MARCO CONCEPTUAL

The PCA results clearly point to a major difference between the reaction series involving an increase in the hydrolysis times for the carbonate samples. The measured hydrolysis times for the marble, limestone and the 1 limestone are the most affected. The

kinetics and mechanisms of the hydrolysis and condensation reactions are known to determine the structure of the final gel (Boonstra and Baken 1990). Therefore, the gels formed in contact with the carbonate stones must be somewhat different from those in contact with sandstone.

Both the 212 and the 413 reactions follow the same relative hydrolysis trends. The highest hydrolysis rates are found in the control, weathered sandstone, 1 marble and sodium hydroxide systems. Sodium chloride, calcium carbonate and sandstone make up a middle category of moderately decreased rates. The marble, limestone and 1 limestone systems have the lowest hydrolysis rates.

Within these general trends there are some exceptions. The sodium chloride system, for example has a great increase in the hydrolysis rate in the 413 solution. Possibly the dilution minimizes the effect that the sodium chloride ions have of stabilizing and isolating (by electrostatic repulsion) the sol species. The results of the 1 marble are also puzzling. According to the amount of soluble matter listed in table 4-3 (see Chapter 4) and the pH values of the solutions (table 6-1) we would expect the 1 marble and the 1 limestone to behave in a similar manner. It clearly does not. Limestone does have a more acidic surface than the marble (see table 4-4 in Chapter 4); in fact, under magnification of the stained surface, the surface shows much greater variation or disorder as to the acidic and basic groups present in the surface layer.

Table 4-3 also shows that all the rocks tested have a variety of soluble components as well as acidic and basic groups on the surface (table 4-4). This makes it difficult to determine the cause of the reductions or increases in hydrolysis. However, by increasing the water content to the 413 system, the hydrolysis times did decrease as expected in all cases except for the limestone, 1 limestone and marble samples. The dramatic increase in

their (limestone, 1 limestone and marble) hydrolysis times suggests that the solubility of the surface, and a slight rise in the pH, are responsible for the effect.

The condensation was shown by the FTIR spectra to begin during the hydrolysis reaction. However, how the condensation mechanism is affected at the closest level, that of the nearest neighbor and subsequent group size, we can not say. The FTIR spectra show differences in the siloxane regions (1025 cm'^ and 770 cm’*) as well as silanol variation (920 cm’*) but this information will vary greatly depending on: the stage of the reaction (i.e. overall time) to the gel time; the type of system (i.e. catalyst, salt content etc.) will affect the structure and the length of time from the increase in viscosity to gelation; solubility - some solvents or pH ranges (over 7) will promote depolymerization processes. Finally, the aging process itself can alter the structure significantly and closed systems, as in the first forty - four days of these systems, will accentuate these effects.

Figure 6-24 shows the spectra of 212 limestone and marble overlaid on the control spectrum. The day (T) was picked arbitrarily and the ratio of the time to Tg^i was used to decide upon the control spectra. The control spectrum is from day 46 and the

limestone and marble spectrum are from day 44 and 43, respectively. While we can not state that the systems are in the same stage of development, we can say that the

systems can not be compared directly. In figure 6-24, the siloxane regions absorb more strongly in the limestone and marble samples than in the control. Why? Maybe the control system has formed more siloxane bonds, or maybe the carbonates are undergoing some depolymerization. Basically, we can not tell how FT-IR spectra relate to the actual structure of the sol clusters.

This does not diminish the importance of the fact that the day before gelation, the FT-

Figure 6-24: 212 MTMOS systems at T:T^^, = 0.8

100L

control - heavy line limestone - normal line

m arble--- 90- 80- 70l S .1

I

ER spectra are very different - particularly in the siloxane regions. The time to gelation in the 212 systems also indicates that the systems have different sol structures. The limestone and marble samples are the first to gel. This is consistent with base or weak acid catalyzed hydrolysis at near neutral pH - the pH point of minimum time to gelation. The calcium carbonate has a greater time to gelation due to its pH of slightly over 7. The sodium chloride and weathered sandstone with their slow time to gelation are very consistent - we expect them to stabilize the sol clusters and slow down gelation. The 1 marble and 1 limestone results are, again, unexplainable.

The 413 system delays the initial increase in viscosity that is considered the beginning of the gel formation. However, it has no effect on the time to gel formation for the

limestone and marble series. In all other cases (except, of course 1 limestone and 1 marble) the 413 increases the time to gelation. The decrease in the time to gelation of the 1 marble and 1 limestone may indicate the effect of an increase in the soluble material that alters the limestone and marble series so dramatically.

The gels and xerogels derived from the different systems show conclusively that two different sol structures developed. The limestone and marble systems gave gels that were weak that, when pierced, gave lumpy surfaces with very little adhesive ability or cohesive strength. The control and sandstone systems gave gels that slowly increased in viscosity until the xerogel was formed. This gel type was y sticky and elastic.

These xerogels eventually all become glassy and brittle. The differences seem to have become minimized to some degree in the monoliths to the naked eye but mechanical testing or solid state analysis must be carried out for any certainty.