pH of which cement remains below 7 even after 1 month after insertion into the cavity?
The incidence of secondary caries is markedly less around silicate restorations. This is surprising when considering that severe leakage takes place at its margins. 
Also, the incidence of contact caries is less when compared to amalgam restorations (contact caries is the term applied to caries occurring on the proximal surface of the tooth adjacent to the restoration). 
The anti-cariogenic property is due to presence of 15% fluoride. Fluoride release is slow and occurs throughout the life of the restoration. 
Silicate cements were classed as a severe irritant to the pulp because of its low pH (acidic). For many years, silicate served as a standard for comparing the pulpal response to other materials. In deep cavities, the pulp had to be protected with varnish or calcium hydroxide.

Ref- Manapallil 3rd Ed P:88
Glass ionomer cement
Zinc phosphate
Resin cement
Silicate cement