microstructural

development to be studied. In order to study similar effects on mortar samples, water

penetration tests were carried out. Both techniques complement each other. While

the degree of capillary porosity does not give any indication as to the connectivity of

the pores, it does give information regarding the microstructure. The water

penetration tests on the other hand, does not give any information on the

microstructure, but can be used to assess the connectivity of the pore system, and in

some cases it is often linked to durability.

Drying or preconditioning of samples is a very important aspect that should be

considered in determining the water transport properties of mortars or concretes.

Drying at 50°C is considered to be less damaging to the microstructure [131],

however, it usually takes longer periods for the samples to reach constant mass

during which hydration and pore refinement will still be going on. On the other

hand, drying at 105°C reaches equilibrium faster but induces microcracks [325] and

creates irreversible microstructural alterations [326], which will also have effects on

the transport properties. While drying at 50°C might seem to be the best option,

there is the possibility that microcracks could also be formed as reported by Wu et

Since the scope of the study did not involve looking at the effect of the drying

technique on water penetrability, two different methods – sorptivity and water

absorption, involving the use of different drying techniques were used to study the

water transport property. In terms of the age of the samples, 7 day old samples were

considered unsuitable for the tests. This was as a result of the longer times required

to dry the samples to constant mass. For 7 day old samples, where the microstructure

is not fully developed and hydration is still on-going, drying would interfere with the

hydration process and microstructural development thus affecting the results. Hence,

the tests were only conducted on 28 and 90 day old samples. Also, mortar samples

were used instead of concrete so as to be able to relate the results to other aspects of

the study.

Sorptivity:

Sorptivity was determined using similar methods as used by Tasdemir [328], and

Gϋneyesi [131]. 50 mm mortar samples were used for the test. The test was conducted on triplicate samples. The samples were cured for 28 and 90 days at 20

and 38°C, after which they were dried to constant mass in an oven at 50°C.

Depending on the mix, it took about 20 – 35 days for the samples to dry to constant

mass, with the slag blends taking longer times to dry than the neat system. After

drying, the sides of the samples were coated with paraffin and weighed to obtain the

initial mass before they were placed in a trough of water at 20°C. The water level

was maintained at about 5mm from the base of the samples (Figure 3.11), all

through the experiment. This was achieved by occasionally topping-up the water in

the trough. The mass of the samples were recorded at predetermined times (1, 4, 9,

16, 25, 36, 49 and 64 mins). At each of these times, the mass of water absorbed by

mass, and from this the sorptivity coefficient (k) can be determined using the

following expression:

𝑘 = 𝑄

𝐴 𝑡 (3.7)

where:

Q amount of water absorbed in m3, which was calculated by dividing the mass of the water absorbed in kg, by the density of water (1000

kg/m3)

t time in seconds

A cross-sectional area of the specimen that was in contact with the water in m2

k sorptivity coefficient in m3/m2s1/2.

Figure 3.11: Schematic of setup used for sorptivity test

In determining k, values of Q/A were plotted against √t, and k was taken as the slope

of the straight line fitted through the origin to the data points as shown below in

Figure 3.12: Linear fit for the determination of sorptivity coefficient for a C1 mortar sample cured for 28 days at 20°C

Water absorption:

Water absorption was determined according to the procedure outlined in BS 1881-

122:2011 [329]. However, mortar samples were used in place of concrete. The

mortar samples were prepared and cured in similar ways as the samples used for the

sorptivity test. At the end of the curing period, the samples were dried in an oven for

72 hours at 105°C to remove all the evaporable water. After drying, the samples

were placed in a desiccator and left to cool for 24 hours at room temperature before

testing. The samples were weighed to obtain a dry mass Md, after which they were

completely immersed in a water tank at 20°C. The water level in the tank was

maintained at 25 mm above the samples throughout the duration of the test by

occasionally topping-up with water. The mass of the samples were recorded at 10,

30, 60 and 120 mins. The water absorbed (Wa) as a percentage was calculated using

Equation 3.8 and plotted against time t.

0 2 4 6 8 0.0000 0.0001 0.0002 0.0003 0.0004 C1 Linear fit Q/A (m 3 /m 2 ) t1/2(sec1/2) Adj. R2 = 0.9853

𝑊𝑎 =

𝐾 𝑀𝑡 − 𝑀𝑑

𝑀𝑑 × 100 (3.8)

where:

Md dry mass of the sample in grams

Mt mass of the sample after time t, in grams

K correction factor for the shape of the samples, which is equal to 0.667 [329]

Wa water absorbed in %