This thesis is submitted in partial fulfillment of the requirements for the degree of Master of Science in Civil Engineering. The properties of fresh concrete, the properties of hardened concrete and the durability characteristics of the produced SCC were investigated.
Introduction
- Statement of the Problem
- Goals and Objectives
- Methodology and Approach
- Research Questions
- Thesis Structure
The focus of the first phase was to investigate SSA feasibility to be used as a partial replacement of OPC. Performance evaluation of the manufactured SCC by reusing SSA as a partial replacement of OPC for different construction applications.
Literature Review
Introduction
Self-Compacting Concrete
- Background
- Advantages and Disadvantages
- Mix Design Principle
- Mix Design Proportions
This was the primary reason for the development of the first practical form of SCC. Therefore, the packing density of the granular skeleton should be high enough to achieve the desired flowability of SCC.
Supplementary Cementing Materials
In addition, Alyamac et al. 2017) consumed maximum amount of marble powder (MP) as mineral additive in SCC. Calcined kaolinite clay, or metakaolin, has a strong history of use as an SCM in concrete (Wang et al., 2018).
Studies on Self-Compacting Concrete with Different SCMs
It was also observed that increasing the replacement level of VP resulted in a gradual increase in the L-box height ratio of SCC mixtures. It was observed that all the mineral admixtures significantly improved the workability properties of SCC.
Sewage Sludge Ash
- Sewage Sludge Ash Characterization
- Sewage Sludge Ash in Construction Industry
The physical properties of SSA have been reported in the literature and presented in Table 5. It was reported that the addition of SSA in mortars resulted in a reduction of consistency rates due to the irregular shape of SSA particles that absorbed part of the mixing water. Sorption isotherms showed that the hygroscopic water vapor absorption of the two types of SSA varied.
It was concluded that the setting time of SSA–cement paste increased with increasing SSA fineness.
Research Significance
The behavior of SSA and metakaolin-based geopolymer concrete was evaluated by Istuque et al. It was concluded that addition of SSA (ie, up to 20%) to replace metakaolin resulted in slightly lower compressive strength for mortars baked at 65oC compared to mortar without SSA. Recent studies showed limited published data on the effect of SSA on cement hydration and the relationship between pore structure and mechanical properties.
Therefore, the proposed research will help to understand how the temperature and time of combustion of SSA particles introduced as a partial replacement of cement in mortars can affect the performance of mortars and the possibility of reusing SSA in the production of SSC.
Investigation Program
Materials
- Aggregate
- Cement
- Water
- Sewage Sludge Ash
- Admixtures
The permeability of SCC mixtures mainly depends on the maximum nominal size of the aggregates used. The chemical composition of the cement used is presented in Table 9 as determined by X-ray fluorescence (XRF). A synthetic high molecular weight VMA copolymer (RheoMATRIX ® 110) was used in the preparation of the SCC mixtures studied in this thesis.
The properties of the used SP and VMA were provided by the manufacturer and are illustrated in Table 11.
Methods
- Phase 1: Characterization of SSA
- Phase 2: Reusing Sewage Sludge Ash to Produce SCC
All samples were placed directly on a carbon tape and gold (Au) coated. The samples were sufficiently dry to avoid the evaporation of moisture in the sample under the high vacuum in the microscope, thereby deteriorating the vacuum in the SEM column and affecting the efficiency of the electron beam. Durability as stated in EFNARC (2005) guidelines is "the ability of a concrete structure to withstand environmentally aggressive situations during its design working life without compromising the required performance". The durability properties of the fabricated SCC were evaluated using the tests described in the following subsections.
A is the cross-sectional area of the sample [cm2], L is the length of the sample (cm), and.
Performance Index Approach
- Tests Used for the Performance Index Evaluation in Phase 1
- Tests Used for the Performance Index Evaluation in Phase 2
The performance index method is adopted to facilitate the selection process of firing temperature and duration to produce the most suitable SSA used in concrete production and the most suitable SCC mix. The second step to complete the PI approach is to calculate the numerical index (Ri). The PI is calculated for each SSA sample/SCC mix as a percentage of the sample/mix index (Sn) relative to the highest index of all SSA/SCC mix samples (Snmax) according to Equation (18).
The energy consumption of the combustion process at different temperature and duration was calculated based on the model and type of use.
Results and Discussion - Phase 1 : Characterization of SSA
Introduction
X-Ray Fluorescence (XRF)
It is well known that pozzolanic activity is directly related to the chemical composition of the materials, since silicon or silicon aluminate materials, if such compounds are present in an amorphous state, chemically react with calcium hydroxide at room temperature and form compounds possessing cementitious properties. (Malhotra and Mehta, 1996). CaO content in SSA can provide some hydraulic properties to the ash, while the presence of SiO2 and Al2O3 can be responsible for some pozzolanic properties, thus supporting the feasibility of being used as a partial substitute for cement (Pekmezci and Akyüz, 2004). If present in amounts of 1–2% in Portland cement clinker, it slows down the hardening rate of the cement.
Since P2O5 is one of the main components of SSA, the addition of SSA to the concrete can contribute to a longer curing time.
