Role of calcium hydroxide in the alkaline activation of coal fly ash

Aiming at reducing the environmental effects generated by the manufacturing of Portland cement, the construction sector perceives significant potential in alkali-activated cements. The use of calcium hydroxide becomes an alternative to modify the properties of these types of cements in a controlled manner. This research work examines the influence of calcium hydroxide at different concentrations on the alkaline activation of a coal fly ash with sodium hydroxide. The cements pastes were evaluated according to their compressive strength at 3 and 28 days, and once the curing process had finished at the initial conditions (3 days, >95% RH at 45 °C), they were characterized using the XRD, FTIR, TG, MAS NMR and BSEM/EDX techniques. The presence of calcium hydroxide increased the compressive strength of the pastes at 28 days from 4.5 MPa to 25.65 MPa. Likewise, the maximum resistance growth rate was 41.03 %. The highest percentages of calcium hydroxide fixation were 74.82%, which modified the type and quantity of cementing gels. The calcium hydroxide forms a layer on the fly ash particles and these, when in contact with the activator, produce absorption and desorption processes of network modifying elements (Na and Ca) and network formers (Al and Si). Finally, the confluence of the chemical constituents (Na, Ca, Al and Si) initiates the formation of cementing gels type C−(A)−S–H and (N,C)−A−S–H. [Display omitted] •During alkaline activation, the fly ash absorbs OH-, Na+ and CaOH+ ions and desorbs aluminium and silicon ions.•CH helps the formation of amorphous phases such as (N,C)-A-S-H and C- (A)-S-H gels, and zeolite-type crystalline phases.•Initially, the C-(A)-S-H gel is formed on the fly ash’ surface, where the formation of the (N,C)-A-S-H gel begins.•An increase in the amount of CH fixed densifies the mixtures, improving their porosity and mechanical resistance.