Determination of water absorption of non-sintered materials is a core laboratory procedure used to evaluate the open porosity, density, and expected durability of construction products that develop strength through chemical binding rather than high-temperature firing. This technical article explains the applicable standards, sample categories, measured parameters, and step-by-step test methods for unfired clay units, concrete masonry, gypsum products, geopolymers, and chemically bonded ceramics.
Technical Overview
Water absorption is defined as the mass of water absorbed by the accessible pores of a material, expressed as a percentage of the dry mass of the specimen. For sintered ceramics, the value reflects the residual open porosity after vitrification; for non-sintered materials, it reflects the pore network created by hydration, carbonation, compaction, or other low-temperature binding reactions.
Non-sintered materials, including unfired clay bricks, compressed earth blocks, concrete masonry units, gypsum-based products, magnesium oxychloride and oxysulfate cements, geopolymers, and chemically bonded ceramics, differ from fired products in several important ways. Their pore structures contain interconnected capillary pores and fine gel pores, they may continue to mature with curing age, and some of their binding phases are thermally sensitive. Consequently, the drying regime, saturation method, and specimen conditioning must be selected carefully so that the test does not alter the material being measured.
The principal results obtained from a water absorption determination are the water absorption percentage, apparent porosity, dry bulk density, and saturation coefficient. Together, these parameters allow engineers to assess quality consistency, predict freeze-thaw performance, estimate capillary suction behavior, and verify compliance with product standards such as ASTM C642, ASTM C140, ASTM C67, EN 772-21, and EN 12390-7.
Testing Samples
Unfired Clay and Compressed Earth Products
This category includes adobe bricks, compressed earth blocks (CEB), stabilized earth blocks, and unfired clay masonry units. These materials are typically bound by clay minerals and, when stabilized, by small amounts of cement or lime. Specimens are usually tested as whole units or as cut halves, and results are strongly influenced by forming pressure, moisture content at compaction, and curing history.
Cement and Concrete-Based Units
Concrete masonry units, concrete bricks, paving blocks, precast elements, mortar prisms, and geopolymer specimens fall into this group. Because hydration continues over time, water absorption testing is normally performed at a defined age, commonly 28 days, on specimens conditioned to a stable moisture state. Full-size units are preferred where practicable; otherwise, sawn sections representative of the unit cross-section are used.
Gypsum, Magnesium, and Chemically Bonded Ceramic Products
Gypsum blocks and boards, magnesium oxychloride floor screeds, phosphate-bonded ceramics, and other chemically bonded materials contain crystalline hydrate phases that decompose or transform at relatively low temperatures. For these products, the drying temperature is restricted, often to 40 to 60 degrees Celsius, or conditioning is performed over desiccants instead of in an oven, to prevent loss of bound water from being misreported as pore water.
Testing Items
A complete water absorption determination for non-sintered materials typically reports the following parameters:
- Water absorption after 24-hour cold immersion, expressed as a percentage of dry mass.
- Water absorption after 5-hour boiling, which accesses pores not filled by cold immersion alone.
- Water absorption after vacuum saturation, which approaches total open porosity.
- Apparent porosity, the volume of open pores expressed as a percentage of the specimen bulk volume.
- Dry bulk density, calculated from dry mass and specimen volume.
- Volume of permeable voids, derived from absorbed water volume and bulk volume.
- Saturation coefficient, the ratio of cold water absorption to boiled water absorption, used as an indicator of freeze-thaw resistance.
- Initial rate of absorption, or sorptivity, where the capillary suction behavior of the material is of interest.
Testing Methods
Step 1: Drying to Constant Mass
Specimens are dried in a ventilated oven until the difference between two successive weighings, taken at least 2 hours apart, does not exceed 0.1 percent of the lesser mass. For cementitious and silicate-based materials, the standard drying temperature is 105 plus or minus 5 degrees Celsius. For gypsum and magnesium-based products, drying at 40 to 60 degrees Celsius, or conditioning in a desiccator over silica gel, is required so that chemically bound water is not removed. After drying, specimens are cooled to room temperature in a desiccator before weighing to obtain the dry mass, recorded as m1.
Step 2: Cold Water Immersion
The dried specimens are fully submerged in clean water at approximately 20 degrees Celsius, with at least 15 mm of water above their top surfaces, for 24 hours. Each specimen is then removed, and surface water is removed with a damp cloth using a single wiping motion to establish the saturated surface-dry condition. The saturated mass is recorded as m2. Where the immersed, hydrostatic mass is required, the specimen is weighed while suspended in water, recorded as m3, and this value also provides the specimen volume by buoyancy.
