1975 Volume 17 Issue 3 Pages 66-72
From the experimental results it is made out that both PAC and ZPC, when soaked in water, show contraction and weight change to a great degree. That is, PAC shows contraction in dimension ranging from 0i20% to 0.28% 30 minutes after the beginning of mixing, while ZPC shows 0.24% of contraction. From the comparison of PAC and ZPC in the velocity of contraction in the initial stage of setting, it is presumed that PAC, as compared with ZPC is very slow in the velocity of setting reaction and unstable, for in the case of PAC, contraction continues for about 10-20 days, while ZPC becomes almost stable after 5 days. That is, it is presumed that contraction and weight decrease appearing in the initial stage of setting are closely related to the setting reaction. It is generally considered that the setting reaction of polyacrylic acid whose molecular formula is (CH2CHCOOH) n is due to the chelate bond between COOH of polyacrylic acid and Zn ion dissociated from ZnO, accompanied with dehydration, but it has not been confirmed as yet. But, at least, it can be presumed that the form of this bond would be -O-Zn-O-C-O- and Zn<OO>C-.
Again, it is considered that as the result of chelate bond, water formed is gradually scattered outside from the set product of cement and consequently the weight decrease proceeds gradually. That is, it is presumed that the setting reaction due to chelate proceeds comparatively slow to continue for a long time. This is obvious from the fact of weight decrease, too. It is indeed a very interesting fact that in all cases except D, the weight change curves are rising from a day after, indicating the weight increase becoming larger gradually, and though there are some possible reasons for this fact such as the association between COOH group, the hydrophilic group existing in the molecule of polyacrylic acid, and H2O, water absorption due to the pores in the surface of set products and the transformation of zinc oxide ZnO into zinc hydroxide Zn(OH)2, still there remain points not yet clarified in this phenomenon. Again, as D, though the weight decrease can be seen up to 20 days after, from the fact that its weight increases slightly after 30 days, it can be presumed that in this case too, like in other cases, weight increase is ultimately continuing. But in this case, the setting reaction is considered_ very slow.
On the other hand, it is a wonder that the dimensional change curves show contraction despite the weight increase. But from the fact that contraction becomes larger as time goes on, it can be presumed that the weight increase, unrelated to contraction caused by the setting reaction, is mainly due to the water absorption at the surface of set products.
Anyway, it is obvious that PAC has contractivity and water absorbability. From this very fact it can be presumed that there is the possibility of the excellent adhesive strength of PAC being gradually deteriorated.
Next, the dimensional and weight changes after 30 days in case the powder-liquid ratio is varied are as follows : in case the amount of powder is increased by 20% as compared with the standard consistency, the contraction rate decreases by approximately 50%, and the weight increase becomes very slow. Contrarily, in case the amount of powder is decreased by 20%, the results become completely reversed.
The above fact suggests that in case the amount of powder is increased, ZnO showing a filler effect in the set products presumably suppresses the contraction. Again, the increase of the amount of powder means the relative decrease of polyacrylic acid, and it is understood that the weight change decreases because of the decrease of the unreacted acid and the decrease of the pore rate.
At any rate, by making the amount of powder increased it becomes possible to make the dimensional and weight changes with the passage of time smallest, and undoubtedly the efficiency of cement can be
