**1. Introduction**

Despite good thermal resistance, cement composites are subject to physical and chemical changes at high temperatures. These changes lead to the worsening of mechanical performance and structural defects. The research described in [1] and [2] has confirmed that a temperature lower than 300 ◦C does not affect the cement composites' strength. A significant strength drop is observed when the temperature reaches 400 ◦C [3], and at 800 ◦C, the strength decreases to be below 30% of the initial strength [4]. The limit temperature is 1000 ◦C, above which the cement composites lose most of their total strength [5]. The worsening of the mechanical performance of the cement composites, including concrete, at a high temperature is connected to the decomposition of hydrates, and mainly, the C-S-H phase and portlandite [6], a non-uniform thermal gradient, which causes shrinkage of the cement matrix and expansion of the coarse aggregate [7], as well as the loss of bound water, which creates voids [8].

Recent studies showed [9–12] that cement concrete damaged by fire can regain its original strength if exposed to moisture for a long time. The process of mechanical property recovery by concrete is called the after-fire hardening technique. The main reason for regaining the strength of the cement composites is the rehydration of the cement matrix dehydrated during the fire [11–13].

The tests on the cement mortars containing 1% to 5% of colloidal nano-silica (NS) have shown that NS can support the partial strength recovery of the cement composite following its heating at a high temperature.
