Proposed Specification for Powdered Glass as a Component in Cement Combinations

A research project carried out by the Concrete Technology Unit at the University of Dundee, has developed specifications [1] and specification clauses [2] geared towards assisting engineers in the use of powdered glass as a fine aggregate or filler for use in concrete.

Overview of proposed specification

Requirements for powdered glass relate to fineness, initial setting time, soundness, chloride content, alkali content and moisture content.

The requirements for soundness and chloride content are identical to those given for ground granulated blast furnace slag in BS 6699, since these are essentially general requirements for acceptable volume stability and durability.

A minimum fineness requirement is defined in terms of a minimum specific surface area determined using the Blaine air permeability method. The limit is based on strength development measurements made on mortars containing glass cullet of different fineness [3] and was selected on the grounds that mortar strengths above this fineness were substantially higher than for mortars containing coarser cullet.

Initial setting time (measured in accordance with BS EN 196-3 on a paste containing 30% powdered glass by mass to 70% CEM I- 42,5 cement) is limited to 30 minutes after the initial setting time of a paste made exclusively with the same CEM I-42,5 cement. This limit is based on measurements carried out with both washed and unwashed samples of powdered glass cullet, which indicated a 25-minute delay in setting time for the unwashed material. The setting time requirement is therefore set to limit the presence of residues on cullet which could delay setting time further. Direct measurement of such residues (using techniques such as measurement of loss-on-ignition) has been found to be impractical.

Alkali content is limited to reduce the risk of alkali release leading to exacerbated expansive alkali-silica reaction (ASR). The basis of the limit is a series of ASR tests carried out in accordance with BS 812-123 using concrete mixes containing aggregate combinations known to be highly reactive, and powdered cullet as a cement component [3]. The tests were carried out to establish whether the sodium which is present in glass cullet is likely to aggravate ASR.

The study found that harmful ASR was not produced by the presence of powdered cullet, and that it seemed unlikely that any sodium released by the glass was having a detrimental effect. The limit therefore reflects these findings in that it:

(a) uses an alkali extraction method that will only extract readily available alkalis (BS BS EN 196-21:1992, clause NA.5.1); and

(b) sets a limit fractionally above what is typically obtained for a washed cullet sample, thus effectively discounting the presence of sodium in the cullet, whilst limiting the levels of sodium present as residues on the glass.

NOTE: The ASR testing programme carried out as part of the study has been extended to at least two years (as opposed to the minimum requirement of 1 year). The above approach to alkali contribution should be viewed as being tentative until the ASR testing programme is complete.

A requirement for a maximum moisture content of 1.0% by mass of dry cullet is also included in the standard. This is a value based on requirements for other similar powders and laboratory tests that showed glass cullet would still flow with 1.0% water.

As for aggregate, sulphate content is not covered due to the low concentrations typically encountered in cullet.

Overview of specification clauses

Project specifications which are to include the option of using glass cullet in concrete should be based on BS EN 206-1 and BS 8500. Additional specification clauses have also been developed specifically for glass [2].

Two additional clauses are provided. The first of these clauses states that the material must conform to Specification for powdered glass as a component in cement combinations [1].

Additionally it is required that powdered glass cullet should be used in combination only with CEM I cement of at least standard strength class 42,5. This requirement is necessary because all research has, to date, concentrated exclusively on such combinations. The clause requires that the proportion of glass cullet used in such a combination be at least 6% and not more than 20% by mass. This range is selected on the grounds that use at quantities less than 6% would constitute use as a minor additional constituent (as defined by BS EN 197-1), and beyond 20% a decline in performance is observed in mortars [4].

Availability

Most collectors and reprocessors of glass cullet are able to process cullet to the requirements of the specifications discussed above. A list of UK collectors and reprocessors is held and updated on the Letsrecycle website.

References

  1. Concrete Technology Unit, Specification for powdered glass as a component in cement combinations, University of Dundee, Dundee, DD1 4HN, 2003
  2. Concrete Technology Unit, Specification clauses for the use of powdered glass as a cement combination, University of Dundee, Dundee, DD1 4HN, 2003
  3. Dhir, R.K., and Dyer, T.D., Realising a high-value sustainable recycling solution to the glass cullet surplus, Final Report for DETR Research Contract No. GW-12.10-108), 2003
  4. Dyer, T.D., and Dhir, R.K., Chemical reactions of glass cullet used as a cement component, ASCE Journal of Materials in Civil Engineering, 2001, pp412-417

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