Mortars with crystalline additive in aggressive environments

The paper describes how a crystalline additive influences the properties of cement- based composites. The investigation was focused on the degradation of mortars with crystalline additive exposed to selected aggressive environments. The specimens were tested for compressive and flexural strength. The influence on the microstructure is also discussed (high pressure mercury intrusion porosimetry, SEM, DTA). The results confirmed the positive influence of a crystalline additive on the values of compressive strength after exposure to all aggressive environments.


Introduction
Nowadays, concrete is the most widely used construction material, and therefore improving its properties remains an important issue [1,2,3].Cement composites usually consist of cement, water, aggregates, admixtures, additives and air. The binder is a continuous phase in the cement composite, thus the fact that it is porous is critical in terms of the transport of water and chemical substances in or out of the concrete [4]. One of the progressive technologies for cement based composites protection is the use of coatings, sprays or additives on so-called "secondary crystallization bases" also known as "crystalline technology" base [3]. The dry crystalline products are powder compounds that consist of Portland cement, quartz sand, and a compound of active chemicals [3,5].
In principle, crystalline materials work so that the chemical components react with the cementitious matrix during hydration, temporarily forming Ca(OH)2 followed by the formation of disilicate and polysilicate anions. Available knowledge suggests that the cumulated process is accompanied by the formation of 3CaO·2SiO2·3H2O together with the production of 4CaO·Al2O3·13H2O [5].
Laboratory study [2] has shown, using X-ray fluorescence spectrometry (XRF) analysis, the resulting needle-like crystals likely comprise calcium or silicon [2]. The fine insoluble crystals seal the pore structure [6]. Authors [7] described a general process, where a general crystalline promoter MxRx reacts with tricalcium silicates and water to produce modified calcium silicate hydrates and a poreblocking precipitate MxCaRx -(H2O)x by the following equation (1): Similar process may exist involving the calcium aluminates. The resulting crystalline deposits become integrally bound with the hydrated cement paste and become a permanent part of the cementbased composites [7].
The objectives of the work was to study the degradation of mortars with crystalline additive exposed to selected aggressive environments.

Experiment
The experiment was focused on evaluating the influence of crystalline additive on physicalmechanical properties and microstructure of mortars containing a crystalline additive and polypropylene fibres. In order to observe the degradation of the cementitious composite, specimens were exposed to an aggressive environment.

Materials
This study used Portland cement CEM I 42.5 R as binder, crystalline additive, quartz sand and polypropylene fibres. The chemical composition of the crystalline additive (CA) is shown in table 1. CA was added in an amount of 0.5 and 1.5 % of cement weight, see table 2.       Figure 3 shows the influence of the crystalline additive on the porosity of mortars containing polypropylene fibres stored in environment A and C. The data shows that the greatest reduction in porosity as a result of CA addition was in specimen C_FCA1.5, which contained 1.5 % of CA and was exposed to an ammonium chloride solution.

Conclusion
The tests confirmed the positive influence of a crystalline additive on the values of compressive strength after exposure to all aggressive environments. Mortars which contained polypropylene fibres also saw an increase in flexural strength. The addition of polypropylene fibres can negatively influence the porosity of hardened mortars, while a higher CA content, i.e. 1.5 %, can bring about a reduction in porosity in mortars exposed to aggressive environments. The research results confirm that CA has an influence on the increase of the amount of portlandite. SEM sampling revealed differences in the microstructure of mortars both with and without crystalline additives after exposure to aggressive environments.