The use of municipal solid waste incineration ash in various building materials : a Belgian point of view

Huge amounts of waste are being generated, and even though the incineration process reduces the mass and volume of waste to a large extent, massive amounts of residues still remain. On average, out of 1.3 billion tons of municipal solid wastes generated per year, around 130 and 2.1 million tons are incinerated in the world and in Belgium, respectively. Around 400 kT of bottom ash residues are generated in Flanders, out of which only 102 kT are utilized here, and the rest is exported or landfilled due to non-conformity to environmental regulations. Landfilling makes the valuable resources in the residues unavailable and results in more primary raw materials being used, increasing mining and related hazards. Identifying and employing the right pre-treatment technique for the highest value application is the key to attaining a circular economy. We reviewed the present pre-treatment and utilization scenarios in Belgium, and the advancements in research around the world for realization of maximum utilization are reported in this paper. Uses of the material in the cement industry as a binder and cement raw meal replacement are identified as possible effective utilization options for large quantities of bottom ash. Pre-treatment techniques that could facilitate this use are also discussed. With all the research evidence available, there is now a need for combined efforts from incineration and the cement industry for technical and economic optimization of the process flow

Reactivity tests for supplementary cementitious materials: RILEM TC 267-TRM phase 1

A primary aim of RILEM TC 267-TRM: “Tests for Reactivity of Supplementary Cementitious Materials (SCMs)” is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well

Processed bottom ash based sustainable binders for concrete

The world is going through a transition from a GDP (gross domestic product) oriented to a sustainable development-oriented way of life. Huge amounts of wastes are generated each day, and smart management of these wastes is an important issue for attaining sustainability. Incineration is an effective method to handle huge quantities of municipal solid wastes which cannot be reused or recycled. This process generates huge amounts of residues which are currently landfilled or used for low value applications such as in construction of road subbases. The municipal solid waste incineration (MSWI) ashes have been widely investigated as a replacement for aggregates, and many studies concluded that this posed the possible risk on alkali silica reaction, which indicates the presence of reactive silica. Having a chemical composition similar to that of coal combustion fly ash, and having gone through heat treatment, these MSWI ashes have the potential to be used as a supplementary cementitious material. Furthermore, they can be used as a corrective agent in raw meal for clinker production. Very few studies in literature have explored these options. One of the main obstacles for implementation as supplementary cementitious materials is the presence of elemental aluminium. The main treatment proposed in literature for elemental aluminium removal when the ashes are used as aggregate, is treatment with NaOH. However, this has the disadvantage of introducing alkali into the ash when treated after grinding, which could alter the hydration kinetics of the binder when the ashes are used along with cement. When ashes are treated before milling, the treatment risks to be slow or ineffective. Three fractions (6/15(50) mm, 2/6 mm and 0/2 mm) of bottom ash from a Belgian incinerator were collected. Visual sorting of the 6/15 fraction revealed the components to be predominantly glass and stone objects, including also ceramics, bricks, metal parts and organics. The 6/15 and 2/6 fractions are predominantly amorphous in nature. Two kinds of treatments which are simple and effective are proposed in this thesis for removal of elemental aluminium and other unwanted residues. One is submerging the milled ash in water (L/S ratio 1:5 was used here) and subjecting the slurry to a temperature of 100°C until the ash is dry. The second type of treatment consists of slow grinding of ashes and subsequent sieving out of the coarse fraction >142.5 µm containing a major portion of the elemental aluminium (which is less easy to grind). Various effects of the first treatment method were investigated in laboratory, including the effect of partial replacement ofxvi Portland cement by MSWI ashes on expansion, strength, hydration kinetics, setting time and workability of cement-bound material. Expansion of mortar reduced considerably after the ash pre-treatment, while strength increased. Workability of mortar with ash replacement reduced slightly on treatment of the ash. Initial setting time increased on replacement of cement with both treated and untreated bottom ash, with treated ashes leading to shorter setting times than untreated ashes. This could be due to a prolongation of the induction period which can be observed also in calorimetry curves. From the preliminary tests, the 2/6 fraction was selected for further investigation at the concrete level and was treated at larger volumes, for which the second type of treatment; the bulk treated ash is named as 2/6 NB. Reactivity tests on the treated ashes showed low to moderate reactivity of the ashes. The 2/6 fraction ashes passed the strength activity index test at both 28 and 90 days, and therefore were selected for further tests at the concrete level. The R3 calorimetry test on ashes indicated that aged ashes (stockpiled) have lower reactivity than young ashes. The compressive strength of mortars after 28 days of curing had good correlation with the cumulative heat released by bottom ash in cement paste during 7 days of hydration at 40°C as determined by isothermal calorimetry, while the correlation with heat release in the R3 paste at 40°C was lower. This indicates that the dominant effect on compressive strength put forth in the paste by bottom ash is not the pozzolanic reaction, but rather an improvement in the hydration of alite. This was confirmed via XRD with Rietveld refinement, on pastes of water-tobinder ratio 0.5 with CEM I 52.5N reference and blended cement with 25% CEM I 52.5 N replaced with 2/6 NB, showed an increase in degree of hydration of alite with ash replacement. Portlandite consumption determined by TGA was minimal. However, this determined value is interfered by the additional portlandite formed due to increased reaction of alite, and the inherent mass loss in that range (400-500˚C) in the ashes. Concrete level tests were conducted on mainly four mixes namely Mix 1, 2, 3 and 4. Mix 1, 2 and 3 had same relative proportions of materials, differing only regarding the binder, CEM I 52.5N (Mix 1), CEM II B-V 32.5R (Mix 2) and 75% CEM I 52.5N + 25% bulk treated ash (Mix 3). Mix 4 was designed using various trial mixes to have similar strength at 28 days as that of Mix 1 and had 80% CEM I 52.5 R with 20% bulk treated bottom ash as binder. It also had a water-to-binder ratio which was reduced by 0.05 compared to the first three mixes. At 28 days, Mix 2 and Mix 3 had comparable strengths, and Mix 1 and Mix 4 had comparable strengths. However, Mix 2 gained morexvii strength than Mix 3 and Mix 4 gained more strength than Mix 1 from 28 to 90 days. At 90 days, Mix 4 had better strength than Mix 1. In terms of durability tests performed (open porosity, capillary imbibition, air permeability, chloride ingress and freeze thaw), both Mix 2 (concrete mix with mainstream fly ash) and Mix 4 (optimized concrete mix with bottom ash) performed the best. Similar to most durability phenomena tested, Mix 2 performed best in creep and shrinkage, followed by Mix 4, and Mix 3 performed the worst. While judging based on these results, it is to be kept in mind that Mix 2 and Mix 1 are concrete mixes with commercially available cements that are optimised in particle size distribution and packing density. This contrasts with Mixes 3 and 4, where the addition of bulk treated ash was done by simple blending during concrete mixing. The prospect for second life of MSWI ash concrete was evaluated by using it as recycled aggregate in new concrete. Concrete Mix 5 was produced with the same basic mix design as that of Mix 1 with around 26 mass-% of the aggregates replaced with crushed concrete from Mix 4 trials. Compressive strength and pore structure of Mix 5 were found to be better than for Mix 1, which could be attributed to a large extent to the fact that the parent concrete from which recycled aggregate was produced, was of a better strength class than the recycled aggregate concrete. Further, trials on production of cement clinker with MSWI bottom ash were conducted. Three raw meal mixes were formulated with around 5% of each of the three fractions of bottom ashes and were fired to obtain three clinkers. The resulting clinkers had comparable mineralogy and hydration kinetics as that of commercial Portland cement. SEM-EDX analysis was also conducted on clinkers and trends visible in presence of minor elements along with major phases were interpreted. Mg and Ti were found to be prominently spotted along with C3A and C4AF phases. Overall, the results of this research exhibit the potential use of bottom ash in concrete as cement replacement and form a basis for further research to optimize and standardise its use for commercial applications

