GEOPOLYMERS WITH GREEN LIQUOR DREGS : An investigation of the possibility tomanufacture a geopolymer based on residual streams
Abstract: The pulp and paper industry make up a large part of the Swedish industry and is alsogrowing worldwide. With its growth, the amounts of residuals that are produced alsoincrease. The estimated annual global amount of residuals generated from pulp millsexceeds 1 million tons. The residuals include fly ash (FA) and green liquor dregs(GLD), which can cause harm to the environment as well as to the human health ifnot taken care of properly. Therefore, new, sustainable uses for these residual streamsare in strong need to be found. The construction sector is one of the most energy-intensive and CO2-emitting sectorssince ordinary Portland cement (OPC) is one of the most manufactured materials inthe world and causes large amounts of CO2 emissions when produced. Research hasfocused on reducing the CO2 generated by OPC. One approach is to include FA andGLD in a so-called geopolymer, which is a cementitious material formed when aninorganic material rich in aluminium (Al) and silicon (Si) reacts with an alkalineactivator such as sodium hydroxide (NaOH). A strong geopolymer including FA andGLD would not only create a use for these residuals, but it would also be a lessenergy craving alternative to concrete. Using FA and GLD from the Metsä Board pulp mill in Husum in various proportions,this study aimed for creating a geopolymer that is suitable as a construction material.The composition of the geopolymer blends differed depending on the proportions ofGLD and FA added, as well as regarding the water contents of the blends, and thequantity of alkaline activator solution added. The compositions also varied regardingthe addition of kaolin, an additional aluminosilicate source. Lignosulfonate, a ligninbasedproduct from the sulfite pulping industry was also evaluated as an additive dueto its water-reducing properties when used in concrete. The geopolymers wereevaluated in terms of blend workability and by uniaxial compressive strength (UCS)tests after 7 and 28 days of curing. The strongest geopolymer, in which GLD constituted 20 wt.% of the dry components(sand and alkaline chemicals excluded), endured a pressure of 2.3 MPa after 28 daysof curing. Increasing the water content made the geopolymer blend more workable,but also resulted in a UCS decrease of the geopolymer. Addition of cement to themixture and an increased quantity of alkaline activator solution both resulted in alower UCS as well. Compared to cement mortar (20 MPa at the 7th curing day) andliterature values of other geopolymers, the strengths of the manufacturedgeopolymers were low overall (0.4–1.4 MPa at the 7th curing day). One reason for thelow UCS could be the use of kaolin instead of a more reactive aluminosilicate source.Moreover, the FA showed to have low Si and Al contents, which can affect thegeopolymer strength. Further investigations are needed to develop a strongergeopolymer.
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