Sewage Sludge Ash used as Supplementary Cementitious Material in Mortar and its Effects on Hygrothermal Parameters and Indoor Environment Quality in Buildings

Abstract

The overexploitation of available natural resources and its possible adverse effects on environment are prevailing and extensive problems faced by the construction industry. Specifically, cement is the second most consumed substance by weight in the world, after water, and during its production 900 kg of CO2 are emitted per ton of cement. The growing demand for reduced emissions of CO2 urges the cement industry to find materials with low CO2 footprint, which calls for cement substitution. In order to achieve feasible rates of cement substitution, it is necessary to introduce new supplementary cementitious material (SCM), which is readily and possibly locally available. Sewage sludge ash (SSA) could be such a viable resource. An assumption of the study has been that SSA, as an industrial by-product, can be applied as a potential cement substitute in cement-based materials without compromising the material performance. In order to corroborate or disprove this assumption, various material examinations were performed together with different methodologies of material characterization. The Ph.D. study investigated partial replacement of cement by SSA in mortar and its effect on hygrothermal properties of mortar and impact on indoor air quality. An overall scope of the study was to examine how mortars of different composition interact with the indoor climate. Additionally, the aim was to compare methods applied when assessing hygroscopic performance of cementitious materials and their impact on indoor air quality. This calls for a cross-disciplinary approach. Three main research tasks were carried out in order to meet the goals: (1) Ash characterization and investigation of the effect of cement substitution on selected physical properties of mortar. (2) Examination of the effect of ash content on hygrothermal characteristics of mortar. (3) Sensory and chemical measurements of mortar emissions and interaction of mortar with typical indoor pollutants. Two SSAs were used as a substitute for cement in the study. They originated from two major Danish incineration plants located in Avedøre and Lynetten in the Greater Copenhagen area. The effects of different ratios of cement substitution in mortar and two pre-treatment methods of ashes were investigated. First of all, the results on characterization of SSAs confirmed previous findings concluding that chemical composition of SSAs is highly variable and even their origin from wastewater treatment plants located in the nearby areas does not ensure comparable quality. Moreover, granulometric examination showed that the SSAs typically consisted of larger particles compared to cement particles. Incorporation of ash in cement-ash-based mortar resulted in more porous mortar structures compared to cement-based mortar, which adversely affected the mechanical properties of mortar such as the compressive strength. The hygroscopic sorption properties of the individual constituents alone and of complete mortar samples were described by sorption isotherms for water vapour and by a capillary water absorption test. Although the sorption abilities of individual constituents were significantly different, the effect of ash content on the sorption isotherms of cement-ash-based mortars was minor. The water absorption coefficient of mortar samples increased significantly with the ash to cement ratio. This indicates that the higher porosity of cement-ash-based mortars have more pores in the capillary range than cement-based mortars. Hygrothermal properties of the cement-based mortar and cement-ash-based mortar were examined by means of various tests, specifically: thermal conductivity, two hygroscopic sorption tests and water vapour permeability measurements. The measurements showed that the higher porosity of mortar was responsible for a decrease of thermal conductivity and an increase of vapour permeability of mortar. This was expected as these are a function of porosity. Sorption was tested by two methods: the climatic chamber method and the desiccator method. The observations proved that the sorption isotherms were influenced by the method applied, i.e. CO2 concentration in the supplied air. The method using sealed glass vessels as desiccators showed minor or no differences between cement-based and cement-ash-based mortar, while differences were observed between different mortar mixtures when the climatic chamber method was applied with its atmospheric CO2 content. Comparison of both methods revealed that the method applied may affect measured parameters to such an extent that the reported results may differ. The assessment of hygroscopic properties of cementitious materials is not clearly defined in any standard and is not easy to evaluate considering that it can be easily affected by various factors. There is a need to delineate standard methodology for hygrothermal assessment of cementitious materials.Additionally, test on presence of calcium carbonate (CaCO3) content was performed to detect the effect of carbonation on mortar samples. CaCO3 was detected in all the mortar samples. With respect to mortar composition, CaCO3 content was higher for cementbased mortar.The possible effect of emissions from cement-based mortar and cement-ash-based mortar slabs on perceived air quality was studied and compared by means of sensory assessments and chemical measurements. The tests were conducted on mortar samples alone and together with either carpet or linoleum. Air discharged from the CLIMPAQlike glass chambers was assessed by twenty-five subjects and sampled on Tenax TA tubes for analysis by the GC/MS method. No significant differences were observed in odour intensity produced by cement-based mortar and cement-ash-based mortar slabs, and neither did they differ in chemical composition of the emissions. Odour intensity increased with the increased exposed area of mortar. A significant sink effect was observed when linoleum was added to any of the two slabs. The sink effect increased with the increased loading of the slabs. The combination of linoleum and mortar slabs resulted in lower odour intensity than that produced by any of the two materials when tested alone. A possible explanation is the uptake of organic acids emitted from linoleum by the mortar slabs, as the mortar is a strong base. A much weaker or no sink effect was observed in the case when carpet was exposed together with the slabs. This was probably because carpet was not such a strong source of acids. The total concentration of organic compounds (TVOC) did not show any reasonable differences in concentrations of pollutants emitted by slabs alone or when they were together with linoleum and carpet. However, distinct differences were observed when the concentrations were considered based on the functional groups, e.g. organic acids, alkanes and alcohols.