Thermally Treated Waste Silt as Filler in Geopolymer Cement

Abstract

This study aims to investigate the feasibility of including silt, a by-product of limestone aggregate production, as a filler in geopolymer cement. Two separate phases were planned: The first phase aimed to determine the optimum calcination conditions of the waste silt obtained from Società Azionaria Prodotti Asfaltico Bituminosi Affini (S.A.P.A.B.A. s.r.l.). A Design of Experiment (DOE) was produced, and raw silt was calcined accordingly. Geopolymer cement mixtures were made with sodium or potassium alkali solutions and were tested for compressive strength and leaching. Higher calcination temperatures showed better compressive strength, regardless of liquid type. By considering the compressive strength, leaching, and X-ray diffraction (XRD) analysis, the optimum calcination temperature and time was selected as 750 °C for 2 h. The second phase focused on determining the optimum amount of silt (%) that could be used in a geopolymer cement mixture. The results suggested that the addition of about 55% of silt (total solid weight) as filler can improve the compressive strength of geopolymers made with Na or K liquid activators. Based on the leaching test, the cumulative concentrations of the released trace elements from the geopolymer specimens into the leachant were lower than the thresholds for European standards.

Thermal, X-ray Diffraction and Oedometric Analyses of Silt-Waste/NaOH-Activated Metakaolin Geopolymer Composite

“…As reviewed in a very recent work [28], the recycling of the waste silt minerals was carried out by employing a geopolymerization process based on metakaolin as the starting raw material. Two commercially available metakaolins, namely Masterlife MK 828 (BASF Construction Chemicals Italia Spa, Treviso, Italy) and Metamax (BASF SE, Ludwigshafen, Germany), were used as fine powders.…”

“…As explained and detailed in the work of Solouki and coworkers [28], the waste silt was collected from S.A.P.A.B.A. s.r.l.…”

“…(Bologna, Italy) sedimentation lakes, where during aggregates' production, undesirable substances such as silt, clay, and dirt are washed, separated and stored. The average chemical analysis (weighted percentages of oxides) of the uncalcinated waste silt, as provided by the cited work [28], is reported in Table 1. The waste silt was thermally treated at 850 • C for 6 h and 30 min using a static furnace before use in the geopolymerization reaction to enhance its surface reactivity [28].…”

“…The average chemical analysis (weighted percentages of oxides) of the uncalcinated waste silt, as provided by the cited work [28], is reported in Table 1. The waste silt was thermally treated at 850 • C for 6 h and 30 min using a static furnace before use in the geopolymerization reaction to enhance its surface reactivity [28]. This recovered material has to be considered a filler, and it was grounded and calcinated to (i) increase its surface area, (ii) deform the particle surface and (iii) remove adsorbed and structural water.…”

“…Secondly, there is a continuing interest in the possibility of replacing fractions of the precursors (mainly metakaolin) with a different type of waste for both recycling and sustainability purposes. As shown in a recent work [28], silt-waste from sedimentation lakes could be a possible, safe source of both precursors (e.g., kaolinite clay) and inert mineral/materials (e.g., quartz), the latter fraction acting as filler. To enhance its possible surface reactivity towards adhesion of the silt particles to the geopolymer matrix, the fine powder of the waste silt was calcinated at 850 • C.…”