Effect of the superplasticizers on

Effect of the superplasticizers on the reactivity of the C3A-gypsum system in a lime saturated solution

The evolution of the electric conductivity during the hydration of a C3A- gypsum system in a lime saturated solution and in presence of 0.5g of superplasticizer that is to say 5% with respect to the C3A weight is represented in





Fig.2. From these curves it appears that the diphosphonate doesn’t modify the duration of ettringite precipitation whereas both PNS and PCP decrease the rate of ettringite formation. Next Figures (Fig.3,4,6) represent the evolution of the ionic concentrations during the C3A-gypsum system hydration in presence of the different superplasticizers.

Diphosphonate.--The C3A-gypsum mixture was introduced in a lime saturated solution containing 5% of diphosphonate with respect to the C3A weight.The results are presented in Fig.3. The reference experiment was carried out with the same C3A sample. As we can see In fig.2 the diphosphonate doesn’t modify the duration needed to precipitate ettringite from gypsum and C3A. The same experiment was also carried out with higher levels of gypsum (2 and 2.5g of gypsum) and again, the time needed to precipitate the ettringite is the same with or without the diphosphonate, as reported in Fig.3. The only noticeable difference is relative to the initial conductivity. The slight increase observed in presence of the diphosphonate can be attributed to the ability of the diphosphonate to form a complex with calcium ions and consequently increases the solubility of lime. [20]


PNS.--The average rate of ettringite precipitation is drastically decreased in the presence of PNS; the time needed is approximately multiplied by four in the experimental conditions (Fig.2). The decrease of the rate depends on the PNS dosage as shown in the Fig.4. Moreover Fig.4 shows the decrease of the precipitation rate when the sulfate concentration is at the plateau level and also when it decreases. Figure 5 suggestes that the time needed to precipitate ettringite in the presence of PNS could be correlated with the initial adsorption level of PNS. The slowing down of the ettringite precipitation is maybe linked to a decrease of C3A dissolution rate.

PCP.--Figure 2 shows a slowing down of ettringite precipitation as a result of the PCP presence. Under the experiment conditions the time needed to precipitate the ettringite is multiplied by 1.5 in presence of 5% of PCP. Moreover an increase of the PCP amount (5 to 6 % ) leads to higher duration (Fig.7). As the slope of the sulfate concentration decrease is linked to the rate of ettringite precipitation we conclude that PCP slows down the rate of ettringite precipitation.
Nevertheless it appears that the rate of ettringite precipitation is not affected when the sulfate concentration is high enough and at the “plateau” value (Fig.7). In the same time, the size of ettringite crystals precipitated in the presence of PCP during this period, is smaller than usually. Indeed in order to determine concentrations of species in solution during the C3A hydration reactions, small portions of the suspensions were removed, and as usually filtered through 0.3 µ m millipore filters. However in the presence of PCP, aluminium, calcium and sulfate concentrations proved that ettringite crystals were present in the filtered solution (Fig.8). In this case a new filtration was done using 0.1µ m filters in order to prevent ettringite from going through. SEM images of the solid collected on the 0.1µ m filter one hour after the beginning of the C3A-gypsum hydration in saturated lime solution in the presence of PCP are presented in Fig.9. PCP clearly decreases the size of the ettringite crystals formed.
Consequently, the presence of PCP induced a slowing down of the rate of ettringite precipitation and a decrease of the size of ettringite crystals. The PCP may act on ettringite precipitation by decreasing its growth rate. Previous studies already showed such a decrease of the growth rate or a modification of the ettringite morphology as a





result of a polymer adsorption [21-25]. However the average rate of ettringite precipitation is not affected when the sulfate concentration is high enough and at the “plateau” level Namely when the sulfate concentration is high enough and at the plateau level, ettringite nucleation might take place and by this way the rate of ettringite precipitation would not be decreased during this period.