# Concrete Composites Based on Quaternary Blended Cements with a Reduced Width of Initial Microcracks
## 1 Introduction
Such modifications consist of replacing part of ordinary Portland cement (OPC) with other mineral materials [30; 31]. Substitutes for cement binder are most often additives, and in recent years also nanoadditives [32; 33]. In the simplest solutions, i.e., concretes based on binary binders, OPC is replaced by only one additional component, e.g., fly ash (FA), silica fume (SF), or metkaolin (MK) [34; 35; 36]. The benefits which have been observed from such solutions include limiting the width of the initial microcracks in concretes of this type, reduction of porosity of such materials, and improving their mechanical parameters [37; 38; 39]. In most cases, however, such effects resulted from strengthening the cement matrix structure through the development of additional C-S-H and C-A-S-H phases [40; 41; 42]. Previous studies have shown, among other things, that a particularly positive effect on the microstructure of the ITZ area between coarse aggregate and paste is the modification of the composition of the cement binder with single additives and nanoadditives, such as:  
* Siliceous fly ash [43];
* Silica fume [44];
* Metkaolin [45];
* Ground granulated blast furnace slag [46];
* Rice husk ash [47];
* Waste glass [48];
* Natural pozzolan [49];
* Limestone powder [50];* Granite powder [51];
* Nanosilica [52; 53];
* Carbon nanotubes [54];
* Graphene [55];
* Reactive powders [56];
* C-S-H seeds [57; 58]. Benefits in reducing the microcracks in the ITZ area have been also obtained when replacing mineral aggregates with recycled aggregates [59; 60; 61] or coral aggregates [62], and using sand concretes [63]. In addition, the work [64] presents the results of tests evaluating the effect of curing time on the size of microcracks in the ITZ area in concretes with the addition of FA. However, more advanced solutions in this field of material engineering rely on the substitution of cementitious binder with a combination of two or even three active Supplementary Cemertitious Materials (SCMs) [65]. Such modifications are referred to as concretes based on ternary or quaternary binders [66; 67]. As previous studies have shown, this type of concrete, in comparison with ordinary concrete, is characterized by:  
* Excellent mechanical parameters, i.e., with the results exceeding the results obtained for unmodified concrete by several dozen percent;
* Fracture toughness increased by several to several dozen percent;
* A more stable crack propagation process;
* A more homogenous structure of the cement matrix. It should also be mentioned that material modification in the form of concretes based on multi-component binders also includes pro-ecological activities. It has been proven that by reducing the consumption of OPC and the possibility of using waste materials as a substitute for the binder, such solutions cause [68; 69; 70; 71]:  
* Significant reduction of CO\({}_{2}\) emission;
* Reduction of electricity and heat energy;
* The possibility of waste management. Due to the fact that material modification of concrete by a combination of several SCMs simultaneously contributes to both the improvement of properties of the composites with cement matrix and the development of sustainable construction, this article presents the results of new experiments assessing the size of microcracks in the ITZ area of such materials. The manuscript contains analyses of:  
* The width of microcracks (\(W\)c) occurring in the ITZ area of concretes based on quaternary blended cements;
* Changes in ITZ morphology in the concretes in question. Based on the results of the experiments conducted, it will be possible to obtain information about the possibility of effective modification of concrete composites in terms of obtaining modern, innovative, and ecological materials with increased durability and increased reliability.