Can We Really Measure Cement Content In Hardened Concrete and Mortar?

We are regularly asked to determine the amount of cement in hardened concrete and mortar. The request is normally made for one of two reasons; the most common being that something has gone wrong and the cause and/or blame for the problem is thought to be related to cement content. The other is that an older structure is being repaired or expanded and it is desired to match the existing materials.There are ASTM standards describing how to do such determinations, but they are based on a number of assumptions that, in some cases, are not valid. This document seeks to describe what is actually measured, the source of potential errors and how big they may be, and what can be done to improve the probability of reaching an answer close to the truth. This is done by describing what mortar and concrete are made of, what can be measured, what assumptions are made, and how the whole puzzle is solved.

By Peter C. Taylor
Contributing Editor
www.concrete.com

Can we really measure cement content in hardened concrete and mortar? We can measure the raw materials in hardened concrete and mortar, but these data do not necessarily give enough information to allow us to state the cement content without some assumptions and qualifications.

WHAT IS IN CEMENT, MORTAR AND CONCRETE?

We start by describing the raw materials that go into mortar and concrete and by defining some terms. Cement is a generic term meaning “glue.” Portland cement is a gray powder that when mixed with water forms a paste that hardens and gains strength with time. This is the glue that holds mortar and concrete together. When sand or fine aggregate is added to paste the mixture is known as mortar which is suitable for thin cross sections. Grouts, plasters and stuccos are generally special mortars and contain much the same raw materials. Stone added to mortar makes concrete which can be used in structural or massive applications.Cement

The cement most often used in construction is known as portland cement. There are other types of construction cements, some used in masonry construction and other special cements used for repairs or high temperature applications. This paper addresses portland cement and its derivatives only.The predominant chemical compounds in portland cement are based upon oxides of calcium (lime), silicon (silica), aluminum (alumina) and iron. There are other compounds present in smaller quantities such as magnesia and carbon dioxide and a number of trace elements. The principal chemical compounds that combine with water (hydrate) to provide strength are calcium silicates. However, in all reported chemical analyses, the constituents of cement and concrete are reported simply as the appropriate oxides. The way in which these compounds combine is extremely complex and outside the scope of this paper. Modern portland cements, by definition, all tend to contain these compounds in a fairly tight range of values even if they come from different manufacturing facilities. Hydrated portland cement has the unusual, and desirable, property that it will continue to gain strength (albeit at a decreasing rate) when in the presence of water. This complicates chemical analysis because the system is continually changing from the time of first mixing to the time of test.

A source of further complication is when historic materials are being tested because the composition and fineness of cement made in 1920 is not the same as that made in 2000. Masonry cements are normally a blend of portland cement, crushed limestone and some polymeric additives. The manufacturers do not publicize the relative amounts of portland cement and limestone but ASTM standards do set out ranges into which the blends should fall. It is these blends that tend to cause the most complicated analyses and the broadest range of assumptions in the method.

Aggregates

The aggregates used in mortar and concrete are built from the same building blocks: lime, silica, alumina and iron oxide. Some aggregates can be physically separated from hydrated portland cement by their differing solubility in acid. Aggregates tend to fall into two very broad categories, those containing mainly silica and those containing mainly calcium and magnesia. Siliceous aggregates are generally insoluble in acid, but not always, and this is the source of one important assumption made by ASTM C 1084. Calcareous aggregates are soluble in acid, but generally do not contain soluble silica - another assumption.Supplementary Cementing Materials

Other materials coming into the market are the so-called supplementary cementing materials such as fly ash and slag. These are often waste materials that contain similar compounds as portland cement, albeit in differing proportions. By virtue of their chemistry, glassy state and fineness they will react beneficially with portland cement. They are either added to concrete to reduce costs, or to enhance properties. It is difficult to distinguish between these materials and cement in a hardened concrete. The range of their chemical compositions is large, further complicating the interpretation of chemical analysis.Unhydrated particles of fly ash and slag can be observed using microscopical techniques, and an experienced analyst can estimate the volume of residual fly ash and fly ash present. The presence of slag can also be qualitatively indicated by testing for the presence of sulfides. The extremely small size of silica fume particles (another supplementary cementing material,) and the low dosage normally added makes definitive detection of this material difficult.

Chemical Admixtures

Chemicals, generally in liquid form, are often added to cementing materials in order to modify or enhance the properties of the plastic or hardened concrete. They are generally added in very small doses and their presence does not usually interfere with cement content determination.WHAT DO WE MEASURE?

