Influence of different artificial additives on Portland cement hydration and hardening

•Tribochemically activated biomass fly ash (AFA) does not have a pozzolanic activity and acts as an inert microfiller.•Synthesized clay–silica gel additive (CSG) is a pozzolanic material.•Both AFA and CSG additives have a positive effect on the compressive strength of cement samples. This study investigated two artificial materials—biomass fly ash (AFA) and a new synthesized composite from clay and silica gel waste (CSG)—as additives for Portland cement. AFA has very weak pozzolanic properties and acts as an inert microfiller, whereas CSG is a pozzolana. It was determined that up to 15wt.% of ordinary Portland cement (OPC) can be replaced with either additive without impairing the strength properties of the samples. The positive effect of AFA additive on the strength properties of OPC is associated with significant acceleration of the calcium silicate hydration process, whereas this effect for the CSG additive is related to the formation of stratlingite (C2ASH8), hydrogarnet (C3ASxH6-2x), and an additional amount of calcium silicate hydrates.