Properties of Cements Containing Fly Ash Together With Other Admixtures


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Title: Properties of Cements Containing Fly Ash Together With Other Admixtures

Author(s): Z. GiergicznyI

Publication: Special Publication

Volume: 132


Appears on pages(s): 439-456

Keywords: admixtures; compressive strength; concretes; flexural strength; fly ash; hydration; phosphogypsum; pozzolan cements; silica fume; Materials Research

Date: 5/1/1992

The admixtures of condensed silica fumes (CSF) and phosphogypsum (neutralized and dehydrated at 400 C) were used together with fly ashes as blended cement components to improve early strengths and other properties. The cements with the initial 15 to 50 percent low-calcium PFA content (SiO2 + AL2O3 + Fe2O3 - 83.3 percent) or 15 to 70 percent high calcium PFA content (22.1 percent CaO) were mixed with the additional components just mentioned. Standard tests at normal curing were made, as well as measurements after the low-pressure steam treatment at 70 C. All cements mixed with CSF showed standard compressive strengths about 13 to 20 MPa higher than the reference mortars. More detailed studies of the hardening process were also carried out using calorimetry, DTA, TG, XRD, and porosimetry, which showed acceleration of the hydration process due to pozzolanic properties of CSF. Reduction of total porosity and pore size was also found. The same positive effect of CSF was observed in the case of mortars treated at 70 C. This additive improves significantly the pozzolanic properties of low-calcium PFA. At standard curing, activated phosphogysum addition brings about a decrease in the hydrated calcium silicates. A substantial amount of ettringite forms and partially inverts into monosulfate after 28 and 90 days of hardening. At accelerated curing, the mortars containing phosphogypsum show a significantly higher degree of hydration than the reference mortar. The results relating to pastes and mortars have been confirmed for concretes. Therefore, one can conclude that the admixtures studied, particularly CSF, have positive influence on the properties of PFA concretes and help to augment the effect of PFA content in these concretes.