Gypsum attack: performance of silicic pozzolans and Portland cements as determined by ASTM C 452-68

An earlier study with metakaolin and 10 Portland cements (PC) suggested that the ASTM C 452-68 test could be used to characterise and differentiate 28-day blended cements prepared with metakaolin. ASTM C 452-68 testing was once again conducted on 67 cements: the above 10 PC and 57 blended cements (POZC) containing 20%, 30% or 40% silica fume or diatomites (D or N) (= Z pozzolan). ASTM C 452-68 specimens were made from all the cements and their daily rate of increase in length was determined. The results showed that in a gypsum and water environment, the pozzolanic reactions induced by the reactive silica, SiO₂^r−, present in Z pozzolans, prevailed over the sulfatic hydration of the pozzolans themselves, reaching completion within 28 days. The explanation is that the formation rate (V_f) of the calcium silicate hydrate gel and silanol groups from the SiO₂^r− present in the Z pozzolan finishes being greater than the V_f observed for the ettringite forming slowly (ett-lf) from the C₃A of ordinary Portland cement, which declines to zero, even though sulfatic hydration continues. The positive 7 and/or 28-day results obtained with the Frattini test (EN 196-5) and the tests set out in standards EN 450, ASTM C 311-07, EN 13263-1 or ASTM 618-95a provided proof that the ASTM C 452-68 test can be validly used to characterise 28-day POZC prepared with any natural or artificial pozzolan, subject, however, to the establishment of suitable standard physical-chemical requirements and specifications.