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Production of Sustainable Self-Compacting Concrete

Final Report Summary - PROSEC (Production of Sustainable Self-Compacting Concrete)

Concrete is the most widely consumed material in the world. It requires slow, heavy, noisy, expensive, energy-consuming and often dangerous mechanical vibration for compaction in order to develop strength and durability. Modern technology; Self-Compacting Concrete (SCC) is now available, where concrete can simply flow freely into the moulds, thus eliminating the need for compaction. Despite of the steady increase in SCC usage in Europe and worldwide, its adoption in general construction has not been as rapid as expected. The full benefits of SCC are far from being exploited. One of the main reasons for this is its high materials costs (mainly the chemical admixtures-i.e. superplasticiser) compared to conventional concrete. This is because SCC requires a substantial quantity of fines to give the concrete the necessary cohesion and prevent segregation. Special chemical admixtures are essential for providing the high filling ability required. The main objective of this project is to produce novel chemical admixtures that are cost effective and environmentally friendly which would allow the production and increasing usage of SCC.

In order to achieve the main objective of this research, the Incoming International Fellow (IIF) collected information on quarry waste fines; mix designs and properties of typical successful SCC mixes in the UK. The materials included a number of cementitious materials (e.g. limestone powder, fly ash and microsilica) and aggregate types (crushed and uncrushed) that are available. There was also a wide range of aggregate grading’s with, for example, the amount passing the 4 mm sieve varying between 40 and 60%. Although SCC requires uniform well-graded aggregates, the local available fine aggregates were used and the experimental trials on typical aggregate size have been carried out with excellent results.

Regarding to environmentally friendly admixtures, the IIF prepared different kind of chemical admixtures including acrylic polymers as polymerization of 2-hydroxy ethyl acrylate with styrene, butyl acrylate, and hydroxymethylacrylate, poly [acrylic-co-butylmethacrylate]; blending polymers as polyvinyl alcohol/starch and methyl cellulose/poly(acrylic acid)-polyurethane dispersion. The addition of acrylic and blend polymers improved physico-mechanical properties of cement paste and mortar mixes. However, the addition of polymer latexes into mortar containing fly ash (FA) improved most of the physic-mechanical properties of mortar at all curing times. Compressive strength and workability of FA mortar premixed with latexes are higher than those without latexes. The work was further extended to include these chemical admixtures in SCC mixes. The results showed that, while these chemical admixtures improved the compressive strength, however, the consistence (i.e. workability) of concrete was not suitable to produce SCC.
The next stage of the research was to produce chemical admixtures that are cost effective and produce SCC. This chemical admixture was molasses which is a by-product of sugar beets industry. Initially no chemical modification was made to the molasses. It was found that the addition of molasses (% by weight of cement) can produce concrete with self-compacting properties but the strength was very low (~zero) at 28 days.
Following this initial test, the molasses was modified by establishing a new and simplified process for producing a non-toxic superplasticizer from waste materials. The preparation of such a superplasticizer was carried out using a simple method involving moderate temperatures, low acidity and without the use of highly corrosive chemicals. A further novelty is to provide a process of treating wastes to produce industrially useful, valuable and economical products. The superplasticizers based on molasses (a waste material) with each of poly(itaconic acid) and poly(maleic acid) were synthesized by blending process using glacial acetic acid as crosslinking in aqueous solution. Other waste materials were also attempted (e.g. lactose) and initial findings were not encouraging and they were not included in further experimentation.
Eleven concrete mixes were used to examine the influence of new superplasticizers (i.e. modified molasses) with different of Molasses and ionic content (code: NJ and NK) on the properties of SCC containing the same amounts of cement, aggregates and water. Details of mixes are given in the other sections). The reference mix (M0) did not include superplasticizer. The water to binder ratio for all mixes was maintained constant at 0.50. A constant dosage of superplasticizer of 0.5% (with different ionic content) by mass of binder was used for all mixes.

The prepared novel superplasticizer by blend method based on molasses and ionic acid [poly(itaconic acid) and poly(acrylic acid)] [NJ and NK] respectively, improved the workability of SCC mixed. However, the consistence (workability) of concrete mixed with NJ is higher than that of mixes containing NK and the reference mix (i.e. no superplasticiser). It is important to note that the consistence of concrete containing NJ superplasticiser was higher than concrete containing commercially available superplasticizer (details may be provided upon request but due to commercial sensitivity the name was not mentioned). Higher flow is thought to be caused by the creation of entrained air in presence of novel superplasticizer. The consistence of SCC mixed with varied amount of FA as partial replacement of cement in presence of NJ and NK is higher than that of SCC mixes containing only superplasticizers, because of the spherical shape of FA particles. Our results on SCC mixing NJ and NK are in agreement with the specifications of SCC (i.e the slump flow is in the range between 500 to 800 mm). Also other criteria were satisfied, namely the filling and the passing ability of SCC mixes when using the NJ and NK chemical admixtures. Therefore, these products (NJ and NK) can be used as novel superplasticizers in SCC mixing.

As mentioned earlier that the addition of molasses without modification (M) increased the workability with hardly any strength (i.e. 0 MPa at 28 days). The modification of molasses by blending with poly(itaconic acid) NJ increased the strength of SCC mix from 0 MPa to 51.5 MPa at 28 days. Replacing cement with up to at least 30%, 60% and 90% FA in presence of NJ causes increase in strength compared to NK and the commercial superplasticiser that is available. The ultrasonic pulse velocity seems to follow a similar trend to the strength. SCC premixed with NJ exhibited lower water absorption than that of other mixes and reference SCC (M0). The novel superplasticizer, which were used in the SCC containing FA as replacement material, played important role in the specific characteristics of the SCC and allowed higher replacements of cement with FA (up 60%). The longer side chains of the superplasticizer (NJ) gave more fluidity, lower water absorption, a higher strength and hydration. Finally the best result of SCC mixes that containing NJ50.

The IIF prepared novel superplasticizer based on waste material in aqueous phase. The admixtures that are used in producing SCC are not expensive and environmentally friendly and these are the main outcome of the proposal work. As promised in the proposal that the outcomes of the research was to produce efficient sustainable construction materials (i.e. SCC) that have a positive impact on the costing, environment and society. Costing is reduced as the new novel admixtures consisted largely of waste materials (i.e. molasses); environmentally friendly and socially acceptable because it is based on waste that is normally disposed of and would reduce the noise in construction.

As will be described and listed later that the research produced a good number of journal and conference papers. At the end of each paper, there is acknowledgment to the EU under the Marie Curie action programme.