Portland Cement Mortar Carbonation Assessment by Using Blue Timol Indicator

It is well documented in literature the carbonation reaction of calcium hydroxide contained in the Portland cement mortars [1-4]. This reaction proceeds in two steps. During the first step, carbon dioxide is absorbed onto the alkaline pore water on the sample surface. The result is the precipitation of calcite crystals where the controlling factor is the dissolution of carbon dioxide and calcium hydroxide in the pore solution. The second step of carbonation is controlled by the carbon dioxide diffusion through the mortar pore network. Given that, the rate of carbonation is defined by the rate of carbon dioxide uptake by calcium hydroxide among other Portland cement constituents.

Carbonation in Portland cements prevents further carbonation from occurring because the CaCO₃ formation causes pore blocking. Nevertheless, blended cement carbonation leads to higher porosities [5,6]. Particularly, ground granulated blast-furnace slag is one of the most frequently used additions in Europe, which are recommended in aggressive environments [7]. However, concretes and mortars made with ground granulated blast-furnace slag have a weak carbonation resistance [8]. Therefore, the knowledge of the carbonation resistance of mortars made with blast-furnace slag will help to design mortars and reinforced concrete structures with an adequate service life.

Materials and mortar preparation

One commercial common Portland cement CEM III/B 32.5NLH/ SR according to the European standard EN 197-1:2011 [9] was used in this study. This cement contains 62.2% ground granulated blast-furnace slag. Chemical analyses of SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, SO₃ and free lime were performed according to the European standard EN 196-2:2014 [10]. Chemical properties are given in Table 1. The testing mortars were prepared according to the European standard EN 196-1:2016 [11]. They were cured under water for zero, one, three, seven, 14 or 28 days. Thereafter, mortars were tested for natural carbonation using a blue timol indicator solution.

Table 1: Chemical compositions of cements determined according to EN 196-2:2014 [10] (%).

*Insoluble residue determined by the Na₂CO₃ method (European standard EN 196-2:2005).

Natural carbonation testing

Natural carbonation testing was performed exposing the mortars to the natural exterior environment under shelter from rain conditions (CO2 concentration of 0.035±0.005%, a temperature of 20±2 °C and a relative humidity of 65±5%). Measurement was taken at 12 months of natural exposure after the curing period under water of zero, one, three, seven, 14 or 28 days. The depth of carbonation was measured on the freshly sawn surface with the blue timol indicator solution.

Figure 1: Example of the carbonation front measured by using the blue timol indicator solution. The easiest and most widely carbonation model used in mortars and concretes is shown in Eq. (1).

Figure 2: Natural carbonation results at 12 months measured by using the blue timol indicator solution.

The depth of carbonation was measured with the blue timol indicator solution and Figure 1 shows an example of the carbonation front measured by using this pH indicator. Natural carbonation results at 12 months of the commercial common Portland cement CEM III/B 32.5 N-LH/SR, according to the European standard EN 197-1:2011 [9,12], are shown in Figure 2. As expected, the influence of the curing period in clear. It is evident that a curing period of seven days is enough to ensure an acceptable carbonation resistance. Consequently, the results obtained after seven, 14 or 28 days of curing under water are quite similar (carbonation depth between 7-8mm).

where : B = carbonation coefficient (mm/year0.5)

x = carbonation depth (mm)

t = natural carbonation exposure time (year)

The natural carbonation exposure time is one year, therefore, the carbonation coefficient (mm/year0.5) and carbonation depth (mm) present the same figure but different units.

Based on the experimental results, the following conclusions can be drawn:

1. Portland cement mortar carbonation is influenced by the time of curing.

2. A higher time of curing have to be defined in mortars with higher ground granulated blast-furnace slag content.

3. Seven days curing time is stated as the minimum time for mortars made with cement containing with more than 62% of ground granulated blast-furnace slag.

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