Step 3: Boiling Method
For materials that are not thermally sensitive in the pore-filling stage, specimens from the immersion stage are boiled in water for 5 hours, then allowed to cool in the water for at least 14 to 24 hours to a temperature of 20 to 30 degrees Celsius. The boiling process displaces trapped air from fine pores and yields the boiled saturated mass. The saturation coefficient is then calculated as the ratio of the 24-hour cold absorption to the 5-hour boiled absorption.
Step 4: Vacuum Saturation Method
In the vacuum method, dried specimens are placed in a vacuum chamber and the pressure is reduced to below about 5 kPa and held for a defined period, commonly 15 minutes to 2 hours, to extract air from the pore network. Water is then introduced while the vacuum is maintained, followed by a soaking period at atmospheric pressure, typically 1 to 2 hours. This method is preferred for dense or fine-pored non-sintered materials where boiling could cause microcracking or where a close estimate of total open porosity is required.
Step 5: Calculations and Reporting
The water absorption is calculated as W equals (m2 minus m1) divided by m1, multiplied by 100 percent, where m1 is the dry mass and m2 is the saturated surface-dry mass after the selected saturation procedure. Apparent porosity is calculated from the volume of absorbed water, using the density of water, divided by the bulk volume of the specimen derived from geometric measurement or hydrostatic weighing. The dry bulk density equals the dry mass divided by the bulk volume. The report must state the material type, curing age, drying temperature, saturation method, individual and mean values, and any deviations from the reference procedure.
Applications
Water absorption data for non-sintered materials serve a wide range of engineering and commercial purposes:
- Production quality control for block plants, earth-block manufacturers, and precast producers, where absorption limits are specified in product certification.
- Durability assessment, since high absorption and unfavorable saturation coefficients correlate with reduced freeze-thaw resistance and increased risk of salt damage.
- Compliance verification against product standards such as ASTM C90 for loadbearing concrete masonry, ASTM C67 as adapted for unfired units, and EN 771 series requirements for masonry elements.
- Research and development of geopolymers, stabilized earth mixtures, and chemically bonded ceramics, where absorption tracks changes in pore structure caused by mix design adjustments.
- Forensic evaluation of failed masonry or flooring, in which abnormal absorption values help identify under-cured, under-compacted, or moisture-damaged material.
- Mix proportioning and acceptance testing of renders and screeds, where sorptivity influences finishing behavior and adhesive compatibility.
FAQ
Why is the drying temperature so important for non-sintered materials?
Unlike fired ceramics, many non-sintered materials contain hydrate phases, such as gypsum dihydrate or magnesium oxychloride salts, that release chemically bound water at temperatures between roughly 60 and 120 degrees Celsius. Drying at the standard 105 degrees Celsius would remove this bound water, inflate the apparent absorption, and potentially damage the binder structure. Selecting a material-appropriate drying temperature is therefore the single most critical decision in the procedure.
What is the difference between boiling and vacuum saturation?
Boiling drives air out of pores by sustained heating and rolling of the water, while vacuum saturation extracts air mechanically by reduced pressure. Vacuum saturation is gentler, faster, and better suited to thermal-sensitive binders and very fine pore structures; boiling is traditional for masonry units and is embedded in many legacy standards. Both aim to fill all accessible open pores, and well-designed comparative testing shows the two methods usually agree within a few percentage points for coarse-pored materials.
What does the saturation coefficient indicate?
The saturation coefficient is the ratio of water absorbed during 24-hour cold immersion to the total absorbed after boiling. A low coefficient means a large reserve of unfilled pores remains after freezing-condition exposure, giving space for ice expansion and generally indicating better freeze-thaw durability. Values approaching 1.0 suggest that nearly all accessible pores can fill with water and signal higher frost susceptibility.
How many specimens should be tested?
Most standards require a minimum of three to six representative specimens per sample lot. Because non-sintered products can be heterogeneous due to compaction gradients and curing differences, testing several units taken from different positions in the production run provides a statistically meaningful result and identifies variability that a single specimen would miss.
Does curing age affect the result?
Yes. Cementitious, geopolymer, and magnesium-based materials continue to refine their pore structure as binding reactions proceed. Testing at 28 days, or at the manufacturer's specified age, ensures comparability between lots. Early-age testing generally shows higher absorption because fewer reaction products have filled the capillary network.