Developing a low embodied carbon-content concrete with conventional concrete properties

Diekmann, Michael Andreas (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.


ENGLISH ABSTRACT: In an ever-developing world, the use of concrete as a construction material, and cement as a main constituent thereof, is at a historical peak and set to increase even further in future. At the same time, the world is confronted with environmental challenges partly due to greenhouse gas emissions, to which the production of cement is a large contributor. In order to decrease the emissions and ensure greater sustainability of the concrete industry, it is therefore critical to reduce the cement content in conventional concrete. This reduction in cement content can however not sacrifice the quality of the concrete, in terms of certain properties that conventional concrete exhibits. It is therefore the main objective of this study to develop a low cement-content concrete, and as such a low embodied carbon-content concrete, with conventional concrete properties. Three approaches of achieving this can be defined. Firstly, cement in concrete can be replaced by more environmentally friendly supplementary cementitious materials (SCM) or fillers. Furthermore, the water requirement of concrete can be reduced in order to achieve a lower cement content by, secondly, using superplasticisers or, thirdly, optimising particle packing. This study establishes reference mixes using the first approach, before separately using the latter two approaches to lower the water requirement of the former mixes at a constant slump and water/binder ratio. The three approaches are finally combined in order to establish what are termed the “optimised” mixes in terms of cement content, with conventional concrete properties being the aim. The concrete properties that all mixes are evaluated for include rheological properties, setting time, compressive strength, permeability as part of durability and the equivalent carbon dioxide (CO2e) emissions. Furthermore, certain indices showing the efficiency of use of cement and the CO2e emissions due to the mixes in terms of compressive strength are determined. It was found that the replacement of various fractions of cement showed a pronounced reduction of CO2e emissions, while resulting in mixes with conventional properties. The inclusion of superplasticisers improved the rheological properties of these mixes and further reduced the emissions of the mix, by significantly reducing the cement content. However, this decreased the compressive strength of the mixes. The optimisation of particle packing improved all the measured properties. The combination of all three approaches resulted in mixes with improved rheological properties, as well as a 40% to 60% decrease in the emissions due to the concrete. The compressive strength was negatively effected and halved compared to the reference mix. However, certain mixes still showed better efficiency indices than the reference mixes, meaning they used less cement and CO2e emissions to develop strength. With regard to cement content, they could indeed be termed the “optimised” mixes.

AFRIKAANSE OPSOMMING: Die gebruik van beton as ‘n konstruksiemateriaal, en sement as ‘n hoofbestanddeel daarvan, het historiese hoogtes bereik en verder groei word voorspel, weens die ekonomiese ontwikkeling van groot dele van die wêreld. Terselfdertyd word die wêreld met uitdagings in terme van ekologiese volhoubaarheid gekonfronteer, as gevolg van kweekhuisgasvrystellings. Hierdie vrystellings word tot ‘n aansienlike deel veroorsaak deur die produksie van sement. Om meer volhoubaarheid van die betonbedryf te verseker is dit van uiterste belang om die sementinhoud van gewone beton tot ‘n groot mate te verminder. Die eienskappe van die beton mag egter nie negatief beïnvloed word nie. Die doel van hierdie navorsing is dus om ‘n beton met ‘n lae sementinhoud, en sodanig ‘n lae koolstofinhoud, met konvensionele beton eienskappe te ontwikkel. Om die doel te bereik kan drie benaderings gevolg word. Eerstens kan gedeeltes van sement met omgewingsvriendeliker aanvullende sementagtige materiale (ASM) of vullers vervang word. Verder kan die waterbehoefte van beton verminder word om sodoende die sementinhoud te verlaag deur, tweedens, van superplastiseerders of, derdens, geoptimiseerde partikel verpakking gebruik te maak. In die navorsing word gewone betonmenge ontwerp deur die eerste benadering toe te pas, en dié menge word dan as verwysingsmenge gebruik. Daarna word die tweede en derde benaderinge afsonderlik gebruik om by konstante versakking en water/binder verhouding die waterbehoefte en, as gevolg hiervan, die sementinhoud van die menge te verlaag. Die drie benaderings word uiteindelik gekombineer om die “geoptimiseerde” menge in terme van sementinhoud te ontwikkel. Die menge word verder ook getoets om te bepaal of dit konvensionele betoneienskappe toon. Die eienskappe waarvoor elke meng evalueer word sluit in: reologiese eienskappe, settyd, druksterkte, deurlaatbaarheid as deel van duursaamheid, en die ekwivalente koolstofdioksied (CO2e) vrystellings. Sekere indekse wat die doeltreffendheid van die sementgebruik en CO2e vrystellings in terme van druksterkte uitdruk word ook bepaal. Daar is bevind dat die vervanging van sement met ASM en vullers ‘n aansienlike vermindering van CO2e vrystellings veroorsaak, terwyl konvensionele eienskappe gehandhaaf word. Die gebruik van superplastiseerder het die reologiese eienskappe verbeter en CO2e vrystellings verder verlaag, maar druksterkte is negatief beïnvloed. Die optimiseering van partikel verpakking het alle eienskappe verbeter. Die kombinasie van die benaderings het die reologiese eienskappe verbeter en die CO2e vrystellings met 40% tot 60% verlaag. Druksterkte is egter negatief beïnvloed en gehalveer in vergelyking met die verwysingsmeng. Sekere menge het egter steeds beter doeltreffendheidsindekse getoon as die verwysingsmeng. Met betrekking tot die sementinhoud kan die menge inderdaad “geoptimiseerde” menge genoem word.

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