X-Ray Diffraction (XRD)
Scanning Electron Microscopy (SEM)
Particle Size Analyzer
The particle size distribution of all ground SSA samples was examined to ensure the consistency of the sewage sludge milling and to ensure that the properties of SCC were not affected by the particle size distribution. It can be observed that all SSA particles had a similar mean particle size compared to OPC ~ 156 µm. Previous work has shown that the particle size distribution and morphology of cement and ash grains have a significant influence on the heat of hydration, the progression of hydration and, consequently, on the porosity and compressive strength of the mortar (Frigione and Marrra, 1976; Monzó et al. 1996). ).
Therefore, the consistency of grinding burnt SSA particles using a disc grinder would greatly contribute to the fineness of the SSA particles being a constant factor.
Pozzolanic Activity
- Frattini Test
- Strength Activity Index (SAI)
The mean values of compressive strength (Fc) of cured SSA mortars including 20% SSA cement replacement and control mortar at 7 and 28 days of curing were measured and the results are summarized in Table 19 and Figure 44. The compressive strength of all mortar samples increased when the curing age was extended from 7 to 28 days. The highest reduction in compressive strength was observed in SSA particles burned at 1100oC, which indicated that increasing the combustion temperature above 900oC could reduce the compressive strength of SSA mortars.
Test results of the SAI after 7 and 28 days of curing for the different SSA mixtures are shown in Figure 44.
Workability and Workability Retention
This improvement is due to the low D50 and average diameter of SSA particles burned at 1100oC compared to the other SSA particles (as shown in Table 18). Another reason for this improvement in workability may be due to the expansion or swelling of materials at high temperature, resulting in a smoother surface and a more compacted pore structure. However, SSA did not expect a significant decrease in consistency in the first hour for SSA particles burned at 1100oC for 6 hours.
A slight decrease in the workability of the paste with time may be due to the loss of water consumed due to hydration or evaporation of the cement.
Mercury Intrusion Prosimetry (MIP)
Heat of Hydration
It can be seen that there was no significant effect of SSA in the initial hydration reaction between 0-40 minutes. However, when the reaction started to accelerate, the reaction was highly exothermic due to the formation of hydration products such as C-S-H, but the sample containing 100% OPC showed a higher heat of hydration than the samples with SSA. Similar findings have been reported by others, where the use of SCM in cement reduces the heat released during hydration (De Rojas et al., 1993; De Rojas and Frías, 1996; Frías et al., 2000).
Therefore, the coarser particle size and smaller surface area of SSA compared to OPC provided fewer nucleation sites for the precipitation of the hydration product and thus slowed cement hydration.
Toxicity Characteristic Leaching Procedure (TCLP)
In addition, five other metals with established limits were examined, including cadmium (Cd), chromium (Cr), lead (Pb), arsenic (As) and barium (Ba), as these are the metals for which permissible limits are specified. For all samples, the leachability concentration of barium (Ba) and strontium (Sr) increased with age. The low concentration of Mg ions after 28 days indicates that all magnesium (Mg) ions were bound and therefore no more Ba or Sr ions will be leached out over the years.
To minimize the leaching of heavy metals in SSA, it was found that adding fly ash to sewage sludge can be considered as an interesting alternative sludge stabilization method (Papadimitriou et al., 2008).
PI for SSA Selection
Pozzolan activity was assessed by three different criteria (i.e. XRD analysis, SAI and CH consumption). However, the SAI results indicated that SSA7-2 had the highest SAI, while SSA9-2 showed the highest CH consumption. Energy consumption increased as combustion temperature and duration increased, therefore the lowest energy consumption value was observed for SSA7-2.
To select the optimal combustion temperature and duration that meets all the selected criteria, multifunctional performance criteria were used (Table 25).
Concluding Remarks
Due to the porous nature of SSA particles, the inclusion of SSA increased the total porosity of the paste at early ages. Also, the use of 20% SSA as a cement substitute slightly decreased the total porosity and therefore increased the microstructure of the paste. Whereas, the incorporation of 20% of SSA in the cement paste reduced the pore size diameter by 5% compared to the control mix.
The addition of SSA reduced the mortar consistency due to the high water absorption, irregular morphology and rough surface texture of the SSA particles.
Results and Discussion – Phase 2 : Reusing Sewage Sludge Ash in
- Introduction
- Fresh Concrete Tests
- Slump Flow
- J-Ring Test
- V-Funnel Test
- Concluding Remarks
- Hardened Concrete Properties
- Compressive Strength
- Pore Size Distribution
- Bulk Electrical Resistivity
- Initial Rate of Absorption
- Permeable Pore Percentage
- Water Permeability
- Drying Shrinkage
- Performance Index for SCC mixtures
- Concluding Remarks
The flowability of the manufactured SCC mixes was evaluated by the drop flow test and the results are shown in Table 26. The test results will give an indication of the flowability of the manufactured SCC mixes. The inclusion of SSA in the control mix at 20% and 40% partial replacement of cement resulted in a significant decrease in the slump flow and T50 cm values due to the rough surface structure and irregular morphology, in addition to the high characteristics of absorption of SSA particles.
Therefore, the pore diameter is an indirect measure of the strength and durability of the concrete produced.
Conclusions and Recommendations
General
Conclusions
- Sewage Sludge Ash (SSA) Characteristics
- Fresh Concrete Properties
- Hardened Concrete Properties
Recommendations
