AbstractMassive construction activities in the state of Qatar have stressed the market of building materials and raised legitimate concerns about the associated impact of construction practices on the environment. As concrete is the most commonly Used building material, reducing the eCO2 of concrete and maximising the use of recycled materials are a necessity for the construction industry to comply with the ambitious sustainability goals of the country.
This thesis describes a 3 year research programme carried out to (i) evaluate the properties of locally produced recycled aggregates, (ii) investigate the properties of a promising non-Portland cement binder and (iii) assess their effect on concrete properties in a performance related approach. The material considered for the study included 2 types of recycled aggregates (CDWA and EWA), two cements (CEM I 52.5N and CSAB 72.5) and two pozzolanic additions (FA-type S and GGBS).
The initial experimental work involved characterization of the test materials. Recycled aggregates were characterized physically and chemically to establish conformity with construction standards. Microstructural, mechanical and durability characteristics of CSAB cement pastes and mortars were investigated in relation to elevated amounts of gypsum aiming to identify an optimum addition. Thereafter, laboratory tests covering fresh, engineering and durability properties were carried out on concrete mixes incorporating various recycled aggregate contents, different binder combinations and encompassing w/c ratios in the range of 0.45 to 0.75. This enabled the measured properties to be also compared on an equal strength basis. Finally, the eCO2 and the practical implications of these concretes were discussed.
The results from the physical and chemical characterization tests on recycled aggregates suggest that the properties of coarse CDWA and EWA were inferior to that of imported natural aggregates and could not meet all the requirements specified in Qatar construction Specifications (QCS5-2, QS, 2014). Among these, of high importance are the fines contents, water absorption and chemical impurities i.e. chlorides and sulfates. However, the properties were generally found to comply with the broader limits of BS EN 12620 (BSI, 2002) and further processing is required before are used in concrete. Fine recycled aggregates were highly absorptive and rich in sulfates and it is very unlikely to be used in any concrete application.
The study carried out to investigate the properties of CSAB cement paste established that ettringite content increased with increasing the gypsum content and hence it can be used to control the ratio of AFm/AFt hydrates. The lowest paste porosity and permeability was observed for the mixture having 23% w/w gypsum which matches that required to fully convert ye’elimite to ettringite. However, the higher porosity did not correspond to higher strengths. Mixtures with gypsum additions above 2% w/w exhibited lower maximum strengths by approx. 40%. The 2% w/w gypsum addition appeared to be alsocritical for setting of cement, above which the setting times became very short. Alongside with ettringite
formation, expansion potential was also increased, but above a gypsum content of 9% w/w it dropped back to lower expansion levels. Regarding the chloride ingress, it was concluded that a test of sufficient duration should be used to allow for binding to form and in this case the binding capacity was found to be inversely proportional to the total gypsum content. Similar to other studies, the pH of the cement appeared to be around pH 12.0 which raises concerns about the risk of carbonation induced corrosion.
Tests on recycled aggregate concrete properties showed that given the water absorption of recycled aggregates was treated properly, there was no requirement for higher SP doses as far as the fines content is not too high. Similar to literature findings, the results showed that up to 30% v/v coarse CDWA or EWA incorporation there was no or little influence on the engineering and durability properties of equal w/c ratio concretes. At higher replacement levels though the detrimental effect increased with recycled aggregate content. However, for an equivalent strength, all recycled aggregates performed comparable or even better than reference concrete and displayed additionally lower eCO2. At a given strength, the addition of pozzolanic materials has also enhanced durability (except carbonation) and sustainability.
Concretes produced with CSABg cement and with or without pozzolanic additions have initially met the target slump but then all mixes lost workability rapidly. Although, as well documented, the early strength development for CSABg-based concretes was faster than that of CEM I-based concretes and achieved maximum strength within 7 days, after 28 days CSABg concretes experience significant strength losses up to 23%. Mixes containing pozzolanic additions at 15% w/w retained their 28-day compressive strength, but no further strength gaining was noted. Given the expansive nature of CSABg-based cements, all concretes exhibited very low drying shrinkages. Similar to CEM I concretes, permeability and durability of CSABg concretes was strongly asssoicated with the w/c ratio of the mix. In comparison with CEM I concretes of equal w/c ratio or strength, CSABg concretes had lower chloride diffusion coefficients, higher resistance to sulfate expansion, lower eCO2, but higher permeability and reduced carbonation resistance. The latter was due to the low alkaline environment of CSAB cements. As expected, the addition of 15% w/w FA and GGBS in the cement blend has only improved performance at an equal strength basis. In comparison to CEM I/FA or CEM I/GGBS concretes, the engineering, durability and environmental performance of all CSABg-based concretes was comparable, if not inferior.
The practical implications of the study have been considered and overall, it has been shown that locally produced recycled aggregates can be successfully used as a replacement of imported aggregates in concrete applications under a range of exposure conditions. This will yield significant environmental and economic beneﬁts to Qatar. Likewise, CSABg-based cements could be potentially used in concrete applications in aggressive environments and where there is no risk of corrosion induced carbonation. In such cases, the use of CSABg-based cements would satisfy design criteria and reduce eCO2 of concrete.
|Date of Award||2019|
|Supervisor||Rod Jones (Supervisor)|
- Recycled Aggregates
- Calcium sulfoaluminate cements
- Gulf Region