Effect of water-binder ratio on properties of phosphogypsum binder

Abstract

Technological, physical and mechanical properties of mineral binders are often defined by the amount of water add to the binder known as W/B ratio. The role of water is contributed to the workability of fresh paste, hydration processes, porosity, mechanical performance and durability of the material. Present research summarizes the relationship between gypsum based binder W/B ratio in mixture composition to properties of hardened gypsum binder, giving a marginal borders of properties affected by W/B. The W/B ratios evaluated were from 0.3 to 1.15. Dihydrate phosphogypsum (PG), which is a waste of phosphorus fertilizer industry and commercial gypsum binder were used as a raw material for production of gypsum samples and results compared to commercial gypsum binder. At the beginning PG was dried at 60°C and afterwards homogenized by milling it in ball mill or by collision milling in disintegrator. Samples were made with plasticizers, mineral additives and foam forming additive. In this work 26 dense and 9 porous mixture compositions was summarized, from which 18 compositions were made from commercial binder for comparison. A series of performance properties of the binders were investigated, such as density, porosity and 14d compression strength regarding to W/B ratio used to prepare the samples. The chemical and mineralogical composition of PG used was determined by XRF and XRD. The results indicate that plasticizers in PG based gypsum binder could reduce W/B ratio from 1.15 down to 0.4. The PG binder bulk density for samples without pore forming admixture was in range from 726 – 1600 kg/m3, total porosity 35 – 71 vol.% and compressive strength from 1.4 to 28.7 MPa at the age of 14 d. Ultra porous gypsum material was obtained with the pore forming agent, which allowed to obtain gypsum based material with bulk density from 213 – 726 kg/m3, total porosity from 67.9 – 90.6 % and compressive strength from 0.2 – 0.8 MPa. Results show that the material density and strength decreases while porosity tends to increases with the increase of W/B ratio. These tendencies are true both for dense and porous specimens. It was concluded, that lowest W/B of dense PG specimens was 0.43, otherwise the workability of a binder decreased dramatically and decrease of compressive strength was observed. With the same W/B ratio the technological properties of PG binder may be affected by gypsum treatment parameters and selected mixture composition, allowing to improve the properties of gypsum binder obtained from same raw material.