Chemical Treatments for Coffee Husks: Application in Mortar for Coating and Laying Blocks

(1) The use of agro-industrial wastes, such as coffee husks, in cementitious materials is a viable way to achieve the sustainable development of new construction materials. Traditionally, these wastes are applied after calcination, spending energy and financial resources. Furthermore, the calcination of these wastes contributes to a more sustainable environment. This is interesting because calcination treatments are usually more expensive than those using chemical treatments. Thus, the objective of this research is to evaluate the possibility of using coffee husks with the use of chemical treatments with sodium and potassium hydroxide in mortars for coating and laying blocks. (2) The methodology of this article consists of producing test specimens of mortar for coating and laying blocks in the composition 1:1:6:1.55 (cement: hydrated lime: sand: water), using coffee husks that will be incorporated in quantities of 2.5% in relation to the mass of the cement in its natural state and after treatment with sodium and potassium hydroxide. Compositions containing calcined coffee husks are evaluated for comparison purposes. The evaluated parameters were the mass density in the hardened state, water absorption by immersion and compressive strength. To complement these analyses, XRD, SEM and calorimetry tests were executed to compare the proposed treatments. (3) The results indicate that the use of treatments with NaOH and KOH allows for the use of coffee husks in mortars, since the parameters obtained were compatible with this type of application. The use of natural coffee husks is not possible due to their low compressive strength and high water absorption, caused by porosity. The use of alkaline treatments, especially with KOH, does not affect the hydration of the cement, allowing for the obtention of values statistically equivalent to the reference composition. The results with calcined coffee husks are even more promising due to the high content of amorphous potassium, which promotes the conversion of ettringite into hydrous calcium aluminate monosulfate. (4) It is concluded that, for the application of coffee husks in mortars for coating and laying blocks, the chemical treatment with KOH meets the necessary parameters—for example, a compressive strength greater than 2 MPa, making sustainable building materials and promoting the use of agro-industrial waste in building materials.

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