The Performance of Locally Produced Supplementary Cementitious Materials When Incorporated In Concrete

Alexandre, Vital Jorge (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The production of cement has a strenuous impact on the environment. Nonetheless, it is required by the construction industry as a key parameter for socio-economic development. Supplementary cementitious materials (SCMs) are a group of materials that can be used to partially replace cement as a binder in concrete mixtures, two of which includes slag and fly ash (FA). These materials are obtained as waste products from iron smelting and coal combustion processes, respectively. These materials have the capability to generate hydration products similar to that of cement when used. The current study’s main objective was to investigate the performance concrete containing slag and fly ash, produced in South Africa, as a means to reduce the dependency of cement as the only binder used in concrete. The goal of the study was to establish to what degree each of the materials can replace the cement in a typical concrete mix and the impacts thereof. Experiments were conducted on concrete samples made from a series of mixes with a constant binder content and water-to-binder ratio. Cement was replaced on a mass basis, with limits defined by the typical construction replacements: slag at 25, 50 and 75% and FA at 15, 25 and 35 %. The performance is based on: reactivity of the SCM, fresh state, mechanical properties and durability. Setting time was found to be sensitive to the SCM quantity and reactivity, however it was accelerated by more reactive materials and low replacement level. In addition, a wider SCM-particle span increased the bleeding capacity and reduced the bleeding rate. FA was found to increase the plastic settlement of concrete more when compared to slag based concrete, with the maximum plastic settlement occurring at 25 % FA content. In addition, the unrestrained plastic shrinkage of all mixes was significantly greater than that of the reference, yet decreased with increased SCM content. The compressive strength of SCM based concrete was lower than the reference at early ages and improved with curing age. At 91 days the control and a few SCM-based concretes had similar compressive strength of approximately 62 MPa. Moreover, the indirect tensile strength per unit compressive strength of the SCMs based concretes were higher than that of the control, signifying the improvement of the interfacial transition zone. The addition of SCMs also significantly improved the concrete microstructure. Additionally, the durability performance of the SCMs based concrete was better than or equivalent to that of the reference. The chloride resistance of slag-based concrete was four times lower than fly ash concrete or the control mix. The knowledge from the current study shows that SCMs can be used, to a great extent, to replace cement in the construction industry. The early age properties may require attention, yet, in the end, the final product of SCM-based concrete is found to be superior to that of the control. Hence, the use of slag and fly ash, as a binder replacement, does provide a solution to reduce the environmental impact of cement production.

AFRIKAANSE OPSOMMING: Die produksie van sement het 'n strawwe impak op die omgewing. Nietemin, is dit 'n belangrike parameter vir sosio-ekonomiese ontwikkeling. Aanvullende gesementeerde materiale (ASMs) is 'n groep materiale wat kan gebruik kan word om sement gedeeltelik te vervang as ʼn binder vir betonmengsels. Twee van die materiale sluit in slagment en vliegas. Hierdie materiale word verkry as afvalprodukte uit die prosesse van yster-smelt en steenkool verbranding, onderskeidelik. Hierdie materiale het die vermoë om hidrasie produkte, soortgelyk aan dié van sement, te genereer wanneer dit gebruik word. Dus is die hoofdoel van die huidige studie om die verrigting van beton met slagment en vliegas, wat in Suid-Afrika vervaardig word, te ondersoek as 'n oplossing van die afhanklikheid van sement, as die enigste binder wat gebruik word in beton, te verminder. Die doel van die studie is om vas te stel tot watter mate elk van die materiale die sement in 'n tipiese beton meng kan vervang asook die gesamende impak daarvan. Eksperimente is op beton monsters gedoen met 'n reeks van mengsels met 'n konstante binder inhoud en water-tot-binder verhouding. Sement is vervang op 'n massabasis, met grense gedefinieer deur die tipiese konstruksie vervangings: slagment op 25, 50 en 75% en vliegas op 15, 25 en 35%. Die verrigting is gebaseer op: reaktiwiteit van die ASM, plastiese-staat, meganiese eienskappe en duursaamheid. Die set-tyd is gevind om sensitief te wees vir die ASM hoeveelheid en reaktiwiteit, maar is versnel deur meer reaktiewe materiaal en teen lae-vervanging hoeveelhede. Daarbenewens is gevind dat 'n wyer SCM-deeltjie span die bloei-kapasiteit vermeerder en bloei-koers verminder. Vliegas is gevind om die plastiese nedersetting van beton meer te verhoog in vergelyking met slagment-gebaseerde beton, met die maksimum plastiese nedersetting by 25% vliegas inhoud. Die plastiese krimp van alle ASM-mengsels aansienlik hoër as dié van die verwysing, maar het nog afgeneem met verhoogde SCM inhoud. Die druksterkte van SCM gebaseer beton was laer as die verwysing by vroeë ouderdomme en het verbeter met ouderdom. Op 91 dae was die druksterkte van die beheer en 'n paar ASM-gebaseerde beton soortgelyk teen ongeveer 62 MPa. Verder, was die indirekte treksterkte per eenheid druksterkte van die SCMS gebaseer beton was hoër as dié van die beheer, en dui aan op die verbetering van die grenslaagsone tussen die bry en aggregaat. Die byvoeging van SCMS verbeter ook die beton mikrostruktuur aansienlik. Daarbenewens is die duursaamheid van die ASMs-gebaseerde beton beter as, of gelykstaande aan dié van die verwysing. Die chloried weerstand van slag gebaseer beton was vier keer laer as die van vliegas-gebaseerde beton en die beheer. Die kennis van die huidige studie toon dat ASMs gebruik kan word tot 'n groot mate om sement te vervang in die konstruksiebedryf. Die vroeë ouderdom eienskappe verg aanda, maar in die lang termyn is die finale produk van ASM-gebaseerde beton gevind om beter te wees as dié van die beheer. Dus, bied die gebruik van slagment en vliegas wel 'n oplossing teen die omgewingsimpak van sement produksie te verminder.

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