Four sands with different compositions, labelled A, B, C and D, and cement CEM I 52,5R (CEM I) were used in this study.

Two superplasticisers were used: one based on polycarboxylate ethers (PCE) and the other on naphthalene (BNS). PCE admixture has a high side chain length and it is commonly used for precast concrete application. Their main physical and chemical characteristics are listed in Table 5. Their dry extracts were determined as European standard EN 480-8-1997 (36) and elemental analysis was conducted on a CHNS PERKIN ELMER 2400 analyser.

The plateau values for each adsorption curve were found by exponential fitting of the data, further to earlier research (38, 39).

Figures 4a and 4b reproduce the isotherms for PCE and BNS adsorption on CEM I and sands A, B, C and D in water, and also show the adsorption plateau values for each curve.

A comparison of Figures 4a and 4b reveals very distinct differences in admixture consumption on both CEM I and the sands, depending essentially on the composition and structure of both the admixture and the substrate. Seventy per cent less PCE than BNS was adsorbed by CEM I (plateau values = 5.99 and 19.40 mg polymer/g cement, respectively). These results are consistent with earlier findings, according to which cement adsorbed BNS and PMS (melanin derivative) admixtures more readily than polycarboxylate ether-based superplasticisers (11, 42, 43).

Considerable differences in PCE and BNS consumption on sand were also observed. Inasmuch as the features that distinguished the sands were their fineness, particle size distribution and chemical composition. The discussion below is organised around the factors that appear to affect consumption: admixture composition and structure, and sand specific surface, particle size distribution and mineralogy.

a) Admixture composition and structure

Figures 4a and 4b reveal differences in PCE and BNS consumption on the sands studied. Their consumption patterns were the reverse of the patterns found for CEM I. The BNS-based admixture adsorbed on the sand either barely or not at all, exhibiting no significantly differential behaviour with changes in the composition or particle size distribution of the substrate. PCE consumption values, in contrast, were clearly higher than BNs ones, and differed visibly depending on the sand. These findings showed that admixture composition is a determinant key for consumption not only on cements and its additions, but also on different types of sand.

b) Sand specific surface

One of the fundamental differences among the sands studied here was their BET specific surface (Table 3). In cements and additions, a higher specific surface and consequently greater fineness has been shown to raise the amount of admixture adsorbed (11, 39, 44).

Admixture PCE consumption (Figure 4a) was observed to rise from sand A to sand D, albeit non-linearly, with the BET values of the substrate (Figure 5). Nonetheless, in sands B and C, with very similar BET values (1.10 and 1.38 m²/g, respectively: Table 3), the degree of admixture adsorption/consumption varied widely (1.34 and 2.10 mg polymer/g solid, respectively). The obviously non-linear relationship between the BET area and admixture consumption observed in Figure 5 attests to the existence of other factors, including sand particle size distribution, that affected the plateau value.

c) Sand particle size distribution

The isotherms in Figures 6a, 6b, 6c and 6d, showing PCE adsorption on the fractions of sand specified above, were used to analyse the relevance of sand fineness and particle size distribution in admixture adsorption.

The theoretical overall plateau value for each sand was computed from the adsorption data for each fraction and the fraction’s percentage by weight to verify the validity of the isotherms recorded and reproduced in Figure 6. The results listed in Table 7 confirm the suitability and accuracy of the experimental results.

Both Figure 6 and Table 7 show that the finest fractions in all the sands (the ones with the highest specific surface) adsorbed the most PCE. The plateau values for all the particle size fractions also exhibited the same ascending order as for the sand overall, i.e., A<B<C<D.

The percentage by weight for each fraction and the percentage of overall adsorption accounted for, along with the respective plateau value or admixture consumption per sand fraction (red line, in Figure 7), were also found (and shown in Figure 7) to assess the weight of each fraction in overall adsorption and compare the findings in each sand and among sands.

An analysis of Figures 6 and 7 and Table 7 revealed that sand particle size distribution is determinant in PCE adsorption and that, generally speaking, the smaller the sand particle the greater is its affinity for the admixture. Moreover, in all the sands, the proportion of admixture adsorbed by the largest two fractions (2 000 and 2000-500 μm) was smaller than their respective percentage by weight in the sample as a whole. That effect was particularly visible in the >2 000-μm fraction in sand B, which adsorbed practically no superplasticiser.

