Improving concrete usage via testing of manufactured sand

The construction industry is facing a growing shortage of natural sand because of depleted reserves, extraction or transportation costs, or environmental restrictions. Although not an urgent issue in some countries, pressure to make the concrete industry more sustainable will increase. On the other hand, material requirements are becoming more stringent and specifications tighter, with demands for higher consistency and quality. This has led to a focus on sand manufactured from crushed rock as the most suitable replacement to natural sand.

As a leading country in the utilisation of manufactured sand (MS) in concrete, Australia has been proactive in specification development and research programs focused on increased use of MS. Many regions, especially highly developed areas such as Sydney, are at a supply/demand imbalance for natural sand. New extraction sites are not available or permitted and the costs of natural sand and associated shipping are rising.

{{image3-a:R-w:200-c:Figure 1c.}}{{image2-a:R-w:200-c:Figure 1b.}} Manufactured sands are angular products generated as a result of the crushing of hard rock. To qualify as MS in Australia, the crushed material must be further processed with common methods including specialised crushing to improve particular shape, intense washing and/or dry screening for deleterious fines removal. Several characteristics such as particle size distribution, shape and surface texture, plus deleterious fines content, must be taken into consideration while using MS in concrete.

The ability to mitigate deficiencies in MS, such as the cost of washing due to deleterious clays and performance variability, can reduce the reliance on natural sand sources. Technology from the Grace Custom Aggregate Solutions? Clarena product family is mitigating the presence of deleterious clays in aggregate materials, enabling increased MS usage in concrete mix designs. This paper focuses on clay mitigating chemical treatment of manufactured sands and its beneficial influence on a range of concrete grades with increased levels of MS.

METHODS IN CHARACTERISING DELETERIOUS CLAYS IN SANDS
All fine aggregates in Australia must comply with AS 2758.1. There have been significant efforts to determine the applicability of this standard to MS. Currently several test methods are used to characterise deleterious clays in sands (both natural and manufactured).

Sand equivalent (SE) tests described in AS 1289-3.7.1 rely on settling movement differences of individual sand fractions. However, it has been determined that there is a poor correlation of SE values and concrete performance, with some sands with a poor SE value performing equal to sands with good SE values in concrete.

{{image4-a:L-w:400-c:Figure 2. Comparison between EN933-9 and Grace RCT method.}}A second method is the clay and fine silt test, as described in AS 1141.33. Similar to the SE test, it takes into account the sand sample?s settlement properties and measures the volumetric ratio of clay and silt compared with sand-sized particles. It has been noted that a comparison between different sources of MS or accounting for variability within a given source with this method is difficult. Both methods can be time-consuming; the settlement period is 20 minutes for an SE test and three hours for a clay and silt test.

The last method is the methylene blue (MB) test. There are several variations of MB in the literature (eg AASHTO T330, EN 933, ISSA 145, NZS 4407-3.5). Most MB methods measure the end point of the titration by visual detection of a blue halo on filter paper after a titration process. The drawbacks of this test are that the halo can be subject to operator error, the titration process can be laborious and only the minus 75 micron fraction is typically used to determine the clay content.

A Grace improvement on current MB tests has been a colorimeter, a device that measures the absorbance of a given solution at a specific light wavelength. The colorimeter provides portability and ease for use in the field. Instead of a titration process, the MB content is correlated to the light absorbed at a specific wavelength, thereby removing the visual interpretation of the blue halo. Another benefit is that the entire sand sample can be used; screening is not necessary to separate the minus 75 micron fraction. The entire process takes 10 minutes, providing a quick and reliable MB value (MBV) that can be measured easily in the field as well as in the laboratory.

{{image5-a:c-w:620-c:Figure 3. Schematic of CMC mitigation of clay particle.}} An illustration of the test is shown in Figure 1. A sample of fine aggregate is weighed and a known volume of MB is added. The slurry is mixed thoroughly and the solution is filtered to remove any particles. The change in the concentration of MB is then determined with the colorimeter.

In comparison with standard MB tests, the improved MB test provides an excellent correlation, as shown in Figure 2. In this figure, the results for 40 sand samples taken from around the world are shown. Each clay, depending on its expansiveness, surface area and layer charge, consumes a different amount of MB dye, so the clay content can be expressed in terms of ?absorptive capacity?, which can be normalised to one specific type of clay: per cent Na-Montmorillonite (Na-Meq).

CHEMICAL TREATMENT OF MS
Deleterious clays can be harmful to concrete mixes by absorbing large amounts of water and admixtures. This can lead to poor workability, reduced strength due to extra water and increased plastic, longer term drying shrinkage or high cementitious content. A new clay mitigating chemical (CMC) has recently been developed that reacts with clay particles, thereby reducing excessive absorption of water and/or admixtures, as depicted in Figure 3.

Deleterious clays involve platelets of tetrahedral silicate and octahedral aluminate layers with different cations between the platelets. When wet, water can be absorbed between the platelets, which swells the clay. The CMC acts to absorb onto the surface and between the layers to minimise water being lost to the clays.

{{image6-a:L-w:620-c:Figure 4. General effects of per cent Na-M values on concrete performance.}}
From the authors? experience, MS with an Na-Meq content below 0.4 per cent exhibit little detrimental effects due to clay. Between 0.4 per cent and 0.8 per cent, effects become moderate; at greater than 0.8 per cent, the manufactured sands become very difficult to use, as shown in Figure 4. This guideline is dependent on mix design parameters and actual impact on concrete can vary.

CASE STUDIES ? TEST RESULTS
A series of concrete tests was conducted in a laboratory with raw materials from seven Australian quarries to see what benefits could be obtained using chemically treated MS in concrete. The mix designs varied in strength grade (20MPa to 50MPa) and the percentage of natural sand substitution with MS (from 25 per cent to 80 per cent).

The study?s first part was focused on determining how the use of chemical treatment influences concrete?s workability. For materials from one of the quarries, the mixes for various strength grades were done by switching untreated MS with a treated version at two doses of 1.5kg and 2kg per tonne of MS.