Development of Shrinkage/Crack-Controlled Alkali-Activated-Oil-Fibre Repair Mortars for Composite Applications

One of the main weaknesses of alkali-activated materials is their higher drying shrinkage behavior compared to Portland cement-based construction materials. This limits their applications as repair materials, especially in composite action with OPC. In this study, an innovative approach to develop sprayed AAM exhibiting low drying shrinkage and no crack formation is presented. In order to develop proper spray formulation, fresh metakaolin-slag AAM is mixed with vegetable oil and reinforced with hemp and basalt fibers. The impact of oil and different fibers on various properties of the AAM composite is investigated experimentally: (i) workability, (ii) mechanical (compressive strength), (iii) porosity (low temperature nitrogen adsorption), (iv) drying shrinkage and (v) the crack formation due to constrained shrinking (analyzed on cylindrical disc specimens). Addition of oil does not degrade the properties of fresh mortars, but shows a beneficial effect on the shrinkage behavior by significantly reducing the porosity; especially of gel pores and small capillary pores below 20 nm of pore sizes. This led to reduced drying shrinkage rates by 25–30%. The addition of hemp and basalt fibers lead to changes in workability of the fresh mixtures, and a reduction in crack formation after 7 days of curing. By the combination of oil, fibers and air-entrainment agents the drying shrinkage related crack formation allowed the crack-free application of thin AAM layers on concrete substrates.