Performance of alkali-activated slag concrete
The environmental concerns related to the production of cement in terms of the energy consumption and the emission of CO2 lead to the search for more environmentally viable alternatives to cement. One of those alternative materials is alkali-activated slag (AAS) where ground granulated blast furnace slag is used not as a partial replacement to cement but as the sole binder in the production of concrete. The performance of alkali-activated slag concrete with sodium silicate (water glass) as an activator was studied. The scope of the work covered seven mixes: a normal strength OPC control mix, a blended OPC/Slag mix of similar compressive strength but of lower water to binder ratio, a second OPC control mix of a water to binder ratio similar to that of the OPC/Slag mix, and four alkali-activated slag mixes of the same binder content and the same water to binder ratio as those of the second OPC mix. The AAS mixes were prepared with slag as the sole binder, activated with water glass at two dosages, 4% and 6% Na2O (by weight of slag). Two types of water glass were used, one in a solution form and the other in a solid granules form. The two forms of the activator used were also of different silicate modulus (Ms); 1.65 for the solution form and 1.0 for the granule form. Different curing regimes were used including normal water curing, air dry curing and accelerated autoclave heat curing. The fresh concrete properties studied were setting time, workability and air content. The engineering properties studied were compressive strength, splitting tensile strength, flexural strength, dynamic modulus of elasticity and ultrasonic pulse velocity and drying shrinkage. The durability potential of alkali-activated stag concrete was investigated by testing for oxygen permeability, chloride penetration resistance, porosity, carbonation, and alkali-silica reaction. The hydration of alkali-activated slag was studied using x-ray diffraction and thermogravimetry techniques. Alkali-activated slag concrete was found to achieve good workability which was, comparable to that of OPC and OPCfslag concrete. The increase of the Na2O dosage resulted in a lower workability and the activator with higher silicate modulus exhibited lower workability. AAS concrete however, sets rapidly if not controlled by the addition of lime. The main hydration products in the AAS systems were C-S-H (I) and hydrotalcite as observed in the XRD patterns with autoclaving resulting in the formation of a more crystalline C-S-H gel and the possible formation of xonotlite. The mechanical properties of AAS concrete are highly influenced by the activator's silicate modulus and the Na2O dosage where strength was found to be higher with the higher modulus and dosage. The AAS concrete is very sensitive to curing and dry curing resulted in a reduction in strength for AAS concrete much more than that for OPC concrete. Accelerated curing (autoclave) increased the initial gain of strength in AAS concrete but eventually gave results close to those of water curing. Using a waterglass activator with Ms = 1.65 and 6% Na2O resulted in the highest drying shrinkage where as it is lower when the dosage is less and the modulus is lower. Autoclave curing of AAS concrete reduces the drying shrinkage as it causes the formation of more crystalline products of hydration. The increase of the Na2O dosage in AAS concrete, where the activator has an M. = 1.0, results in a decrease in porosity, but in the case of the AAS concrete, with the activator having Ms = 1.65, the porosity increases with the increase of the Na20 dosage. Dry curing increases the porosity of all the concrete mixes. The porosity test results are influenced by the sample preconditioning prior to testing. The alkali-silica test results show that replacing 60% OPC by slag reduces the expansion of concrete prisms containing reactive aggregates. They also indicate that AAS concrete has low susceptibility to ASR expansion because of stronger binding of alkalis in the hydration products. The carbonation test results show that OPCIslag concrete undergoes higher carbonation than OPC concrete with the same w/c ratio. AAS concrete with low compressive strength around 40 MPa has higher carbonation compared to OPC concrete of the same grade while the carbonation is lower with higher strength.