Two factorial designs were developed (19–21) to analyse the effect on two response variables:

porosity accessible to water (PAW)

of four factors: A=presence of a superplasticiser; B=water/cement ratio; C=aggregate type; and D=cement type. With two levels for each factor, listed in Table 1, a total of 2⁴=16 types of concrete were prepared, as summarised in Table 2. The design factor are listed under columns A, B, C and D, while each row gives the combination of factors defining each concrete (listed under the left-most column).

The specimens were prepared in a precast concrete plant in 16 consecutive weeks in random order pursuant to the experimental design (order as shown in Table 2) and subsequently shipped to the laboratory for testing.

A total of 400 kg of cement was used per cubic metre of concrete. The grey and white cements chosen to prepare the concretes were CI-52.5R and BL II-42.5R, respectively. The commercial superplasticiser (“Melcretet”) used was added at a rate of 1.5% of cement weight. A total of 1 100 kg/m³ of 0/5 river sand and 750 kg/m² of 6/13 crushed granite was added to the siliceous aggregate concretes. The concretes with limestone aggregate contained 1 100 kg of 4/12 crushed limestone and 750 kg of 0/4 crushed sand per cubic metre of concrete. All the specimens were cured in a climatic chamber for 28 days prior to shipping and testing.

All but one of the surfaces of the specimens studied (15×15×7.5-cm³ parallelepipeds: see Figure 1) were covered with plastic paint to prevent air from penetrating into the pores on those sides. The casting surface, which was left unpainted to favour CO₂ ingress, measured 15x15 cm². Twelve specimens were prepared per batch (type of concrete). Two were tested upon receipt as 28-day control samples. The other 10 were exposed to accelerated carbonation as described in item 2.3.

The response variables analysed, as noted above, were porosity accessible to water (PAW) and carbonation rate.

Porosity accessible to water (PAW) is defined as the ratio between the volume of water-accessible pores and the total volume of the sample tested. This test was conducted as recommended by RILEM (22) using 7.5×5×5-cm³ specimens. The specimens were first dried at 100 °C for 24 h to eliminate any possible water in the pores before determining the dry weight (M1). The specimens were submerged in water in a vacuum for one week and subsequently weighed first underwater (M2) and then under normal laboratory conditions (M3). The porosity accessible to water was determined as a percentage as shown in Equation [1]:

PAW=[(M3-M1)/(M3-M2)]×100 [1]

M1=dry mass; M2=underwater saturated mass (hydrostatic weight); M3=water-saturated mass in air

Carbonation rate was calculated in terms of the variation in PAW after accelerated carbonation, using Equation [2]:

carbonation rate (%)=[(PAW_n – PAW_i / PAW_i]*100 [2]

PAW_n=porosity accessible to water at age “n” after the accelerated carbonation test (n=1, 2, 3, 6 or 12 months); PAW_i=initial porosity accessible to water (before the accelerated carbonation test).

The concrete specimens were exposed to an accelerated carbonation cycle with compressed air, in which they were submerged in water and subsequently dried to stimulate CO₂ penetration. The procedure, which was similar to the methods commonly described in the literature (23, 24), consisted of 24-hour cycles as described below.

The specimens were submerged in the curing chamber baths for 18 hours.

The specimens were exposed to accelerated carbonation for 1, 2, 3, 6 or 12 months, and subsequently tested.

The carbonation rate was found with phenolphthalein as described in Spanish standard UNE 112-011-94* (25). This procedure detects the reduction in alkalinity prompted by carbonation from the change in colour of the material tested. Phenolphthalein turns reddish purple at pH values of over 9.5 (non-carbonated concrete) but is colourless at pH of under 8 (carbonated concrete). Figure 2 shows a number of concrete specimens selected by way of example, before and after accelerated carbonation. As the photographs show, the intense purple colour gradually faded after carbonation, providing evidence that the accelerated carbonation test is effective.

A full factorial analysis was conducted for each age studied to determine the joint effect of the four explanatory variables described above (see Table 1). Age was not regarded as a fifth factor in the designs, given that the existence of a correlation among the data for the various concretes at different ages ruled out the independence assumed and necessary to apply analysis of variance techniques. Two factorial designs were developed, one for each response variable, at the four ages studied.

The significance assigned to each explanatory variable and their interactions was determined with the F-test, using ANOVA methodology (19–21).

All the factorial analyses were conducted in accordance with the following criteria.

The fourth order interaction, ABCD, was regarded as irrelevant when calculating the degrees of freedom for estimating experimental variability and drawing comparisons.

