In this study, the results of prior XRD and Le Chatelier–Ansttet test experiments were re-verified in terms of a relative parameter, the Length Growth Rate, Vcl(=ΔL (%)/day). For this purpose ASTM C 452-68 testing was conducted for 2 years on 40 cements, 10 Portland cements–6 OPCs and 4 SRPCs–and 30 blended cements containing 20%, 30% and 40% metakaolin (M pozzolan). ASTM C 452-68 specimens were manufactured with all cements and their daily length growth rate was calculated by dividing the measured increase in length, ΔL (%), by the number of days lapsing since the preceding measurement. Additional experiments were also run: chemical analysis, XRD and SEM analysis of ettringites and tests to determine specific properties of some cements tested. The results have once again borne out that:(a 1 )The formation rate of the ettringite deriving from the reactive alumina, Al 2 O 3 r− , present in pozzolans is considerably higher than the rate for ettringite forming from the C 3 A (and, logically, much higher than for C 4 AF ettringite) present in OPC. Owing to this, these ettringites were termed: “rapid formation” (ett-rf), “slow formation” (ett-lf) and “very slow formation” (ett-vlfʺ) ettringites, respectively.(b 1 )The foregoing directly affects the size of the respective crystals, which are much smaller in ett-rf than in ett-lf. These results have also shown that:(a 2 )In a gypsum and water environment (ASTM C 452-68 specimens), the pozzolanic reactions involving the reactive alumina, Al 2 O 3 r− , present in pozzolans take place during the first 28 days of age–and, sometimes, even during the first 7 days (this occurred for most POZCs prepared with this M pozzolan and PC)–, and ettringite from both, Al 2 O 3 r− and C 3 A, origin, are the reaction products in all cases.(b 2 )Ett-rf and ett-lf formation does not take place independently from one another, but inter-dependently or in a joint way or interactive way. Consequently, both ettringites appear, to a greater or lesser extent, when dealing with gypsum and water environments. Nevertheless, the reaction products are closer to ett-rf than to ett-lf, when more M pozzolan is added (40%) and vice versa (20%), and in any case, topochemical mechanism with prior dissolution must be preponderant over through-solution mechanism.(b 3 )According to ASTM C 150-95 standard, the optional physical requirement to characterize SRPCs is ΔL 14 days ≤0.040%. Unlike the requirement formerly used, i.e., ΔL 28 days ≤0.054%, this requirement “predicts” rather than “assures” the most probable behaviour of a PC under gypsum attack. Moreover, if the results of the tests set out in standards EN 196-1, EN 196-3, EN 196-5 and EN 450 and/or ASTM C 311-94b, mainly, are positive, the ASTM C 452-68 test can be validly used to characterize and differentiate at 28 days, POZCs prepared with M pozzolan–or any other natural or artificial (fly ash) pozzolan–, which would give rise to low, moderate or high sulfate resistance. The only requisite in this regard would be to establish suitable standard and chemical–physical requirements and specifications.