Impact of Water Quality on Concrete Mix and Hardened Concrete Parameters

Abstract The subject of the present article is the evaluation of the use of different water types in the production of concrete mix and C20/25 class concrete (assuming the same composition). Taking as an example a selected Subcarpathia-based concrete production plant, equipped with a process water management system, the research analysed the quantity-quality parameters of drinking water, sewage water, and groundwater and evaluated them for their accordance with mixing-water quality standards. It should be emphasised that the majority of specifications recommend the use of drinking water for concrete production. The paper presents the results of research which analysed the impact of water quality on selected properties of concrete mix and concrete (consistency; compressive strength after 7, 14, and 28 days; density). The results obtained confirmed the findings of the research on the suitability of recycled water for concrete production.


INTRODUCTION
Due to its major impact on the environment, the building industry is closely linked to the notion of sustainable development. The building industry:  uses over 40 per cent of global energy production,  is the cause of about 35 per cent of global greenhouse gas emissions,  accounts for about 50 per cent of the total mass of processed materials.
Alongside production and sales, the protection of natural resources is currently one of the most important goals of a socially responsible company. Concrete production plants have long been implementing solutions and technologies that do not cause harm to our planet's natural resources. Production plants establish objectives that aim to minimise the environmental impact of their business activities. One such task is to reduce the use of drinking water for production purposes and implementing a special water-saving system is one of the steps to achieving the set objectives. For a concrete production plant, however, this remains difficult to achieve as there are specific requirements that determine the quantity of water necessary for the concrete binding process. Therefore, excessive water saving could lead to deterioration of the product's compressive strength. This is why concrete production plants aim not to minimise the use of water but rather to obtain it from a source other than the water supply system. The first alternative is to modify the production process by replacing drinking water with groundwater; another solution is to use sewage water.
Water has an important role in the production of concretes and mortars (as mixing water) and in cement binding processes by helping to obtain the appropriate consistency of concrete or mortar; it is also essential for the curing of concrete or mortar in the process of hardening [17][18][19]. During the technological process, cement and aggregate quality control is regularly carried out, whereas water quality control remains neglected.

SEWAGE WATER IN THE PRODUCTION OF READY-MIXED CONCRETE
Sewage is the main source of waste generated by concrete production plants. It is generated as a result of the cleaning of machines used for concrete mix production and transport and during the disposal of concrete mix residues. One might, therefore, consider its further use in production as a substitute for drinking water [20,21]. However, a major problem is posed by the quality of untreated water, which is polluted with a number of chemical compounds [22]. The pollution, including the presence of sulphates, phosphates, zinc, lead, and suspended solids, coupled with high alkalinity, makes it impossible for untreated water to be disposed of into the municipal sewage system. This problem can be resolved by building an in-house sewage treatment unit, but not without generating high investment costs [21]. A more cost-effective solution, implemented in readymixed concrete production plants, is to reuse the recycled, post-production water as mixing water [3,8,20,21,[23][24][25][26].
There are numerous technological solutions for the recycling of production sewage which consist of the separation of washings and their treatment for reuse in concrete production. Particular recycling systems differ mainly in the construction of these separators.

INDIVIDUAL INVESTIGATIONS
The aim of the research was to apply different water types in the production of concrete mixes and concrete. In the first stage of the research, groundwater and sewage water were evaluated in terms of compliance with PN-EN 1008 [18,19]. This phase consisted of a preliminary (organoleptic) and a detailed assessment. The detailed water assessment investigations involved an extensive chemical analysis whose results are presented in Table 1. Groundwater and sewage water meet the required standards, yet the results for sewage water significantly differ from those for drinking water and groundwater. Most sewage water properties appear on the low side of allowable values as set out in PN-EN 1008 [19]. Sewage water density amounts to 1.028g/cm 3 , exceeding the value of 1.001 g/cm 3 . Thus, in accordance with  Table 2 and Fig. 2. The concrete slump test is an on-the-spot test to determine the consistency as well as fluidity of fresh concrete. This test plays a vital role in ensuring immediate concrete fluidity in a construction project. According to the PN-EN 206: 2014 Concrete -specification, performance, production and conformity standard, workability for all fresh concrete mix is classified as S3 category (table 3).  Table 4 shows densities for concrete with all types of water.

DISCUSSION
All results from the laboratory tests of the obtained concretes and concrete mixes are of lower values than those set out in PN-EN 1008. Having measured the impact of sewage water on the compressive strength of concrete after 7, 14, and 28 days, it was established that compressive strength lowered by 29.8 per cent, 25 per cent, and 33.4 per cent, respectively when compared to the concretes made with drinking water (Fig. 2). The results of the compressive strength test meet the criteria for the assumed compressive strength class C25/30. It should be emphasised that the sewage water used in the research was sampled in the concrete production plant on a regular production day. For concretes made with groundwater, differences in compressive strength amounted to 2-8 per cent, remaining within the limits of the typical variability of concrete properties.

CONCLUSIONS
The sewage water used in the research proved to have an impact on the quality properties and parameters of both concrete mix and hardened concrete with the sewage water parameters appearing on the low side of allowable values. Following a water quantity adjustment resulting from the presence of suspended solids in the sewage water, no significant impact of sewage water on concrete mix consistency was found. Consistency was evaluated as S3.
On the other hand, it was found that sewage water has a major impact on the compressive strength of concrete, which impact is clearly noticeable after 7, 14, and 28 days, and the values obtained are about 1/3 lower than the values obtained for concretes made with drinking water. This is probably the result of higher porosity; an increase in aeration under the effect of sewage water considerably affects the compressive strength of hardened concrete. Groundwater has a significantly lower impact on the decrease in compressive strength, with the difference not exceeding 9 per cent. Thus, concretes made with tap water/drinking water and groundwater reach comparable levels of compressive strength.
The presented results may constitute the basis for further research that would include an evaluation of the changes in concrete workability over time, the evaluation of the impact of recycled water on concrete durability, and the evaluation of concrete and concrete mix physico-mechanical properties such as porosity. Today, as both global and regional drinking water resources become scarcer, this type of research gains in importance since it allows us to minimise the use of drinking water for technological and industrial purposes and, therefore, contributes to countering water deficiency. It is certainly in line with the longterm strategy for sustainable natural resource management.