Mortars added with chemically inert sediment aggregates: performance assessment and microstructure changes

摘要

This study investigates the influence of treated sediment aggregates (TSA) used as substitutes to natural aggregates in cement-based building materials. Microstructure in presence of TSA is identified, in order to justify observed changes in macroscopic performance (mainly mechanical properties and durability). TSA are valorized wastes derived from initially polluted marine sediments. They are subjected to a treatment patented by Solvay SA. It is composed of (1) a phosphatation phase, which creates, from raw sediments, an apatite-like mineral able to capture heavy metal components, and (2) a calcination at 650 degrees C, which eliminates organic pollutants. Efficient pollutant retention capability of TSA has been attested in former work. Their high water retention ability, low strength, high fine particles content, low pozzolanic activity and low chloride content are also known. In this study, mixing of the cement-based material (a normalized mortar) induces unavoidable TSA crushing and fine particles creation. In the introduction, we recall our main results as regards the potential effect of TSA content, curing conditions and age upon the performance of mortars substituted with water-saturated TSA. These are substituted to pure silica sand (0, 33, 66 and 100% sand volume). With reference to industrial practice, two extreme curing conditions are used, either water immersion (most favorable to cement hydration), or air curing (highly deficient curing). With more amplitude after water curing, porosity is shown to increase hugely with TSA amount, while apparent density decreases linearly. Up to 33% substitution, elastic Young’s modulus, uniaxial compressive strength and apparent gas permeability are all improved as compared to reference 0%-substitution mortar, all the more so when mortar is air-cured. Maturity is attained from 28 to 60 days curing only. Observed positive changes are attributed to the internal curing effect of watersaturated TSA, whereas negative performance evolutions, significant above 33% substitution, are attributed to TSA brittleness. Secondly, microstructure changes are investigated, in order to validate our interpretation of the observed changes in mechanical and durability performance. To this purpose, SEM observations and EDS analysis have been performed. Noticeable changes in silica sand/cement paste interface are shown qualitatively, whereby portlandite amount is lowered in presence of TSA. We also provide qualitative assessment of the sediment aggregate/cement paste interface and of the microstructure of cement paste in presence of TSA