Potential of waster materials as pozzolans and their influence on the quality of building materials

Abstract

This research investigates the properties of industrial and agricultural waste ashes to evaluate their potential as partial substitution for natural aggregates and cement; in an effort to make building materials more sustainable and recycle waste into construction. The partial replacement of limes/cements with waste would lower the embodied energy of building materials, their CO2 emissions and the consumption of the non-renewable resources used to produce them. Thirteen ashes were investigated comprising sugarcane bagasse ashes and incinerator ashes. Properties including particle size distribution, specific surface area, chemical composition, mineralogy and amorphousness were investigated and related to the pozzolanic activity of lime: ash solutions. The formation of hydrates was monitored with Scanning Electron Microscopy coupled with Energy Dispersive X-Ray Spectroscopy (SEM-EDS). The chemical and mechanical methods evidenced reactivity for the ashes and this was further evidenced with SEM, as hydrates appeared as early as 7 days (in sugarcane bagasse ash 1, incinerator bottom ash 1 and fly ash 1 pastes). The specific surface area and fineness of the ashes are comparable to traditional pozzolanic materials however, despite their reactivity, their amorphousness and silica content are low and their loss on ignition high. The chemical and mechanical methods disagree on the reactivity rating. The chemical method overrates pozzolanic reactivity probably due to the aluminium content which is responsible for the quick consumption of Portlandite in solution. As a result, the incinerator bottom ashes, IBAs, with greater alumina content (11-15%) are rated chemically as the most reactive. However, according to the mechanical index, the sugarcane ashes (SCBA 2) are the most reactive. Taking into account all the properties measured, the assessment of reactivity by strength development (mechanical index) seems a better predictor of pozzolanic activity than the chemical test. The seven most reactive ashes were selected for further investigation as partial Portland cement (PC) replacement in composites. Additionally, the 2 least reactive sugarcane bagasse ashes were investigated as sand substitution ? sugarcane bagasse ash sand (SBAS 1 and 2). 27 ash composites were produced, with levels of cement/sand replacement of 5, 10 and 20%. The composites were investigated and the results compared with a PC control mix. There are correlations among the composite properties. The ashes that caused a larger refinement of the pore structure and lower capillarity resisted frost action the best; general linear relationships exists between compressive strength and bulk density and a good correspondence exists within the thermal properties: all the ashes significantly lower the thermal conductivity of the cement composite (by c.30%) which agrees with the tendency of the ashes to reduce the density of the PC composites. The porosity and capillarity of the composites reduced using ashes from the sugarcane industry, while incinerator ashes tend to increase porosity at high levels of replacement and increased capillarity and vapour permeability (FAs 1 and 2). The hygroscopic characteristics of their chlorides may enhance these effects. As expected, the finest and most reactive ashes (SCBA 1, SCBA 2 and FA 3) with greater silica content and mechanical reactivity, used as cement replacement, lowered porosity, capillarity and vapour permeability the most. However, the coarse sugarcane ashes (SCBA 3 and 4) used as sand replacement ? designated SBAS 1 and 2 - substantially lowered porosity and capillarity despite their poor reactivity and produced denser composites, likely due to their higher cement content. All the ash composites reached significant compressive strength, only three mortars (IBA 1 20%, IBA 2 20% and FA 2 20%) did not attain the lower limit of 30 MPa at 28 days in EN 197-1, however, they still reached high strengths (24-29 MPa). The fly ashes significantly increase flexural strength surpassing the reference material. Some of the sugarcane bagasse ashes (1 and 4) and the fine incinerator bottom ashes (1 and 3) also exceeded the reference flexural strength but most ash composites reached c. 70% of the reference strength. The ashes lowered the stiffness of the reference mortar, with the exception of FA 3 5%, which exceeded the reference elastic modulus by 18%. The ashes used as sand replacement increased mechanical resistance in compression, however maintaining plasticity. Most composites produced with incinerator ashes have lower strengths however most comply with the lower strength requirement for PC mortars at 28 days in EN 197-1. Their durability is lower than the sugar ash composites probably due to their Cl and alkali content. The incinerator fly ashes FA 1 20% (with the highest strength loss at 26%) was the worst performer against salt attack probably due to their high Cl and alkali content. The ash composites performed well against frost, at the end of freeze-thawing cycles no external damage was visually apparent however, their strength decreased likely due to the development of frost-induced microcracks. The ash composites show outstanding thermal properties. The ashes lowered the thermal conductivity, specific heat and thermal mass of the PC. The bottom ashes were more effective at lowering thermal conductivity and providing better insulation properties. The lowering of the thermal conductivity by the ashes is interesting for material design, as it can lower a U-value of 3.44 to 2.11 W/m2K in a 300 mm wall of PC concrete, just by replacing 10% of the cement with IBA 2 (the ash with the lowest thermal conductivity 0.63 W/mK). In addition, using this ash, the standard U-value requirement of 0.21 W/m2K for a typical cavity wall can be reached with a block 40% thinner than the standard. The ashes also lower the thermal effusivity of the PC, sometimes substantially (e.g. FA 2 5% with a 29% reduction) which adds to the increased insulation ability of the ash composites. The sugarcane ashes show great potential for PC and fine aggregate replacement in cement-based composites. The sugar ash composites have good resistance to frost and salt action, they reached the highest strengths and increased bulk density, lowering porosity, capillary suction and thermal conductivity while the water vapour ability is little changed. These combinations of properties have the potential to produce strong materials with a greater insulation ability and a lower moisture transport that enhance durability and water vapour properties adequate to maintain indoor air quality.