Statistical analysis revealed that the admixture and type of aggregate (especially the former) are the factors that affect the initial (28-day) porosity of the material most intensely. These findings are consistent with the many studies conducted on the effect of admixtures on concrete pore structure (12–14).

Today, admixtures are essential components in concrete (12–14, 27–30), and superplasticisers are the type most commonly used (accounting for 70% of the total) (30). The inclusion of small amounts of admixture (less than 1 wt%) substantially improves both the workability of fresh concrete and the properties of the hardened material. Their use lowers the amount of mixing water needed, which in turn reduces pore size and hence system porosity (see Figure 3). That the admixture variable (A) proved to be statistically significant is therefore unsurprising. Moreover, these findings concurred with the several other authors’ reports (6, 31), whereby the addition of superplasticisers in mortars and concretes lowers porosity and consequently raises mechanical strength.

The other factor that was statistically significant in its own right was the aggregate. Aggregate is the majority concrete component in terms of both volume and mass and is regarded, a priori, as an inert filler for the intents and purposes of carbonation. Nonetheless, as the aggregate is known to affect permeability and porosity, all concrete carbonation models take total concrete mass as their reference (2, 32). If aggregate is regarded, a priori, to be chemically inactive, the sole significant change it can only induce changes in concrete porosity and permeability via physical processes (6). Limestone aggregate is known to induce higher capillary water absorption and greater drying shrinkage in concrete than siliceous aggregate (33). That would largely explain the higher porosity accessible to water values in the concretes prepared with this type of aggregate (see Figure 3) and the statistical significance of this factor both individually and in its interaction with other explanatory variables.

Taken individually, neither the w/c ratio nor the type of cement appeared to have any direct effect on concrete porosity. Nonetheless, the latter proved to be of vital importance at all ages of the study in its interaction with the admixture, the aggregate and the w/c ratio (interactions AD, BD and CD). Interaction AD (admixture-cement) consistently appeared wherever binary interactions were significant. The mechanism governing admixture action in concrete depends on the type of cement used. As Figure 3 shows, the presence or absence of an admixture was particularly important in white cement. The explanation lies in the fact that the fluidising molecules in the admixture are adsorbed intensely onto C₃A (27, 28), which normally accounts for a higher share of white than grey cement. The greater contact between the anhydrous cement particles and the water induced in white cement favours speedier hydration and precipitation of reaction products, and consequently lower porosities.

The most statistically significant factor in the analysis of variations in the carbonation rate was cement , both individually and in its interaction with the w/c ratio (see Table 4).

Concrete carbonation takes place in the cement paste, for portlandite (Ca(OH)₂) and C-S-H gel, which account respectively for 25 and 60 wt% of the paste content, are the products most liable to be carbonated (2). Obviously, then, the type of cement used may be closely related to carbonation, for this response variable would depend directly on the formation of the aforementioned hydration products. An overview of Figure 4 clearly shows that the type of cement used consistently affected the carbonation rate. This fact was corroborated by the existence of significant interactions between cement and the other explanatory variables, including the w/c ratio.

Surprisingly, the w/c ratio was not statistically significant in its own right with respect to either porosity or carbonation. Water, which is always present to a greater or lesser extent in hardened cement paste pores, is known to play an essential albeit contradictory role in carbonation. It blocks pores and consequently hinders CO₂ diffusion in concrete, rendering the smallest and generally moisture-saturated capillary pores inaccessible to carbonation, while at the same time favouring the process by providing a medium in which CO₂ can react with the C-S-H gel and portlandite (10, 34). The net result is that the carbonation front deepens with rising relative humidity and water content in the pore solution. The lack of statistical significance of this variable by itself may be attributed to the fact that the two w/c ratios used were not sufficiently different to observe any material variations in the carbonation measurements: i.e., the values used delivered similar workability and induced no differences in concrete consolidation or the amount of water present in the pore solutions. The statistical significance of the interaction between the w/c ratio and cement over time is logical, however. As the system evolves, its degree of hydration rises, reducing the amount of water available in the pore solution (which initially depends on the w/c ratio). That has a direct effect on the carbonation rate in the concrete.

The present findings showed that while the presence of an admixture and the type of aggregate affected initial system porosity, the type of cement, both individually and in its interaction with the other factors, had the greatest impact on the two response variables studied. Therefore, while initial porosity conditions the carbonation rate, the initially most porous concretes need not necessarily be the ones most intensely carbonated, for that process depends largely on the type of cement and its interactions with other factors.