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

Abstract

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, SiO2r−, 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 (Vf) of the calcium silicate hydrate gel and silanol groups from the SiO2r− present in the Z pozzolan finishes being greater than the Vf 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.