Properties of Concrete with Recycled Aggregates Giving a Second Life to Municipal Solid Waste Incineration Bottom Ash Concrete

Economic and environmental factors call for increased resource productivity. Partial or full replacement of Portland cement by wastes and by-products, and natural aggregates by construction and demolition wastes, are two prominent routes of achieving circular economy in construction and related industries. Municipal solid waste incineration (MSWI) bottom ashes have been found to be suitable to be used as a supplementary cementitious material (SCM) after various treatments. This paper reports a brief literature review on optimum use of recycled aggregates in concrete and an experimental study using replacement of natural aggregate by demolished concrete having MSWI bottom ash as partial replacement of Portland cement, and compares its properties to that of completely natural aggregate concrete. Additional water was added as a compensation for the water absorption by the recycled aggregate during the first 30 min of water contact during concrete mixing. Also the fine fraction of crushed concrete (<250 µm) was removed to reduce the ill-effects of using recycled aggregate. The replacement of aggregates was limited to 23% by weight of natural aggregate. The results prove environmentally safe and comparable performance of concrete including recycled aggregate with bottom ash to that of natural aggregate concrete

Reactivity of municipal solid waste incineration ashes as a supplementary cementitious material

The high carbon and energy footprint of cement calls for substitution by supplementary cementitious materials (SCMs), and for various waste streams or by-products, cementitious products have been identified as an effective sink. The variability of the composition of these SCMs makes it necessary to identify a fast and reliable test to predict concrete strength and facilitate their utilization. There are some conventional reactivity tests such as the Modified Chapelle test and Frattini test, that measure reactivity based on indicators such as consumption of calcium hydroxide in accelerated conditions. Furthermore, new tests called R3-tests, applicable for SCMs from various sources, have been developed recently and are currently tested within RILEM TC-267 TRM. Municipal Solid Waste Incineration (MSWI) ash is identified as a potential SCM, the composition of which varies with time and location. In the current research two size fractions of MSWI ash were selected, and conventional and R3 reactivity tests were conducted on this material. This paper reports the results of all reactivity tests and compares them with the strength development of mortar bars with 25% Portland cement replacement by MSWI ash

Properties of concrete with recycled aggregates giving a second life to municipal solid waste incineration bottom ash concrete

Economic and environmental factors call for increased resource productivity. Partial or full replacement of Portland cement by wastes and by-products, and natural aggregates by construction and demolition wastes, are two prominent routes of achieving circular economy in construction and related industries. Municipal solid waste incineration (MSWI) bottom ashes have been found to be suitable to be used as a supplementary cementitious material (SCM) after various treatments. This paper reports a brief literature review on optimum use of recycled aggregates in concrete and an experimental study using replacement of natural aggregate by demolished concrete having MSWI bottom ash as partial replacement of Portland cement, and compares its properties to that of completely natural aggregate concrete. Additional water was added as a compensation for the water absorption by the recycled aggregate during the first 30 min of water contact during concrete mixing. Also the fine fraction of crushed concrete (<250 µm) was removed to reduce the ill-effects of using recycled aggregate. The replacement of aggregates was limited to 23% by weight of natural aggregate. The results prove environmentally safe and comparable performance of concrete including recycled aggregate with bottom ash to that of natural aggregate concrete