In other words, as it is sometimes possible "to fight fire with fire", it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with larger siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity. Beside initially lowering the pH value of the concrete pore water, the main working mechanism of silica fume addition is to consume portlandite (the reservoir of hydroxyde (OH–) in the solid phase) and to decrease the porosity of the hardened cement paste by the formation of calcium silicate hydrates (C-S-H). However, silica fume has to be very finely dispersed in the concrete mix, because agglomerated flakes of compacted silica fume can themselves also induce ASR if the dispersion process is insufficient. This can be the case in laboratory studies made on cement pastes alone in the absence of aggregates. However, most often, in large concrete batches, silica fume is sufficiently dispersed during mixing operations of fresh concrete by the presence of coarse and fine aggregates.