## 4 Hydrotalcites as a Part of the Building Materials
### Hydrolcalcites in Cements and Mortars  
An interesting application of hydrolcalcites is in isolation of dangerous heavy metals in cement prepared from industrial waste. Hydrolcalcites can be used for double barrier technique effect in cement-based mortars prepared for the immobilization of lead contained in electric arc furnace dust. Double barrier mortars with dimercaptosuccinate in the interlayer of the hydrolcalcite reduced lead release by approximately 50% in comparison with the conventional immobilization mortar [101].  
Hydrolcalcites often form in mortars and cements, where supplementary cementitious materials are used as a substitute for cement clinker in order to reduce the emissions of cement production. This replacement was studied recently by using magnesium oxide [57], metakaolin [102], or dolomite [103]. The presence of alumina from a raw aluminosilicate may improve the mechanical behavior of resulting mortars. It may interact with hydrated magnesium oxide and form a hydrolcalcite-like phase [58], while effectively improving the mechanical properties by an enhanced bond with the cementitious matrix [59]. In addition, a material containing hydrate water molecules can decrease the materials' overall porosity [60].  
Hydration kinetics can be also modified using hydrolcalcites, since it is believed, that the addition of nanoparticles lowers the energy barrier of liquid-state precipitation reactions. Hydrolcalcites also provide additional nucleation sites, which are essential for the cement hydration reactions [104; 105].  
The presence of hydrolcalcites in concrete often affects the values of the compressive strength [106]. Many research studies show an increase of compressive strength, varying from units of percents to tens of percents [107; 108]. [PERSON] et al. referred to a 46,2% increase in compressive strength, which was attained by the addition of 1% LiAl layered double hydroxide [109]. However, it has been also reported that the presence of hydrolcalcites leads to opposite effects [101]. [PERSON] et al. used a 10% addition of a MgAl NO\({}_{2}\) layered double hydroxide, which led to the decrease of compressive strength by 14,2% and the flexural strength by 19,1% [110]. [PERSON] et al. observed a 25% compressive strength decrease after the addition of 1-3% of MgAl layered double hydroxide [111]. The differences between the positive and negative influence on the compressive strength depend on the microstructure of both the hydrotalcites used and the concrete. The usage of inappropriate hydrotalcites may cause the increase of porosity, which can also lead to faster ingress of chlorides or other undesired substances [111]. This porosity is also growing with the amount of hydrotalcites added to the concrete [112]. Additionally, the presence of hydrotalcites can lead to an increase in the compressive strength [106].  
Hydrotalcites were also studied as additives in slag cements, where their presence allows to control kinetics and formation of the C-A-S-H phase [113] and also helps binding chlorides [114]. Another approach to hydrotalcites in cementitious materials was shown in the study of [PERSON] et al., where the hydrotalcites were used as hosts for intercalation of organic admixtures in nanocomposites [115].  
Hydrotalcites are also present in non-cement binders, such as in ground granulated blast furnace slags, where they are beside the C-S-H and Ca(OH)\({}_{2}\) phase part of the reaction products, which define the strength of the material [116].