Other findings included the following.

d) Chemical and mineralogical composition of the sands

The quartz content of each sand was plotted against its plateau value (Figure 8) to determine whether, as the preliminary findings appeared to indicate, the sand mineralogy was a significant variable.

First of all, Figure 8 reveals that sand D is the one with lower quartz content and therefore higher clay content, which explains its high BET surface (4.46 m²/g) (Table 3).

The figure shows that the lower the quartz or the higher the clay content in a sand, the higher was its admixture consumption due to its adsorption on or intercalation in the clay (20).

Overall and fraction-by-fraction adsorption was greater in sand C and D than in any of the others. That behaviour would be explained by the greater clay content (specially in fractions < 125 μm) in these sands and the presence of montmorillonite in sand D, which has been shown to have a greater capacity to intercalate admixtures among its layers (20, 21). In order to check this hypothesis, Rietveld analysis of the fine fractions (< 125 μm) of sands C and D were performed. Results are shown in Figure 9 and Table 8.

Table 8 reveals that quartz amount decrease substantially in fine fractions of sands C and D, whereas clay quantity increases specially in sand D, with valuable amounts of montmorillonite.

Nonetheless, while the correlation between quartz/clay content and adsorption (Figure 8) was high, it was not linear, a further indication that admixture consumption by these sands was the outcome of all the factors analysed.

However, it is important to note that PCE additive used in this paper has high side chains (see Table 5), which it is determinant in terms of lower intercalation in clays with respect to other PCE admixtures with shorter side chains more commonly used in practice.

In light of the findings, the factors found to affect the adsorption of admixture PCE mixed in water on sands of different composition included surface area, particle size distribution and the clay content. None of these factors was the sole determinant of the adsorption values found, however. Two further conclusions can also be drawn.

Figure 10 shows PCE adsorption on sands A, B, C and D suspended in an cement pore solution, along with the respective plateau values.

As Figure 10 clearly shows, the adsorption values were 40-70 % lower when the sands were suspended in an cement pore solution than when they were suspended in water.

The zeta potential of SiO₂ and more spcecifically of quartz in water is highly negative (45, 46). However, despite this negative zeta potential, PCE may absorb on to SiO₂ particles via hydrogen bonds (46). Furthermore, the surface charge of clays in water at pH values of around 7 is also negative, ranging from -15 to -30 mV (24, 47). As these were the conditions at which admixtures PCE and BNS suspended in water were adsorbed by the sands (Figures 4a and 4b), low adsorption values would be expected. The values found for admixture PCE, however, were 1.10, 1.34, 2.10 and 2.64 mg polymer/g sand for sands A, B, C and D, respectively. Despite the negative zeta potential, PCE may absorb on to sand particles. Given the large specific surface of the sand, physical adsorption may have also taken place (11). Lastly, as mentioned earlier, part of admixture PCE may have been interlayered in the clay structure, raising its consumption. For that reason, the higher the clay content in sand, the greater was the admixture PCE consumption observed.

Nonetheless, adsorption declined drastically in the presence of the cement pore solution, despite expectations to the contrary. Ca²⁺ and Na⁺ cations provided y cement pore solution may induced a change in pH to values of around 13. Under these conditions, the zeta potential in the clay and in the sand in general would be expected to move to more positive values, which would entail a rise in admixture PCE adsorption (25, 27). Moreover, Plank et al. (48) shown that silica fume (SiO₂) in model pore solution, adsorbed PCEs via Ca²⁺ ions adsorbed on the surface of initially negatively charged SiO₂ particles.

The findings revealed the contrary, however. The explanation may lie in the fact that it was sand, not pure clay, that adsorbed the admixture and that the abrupt sag in adsorption would be attributable to the differences in ionic composition of liquid phase and changes in surface area. In addition to supplying cations, primarily Ca²⁺ and Na⁺, may induce changes in basal space of clays which would have a negative effect for admixtures intercalation (21). Further research is needed in this sense.

Lastly, results for fluidity (normalised slump) changes in mortars without and with admixtures is shown in Figure 11.

As seen in Figure 11, the presence of both types of superplasticizers increase the fluidity of mortars regardless of the type of sand used, which is an indication that despite the adsorption / consumption of admixtures observed , the effect is not relevant in the flowability of mortars.