It is not sufficient to just measure the chemical composition of the hardened material to determine cement content because all the constituents of hardened concrete contain the same chemical elements. This section describes what other means can be used to complement the chemical analysis.Chemistry

The basic procedure is to take a representative sample of the mortar or concrete, crush it to a fine powder, dissolve it in acid and then use standard chemical analytical techniques to measure the relative proportions of calcium, silica, alumina, magnesia. The amount of insoluble residue is also determined and assumed to be aggregate. A portion of the sample is also heated to 1000°C and the loss in mass measured at certain temperatures. These losses represent different phases of the material (including water and carbon dioxide) breaking down into gas and leaving the sample. The original unit weight of the sample is also a useful parameter that is regularly determined. The reliability of these analyses is strongly influenced by the sampling techniques used. The size and number of pieces of mortar or concrete taken from the structure have to be sufficient to represent the concrete being tested. The ASTM methods specify minimum sample sizes, but it is not uncommon to receive much smaller samples from the field. These analyses are often the ones that cause problems.Petrography

Petrographic (microscopical) analysis of the sample is invaluable in addressing a number of questions:

• What type of cement has been used?
• Does the sample contain fly ash, slag, ground limestone or other mineral admixtures, and if so, approximately how much?
• What is the aggregate type and is it possibly soluble in acid?
• What is the water - cement ratio?
• What is the extent of hydration?
• What is the condition of the sample?
• Are there deposits or contaminants?
• Has leaching removed constituents?

Not all of these questions can always be answered, and often the answers are given as ranges of values, all of which have to be built into the final interpretation. Microscopical point-count methods can be useful in determining the presence and amount of fly ash, but this approach requires refinement.ASTM C 1084 - STANDARD TEST METHOD FOR PORTLAND-CEMENT CONTENT OF HARDENED HYDRAULIC CONCRETE

The broad approach in C1084 is to use analytical chemical means to measure soluble calcium oxide, silica and insoluble residue. Allowing for the aggregate type and composition, the amount of soluble oxide is attributed to the cement, and used to calculate the total cement content. Similarly the insoluble material is attributed to the aggregate and used to calculate the aggregate content.The method makes the following assumptions (and qualifies itself accordingly):

• There are no supplementary cementing materials.
• Soluble calcium oxide and silica contents of cement are assumed as fixed values unless given from another source.
• Soluble silica and calcium in aggregate is assumed to be negligible (where appropriate.)

If any of these assumptions are not correct the results of the analysis are likely to be inaccurate. Many aggregates contain soluble calcium and / or soluble silica, while supplementary cementing materials are soluble. The ASTM method recommends that the type of aggregate be assessed but does not require a petrographic examination. This means that even strict compliance with the method is no guarantee of finding out what went into a given concrete sample.ASTM C 1324 - STANDARD TEST METHOD FOR EXAMINATION AND ANALYSIS OF HARDENED MASONRY MORTAR

Tests on mortar are complicated by the larger range of cementing binders used, and by the frequent addition of ground limestone or hydrated lime into the mix. The basic chemical analysis of the sample is similar to that conducted on concrete. The method also requires that a petrographic examination be carried out in order to ascertain what components have been used in the mortar, i.e. masonry cement, masonry lime, and type of aggregate. Estimates are also made of the air content, water - cement ratio and degree of hydration. All of these are used as inputs into the matrix when solving the chemical calculations. This method is somewhat empirical in that estimated values are compared with results from calculations based on assumptions and measured data. The assumptions are modified based on the observations in order to bring the two sets of information into agreement. This type of iterative practice is at the heart of engineering calculations, but is unsettling to pure scientists. What it does mean is that any set of reported results are open to some variation, the extent of which is difficult to assess, and may be large. Again, the method is not a black box that takes a limited set of inputs and returns a neat, absolute, result.WHAT IS NEEDED?

It is important that a sufficient number of concrete samples are extracted so that at least 1 kg (2.2 lb) is available for chemical analysis, with sufficient remaining for petrographic analysis. Two 4 x 8 inch cores are a minimum when concrete is being assessed.For mortar, ASTM C 1324 requires a minimum sample of 10 g. Samples extracted from at least two zones are desirable - one set from the concrete in question and another from similar concrete that is considered acceptable. It is then possible to report on differences between the concretes with some confidence, even if the absolute answers are difficult to extract.

Ideally, the concrete batch materials (cement, supplementary cementing materials and aggregates) should be provided, in which case the amount of each material in the mix can be solved as a set of simultaneous equations.

All information about the aggregate source, mill test certificates for the cements, the use of supplementary cementing materials and the age of the concrete will assist the determination. The more data and material that are provided, the narrower will be the range of error reported at the end of the analysis.