In this section, we first detail the pH variations with the corresponding chemical analysis and then the effects of each species on the elastic properties of calcite paste.
B.3.1 Variation of pH and chemistry of the pastes
In Table B.1 are listed the tested samples with the corresponding values of pH and calcium ions calcu-lated by Visual MINTEQ (φ =20 %). Figure B.1 shows the measured pH as a function of the molar equilibrium calcium concentration [Ca2+]. Each color corresponds to the main additive used.
Samples pHmeas [Ca2+] ( mM)
Pure
◦
8.8 0.243 mMCa(OH)2
•
10.2 0.151 mMHCl+ 3 mM Ca(OH)2 9.6 0.55
5 mMHCl+ 3 mM Ca(OH)2 9.1 2.5
50 mMHCl+ 3 mM Ca(OH)2 6.9 24.6
100 mMHCl+ 3 mM Ca(OH)2 6.5 47.3
47 mMCaCl2+ 3 mM Ca(OH)2 9.4 43.6 97 mMCaCl2+ 3 mM Ca(OH)2 11.2 85.9
Table B.1 –List of all the tested samples with a calcite concentration ofφ=20 %. The concentration of calcium ions are calculated with MINTEQ at the measured pH.
Taking as a reference point the 3 mM of Ca(OH)2(
•
), we can look at the effect of HCl and CaCl2.B.3. RESULTS 151
Figure B.1 –Analysis of pH for all the tested samples. Thex-axis, in a logarithmic scale, represents the molar concentration of Ca2+ calculated by Visual MINTEQ. Following the colored arrows the amount of each additive increases. All the samples are atφ=20 %.
For hydrochloric acid (), we can notice that this addition results in a pH decreasing, as expected.
Under pH=8, we observe an increase in calcium ionic concentration, due to significant calcite dissolu-tion, as reported in the literature [6].
We now turn to the role of calcium chloride (), which has been added directly at rather large concentrations. For an addition of≈50 mM of CaCl2, the calcium concentration increases and the pH is quite stable.
Adding CaCl2 up to≈100 mM results in an increase of [Ca2+] but also of pH. The difference of pH between the two samples is surprising because pH values calculated for the equilibrium with and without atmosphere (resp. 7.4 and 11.4) depend little on the concentration of calcium chloride. This suggests that the carbonation dynamics is different in the two samples.
B.3.2 Elasticity and yielding results
In this section, the effects of additives addition on the elasticity and yielding of calcite paste is examined.
The values of Glin and γcr as a function of calcium ions concentration are displayed in Figures B.2 and B.3.
Influence of HCl ()
The addition of hydrochloric acid decreases the pH and dissolves calcite. This calcite dissolution does not affect too much the volume concentration of the paste, which decreases by less than 1 %. However, it increases the quantity of Ca2+, Cl− and CaCl+ ions in the suspension. Starting from the sample containing only 3 mM of Ca(OH)2(
•
) and follow the blue arrow on figures B.2 and B.3, we find that the addition of HCl results in a progressively decrease of Glin; The γcr increases at the beginning and after is stable.For the sample with 100 mM of HCl and 3 mM of Ca(OH)2 we also look at its evolution in time, such as in Chapter 4 for calcium and sodium hydroxide samples. In particular, we follow the storage modulus in time, as result of the time structuration step in protocolβ (10 h atγ=0.01 % and f=1 Hz),
Figure B.2 –Linear storage modulus,Glin, as a function of the Ca2+ions concentration. Following the colored arrows, the amount of each additive increases. All the samples are atφ=20 %.
Figure B.3 –Values of the critical deformation,γcr, as a function of the Ca2+ion concentration. Follow-ing the colored arrows, the amount of each additive increases.
as shown in Figure B.4.
The sample with HCl is compared with the pure one. The initial elastic modulus is lower for the sample with HCl. This is surprising because the DLVO calculation predicts a strong attraction for this system. This is due to the high ionic strengthI=143 mM (screening) although the measured Zeta po-tential isζ = 15.6 mV. We attribute this effect to the presence of CO2bubbles due to the initial calcite dissolution, as shown in reaction (3):
(3) CaCO3+2HCl CaCl2+CO2+H2O
The elastic modulus of a yield stress fluid has been shown to decrease in the presence of CO2bubbles with the gas volume fraction [5].
Second we find a strong evolution of the linear storage modulus in the first 15 min. This is probably due to the ejection of CO2 bubbles from the sample. This temporal evolution however remains to be fully understood and has also been observed in flow measurements (Section 5.4).
B.3. RESULTS 153
Figure B.4 – Time evolution of the storage modulus of calcite suspensions atφ =20 % for the pure system and the one with 100 mM of HCl and 3 mM of Ca(OH)2.
Influence of CaCl2()
Figures B.3 shows that the yield strain increases upon calcium chloride addition. This is in good agree-ment with the effect observed adding calcium hydroxide (Section 4.2.5). The effect on the elastic mod-ulus (figure B.2) is less clear: it first decreases then increases. For both 47 and 97 mM of CaCl2 the high ionic strength (resp. 134 and 270 mM) screen completely the electrostatic repulsion. We do not understand this behavior.
Appendix C
Organic additives: role of dicarboxylic acids
C.1 Rheological results
In this appendix, the role of small organic molecules i.e. dicarboxylic acids on elasticity and yielding of calcite paste is detailed.
Dicarboxylic acids are organic compounds containing two carboxyl functional groups -COOH, vary-ing the number of carbons nC in the structure between them. We tested small dicarboxylic acids, from oxalic acid— nC=2 (C2H2O4)— to adipic acid —nC=6 (C6H10O4)— and a wide range of molar con-centration —from 5×10−4 to 0.1M. The initial calcite paste volume concentration isφ=20 %. The sample preparation and the rheological measurements are performed following the usual protocol also detailed in Section B.2.
The results obtained with the addition of dicarboxylic acids without any pH or ionic strength control are presented in Figure C.1 and Figure C.2.
Both figures show the values ofGlinandγcr, normalized by the values of the pure calcite pasteGlin(0) andγcr(0)and plotted versus the number of carbons nC present in the dicarboxylic acid structure.
As we can see most of the values lie between the two dashed lines, representing the dispersion range of the pure calcite values (orange solid line). We can notice an increase ofγcrfor the two extremes nC-values. However this effect is not monotonic with the molar concentration. Further studies need to be made to link these points with values of pH and ionic strength, and to clarify the influence of chemical and physical parameters as we did for the ionic species presented in Chapter 4.
155
Pure CaCO3
Figure C.1 –Normalized linear storage modulus,Glin/Glin(0), as a function of the number of carbons in the dicarboxylic acid structure. Dashed lines represent the error-bar of the pure calciteGlin(0)value (orange solid line). All the samples are atφ=20 %.
Pure CaCO3
Figure C.2 –Normalized values of the critical deformation, γcr/γcr(0), as a function of the number of carbons in the dicarboxylic acid structure. Dashed lines represent the error-bar of the pure calciteγcr(0) value (orange solid line). All the samples are atφ=20 %.
Appendix D Solid calcite
D.1 Interest of solid calcite
In this side study we investigated the mechanical properties of a calcite-based cement. This study on solid calcite demonstrates the difficulties in producing a cement starting from rhomboedrical nanocalcite particles. This subject was developed mainly during the internship of Benjamin Thominet, who also designed the molds described in the following sections.