Comparison between the effects of phosphorous slag and fly ash on the C-S-H structure, long-term hydration heat and volume deformation of cement-based materials

Fly ash (FA) has been widely used in hydraulic projects. The utilization of phosphorous slag (PS) to partially or completely replace FA in hydraulic concrete has gained much attention in China in recent years. In this study, the effects of PS and FA on C-S-H structure, long-term hydration heat, hydration products, mechanical properties and volume deformation of cement paste/concrete were investigated and compared via Si-29 NMR nuclear magnetic resonance (29Si NMR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal analysis and the dissolution method. The retarding effect of PS at early age hinders the generation of C-S-H, whereas the pozzolanic reaction of PS largely occurs at middle and late age. More than half of the Ca(OH)(2) content in cement paste is consumed and lots of C-S-H gels with high polymerization degree are produced at middle and late age, leading to a dense microstructure of cement paste. FA has a more noticeable effect on the enhancement of the polymerization degree and Al content of C-S-H than PS. There is a reduction of 25.3% and 18.6% in the cement hydration heat within 3 days and 180 days when 30 wt% FA is blended. By contrast, PS is more helpful for lowering the hydration heat at early age, the incorporation of 30 wt% PS reduces the 180-day hydration heat by about 9.2%. Additionally, after the first few days, PS concretes possess stronger mechanical properties in comparison with FA ones, confirming PS has a higher pozzolanic activity than FA. The adiabatic temperature rising of plain cement concrete at 28 days was reduced by 4-5. due to 30 wt% PS incorporation. The addition of 30 wt% PS reduces the early autogenous shrinkage significantly and declines the 180-day drying shrinkage and autogenous shrinkage by about 7.2%-9.0% and 8-11%, respectively. However, FA is more effective in reducing the adiabatic temperature rising, long-term autogenous shrinkage and drying shrinkage of concrete than PS. The results in this study could provide useful experience for the utilization of PS in hydraulic projects. (C) 2020 Elsevier Ltd. All rights reserved.